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5G HANDBOOK
WELCOME TO OUR FIRST 5G HANDBOOK 5G is more than another wireless technology. It’s changed the radio, the access network, the spectrum, and the network core. It might even change our lives.
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That’s why EE World created 5G Technology World. We cover 5G technologies on engineering terms. The site launched in January 2020, just as COVID-19 emerged. Since then, we’ve grown into a resource you can trust for solid technical information relating to most aspects of 5G. Why most? Because we hear much about how 5G covers computing, software, IT issues, and business applications. That’s not us. People who work higher up the protocol stack assume that the technology below it works perfectly all the time. We don’t. The articles in this ebook cover a wide range of 5G topics. We look at the practical issues of radio and network design and test. From the wireless perspective, “How to improve 5G coverage and capacity” looks at how engineers can overcome signal limitations imposed by mmWave signals on the user side. Available bandwidth limits capacity. That’s where dynamic spectrum sharing helps, explained in “DSS lets 5G and LTE share spectrum.” When it comes to base stations, “The challenges of building a 5G base station” covers the physical layer of 5G radios. That layer includes the radio’s DSP, often called the higher physical layer. Two articles in this handbook cover timing, which gained importance in 5G. “Meet timing requirements in 5G networks” addresses RF timing, which must be stable enough to meet network time error requirements. “How timing sources synchronize open RAN networks” explains that because timedivision duplex (TDD) depends on an understanding of timing, the network uses IEEE 1588 Precision Timing Protocol to synchronize transmitted and received signals. While timebased synchronization is becoming more common in the network, frequency-
based synchronous Ethernet (SyncE) is also critical to system synchronization. In Open RAN, some functions need not reside at the tower. They can reside elsewhere. That’s the functional-split concept. “Functional Splits, the foundation of an Open 5G RAN” explains the tradeoffs in locating functions in different locations. “Open RAN functional splits, explained.” And article not found in the handbook but available on 5G Technology World and EE World, continues the discussion. “Deploying and maintaining an Open RAN network” takes you through the components of Open RAN. Base-station electronics use power, generate heat, and add weight to towers. “Size, weight, power, and heat affect 5G base station designs” takes you into the radio, stressing why these issues pose new design challenges with 5G. IoT devices are coming to 5G and they need antennas. “5 tips for designing with embedded antennas” shows you how to connect and position antennas in your device. 5G’s wireless technology differs from that of 4G and “IoT: How 5G differs from LTE” explains those differences. Remember that those who work at the upper protocol layers assume everything below works perfectly. To assure that, engineers need to test components, subsystems, systems, and networks. Devices that use mmWave signals require over-the-air testing, as “Why 5G needs over-the-air testing” explains. The 5G New Radio must comply with industry standards, covered in “5G radios increase emphasis on compliance testing.” Finally, “Simulate, test, and verify to solve 5G RF design Problems” brings the mmWave design cycle together. You can read these articles, plus others, at 5gtechnologyworld.com and eeworldonline.com. We always welcome contributed technical articles. Contact me at mrowe@ wtwhmedia.com. Let’s talk.
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CONTENTS 5G HANDBOOK • MAY 2021
02
Welcome to our first 5G handbook
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Why 5G needs over-the-air testing
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Open radio access networks offer advantages in locating network functions of proprietary RANs. Automation and orchestration let telecom networks do what computing networks have dome for years.
Over-the-air testing requires different antenna setups for 5G FR1 and FR2. Temperature is a significant factor in calibration and validation.
Simulate, test, and verify to solve 5G RF design problems
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The challenges of building a 5G base station
41
5 tips for designing with embedded antennas
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5G radios increase emphasis on compliance testing
Performing these three steps can improve mmWave and beamforming performance.
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IoT: How 5G differs from LTE
5G extends its scope beyond consumer to many new vertical and enterprise markets. Thanks to its flexibility and improved performance, 5G opens the door to many industrial applications.
Size, weight, power, and heat affect 5G base station designs
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To meet 3GPP specifications, a 5G New Radio (NR) implementation must meet demanding processing requirements and RF capabilities. Compared to LTE, this results in a need for higher performing, more flexible 5G NR hardware.
Designing an antenna into a wireless embedded or IoT device requires special care to maximize performance.
Compared to 4G and previous generations, 5G’s mmWave frequencies and tight integration increase the complexity of both performance and regulatory compliance testing.
Engineers designing 5G base stations must contend with energy use, weight, size, and heat, which impact design decisions.
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Deploying and maintaining an Open RAN network
DSS lets 5G and LTE share spectrum
47
Meet timing requirements in 5G networks
How to improve 5G coverage and capacity
50
How timing sources synchronize open RAN networks
Dynamic spectrum sharing of 5G and LTE networks addresses the need for spectrum, particularly at mid-band frequencies. Here’s how it works.
The laws of physics work against RF engineers, forcing design tradeoffs in mmWave systems. Beam steering, frequency reuse, and greater spectral efficiency can help.
5G needs tighter timing requirements than do 4G networks. The timing must perpetuate from the radio throughout the telecom network core.
IEEE 1588 PTP and SyncE protocols keep radio units, switches and distribution units in sync.
Functional Splits, the foundation of an Open 5G RAN
The open standards for radio access networks offer options for locating network functions. These functional split options depend on network services and available transport links. 5Gtechnologyworld.com
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WHY 5G NEEDS OVER-THE-AIR TESTING
Over-the-air testing requires different antenna setups for 5G FR1 and FR2. Temperature is a significant factor in calibration and validation. As 5G is brings changes in wireless communications to consumers and businesses, it also brings changes in how engineers perform tests. The tight integration in 5G user equipment means that wired testing is no longer viable. Thus, over the air (OTA) testing is now essential. How you set up these tests greatly affects test results. Temperature plays a role, too. In 3G and earlier wireless networks, wireless terminals, handhelds, cell phones, and UEs were tested using hard-wired or cabled connections. This was sufficient because the main cause of transceiver failures were high mobile speeds (also known as Doppler effects) causing shorter coherence times (the time when a channel is constant). This produced estimation errors and a high delay spread, which caused inter-symbol interference due
to large echoes of the signal. None of these issues are antenna dependent. 4G introduced multiple input-multiple output (MIMO), an antenna technology where multiple antennas appear at both the source (transmitter) and destination (receiver). On top of the Doppler and delay spread, a device’s transceiver needed testing against correlation: a metric of how well two or more signals can be separated in the same frequency band. Correlation is a function of the antenna and propagation, thus in addition to propagation, tests must include the antennas. Therefore, over-the-air methodologies (Figure 1) became mandatory to test these devices. In 5G, the antenna is even more predominant because of massive MIMO, which uses the spatial domain to deliver the signal. Therefore, antennas (now an antenna array) are an integral part of the transceiver performance.
JP Nuutinen, Spirent Communications
WHAT IS MIMO OTA TESTING? MIMO over the air (OTA) testing lets engineers test a device in a controlled and accurate environment by subjecting it to a realistic propagation environment that encompasses temporal and spatial dispersion. MIMO OTA is the only way to holistically test a device. 5G MIMO OTA is subdivided into two main categories: FR1 (Frequency range 1, fc at or below 7.125 GHz) and FR2 (Frequency range 2, fc at or above 24.25 GHz). In both cases, a MIMO OTA test system consists of the following components: • A network emulator, capable of making an active connection to the device under test (cell phone, tablet), • A 5G channel emulator, capable of creating the desired propagation environment, • Power amplifier modules, • Frequency converters (for FR2) • An anechoic chamber. A simplified schematic is shown in Figure 2.
Figure 1. Device tests: OTA tests the entire device performance whereas conductive tests are a subset of OTA tests, omitting antennas and antenna arrays.
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OTA TESTING
Figure 2. Schematic of an FR2 MIMO OTA system, which includes a gNB emulator, channel emulator, and anechoic chamber with the device under test.
Critical to the design of such a chamber is correct radiator antenna probe placement. Probe locations (and power weights) are defined by propagation models. In industry standards, a subset of 5G channel models is used to optimize the probe locations to create the appropriate signal distribution in space [Ref. 1]. Thus, designing a MIMO OTA system always requires a deep understanding of radio channel modelling [Ref. 2].
TESTING 5G NR FR1 VS. 5G NR FR2 In 5G NR FR1, MIMO OTA tests are compatible to 4G tests; the only difference is that the probe layout is a 2D ring using 16 dual polarized probes in a circle, while 4G MIMO OTA uses eight dual polarized probes in a 2D ring (Figure 3). The main motivation of this choice was the compatibility to 4G and the fact that 5G NR FR1 devices are
primarily used in MIMO mode, not beamforming mode. The key device performance characterization metric, or figure of merit (FoM) used in both 4G LTE and 5G NR FR1 is spatial correlation. FoM is function of angular spread and antenna array element orientation and separation. In 5G NR FR2, the probe layout is completely different due to design around a different FoM called power angular spectrum similarity percentage (PSP), which expresses the beamformer’s ability to estimate the appropriate power angular spread. PSP characterization can be achieved in a much smaller footprint and results in the configuration of a 3D wall optimized to support two channel models from the 3GPP 5G channel model definitions. The layout is heavily affected by gNodeB beamforming. A typical gNodeB is an 8x16 array, which results in high spatial filtering when applied to any channel model. After this spatial filtering occurs, only 1-2 clusters remain. Therefore, only a small wall is sufficient for probe placement and the size of the associated chamber is also relatively small. If a device under test is placed in radiated near field conditions, only nulls are affected. Many studies show that a 75 cm distance is sufficient to accurately test a handheld device with this method [Ref. 3].
Figure 3. 2D probe layouts for 5G NR FR1 and 4G LTE MIMO OTA testing.
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Figure 4. 3D probe layout for 5G NR FR2 testing.
MIMO OTA TEST SYSTEM DESIGN
In the design of any test system, the first thing to consider is link budget. That is, does the device under test and the network emulator get sufficient power levels to receive and decode the signal and is there enough margin to vary the signal levels? In OTA tests, the main consumer of the link budget is the air link between the probe antenna and device under test. Thus, you need a power amplifier to compensate for the large air loss. This is specifically important in a 5G NR FR2 system, where path loss is very high. Next, you must choose propagation models. The propagation models define how the probe antennas are distributed and weighted in the chamber. The original model needs mapping to a spatially discretised probe-antenna constellation. This requires a deep
understanding of the channel models and how to translate that into chamber design. The system needs calibration and validation. Calibration ensures alignment of phase and amplitude in the test volume (area where the device under test is placed). Calibration is time consuming and requires expertise on using the instrumentation. Validation, on the other hand, is the process that verifies the system creates the desired propagation conditions (level, polarization, temporal, and spatial aspects) in the test volume. Both system understanding as well as propagation testing skills are needed for this exercise. Typically, a MIMO OTA system is unidirectional, meaning only downlink signals are faded. That’s because MIMO OTA stresses the device receiver. Uplink signals aren’t faded, simply because your goal
Figure 5. Throughput vs power level results for a selected channel model.
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OTA TESTING
isn’t to test the network emulator. It is, however, mandatory to have an active connection to the device under test. Thus, both the uplink and downlink must be part of the test system. How, then, do you separate the uplink from the downlink in the system? For FR1 tests, you should use a communication antenna located close to the device under test. Engineers have used this method for many years with 4G MIMO OTA testing. For 5G NR FR2, however, the device under test uses beamforming. Signals from the DUT are spatially selective and some directions are less favorable for establishing a connection than others. Thus, you can no longer use a single (or dual) communication antenna for the uplink. Instead, you must use multiple antennas to establish the uplink connection. Alternatively, you can use the same antennas for the uplink and downlink, but that requires precise timing to switch between the two directions. In principle, the switch should be synchronized to transmitted slots and must be configured to different uplink and downlink transmission structures (i.e., heavy downlink traffic, heavy uplink traffic, balanced traffic). Next, you must consider phase drift over the time. All instruments in the test system should connect to the same master clock (or 10 MHz reference signal). Doing so becomes important in 5G NR FR2 because the DUT uses a beamformer. Beamforming results from controlling the phase (and amplitude) of the transmitted signal, thus any phase change in the test system will modify the received signal. Therefore, you must ensure that phase is stable over time, with no drift introduced; this is addressed as part of the system validation and bring up. At Spirent, we found that temperature is the most important factor in maintaining phase stability. If the room temperature remains constant, the phase will remain stable for days. Temperature is the main reason for abrupt phase changes in instruments. Lastly, all MIMO OTA test systems need software, commonly known as the test executive. The test executive not only takes care of running the test cases, but also calibration, validation, progress monitoring, reporting test results, and informing the user of any failures.
REFERENCES 1. Standard 5G channel propagation models are defined in several 3GPP specifications, dependent upon the frequency range and environment of interest - https://www.3gpp.org/specifications-groups/ran-plenary/ran4radio-performance-and-protocol-aspects 2. J.P. Nuutinen, D. Reed, A. Rodriguez-Herrera,”5G MIMO OTA Testing on Frequency Range 2 (FR2)”, URSI GASS 2020, Rome, Italy http://www.ursi.org/proceedings/procGA20/papers/ URSIGASSSummaryPaperNuutinenReedHerrera.pdf 3. 3GPP RAN4, “R4-1915062 Range length and probe layout considerations in 5G NR FR2” Spirent Communications, November 2019, Reno, TX https://portal.3gpp.org/ngppapp/TdocList. aspx?meetingId=32851
JP Nuutinen is an industry-renowned subject matter expert in radio channel emulation with over 20 years of experience in engineering and research and is currently focused on cellular base station (gNodeB) and device test methodology for 5G technologies. Born in Valkeakoski, Finland, JP holds both Masters and Licentiate Technology degrees from Tampere University of Technology. He has more than 20 granted patents in addition to over 30 authored or co-authored publications.
TESTS AND RESULTS Typical FR2 MIMO OTA tests cover throughput vs. power (i.e., energy per resource element or EPRE) and throughput vs. signal over interference and noise ratio (SINR) with varying channel models. The plot in Figure 5 shows how the throughput increases as power level increases until reaching a plateau. The architecture of a MIMO OTA test system is determined by the target frequency range. With 5G NR FR1, the test chamber size is large (roughly 16 ft W x 10 ft H x 16 ft D or 5 m x 3 m x 5 m) and typically requires customized construction and a great deal of space to be accommodated. Therefore, a system integrator typically needs to lead the delivery. However, with 5G NR FR2, the chamber size is relatively small (roughly 3 ft W x 4 ft H x 2 ft D or about 1 m x 1.2 m x 0.6 m) so that the complete system can fit into a standard lab space.
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SIMULATE, TEST, AND VERIFY TO SOLVE 5G RF DESIGN PROBLEMS Performing these three steps can improve mmWave and beamforming performance. Xiang Li, Keysight Technologies
The year 2020 saw the first deployed 5G networks that use millimeter-wave (mmWave) technology. Tests have found that mmWave achieved greater than 1 Gbps downlink speed and up to 3 Gbps peak speed [Ref. 1]. The good news: download
Figure 1. Signal output power from an amplifier can include spurious harmonics interference.
speed on a 5G mmWave network is 20 times faster than on a 4G LTE network. Tests have also found that 5G mmWave can cover an entire football stadium - including the parking lot - with only one frequency band to deliver ten times throughput capacity than LTE networks. The downside: mmWave signals have a shorter range and don’t penetrate obstacles compared to LTE and 5G low-band and midband frequencies. The benefits of mmWave, however, far outweigh these drawbacks. Massive worldwide deployment of mmWave is happening. The real question is how do we minimize these impediments of mmWave when it comes to 5G device designs?
SPURIOUS HARMONICS AND INTERMODULATION INTERFERENCE Nonlinearities in power amplifiers and mixers crate unwanted signals that appear outside the assigned channel, that interfere with other channels. We refer to this intermodulation interference as spurious harmonics. In Figure 1, all frequencies outside of the frequency band of interest (the fundamental frequency) become unwanted spurious harmonics. Removing spurious harmonics from mmWave signals is difficult because of the short wavelength. Ignoring this issue in device design could violate FCC emission rules on allowable effective isotropic radiated power (EIRP) [Ref. 2]. From an operational point of view, interfering with nearby antenna
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signal beams could cause other wireless communication devices to malfunction. Just as with other noise signals in wireless communication, you can’t eliminate spurious harmonics. Identifying the source of harmonics and the interference they cause is difficult. Even once you identify the source, you may have a difficult time reducing its impact.
PERFORMANCE VERIFICATION WITH BEAMFORMING The advantages of mmWave come not only from wider spectrum availability, but from more intelligent radio resource management methods such as beamforming. The complexity of the phased-array antenna system that supports multiple connections simultaneously requires a reliable performance verification process. Engineers need to consider all realworld scenarios and verify that a design works properly before deployment. One specific example involves accurately verifying the performance of the beamforming signal and antenna in the presence of proper 5 • 2021
channel models. Because base stations may use a complicated phased-array digital, analog, or hybrid beamforming technology, the connecting 5G devices may use different types of antennas depending on the applications. Therefore, verifying the connection process to find the optimal transmission channel between a base station and a 5G device is crucial, although difficult. With the ultra-short wavelength of mmWave, base stations must perform a computationally intensive process of baseband precoding to select the optimal precoding modulation to apply to the signal streams for each user. To achieve the best performance, engineers also need to verify 4G and 5G compatibility. Both the 5G infrastructure and devices must support dual-mode 4G and 5G operation to provide a quality user experience in mixed-deployment networks. Furthermore, we still have pre-coding algorithms, RF phased-array multiple-input and multiple-output (MIMO) system architecture
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mmWAVE DESIGN PROCESS
manufacturing techniques without using exotic packaging processes and materials.
PROTOTYPE TROUBLESHOOTING COMPLEXITY
Figure 2. This lab environment lets engineers test mmWave device designs.
at the base station, and multiple handset antenna placement and radiation patterns to verify. If any of these aspects of 5G mmWave were measured incorrectly, the whole design will fail. Once again, due to the short wavelength of mmWave, setting up the verification can be very challenging as well. A slight misalignment of the equipment can make a significant difference in the results. Figure 2 shows a common mmWave device laboratory testing environment. The device under test is in the middle box, which will radiate a signal to the reflector (curved object on the right), then arrive at the receiving antenna, not shown in this photo. The red line represents the signal path. Assume the test is under 100 GHz. That means the wavelength of the signal is around 3 mm, and any misalignment around 3 mm will have a big impact on the test result’s accuracy. In practice, one unnoticeable dent on the reflector, or if the curvature of the reflector is off by a small fraction can lead to a false measurement. Thus, it usually takes a long time to setup mmWave verification testing and calibrate all the testing equipment. A real-world phaselocked loop measurement can take six hours to set up and calibrate the hardware. Completion of all verification tests can take considerably longer.
COMPACT PACKAGE ISSUES Recall that mmWave signals have a very high path loss, meaning that they cannot travel far, nor can they penetrate obstacles such as walls or trees very well. To provide high data rates and support more traffic, 5G base stations use large amounts of energy. More cost and energyefficient 5G base stations ease the burden. Producing more cost-efficient and energy-efficient base stations sounds relatively easy. Not so. 5G mmWave base stations support much wider spectrum than 4G base stations. Plus, 5G mmWave base stations must support many more features than 4G base stations. Our goal as engineers is to make 5G mmWave base stations as compact and as cost effective as possible, without sacrificing any performance. To provide a better sense of the dimensions involved, Figure 3 is a size comparison between a phased-array chipset and a coin. The parts in Figure 3 represent only a small part of a 5G picocell. Ultimately, a fully functional 5G mmWave picocell should achieve overthe-air rates of 100 Gb/s with kilometer coverage. In terms of reducing deployment cost, designers should leverage low-cost and traditional 5Gtechnologyworld.com
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This challenge may not sound as technical as any of the previous challenges, but in practice it is just as prevalent, and the impact could be worse than any of the technical challenges. As mentioned earlier, mmWave device design requires engineers to measure and verify as many RF device characteristics as possible, and there are already many technical design challenges you need to consider before a prototype device goes to the verification phase. Furthermore, as the complexity of wireless technology and application grows, it requires different skillsets to do device design and device verification. Many companies these days often have designated - but separate - departments to work on design and testing. In some cases, design teams and testing teams are even located in different countries as Figure 4 shows. Therefore, exchanging ideas, data, and knowledge between design and test teams might not be very straightforward. Additional time maybe required due to knowledge, functional, and even geographical gaps. For instance, test engineers can perform antenna measurements, RF parametric, and function/protocol testing. Performance issues not caused by the testing setup can prove difficult to troubleshoot. Design engineers and test engineers to must collaborate to resolve any design issue.
SIMULATION CAN HELP Simulation lets engineers tackle the challenges posed by mmWave. Our dependence on simulation will grow as mmWave devices become more popular. Simulation may not solve all the challenges outlined above, but it can certainly simplify the design process. Beginning with spurious harmonics and intermodulation interference, designers can use simulation tools to predict the frequency and direction. Simulation can also help to identify the root cause of spatially radiated spurious harmonics, including the components that generate them and the signal paths used in the RF chain. Designers can also simulate characterization of the spurious harmonics to assess their impact. Regarding performance verification with beamforming, simulation tools can greatly reduce development time by addressing the signal visualization and verification of many different aspects of 5G end-toend systems. For example, a simulation tool can simulate 5G link-level validation with a proper phased array antenna to verify the performance of the beamforming design. 5G simulation tools can be used to optimize time, frequency, and spatial resources. For base station cost efficiency and footprint, designers can use simulation tools to simulate major component blocks of their design, then do a proper linearity and noise figure and gain simulation for the entire system. System-level modeling and simulation allows designers to test their designs against multiple variables, without physically setting up testing equipment. Simulation can also help with prototyping complexity. Design engineers can simulate their design under over-the-air conditions to find discrepancies and predict results. If the simulation and the testing
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environments are built on a common platform, it is much easier for test engineers to tell if the simulation agrees with the tested results. This makes it much easier to troubleshoot remotely if the design engineers and test engineers are not in the same physical location.
CONCLUSION Higher frequency mmWave spectrum can deliver much faster data rates with low latency, while offering greater traffic capacity. These advantages of mmWave unleash the true potential of 5G. In the 5G era, mmWave will play a very important role. It will be heavily used in urban cities, indoor office spaces, transportation hubs, and the industrial Internet of Things. While mmWave does bring significant design challenges to the table, its widespread deployment is inevitable. Device makers and network equipment manufacturers can accelerate their mmWave design cycles by adopting more simulation solutions in their design workflows. At the same time, they also need to improve simulation accuracy, and connect design simulation and prototype test workflows. Simulation provides the shortest path to mmWave market-ready products without extensive investment or sacrificing performance.
Figure 3. A phased-array chipset makes up some of the components in a picocell.
REFERENCES 1. Deploying mmWave to unleash 5G’s full potential, Qualcomm, Nov. 10, 2020. https://www.qualcomm.com/news/onq/2020/11/10/ deploying-mmwave-unleash-5gs-full-potential 2. Guidelines for Determining the Effective Radiated Power (ERP) and Equivalent Isotropically Radiated Power (EIRP) of a RF Transmitting System, Federal Communications Commission Publication 412172, https://apps.fcc.gov/eas/comments/ GetPublishedDocument.html?id=204&tn=255011
Xiang Li, Industry Solution Marketing Engineer, Keysight Technologies. Xiang is an experienced wireless network engineer with bachelor’s and master’s degrees in electrical engineering from the University of Manitoba.
Figure 4. Gaps in mmWave development workflow can hinder communication between test engineer and design engineers.
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5G VS. LTE
IoT: HOW 5G DIFFERS FROM LTE 5G extends its scope beyond consumer to many new vertical and enterprise markets. Thanks to its flexibility and improved performance, 5G opens the door to many industrial applications. Guillaume Vivier, Sequans Communications When researchers and engineers began developing 5G in 2012, they began to look at use cases. The primary motivation for launching a new generation of wireless technology was insufficient spectrum. Most industry analysts predicted an explosion of data traffic that would result in saturation of existing spectrum resources. Another motivation arose from the expected tremendous growth in the number of connected devices including many new device types for machine type communications (MTC) and Internet of Things (IoT) applications. This expansion gave rise to a variety of requirements that 4G missed. Here’s how 4G and 5G compare in relation to IoT. 5G’s definition took the shape of a now famous triangular icon with three sides depicting the three main 5G components. The triangle was subsequently modified, reused, and adapted by many companies throughout the wireless industry. Figure 1 summarizes the three 5G use cases. • eMBB: enhanced Mobile Broadband. Somewhat the same as 4G, but with faster speed and larger capacity. eMBB supports the accelerating growth in the number of consumer devices and to mitigate the expected saturation of 4G networks. • URLLC: Ultra Reliable, Low-Latency Communications. URLLC fulfills requirements of vertical market segments such as industrial, health, transportation, and aviation that have high demands for low latency and high reliability. These new use cases came from stakeholders outside of the traditional telecommunications world, such as automotive and energy. • mMTC: Massive Machine-Type Communications. mMTC supports a massive number of connected objects. While not necessarily requiring high data rates or low latency, these connected objects have other demanding requirements such as ultra-long battery life, small footprint, and simplicity needed to enable connections for almost any kind of object. With so many use cases and requirements, 5G needed versatility and support for these generally non-compatible requirements. These requirements were key in driving innovation in 5G design. The new requirements imposed on 5G include not only a new radio (NR) interface. 5G adds an evolution in core network principles: convergence of wireless and wireline systems, new radio access networks (RANs) and new telecom network core architectures. These aspects are beyond the scope of this article.
5G DESIGN PRINCIPLES To meet the three main use case requirements, 5G NR needed more flexibility and higher efficiency than 4G while providing more capacity,
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higher speed, and lower latency. To improve the capacity, concluded Shannon, one must either increase the bandwidth or improve the signalto-interference-and-noise ratio (SINR) [Ref. 1]. 4G and 5G differ in their use of spectrum. To increase the bandwidth for cellular, regulatory agencies look to repurpose spectrum from other uses. In the U.S., for example, frequencies formerly used for broadcast TV are allocated for cellular. While 4G resides mainly below 3.8 GHz, 5G uses bands below 6 GHz (frequency range 1, FR1) and 24.25 GHz to 52.6 GHz (FR2). In 4G, the use of unlicensed spectrum was introduced later in development with LTE assisted access (LAA) and LTE in unlicensed bands (LTE-U). In 5G, the use of unlicensed spectrum was considered early, under the NR-U (new radio unlicensed) name. More efficient modulation schemes can increase spectral efficiency, resulting in the delivery of more bits per hertz. 5G uses 256 QAM and 1024 QAM, which provides greater spectral efficiency than lower-order modulations. New waveforms, generalization of multiple-input, multipleoutput (MIMO) antenna schemes, and the introduction of improved forward error correction (FEC) techniques help to improve SINR. Many of 4G’s principles continue in 5G. For example, OFDM, OFDMA, and MIMO all came from 4G, and the protocols are almost identical [Ref. 2,3,4].
5G USAGE SCENARIOS
Figure 1. The original 5G triangle, from Recommendation ITU-R M.2083 [ITU-R, IMT Vision—Framework and overall objectives of the future development of IMT for 2020 and beyond]. Recommendation ITU-R M.2083, September 2015, https://www.itu.int/rec/R-REC-M.2083/en]
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5G HANDBOOK LTE VS. 5G Because flexibility was deemed necessary to meet the needs of a wide variety of new use cases, the time-frequency grid must accommodate different numerologies µ (from 0 to 4), corresponding to the subcarrier spacing (SCS) of OFDM symbols. Numerology 0 refers to a subcarrier spacing of 15 kHz (same as LTE). Numerology 1,2,3 and 4 correspond respectively to 30 kHz, 60 kHz, 120 kHz, and 240 kHz), resulting in different slot durations (the number of OFDM symbols in a slot is kept constant at 14). Table 1 summarizes the numerology. With this flexibility, the NR frame design can accommodate low latency traffic (using very short slot durations), as well as variety of frequency bands (the higher the frequency, the higher the SCS).
TABLE 1. VARIOUS NUMEROLOGIES IN 5G NR Numerology (µ)
0
1
2
3
4
Subcarrier Spacing (kHz)
15
30
60
120
240
slot duration (ms)
1
0.5
0.25
0.125
0.0625
Nb slot per subframe
1
2
4
8
16
Nb slot per frame
10
20
40
80
160
OFDM symbol duration (µs)
66.67
33.33
16.67
8.33
4.17
Cyclic Prefix duration (µs)
4.69
2.34
1.17
0.59
0.29
OFDM symbol duration + CP
71.35
35.68
17.84
8.92
4.46
4G LTE protocol has two main frame structures: FDD and TDD. In contrast, 5G NR has 56 slot formats currently defined that operate in either duplex mode, FDD, TDD, or even in self-contained slots that contain downlink and uplink symbols. Such self-contained slots enable fast communication on the air interface, minimizing the transmission time interval (TTI). Beamforming shows another difference. In 5G NR, all signals are beamformed, which provides better reach and limits the overhead (pilots are transmitted only when needed). The pilot structure is flexible, allowing adaptation to the channel characteristics. Front-loaded pilots let channel estimation occur first, and then demodulate received data symbols on the fly, for faster demodulation. Additional innovations accommodate operation in mmWave bands. For instance, dedicated pilots such as phase reference symbols (PRS) counteract harmful phase noise. 5G also introduces low density parity codes (LDPC) as forwarderror-correction codes for the data channels and polar codes for the control channels. Though polar codes are quite novel, LDPC were already being used in Wi-Fi. While NR design didn’t introduce anything revolutionary, it is a better version of 4G. It can deal with larger bandwidths and higher frequency bands that LTE. The first definition of 5G NR in 3GPP was made in the context of Release 15, completed in December 2017. For this release, the focus of standardization was on the eMBB use case, with some enablers for URLLC. 3GPP identified solutions for mMTC as LTE-M and NB-IoT, which were defined in Release 13. This raises more general questions about support of IoT in 5G.
5G IOT SUPPORT 4G LTE introduced MTC, which refers to two non-smartphone objects
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communicating with each other. MTC was originally considered only for low-data-rate devices and applications, generally known as IoT. 5G NR opens the door for communication of more sophisticated and higher data rate objects that must meet stricter latency and reliability requirements. This corresponds to the URLLC side of the ITU triangle (Fig. 1). These more demanding objects are sometimes referred to as industrial IoT or critical IoT objects to distinguish them from low profile IoT objects, mMTC. 3GPP defined 4G LTE in 2012 with Release 8. It was then improved in subsequent releases, adding higher throughput and more features. Release 13 (2016) added two new flavors specifically defined to address IoT: Category M (LTE-M) and narrowband IoT (NB-IoT, category NB). The former operates in regular LTE deployments, using the smallest possible channel size (1.4 MHz) and the latter operates in a 180 kHz channel. That lets it be deployed in standalone mode (typically reusing GSM channels), in regular LTE bands, or within LTE guard bands. Think of LTE-M and NB-IoT as stripped-down versions of regular LTE, with the design target being low cost, improved (indoor) coverage, and very long battery life. That’s needed for battery-powered IoT applications - utility meters, wearables, alarm panels, and asset trackers. Primary design objectives of LTE-M and NB-IoT include: • Reduced cost, smaller footprint: LTE uses two antennas on the device side. LTE-M and NB-IoT use one, simplifying signal processing. Smaller channel sizes further simplify processing. Eliminating the duplexer (the specific filter that protects the receive path from the transmit signal) also simplifies the design in half-duplex FDD (HD-FDD), the mode used in LTE-M and NB-IoT. This simplified design lets a single hardware design operate globally. • Improved coverage. Removing one antenna negatively impacts receiver sensitivity. To compensate for this loss and improve the coverage (as necessary for deep indoor deployments such as smart meters), coverage enhancement (CE) modes were introduced. CE modes are simply signal repetitions, a low-cost technique for improving SINR. • Long battery life. New power-saving schemes and protocol optimizations let IoT devices enter deep sleep as fast as and for as long as possible, resulting in reduced power consumption. LTE-M is richer in capability than NB-IoT. It supports mobility, VoLTE, and a data rates up to 1 Mb/s while NB-IoT is limited to 30 kbps. NB-IoT achieves theoretically better coverage and lower power consumption. In Releases 14 and 15, 3GPP improved LTE-M and NB-IoT from their initial release. When 3GPP submitted its proposal to ITU for 5G, it submitted NR for eMBB and URLLC, while LTE-M and NB-IoT were accepted as already meeting the requirements for the mMTC aspects of IMT-2020. 3GPP introduced efficient solutions to connect classical objects. Depending on the application, the user can select the most appropriate cellular technology to connect objects, per application, as illustrated in Figure 2. LTE-M and NB-IoT are officially part of 5G. How can we ensure that all deployed objects using LTE-M, NB-IoT, or even LTE Cat 1 have support going forward? In many applications, IoT connected objects are expected to live in the field for many years (utility meters) and some were originally designed to operate with a 4G core network. The 5G core brings improvement, especially with respect to high-end quality of service, but does not bring any specific benefit to low-end IoT (and more problematic, some power optimization features
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5G VS. LTE 5G & 4G IoT APPLICATIONS BY CATEGORY
REFERENCES
NB-IoT | LTE-M | CAT 1 | CAT 4
1.
Michelle Effros and H. Vincent Poor, “Claude Shannon: His Work and Its Legacy,” EMS Newsletter, March 2017, p.20. https://www.itsoc.org/resources/ShannonCentenary/shannon-work-legacy-paper.
2.
Bob Witte, “The basics of 5G’s modulation, OFDM,” 5G Technology World, April 16, 2020. https:// www.5gtechnologyworld.com/the-basicsof-5gs-modulation-ofdm/
3.
Bob Witte, “OFDMA improves spectrum use in Wi-Fi 6,” 5G Technology World, June 4, 2020. https:// www.5gtechnologyworld.com/ofdmaimproves-spectrum-use-in-wi-fi-6/
4.
Bob Witte, “More antennas, faster data transfer,” 5G Technology World, July 7, 2020. https://www.5gtechnologyworld. com/more-antennas-faster-data-transfer/
5.
See RP-200797.zip available at: https:// www.3gpp.org/ftp/TSG_RAN/TSG_RAN/ TSGR_88e/Docs/RP-200797.zip
6.
Packet Data Convergence Protocol (PDCP) specification (3GPP TS 38.323 version 15.2.0 Release 15), https://www.etsi.org/ deliver/etsi_ts/138300_138399/138323/15 .02.00_60/ts_138323v150200p.pdf.
7.
Dave Cavalcanti, “Five reasons why TSN over 5G promises timely deliveries,” 5G Technology World, February 2, 2021. https://www.5gtechnologyworld.com/ five-reasons-why-tsn-over-5g-promisestimely-deliveries/
8.
See RP-201310 available at https:// www.3gpp.org/ftp/tsg_ran/TSG_RAN/ TSGR_88e/Docs/RP-201310.zip
Figure 2. Cellular IoT offers a solution for every use case. were not supported in the Release 15 5G core network). There are three options for this problem: • IoT devices can support both 4G and 5G core networks, which leads to additional cost and complexity, thus wiping out the low-cost advantages. • Upgrade the devices over the air with new firmware when switching from a 4G to a 5G core, assuming an immediate transition on the network and the possibility to upload a complete firmware over the air despite a thin pipe. • Network operators can continue to support 4G core functionalities within the 5G core, allowing easy support of legacy LTE devices. Option three is the most realistic.
5G: CRITICAL AND INDUSTRIAL IOT
5G NR brings significant improvements in latency and data rate compared to 4G, and these improvements are key in meeting the strict requirements in vertical markets such as factory automation (industry 4.0), transport, energy, or entertainment, including augmented and virtual reality. Most of these improvements are defined within the context of the URLLC side of the 5G triangle. URLLC services are enabled by the flexible frame structure (allowing a very short TTI), preemptive scheduling, and anticipated retransmission for fast turn-around, grant free transmission, etc. Cellular connectivity, especially in harsh industrial environments, has an inherent advantage over Wi-Fi and even wired 5Gtechnologyworld.com
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technologies. Wi-Fi is less secure and more susceptible to interference than cellular by design and wired technologies are less flexible and more difficult to update or change in a factory layout. Thus, 5G will be a key technology for industrial applications, especially when deployed as a private network where the network owner has full network control. In Release 16, 3GPP introduced a dedicated working group to address Industrial IoT. The work item introduced improved reliability thanks to enhanced packet data convergence protocol (PDCP, an upper layer of the protocol stack) duplication, mechanisms to prioritize traffic between UEs and within a UE, and a means to support time sensitive networking (TSN) [Ref. 5, 6]. TSN is a technique introduced by IEEE 802.1 group for an Ethernet wired network that provides deterministic transmissions by synchronizing various equipment components to a single master clock [Ref. 7]. Mechanisms to ensure deterministic delays and synchronization were defined by IEEE and the objective of 3GPP was to adapt these mechanisms to the wireless and 5G world. Industrial IoT and its subsequent ongoing work item in release 17 called in 3GPP Enhanced IIoT complements URLLC and is expected to fully support the most stringent requirements of critical and industrial connected objects [Ref. 8]. 5G will not replace 4G. Both will coexist for a long time, especially for the LPWA side of IoT for which LTE-M and NB-IoT will remain the solution of choice. With 4G LTE and 5G NR, 3GPP defined a unified toolbox to support professional IoT and a wide range of applications from very simple, low data rate types of connected objects to high performance industrial and critical IoT. 5 • 2021
Dr. Guillaume Vivier graduated from Telecom Paris Tech and received his PhD degree from Sorbonne University. After various positions in Alcatel and Motorola, he joined Sequans Communications to drive innovation into products. Early 2014 he initiated 5G activity to anticipate the ongoing development of Sequans 5G products. As CTO, he is constantly identifying new technological trends and opportunities to support Sequans future growth. DESIGN WORLD — EE NETWORK
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5G HANDBOOK
SIZE, WEIGHT, POWER, AND HEAT AFFECT 5G BASE STATION DESIGNS
Dib Nath, Advanced Energy
Engineers designing 5G base stations must contend with energy use, weight, size, and heat, which impact design decisions. 5G New Radio (NR) uses Multi-User massive-MIMO (MU-MIMO), Integrated Access and Backhaul (IAB), and beamforming with millimeter wave (mmWave) spectrum up to 71 GHz. These capabilities provide massive connectivity, multi-gigabit speeds, and single-digit-millisecond latencies that help distinguish 5G from 4G and older generation wireless technologies. Unfortunately, they present design challenges focused on power, heat, size, and weight.
Figure 2. Streetlights support small cells but provide little space for radios and cables. 16
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5G NR brings fundamental changes to the gNodeB’s power amplifier (PA) and power-supply unit (PSU). These changes directly affect operators’ capital expenditures (capex), operational expenditures (opex), and their ability to provide the coverage and quality that customers demand. In 2G, 3G and 4G, the PA and PSU were separate components, each with its own heatsink (Figure 1). For a variety of reasons, many infrastructure OEMs are considering integrating the PSU into the gNodeB, where it will share a heatsink with the remote radio unit (RU) PA in so-called active antenna units (AAU). This change creates a host of design considerations and challenges.
REDUCE ELECTRICITY USE
Power consumption is one major reason for these changes. Electricity currently is 5% to 6% of a mobile operator’s opex, according to MTN Consulting [Ref. 1]. 5 • 2021
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5G BASE STATIONS Energy use will increase dramatically with 5G because a typical gNodeB uses at least twice as much electricity as its 4G counterpart, MTN says. Higher opex makes it difficult for operators to price their 5G services competitively and profitably. Some operators have tried to rein in their 5G electricity opex by using 8T8R and 32T32R MIMO systems rather than 64T64R — a compromise that can undermine performance. Even so, the additional PAs and additional signal processing needed in these MIMO AAUs drive up the power requirements, yet additional space and cooling aren’t provided. These challenges might come as a surprise because 5G is promoted as being more energy efficient than 4G. This comparison, however, is based on the number of bits of data delivered for a given unit of energy consumed. Using mmWave will require multiple small cells, which will result in higher overall energy consumption even though it’s more efficient in transmitting data than previous generation wireless technologies. Equipment manufacturers have been looking at ways to reduce this energy consumption to help operators lower their carbon footprint. For example, 4G radios are always on (e.g., transmitting reference signals to detect users), even when traffic levels don’t warrant it, such as in the middle of the night. 5G base stations can analyze traffic patterns and determine periods of low data-traffic, when it may be suitable to shut down into a “sleep mode.” An example being considered during this time is to power down the radio in the range of 5 msec to 100 msec, and then enable it to see if there are any active devices within range, ensuring that the network is always available for 911 calls and timesensitive IoT transmissions. Known as “pulse power,” this technique reduces opex by minimizing energy consumption as only the essentials of the cell site remain powered during the sleep mode. This technique will result in lower average energy consumption and result in lower operating costs for the operators. Infrastructure OEMs focus on two aspects of pulse power. First, they want to understand how these power cycles affect the overall life of the PSU. The typical expected life of an RU is in the range of 7 to 10 years. A failure of an antenna results in network downtime, compromise in network reliability, and could result in revenue loss. Second, they want to know how low power consumption can drop when the PSU is in quiescent mode. For example, when the PSU stops powering the PA, which is the main power draw, but still needs to power other electronics. The current target for low-load efficiency is about 30%. Some OEMs would like to see it closer to, for example, 10%. Equipment providers must find the minimum power required to support radio functions during the quiescent period. PSU manufacturers must minimize power consumption during this quiescent period. The PSU must immediately power-up and provide the necessary power for the radio to resume normal operation and provide this power with minimum voltage transient effects. Plus, it must survive being repeatedly switched from quiescent and normal-power modes and still maintain reliability and life specifications. During quiescent periods, the PSU must minimize all load power. It must keep basic antenna functions ready, then then go to full power when the antenna checks for active users within range, typically from 5 msec to 100 msec.
spectrum in massive MIMO antennas to deliver gigabit speeds. The higher the frequency, the shorter the signals travel, which means mmWave 5G will require a much higher density of small cells. Many of them also will need to be close to street level and thus close to people. Small cells are being deployed on utility poles and streetlights, which have limited space for radios and cables (Figure 2). Meanwhile, similar constraints on macro sites operating at traditional sub-6 GHz frequencies play a role. For example, many towers are already jammed with cables, whose weight affects their wind load and thus antenna capacity. An operator’s gNodeB product choice directly affects its ability to get the sites it needs to provide seamless coverage, which in turn affects its competitiveness. OEMs also want to limit the weight of the AAU (e.g., to less than 50 lbs/23 kg.) to ensure a single person can install it. This situation creates opportunities for engineers to design gNodeB products that minimize radio size, reduce weight, and reduce accessory weights such as those from power cables. Multiple pairs of low-gauge cables are used to distribute -48 V power to the RUs on the tops of cell tower antenna masts; they’re also used to minimize voltage drops (Figure 3). These cables are expensive, heavy, and must be supported by the cell tower in addition to the multiple antennas and other equipment. Technicians must place 5G radios supporting mmWave higher than other antennas to minimize attenuation from obstacles. Using higher voltages to distribute the power to these antennas could reduce cable weight. Higher gauge cables could distribute 120 VAC or 240 VAC, or even 400 VDC, thereby lessening the load on the antenna masts and minimizing voltage drops. Higher gauge wires lower both purchase and installation costs. As with pulse power, making this change requires understanding how the higher voltages would affect PSU designs and component life. Server OEMs perform similar research, much as the data center world considers a shift to higher voltages to lower their current consumption and opex. Mobile OEMs may be able to learn from their IT counterparts — albeit with a few caveats. For example, mobile operators typically want PSUs to be designed to last about 10 years, whereas a data center server usually is retired after about four years. Personnel safety is another consideration. Cell site installers work
Figure 1. A power-supply unit suitable for 5G gNodeB installations requires a heatsink.
DON’T GET TOO HEAVY Siting is another major reason for the PA and PSU changes. For example, in dense urban areas, 5G networks will rely heavily on mmWave 5Gtechnologyworld.com
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5G HANDBOOK
with -48V DC, so they’ll need training to safely work with higher voltages. Operators’ drive for lower opex costs coupled with meeting their climate change goals may hasten this transition.
SIZE AND HEAT Another design under consideration looks to integrate the PSU within the RU, which reduces weight and shrinks the size of the RU (AAU). In this architecture, the PSU will share the heatsink with the PA. This combination creates several design challenges, starting with heat. PAs have much lower efficiency than PSUs. That heat dissipates into the shared heatsink, raising its temperature and resulting in less available cooling capability for the PSU. PSUs that traditionally operated at 85°C will now need to endure temperatures of 95°C to 100°C, an increase that could affect component life and performance. Integration also increases the risk of signal interference, which results in poor network quality. That raises two issues with integrated PSUs:
and in electric vehicles. Power FETs designed with these technologies may allow for operation in higher baseplate temperatures and higher frequency operation, resulting in smaller designs. PSUs often get sandwiched with other components inside an AAU. Thus, engineers need low-profile components, typically under 22 mm. The challenges and opportunities surrounding embedded PSUs highlight how 5G NR compares to previous wireless technologies. OEMs that help their customers overcome these challenges will position themselves for success in the burgeoning 5G market.
REFERENCES 1. Matt Walker, “Operators facing power cost crunch,” MTN Consulting, https://www. mtnconsulting.biz/product/operators-facingpower-cost-crunch/
• Being close to the PA means the PSU will must be immune to the E-fields that the PA generates. The PSU also generates E-fields. These fields must fall within limits and not interfere with PA and other RU electronics. • Integration must not cause passive intermodulation (PIM) interference with the radio frequencies.
Dib Nath is a senior director of technical marketing, telecom and networking with Advanced Energy. He has more than 20 years’ experience in product line management and strategic marketing, as well as applications engineering for power conversion products and power systems used in telecommunications and the datanetworking market space. He has both technical and marketing background and holds two U.S. patents. Dib has a bachelor’s and master’s degree in electrical engineering, as well as an MBA.
Figure 3. Low gauge cables distribute -48 V power to cell tower antenna masts.
PIM can occur when two or more signals pass through junctions of dissimilar materials — such as loose cable connections, contaminated surfaces, poor performance duplexers, or aged antennas — and mix to produce sum and difference signals within the same band, causing interference. Engineers must make careful design and manufacturing considerations to ensure the PSU will not cause PIM interference during its useful life. To reduce weight, OEMs want physically small PSUs. Meeting this goal will require the use of new switching technologies, such as gallium nitride (GaN) and silicon carbide (SiC), widely used in solar system inverters
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CALL FOR NOMINATIONS:
THE 2021 R&D 100 AWARDS What is the R&D 100 awards program? Established in 1962, the R&D 100 Awards is the only S&T (science and technology) awards competition that recognizes new commercial products, technologies and materials for their technological significance that are available for sale or license. There are six categories in the R&D 100, listed below. There are also five special recognition categories, which follow. A given innovation can be entered in both a regular category and any of the special recognition categories — but please note that a separate entry fee is required for each nomination. Special recognition categories are awarded separately from the 100 winners that comprise the R&D 100. In addition, the judging panel will award finalist designations to selected top nominations. This announcement of finalists is made first, followed by the actual R&D 100 winners several weeks later. This allows all finalists and winners plenty of time to make arrangements to attend the awards banquet and/or conference.
SUBMIT YOUR ENTRY Deadline for submissions May 7, 2021 Late deadline for submissions June 21, 2021 To be eligible for R&D 100 Awards consideration, your product or service must have been made available to the marketplace between January 1, 2020 and March 31, 2021.
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5G HANDBOOK
DSS LETS 5G AND LTE SHARE SPECTRUM Dynamic spectrum sharing of 5G and LTE networks addresses the need for spectrum, particularly at mid-band frequencies. Here’s how it works.
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5
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Initial 5G network deployments took advantage of underutilized unpaired spectrum that use time-division duplex (TDD) at midband frequency ranges such as 3.5 GHz. Unfortunately, most of the spectrum below 6 GHz is paired spectrum using frequency-division duplex (FDD) that network operators use for their 4G LTE-Advanced networks. Because LTE’s FDD-based spectrum assets must remain in place, network operators must choose between acquiring new spectrum or re-farming spectrum already in use. Both options are costly. The 5G NR standard lets radios adapt to existing LTE deployments and share the spectrum originally dedicated for LTE. The enabling feature, dynamic spectrum sharing (DSS), is part of the overall mechanism that lets 5G NR and LTE coexist while in the same frequency band. Short-term DSS enables network operators to deploy 5G NR using their lower frequency bands, typically targeting frequencies
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Figure 1. MBSFN subframe
below 1 GHz. To date, this is still nonconfiguration in SIB Type 2. The standalone (NSA) mode and requires first three subframes (blue) the combination with an LTE anchor, of the upper 40 subframes are typically mid-band 1 GHz to 3 GHz. MBSFN subframes. For DSS to work properly, 5G radios must yield a frequency to LTE radios because LTE radios in service today were not designed for sharing. Therefore, it’s up to the smarter 5G radio to get out of the way when LTE radios need the spectrum.
THE MBSFN AWAKENING As in every wireless communication standard, receivers need to synchronize to downlink signals. 5G NR is no different and uses synchronization signal blocks (SSBs) for that purpose. When sharing the spectrum with LTE, the 5G NR needs a “gap” in time to transmit these SSBs. LTE configures Multimedia Broadcast Single Frequency Network (MBSFN) subframes to periodically allow 5G NR’s SSBs. 5Gtechnologyworld.com
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DYNAMIC SPECTRUM SHARING
The network can configure six out of ten subframes forming the LTE radio frame to become MBSFN subframes. Based on the 3GPP standard, this could be subframes #1, #2, #3, #6, #7, and #8 within one radio frame. The LTE network broadcasts the applied configuration with system information in block Type 2 (SIB2). A standard LTE terminal reads in the MBSFN configuration from SIB2 and ignores the subframes configured for broadcast. Figure 1 shows an example of the MBSFN configuration in a commercially deployed LTE network that uses DSS. The LTE network configures the allocation mode to be four radio frames, which means the given configuration repeats across a total of 40 subframes. Three of these 40 subframes are MBSFN, which reduce the LTE system capacity by 7.5%; the LTE network can no longer use these subframes for standard data transmissions. These subframes can now be used to carry 5G signals and channels. DSS initially aims at low-band deployments with carrier frequencies below 3 GHz, which results in a maximum of four SSB indices transmitted. The general guideline conveys the transmission of these four SSBs to the first half-frame (5 msec) of a radio frame. The mapping principle depends on the physical layer numerology. For initial DSS deployments, 5G NR operates at the same 15 kHz subcarrier spacing as LTE. For a subcarrier spacing of 15 kHz and carrier frequency below 3 GHz, the start symbols for mapping the four possible SSB indices are symbols #2, #8, #16, and #22. The first subframe within an LTE radio frame that is an MBSFN subframe candidate is subframe #1. Due to the SSB mapping principles for 15 kHz subcarrier spacing, known as Case A, SSB indices #0 and #1 cannot be transmitted, as their mapping occurs in LTE subframe #0. That leaves SSB indices #2 and #3 as their mapping occurs in symbols 16 and 22. The default transmission periodicity for SSBs is 20 ms. Thus, an MBSFN subframe is required every 20 msec, which is fulfilled in Figure 1. In a real deployment scenario, the network sends only one SSB index. The reason is hardware-related. Leading infrastructure providers claim to enable DSS by a simple software update. The antenna configuration of already deployed hardware does ,however, necessarily support beamforming; it is even more unlikely for equipment that uses low5Gtechnologyworld.com
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Figure 2. 5G NR transmission in LTE MBSFN subframes includes a non-MBSFN region defined at the beginning of each subframe.
band spectrum. This means creating several coverage beams, where each beam carries an SSB index directed to a specific area of the serving cell, is not feasible at all.
THE NEED FOR MBSFN SUBFRAMES In Figure 2, the transmitted SSB index is #2 and the diagram further gives a detailed overview of 5G NR signals and channels mapped within an MBSFN subframe. Figure 2 shows a non-MBSFN region defined at the beginning of each subframe, that can be one or two OFDM symbols long, depending on the overall available signal bandwidth. This region carries the required control channels for LTE, such as physical hybrid ARQ channel (PHICH), physical control format indicator channel (PCFICH), and the physical downlink control channel (PDCCH), including LTE’s cell-specific reference signals (LTE CRS). Therefore, any 5G NR transmission can only start at OFDM symbols #1 or #2 within an MBSFN subframe. The second OFDM symbol (symbol #1) in the slot carries the Control
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Resource Set (CORESET), and the initial symbol l0 for the first transmission of the demodulation reference signal for the data channel (PDSCH DMRS) is the fourth symbol (symbol #3). The support of mobility requires the transmission of an additional symbol for the demodulation signal. Based on the definitions in the standard, this is the twelfth symbol (symbol #11). Figure 2 also shows the position of SSB index #2 and #3, but again only index #2 is transmitted. That has to do with the second signal component, which requires the availability of MBSFN subframes: tracking reference signals (TRS). Index #3 would collide with the mapping of these 5G signal components. As there are no cell-specific reference signals in 5G as there are in LTE, the device maintains synchronization based on channel state information reference signals (CSI-RS) configured for time-frequency tracking. Enabling 5G NR in standard LTE subframes The 5G NR standard requires the transmission of channel state information reference signals (CSIRS) over two consecutive slots, when configured
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as tracking reference signals (TRS). This corresponds to two consecutive subframes when the subcarrier spacing is 15 kHz. The second requirement is that two resources (= symbols) within the slot are available to transmit this CSI-RS configuration. These two resources per slot could be symbols {4, 8}, {5, 9} or {6, 10}, where the latter is the mapping that avoids collision with SSB transmission if SSB index #2 is transmitted. The periodicity for TRS as CSI-RS is flexible. However, 40 ms provides adequate accuracy and results in the requirement of having two MBSFN subframes available every 40 ms, which the network enables with the MBSFN configuration shown in Fig. 1.
RATE MATCHING FOR LTE CELL-SPECIFIC REFERENCE SIGNALS Transmitting 5G NR in three out of 40 subframes (see Fig. 1) does not provide any substantial capacity for any 5G NR deployment. To overcome this situation, DSS enables the transmission of 5G NR in standard LTE subframes not configured for MBSFN. In standard LTE subframes, there are still signal components that need to be avoided by any 5G NR transmission, even if the scheduler in the LTE base station does not intend to use this subframe for data transmission. Besides the control channels at the beginning of every LTE subframe, cell-specific reference signals are always present in each subframe. Their mapping in the time-frequency resource grid depends on the physical cell identity and the MIMO mode (2x2 or 4x4). Figure 3 gives an overview of these signals’ mapping up to four antenna ports. The 5G NR base station scheduler uses a rate-matching algorithm to puncture these resource elements, thereby avoiding any transmission on the resource elements carrying LTE CRS. The network signals the required information of frequency, bandwidth, and the number of antenna ports to the device. Besides this necessary information, the system indicates further the MBSFN configuration and a frequency shift parameter vshift. The shifting parameter depends on the physical cell identity and follows a modulo-6 operation. The MBSFN configuration is required, as the scheduler does not need to avoid any LTE CRS in MBSFN subframes, as there aren’t any transmitted. The Abstract Notification Syntax 1 (ASN.1) in Figure 4 gives a summary of the transmitted parameters.
ALTERNATIVE POSITION OF ADDITIONAL PDSCH DMRS SIGNAL The second required feature is the support of an additional position for the mapping of 5G NR’s Physical Downlink Shared Channel
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(PDSCH) Demodulation Reference Figure 3. LTE cell-specific Signal (DMRS). Based on a standard reference signals map to antenna LTE subframe, with the LTE control ports, four in this instance. channel and CRS present, assuming the scheduler does not schedule any PDSCH, the remainder of the subframe is available to 5G NR. Therefore, the CORESET and the 5G NR PDSCH with rate matching active, including its DMRS, are mapped on the LTE subframes’ available resource element. As explained earlier, a second PDSCH DMRS is required to enable mobility. The symbol position depends on the number of available symbols for the 5G NR transmission. If we assume that the LTE control channels occupy only the first symbol of the slot, then 13 OFDM symbols are left for transmission with symbol #11 carrying the additional PDSCH DMRS. The PDSCH DMRS transmission would, however, collide with LTE CRS already occupying certain resource elements within that OFDM symbol; see Figure 5. In this case, the solution is to move the transmission of the additional DMRS position by one symbol to symbol #12. This change only applies if the 5G terminal has indicated its support of this feature when transmitting its UE capabilities towards the network, and the network configuration itself meets two specific settings: • The position of the initial PDSCH DMRS transmission is l0 = 3. The devices decode this information from the Master Information Block (MIB) transmitted via the Physical Broadcast Channel (PBCH) as part of the SSB transmission. • The network has configured the device via dedicated RRC signaling with the LTE-CRS rate matching algorithm that we discussed earlier.
Figure 4. This syntax describes an LTE CRS rate matching algorithm.
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DYNAMIC SPECTRUM SHARING
Figure 5. An alternative position for 5G NR’s additional PDSCH DMRS (symbol 12 instead 11) avoids a collision.
WHERE IS THE “DYNAMIC” IN DSS? So far, we have discussed a semi-static configuration of both LTE and 5G NR to enable the use of specific subframes for 5G NR when LTE is not present at all, or mechanisms that allow 5G NR to transmit in LTE subframes that are not used by LTE. The question remains, is there a way for LTE and 5G NR to share a subframe, and for both to transmit control information and data? The answer is yes, of course, but it very much depends on the infrastructure vendor. To put the dynamic to DSS, the infrastructure vendor typically requires the network operator to co-locate 4G LTE and 5G NR radios and connect them to the baseband processing unit that controls a cell and thus its frequency band. In that case, the scheduler — while monitoring the cell load, type of services requested by the terminals, and ultimately knowing how many LTE and 5G devices connect to this cell - can use this information to assign resources towards LTE or 5G dynamically. According to public announcements of infrastructure vendors, this situation is assessed on a 1 ms basis and can, therefore, change rapidly depending on the load situation in the cell. Nonetheless, the dynamic aspect of spectrum sharing is a proprietary characteristic of the used infrastructure vendor.
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Andreas Roessler is a technology manager for Rohde & Schwarz. He focuses on 3GPP’s 5G New Radio (NR) standard and advancing 6G research topics. His responsibilities include strategic marketing and product portfolio development for Rohde & Schwarz test and measurement division. Andreas has more than 15 years of experience in the mobile industry and wireless technologies. He holds an MSc in electrical engineering with a focus on wireless communication.
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5G HANDBOOK
HOW TO IMPROVE 5G COVERAGE AND CAPACITY The laws of physics work against RF engineers, forcing design tradeoffs in mmWave systems. Beam steering, frequency reuse, and greater spectral efficiency can help.
Reza Rofougaran, Movandi
5G provides data rates greater than 2 Gb/sec, mmWave BANDS latency below 5 msec, high capacity, and network slicing. These advances bring new opportunities to further improve spectrum efficiency, RF coverage, and access networks. How? To reach high capacity, 5G will need to use existing C-band spectrum and mmWave frequency bands. The mmWave signals impose challenging propagation conditions that large antenna arrays can alleviate. Antenna array need RF circuits behind each radiating element. 5G systems will deploy a mix of access points and even smart relays to reach users. The architecture of these access points should scale with coverage area. For example, a nanocell may use a 2x2 antenna array, while a macrocell may use a 16x16 array. Under ideal conditions, both the nanocell and macrocell access points would use identical RF components behind each antenna element. Such a design would require a novel RF architecture.
Most cellular frequency bands operate from 700 MHz to 3.5 GHz. The bandwidth allocated in these frequency bands is typically no wider than 20 MHz, which can support data transmission rates of 100 Mb/sec. The tradeoff comes with greater range from lower frequencies. To achieve wider bandwidth and higher data rates, network operators use mmWave frequencies at 24 GHz to 30 GHz, 37 GHz to 40 GHz, and 60 GHz. These mmWave frequencies are often called frequency range 2 (FR2). mmWave bands offer two advantages: • Higher bandwidth: The 28 GHz band offers 400 MHz of bandwidth, which supports 2 Gb/sec transmission data rates. This data rate is 10 to 20 times offered by 4G systems. • Lower cost: Many mmWave bands cost the operator less to acquire. These bands can cost two orders of magnitude less per hertz when compared to the cost for licenses in the sub-1 GHz bands.
Figure 1. A narrow beam can miss possible blockages, letting its full power reach the receiver.
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Although mmWave bands offer terrific bandwidth, they suffer from severely limited propagation distance. Thus, operators use a mix of lower and higher frequency bands to get coverage and capacity. They can, for example, service a campus, sports arena, or similar hot spot with mmWave base stations and offload their lower band frequencies that cover wider areas. As a radio wave’s wavelength shortens, it becomes more susceptible to blockage by objects, foliage, and even heavy rainfall. A signal’s ability to penetrate an object depends on its wavelength. As a rule of thumb, an object that has the width of a wavelength can block that radio frequency wave. For example, at 700 MHz the wavelength is around 42 cm whereas at 30 GHz the wavelength is 10 mm. A tree leaf’s dimension is closer to 10 mm than 42 cm. Thus, trees block millimeter waves. Radio waves travel from transmitter to receiver through four modes: they propagate in a straight line (line of sight), get reflected from a building, get diffracted or bend around obstacles, and get scattered from rough surfaces. When a transmitted signal propagates, the received signal varies in amplitude, a condition called fading. Measurements show that millimeter waves, on the other hand, get blocked and experience far less diffraction, reflection, and scattering. Therefore, mmWave RF waves are restricted to line-of-sight (LOS) travel. They don’t fade but undergo severe path loss. As mmWave signals travel along a lineof-sight path, they lose power with distance, called path loss. For example, at 900 MHz, the path loss is around 91 dB/km but can reach 121 dB/km at 28 GHz. That 30 dB difference 1000 times more loss than at 900 MHz. Additionally, these millimeter waves further attenuate from foliage, windows, and rain. 5Gtechnologyworld.com
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mmWAVE DESIGN TRADEOFFS
A tree can attenuate a signal by 6 dB/km, and a tinted glass window can attenuate a transmitted signal by 40 dB. Fortunately, antenna technology mitigates loss at these frequencies. Antenna arrays adjust amplitude and phase to radiate power in a narrow beam, called beamforming or beam steering. Narrow beams maximize line-of-sight travel and eliminate diffraction, scattering, or reflections from objects. With wide beams, the radiated power has a higher probability of hitting a building and getting blocked, as shown symbolically by the yellow beam in Figure 1. The concentration of an antenna’s radiated power in a direction is called its directivity, defined by its half-power beamwidth (HPBW). This value is the beamwidth measured where the power is half the peak power (3 dB down from the peak). The HPBW of an MxN element antenna array is inversely proportional to the number of elements and the element spacing. Hence, by controlling the number of antenna elements and their spacing, you can control the beam width and directivity. Each phased array antenna element needs electronics such as a beamformer, power amplifier (PA), and low noise amplifier (LNA), etc. Hence, as the number of antenna elements increases, the cost of the system increases.
CAPACITY AND SPECTRAL EFFICIENCY Capacity can be defined as Data Rate/MHz/Unit Area
Figure 2. Frequency reuse patterns have changed with each generation with wireless technology. sec/Hz, which means GSM allows transmission of 272 kb/sec in a 200 kHz channel. 3G using 16QAM modulation with, say, a rate of ¾ coding gives a spectral efficiency of 3 bits/sec/Hz. 5G uses 64QAM, and a coding rate of approximately 9/10 coding gives around 5.5 bits/sec/Hz. Therefore, with 400 MHz bandwidth, you can achieve 2.2 Gbps. Spectral efficiency determines the data rate in a channel while frequency reuse determines how often a network can reuse the channel to accommodate users.
MIXED CELLS OR ACCESS POINTS? To optimize coverage and capacity, operators will use mixed cells. For wide coverage, network operators will use gNodeBs (gNBs, base stations) to serve many users over an area. gNBs radiate higher power than small cells. To cover a hotspot with many users such as a campus or sports arena, however, a network could use smaller cells, each with lower power. The large coverage area could use C-band spectrum while the hotspot could use mmWave spectrum. This offers the operator the flexibility of using hotspots to offload the large cells. Hence, by using different size cells operating on different frequency bands, the operator could optimize coverage and capacity. Figure 3 illustrates the point. A cell’s antenna pattern covers an area. Each cell can be successively split, using lower height antennas and lower power so that cells don’t overlap in frequency. If they do
Figure 3. Cell splitting and nested cells results in higher capacity than with a macro cell alone.
Therefore, to secure higher capacity, the reuse factor must be reduced, and the spectral efficiency must increase. Figure 2 illustrates the concept of frequency reuse. The sectors or cells marked by the red “>” shape and the yellow arrow illustrate how far away the same frequency can be reused. The capacity of cellular communication is limited by interference on the downlink (communication from base stations to mobile devices). Separating frequencies by distance avoids interference. Mitigating or canceling interference increases capacity by reusing frequencies at closer separation distances. Massive MIMO, using multiple antennas and signal processing, allows tighter reuse of the same channels to increase capacity. An increase in spectral efficiency — measured as bits/second/Hz — also increases capacity. In GSM (2G) the spectral efficiency is 1.36 bits/
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in reductions in power and cost. For example, the synthesizer must have low phase noise to minimize jitter. As the frequency increases, however, the power required to keep the phase noise low also increases. If the synthesizer uses a lower frequency to generate the reference and the mixers had on-chip frequency multipliers, you can save power and generate an accurate reference signal with low jitter.
SUMMARY Figure 4. A scalable RF architecture can support a variety of antenna array sizes.
overlap, they are differentiated in power and interference cancellation. By splitting a cell, the operator can reuse the same frequency, resulting in frequency reuse gains. Splitting the frequencies between bands reduces interference. Assume the system supports 100 users in a 5 MHz channel in the macrocell. This macrocell is split successively into smaller cells using less power with each split. If the microcell supports 40 users, the picocell supports 10 users, and the femtocell supports five users, the network now supports 135 users versus the original 100. These users do interfere with each other but using antenna arrays (massive MIMO) and advanced signal processing, the user signals can be separated. For 5G to achieve the best coverage, operators use an increasing mix of gNBs, small cells, and smart repeaters. Each of these access points use antenna arrays, resulting in an increased opportunity for RF chips and products. Furthermore, 5G mmWave technology offers an opportunity to provide cost-effective broadband to residences without the need to lay fiber. An exterior 5G repeater or small cell can beam data at 2 Gbps to customer premises equipment (CPE) that can receive and redistribute the signal throughout the house. Such systems provide data rates that rival current fiber offerings but also provide very low latency.
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RF ARCHITECTURE RF front ends for 5G mmWave should address: • Large antenna arrays to overcome the propagation losses and handle interference mitigation. • Support for scalable mixed-size cells along with wide operating frequencies and interference cancellation. Figure 4 shows an example of an RF architecture that supports any size antenna array as a tiled tree of three IC building blocks. The first is a beamformer that contains phase shifters, switches, a power amplifier (PA), and a low-noise amplifier (LNA) to drive, say, a 4 x 4 or 8 x 8 dual-polarized antenna array. A number of these devices combine to drive multiple antenna elements as needed. This device is connected to a second IC comprising mixers for up-and-down conversion. A single mixer chip can drive four beamformer chips. The final IC is the multiband synthesizer, which provides a precise reference frequency reference for the mixers. The critical RF parameters that determine performance are the output power levels of the PAs, noise figure of the LNA, phase noise (jitter), and distortion measured by error vector magnitude (EVM). Optimizing these parameters across the three chips creates a high-performance system resulting 5 • 2021
mmWave bands will find increasing use to meet the requirements of higher capacity through their wider bandwidth. mmWave spectrum in many countries is also much more affordable than lower band spectrum. These bands are, however, challenged by their propagation conditions. To solve this large issue, antenna arrays can be used to enhance coverage as well as mitigate interference. This technique helps increase capacity by using mixed cells (gNBs, small cells, and even smart repeaters) for both capacity and coverage.
Reza Rofougaran is CTO and co-founder of Movandi and is a leading pioneer, engineering executive, and entrepreneur in wireless system design. He co-founded Innovent Systems in 1998 and is one of the top ten patent holders in U.S. and top twenty patent holders in the world. While at Broadcom, Reza was influential in starting and building the wireless business unit that shipped over 1.5 billion radios per year. Reza holds over 800 issued patents.
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OPEN RAN FUNCTIONAL SPLIT
FUNCTIONAL SPLITS, THE FOUNDATION OF AN OPEN 5G RAN The open standards for radio access networks offer options for locating network functions. These functional split options depend on network services and available transport links.
Olli Andersson, Benetel
C-RAN ARCHITECTURE
Disaggregation of the 5G Radio Access Network (RAN) opens
Backhaul
the door to new entrants into the ecosystem. For disaggregation
Fronthaul
to work, the telecom industry defined transport interfaces between three logical nodes of the RAN. Prior to Open RAN, networks needed one transport technology: backhaul. Open RAN adds fronthaul and mid-haul transport. This openness adds flexibility, which brings on new options and decisions in network architecture. Open RAN distributes baseband processing across the logical nodes, thereby setting the characteristics of these links, particularly for the fronthaul. 3GPP defines several options called functional splits between the nodes. Each split describes how the logical nodes interrelate to one another, and what specific activities each undertakes. When rolling out a network, operators must choose the best functional split based on the services they provide and the economics of the available transport networks. Telecom equipment manufacturers must decide which option is best for the markets they target. No single functional split will suit all scenarios. Knowing which option to choose isn’t always straightforward. We will now review the functional split options and cover their respective merits and drawbacks, with particular emphasis on the lower level (option 7.x) splits, which are gaining most traction within the industry.
THE 3GPP 5G NR ARCHITECTURE MODEL In Release 15, 3GPP, defined a new, flexible architecture for the 5G RAN, where the base station or gNodeB (gNB) is split into three logical nodes: the Central Unit, (CU), the Distributed Unit, (DU) and the Radio Unit, (RU), each capable of hosting different functions of the 5G NR stack. Figure 1 shows the Baseband Unit (BBU) – a proprietary box – dividing into the three units. This architecture recognizes that transport networking availability, along with specific 5G use cases, will dictate the optimum network configuration. 3GPP specifies eight options for distributing the functionality of the 5G NR RAN stack across the fronthaul network − the functional splits. In parallel to 3GPP’s work, several standardization bodies have been looking at the definition of the new transport interfaces − fronthaul, midhaul, and backhaul − with the latter two particularly impacted by the choice of functional split. The fronthaul network is key to flexible RAN deployment. The enhanced CPRI (eCPRI) standard has emerged as a successor to CPRI, which was defined for the original Centralized RAN, (C-RAN)
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Backhaul
Midhaul
Fronthaul
Figure 1. 3GPP’s 5G RAN architecture distributes configuration. Developed the stack functions across three logical nodes. by the eCPRI Forum, this Source: https://www.nctatechnicalpapers. protocol makes more com/Paper/2019/2019-5g-backhaul-fronthaulefficient use of bandwidth than its predecessor and opportunities-and-challenges is packet-based, meaning that it can be framed within Ethernet. This brings enormous advantages to the fronthaul network which, depending on the functional split, can now use Ethernet connectivity instead of relying on the availability of fiber. eCPRI is also an open interface, enabling operators to mix and match vendor equipment.
FUNCTIONAL SPLITS When describing the functional split options, operators consider the CU and DU as a single logical unit because the splits apply to the fronthaul network. They can distribute higher Level functions of the stack between CU and DU over the midhaul network. Bit rates and latency requirements between higher layer stack functions provide more flexibility on choice of midhaul transport. In practice, the industry has settled on a split between CU and DU where CU hosts the network layer functionality (RRC) and the PDCP functionality from the Data Link Layer. Within the eight main functional split options that 3GPP defines, option 7 further divides into sub-options 7.1, 7.2 and 7.3, discussed in more detail later. Figure 2 shows the eight major splits, with each one offering a different trade-off between centralization benefits and fronthaul network requirements. In Option 8, all stack functionality is centralized, and this split corresponds to the original C-RAN configuration. Option 8 maximizes the benefits of centralized baseband processing enabling 5 • 2021
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5G HANDBOOK FUNCTIONAL SPLIT OPTIONS FOR 5G load-balancing and sharing of the processing capability across the RUs. Centralization of the baseband functionality also enables virtualization of many network functions, with the software hosted on COTS servers. The result: a simplified RU because only the RF functions remain at the remote site. This small RU costs less, uses less power, and more easily fits on a tower. Operators can perform most network upgrades at the CU, requiring fewer site visits. The simplified RU can handle multiple Radio Access Technologies (RAT) further reducing the footprint of the remote mast, which must support multiple cellular generations. By fully centralizing the functionality of the 5G RAN stack, Option 8 places the highest demands on the fronthaul network, with high bit rates and strict latency requirements. Information flows at the higher levels of the stack are less data intensive than those at the lowest, PHY, layer, which essentially converts digital bits into radio waves in the Downlink (DL) direction and vice-versa in the Uplink (UL) direction. Functions in the PHY layer such as Cyclic Redundancy Checking (CRC), modulation, and mapping and encoding add information to the data blocks received from the higher MAC layer. Thus, progressively higher bit rates occur as the information flows towards the RF functionality. Additionally, the time-sensitive nature of communications between PHY functionality and certain higher-level processes such as Hybrid ARQ (HARQ) require round-trip delays as low as 5 ms. The fronthaul requirements of Option 8 effectively limit its use to scenarios where fiber
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is economically justifiable, e.g., in urban areas or where operators already own it. At the other end, an Option 1 split places all baseband processing within the RU, which becomes large, complex, and requires more power than the simplified RU of Option 8. The fronthaul network demands of functional split 8 are, however, much simpler because the entire protocol stack resides in the RU. That means more processing must occur before data can travel between RU and CU, leading to much lower bit rates and higher latency tolerances on the fronthaul network. The remaining functional split options, from 2 to 7, vary in the level of baseband processing left in the RU as opposed to being hosted in the DU/CU, as illustrated in Fig. 2. The higher the split level, the lower the demands on the fronthaul link, balanced against diminishing benefits from centralization. Coordinated Multipoint (CoMP),
LAYER 3 FUNCTIONS
Figure 2. Eight Functional Splits define how the 5G NR Stack is allocated to the logical nodes. for example, is a key technology in the 5G RAN, improving service at the cell edge by allowing connections to several gNBs at the same time. Split level 5 affects the CoMP’s performance. By split level 2, this capability is effectively lost. Figure 3 shows the results of calculations of fronthaul bit rates for each functional split. Although the calculations were done on an LTE network, the findings are relevant for 5G NR. This chart shows that fronthaul bit rates are highest for the low-level split options, 7 and 8, dropping significantly for higher level splits and being fairly consistent across options 1 to 5. Fig. 3 clearly illustrates why there is so much industry interest in the Option 7, or PHYlevel functional splits. Option 7 offers a good balance between RU complexity, fronthaul bandwidth, and inter-cell cooperation.
THE LOW-LEVEL SPLITS Figure 3. The Fronthaul bit rates vary significantly across the functional splits. Source: https://ieeexplore.ieee.org/ document/8479363
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The telecom industry effectively considers functional split Options 6, 7 and 8 as the low-level splits (even though they use higher numbers), with each option supporting network centralization benefits while generating different bit rates on the fronthaul network. Option 6 splits the baseband functionality at the boundary of the MAC and PHY layers, leaving all PHY functions in the RU. This split results in a significant reduction in the fronthaul bit rate as the payload of the link consists of the transport blocks transmitted between the MAC and PHY layers. This bandwidth reduction forces a tradeoff: a reduction in the benefits achievable by a centralized pooling of baseband functionality. Option 6 limits pooling to the 5Gtechnologyworld.com
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OPEN RAN FUNCTIONAL SPLIT LOWPHY
RF
HIGHPHY
8
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RLC
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Figure 4. The 7.X splits differ in how they allocate the PHY functionality between RU and DU. Source: https://ieeexplore.ieee.org/document/8479363
IP
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functions in the Data Link and Network layers, giving only 20% of the total pooling potential − the remaining 80% based on PHY layer functions. The three Option 7 functional splits, 7.1, 7.2 and 7.3 vary in the way that they divide the PHY layer of the stack, between the DU and the RU, Figure 4. The level 7 splits all support the key centralization benefits, including carrier aggregation, MIMO and CoMP, the main difference between them being the data-rates on the fronthaul network. All three options leave the Fast Fourier Transformation (FFT) functionality in the RU, significantly reducing the fronthaul bit rate. By allocating slightly more functionality (pre-coding and resource element mapping) to the RU, Option 7.2 further reduces the fronthaul bit rate, when compared to Option 7.1. Because the resource element mapping detects unused subcarriers from the RF link, locating this function in the RU leads to a variable bit rate on the fronthaul network -- Option 7.1 gives a constant bit rate. Option 7.3 is a downlink-only option and achieves further fronthaul bit rate reduction by allocating even more functionality to the RU, which becomes more complex. All three splits provide a good compromise between centralization and fronthaul requirements, allowing relatively simple RU configurations. These options become candidates for high-capacity networks in dense urban areas. Option 7.2, however, has increasingly gained traction within the industry with its lower and variable fronthaul bit rate being compatible with the eCPRI protocol. There is no single, ideal functional split because different options will suit different applications. It is, however, unlikely that the industry could practically supported all eight options. To ensure scale and openness, various industry alliances are working to gain consensus on the best options to be adopted. 3GPP has recommended Option 2 for highly centralised applications such as fixed wireless access, (FWA), where cell-site coordination is not required and latency and bandwidth requirements on the transport network are relatively relaxed. At the same time, Option 6 is being pushed by the Small Cell Forum (SCF) as the optimal split for low-cost, low-capacity deployments). SCF, whose membership includes leading players in both the operator and equipment manufacturer communities, has developed specifications for this split known as nFAPI, (network Functional API). Meanwhile, the equally influential O-RAN Alliance supports option 7.2 for networks with high-capacity and high-reliability requirements. This functional split enables a relatively simple RU whose size and power 5Gtechnologyworld.com
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Coded words
Scrambling
Coded block
Coding Block Segment
Transport Blocks
CRC attach
Transport Blocks
consumption supports network densification and enables sharing by multiple operators, facilitating the developing neutral host market. The ability of eCPRI to run on Ethernet is a significant advantage in urban areas and in indoor environments such as factories and office blocks where 5G coverage will be required.
CONCLUSION The success of 5G depends upon the current industry drive towards an open RAN, supported by a transformed supply chain, where new entrants bring innovation and drive “cloud-level” economies. Disaggregation of the RAN brings more flexibility to network deployment and, potentially, enables vendor specialization, breaking the traditional, vertically integrated supply chain. This is only possible, however, if the interfaces within the disaggregated RAN are truly open and 3GPP’s functional split definitions are a key step towards the open RAN. 3GPP’s work has been consolidated by the efforts of key industry bodies working to gain consensus on the best subset of options for practical deployment. With this level of standardization and industry support, new entrants to the 5G eco-system can make better informed decisions when developing their products.
Olli Andersson is Senior Vice President Americas at Benetel. Andersson headed up Nokia and Nokia Bell Lab’s Innovation Centre in Sunnyvale, Calif. During his career, Andersson has led laboratories and teams of managers and engineers across the globe on a wide range of advanced telecommunications projects. Prior to joining Nokia, Andersson spent three years in South Korea heading Nokia Siemens Networks’ Smart Lab and CTO team. He holds an MBA in Strategic Management from the Helsinki University of Technology and a B.Eng in Telecommunications from the Mikkeli University of Applied Sciences.
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5G HANDBOOK Figure 1. Traditional RAN Deployment Interfaces have remained proprietary and fundamentally unchanged.
DEPLOYING AND MAINTAINING AN OPEN RAN NETWORK Open radio access networks offer Open radio access networks (Open RAN) advantages in locating network significantly enhance how mobile functions of proprietary RANs. networks operate. The telecom industry, Automation and orchestration let and especially the RAN segment, is telecom networks do what computing several years behind the innovation curve networks have dome for years. due to tightly integrated hardware and André Silva Biscoto, Altiostar
software from just a few vendors. Open RAN can change that.
The IT industry has long benefited from a software-centric approach and transitioned to an open model where software companies compete and innovate. Now the telecom industry is going through the same transformation in many aspects of the network, but especially in the RAN. This change began with the deployment of networks such as Rakuten Mobile in Japan and the DISH Open RAN 5G network in the US. Mobile network operators (MNOs)
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throughout the world, including many brownfield networks, are now trialing and deploying Open RAN. Open RAN is part of a shift in deploying and maintaining software defined networks (SDNs) and automation. SDN approaches associated with virtualization and vendor neutrality offer new ways to manage the mobile network. Orchestration and automation play an important role in demonstrating the potential of Open RAN by reducing human intervention in maintaining the network. Long-term transformations are also occurring in the RAN, including how Open RAN will enable new 5G use cases and the overall deployment of 5G networks.
DEPLOYING OPEN RAN HARDWARE Traditional RAN deployments had all intelligence located inside an environmentally conditioned cabinet designed for an ASIC server appliance 5Gtechnologyworld.com
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OPEN RAN TELECOM NETWORKS
Figure 2. Traditional RAN interoperability limitations result from proprietary interfaces. called a baseband unit (BBU). BBUs process Layer 1, Layer 2 and Layer 3 IP protocol stack functions before sending the processed signals over the backhaul interface to the operators’ 4G evolved packet core (EPC). As Figure 1 shows, such transport uses conventional network methods such as multiprotocol label switching (MPLS). This method has been in place since the advent of 1G and 2G networks. Today, two trends allow virtualization in Open RAN networks. One comes from wide availability of low-latency transport options with higher bandwidth from fiber-optic transport networks. The second is a concept that was widely adopted in IT platforms but made few strides into RAN until now: network functions virtualization (NFV). With NFV, key processing elements and intelligence previously managed by the BBU can now disaggregate and move to an edge cloud data center away from the cell site. Cloud-native containerization followed NFV as another virtualization tool. 5Gtechnologyworld.com
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ENTER OPEN RAN
Open RAN is a set of standards that specify interoperability between RAN hardware and software elements from different vendors. While enabling this interoperability, Open RAN has incorporated virtualization, using NFV and containers that let RAN workloads operate as virtual network functions (VNFs) and containerized network functions (CNFs), which run on commercial off the shelf (COTS) servers. This development eliminates ASICbased appliances, enabling BBUs to run on conventional servers that meet the memory and processing requirements demanded by RAN functions. In traditional RAN deployments, software and hardware of a single vendor are tightly coupled using proprietary interfaces, as shown in Figure 2. Any mix of components of different companies risks incompatibility, although radio and basebands served the same purpose. The radios connect to the baseband over proprietary interfaces using the Common 5 • 2021
Public Radio Interface (CPRI) standard. Each manufacturer implemented its own CPRI interface version, thus making radios and basebands of different companies incompatible. When 3GPP standardized 4G LTE, the overall network architecture became flatter. An expectation arose that, to enable optimal subscriber experience, base stations would use the X2 interface to communicate with each other to handle resource allocation. This initiative didn’t gain traction. Traditional RAN companies created lock-in by implementing their own flavors of the X2 interface, creating difficulty for operators to use more than one RAN company’s products in a given location. Operators became locked into deploying equipment from a single RAN company per geographic area. Assume company A’s products were deployed in the north of a city or state, whereas company B’s was deployed in the south. Here are some of the hurdles that exist even with geographical diversity: DESIGN WORLD — EE NETWORK
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5G HANDBOOK • Spares rationalization was impossible: the operator had to keep a pool of spare BBUs for both company A and company B, and similarly a pool of radio spares for all bands for both company A and company B. • Border Performance between two RAN companies: because the handovers should use the X2 interface, which were not interoperable, operators instead had to use the S1 interface. This increased signaling, adding delay and ultimately negatively affecting the performance. Operators mitigated this limitation by planning the RAN borders so that they would overlap in areas of low traffic and reduced mobility such as natural geographical borders, such as a mountain, river or forest. It would affect fewer people if the borders crossed in a forest rather than a densely populated city, for example. • Operations: supporting two networks increases the number of people needed to operate the networks. Mobile operators needed a technician specialized and trained for company A’s RAN software and hardware. This same person sometimes lacked the skill and knowledge to troubleshoot the same issues on company B’s equipment. Hence the operator needed to have specialized pools of resources trained to operate both networks. • Support: operators pay an annual fee to each RAN vendor to support its hardware and software. This fee is set partly on the number of cells, radios, and nodes deployed in its geographic area. Often times, the benefits of scale did not apply to support fees as the infrastructure was divided into multiple vendors. • Swap: should the operator decide to modernize or swap its network and change from one vendor to another in a given geographical area, the operator must rip and replace the existing hardware. That requires a truck roll and a tower climb to remove existing radios and install new hardware to do essentially the same thing. While costly, this effort would not yield new functionality or benefits.
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OPEN SPECIFICATIONS The O-RAN Alliance designed its specifications to solve these hurdles and let operators benefit from best-of-breed hardware and software that are interoperable as illustrated in Figure 3. The operator can now choose a radio based on its own technical and or procurement criteria. Open RAN also opens the door to custom radio hardware adapted to the operator’s needs. For example, assume that an operator had spectrum assets in 3GPP band 3 (1800 MHz FDD), band 7 (2600 MHz FDD) and band 1 (2100 MHz FDD). They could design each cell site with three remote radio units (RRUs) one for each band multiplied by the number of sectors, typically three per site. So, a total of nine radios would be required. With Open RAN, the operator could ask an Open RAN radio manufacturer to design and build a radio with all three bands as shown in Figure 4. This can dramatically reduce the costs operators pay for tower rental space and power consumption while reducing load on tower structures. This practice also lets the operator have hardware tailored to its engineering principles. For example if vendor A only had in its portfolio a radio that can transmit 120 W, but the operator would like a radio capable of transmitting 160 W, the operator can now search for other vendors who support their preferred power levels. Open RAN introduces competition that pushes companies to better support network operator needs. This competitive scenario is also available for baseband hardware, where an operator can choose a hardware server platform that fits its individual needs. Because these are COTS systems, there may also be synergy with hardware that its IT department uses reducing the number of spares and the tapping into a trained pool of management and troubleshooting experts. For the software, an operator can choose from an array of different vendors, but there is no limitation, as O-RAN standards have provisions for software of different providers to interoperate and all interfaces are open and standardized. 5 • 2021
Figure 3. Open RAN achieves interoperability through standardized interface.
TRANSPORT 3GPP standardized BBU disaggregation with a central unit (CU)-distributed unit (DU) split. The non-real time functions of the CU can be virtualized in a cloud or central data center, while the real-time functions provided by the DU reside in an edge cloud server or installed at the cell site. So, the functions inside the IP stack that before resided within the BBU and hence had very little delay from the RU to the BBU now will now be flexibly and more cost effectively deployed in the edge cloud data center using a low latency transport medium. Real-time functions such as forward error correction (FEC) that now reside in the DU need to work properly and without additional delay or quality of service. The initial solution to this delay involves replacing the BBU with DU on site. See Figure 5. Because DUs run on COTS servers in cell sites, they connect through a new interface called midhaul. Midhaul interfaces operate with a relaxed latency requirement (compared to fronthaul but more strict than backhaul), typically in the order of 50 ms round-trip time (RTT). Many DUs on the cell sites can connect to a CU using COTS servers and also reside in a centralized data center. The CU will connect to the mobile core over a backhaul link. Open RAN can expand to deployments that use low-latency fiber in densely populated cities and cover small site-to-site distances. The DUs can operate in an edge cloud data center that typically manages a small number of cell sites covering an area where latency 5Gtechnologyworld.com
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OPEN RAN TELECOM NETWORKS
will not exceed 150 µs to 200 µs RTT from air interface (or input port of radio) to the edge cloud data center. These low-latency figures can be obtained by typically using dark fiber but any Layer 2 (LLS Ethernet/ eCPRI) connection that fulfills the latency budget between the nodes could allow full virtualization. This configuration is commonly called split 7.2x. The fully virtualized deployment model, shown in Figure 6, provides rationalization and pooling of processing resources in a data center. It also expands the use of edge computing, improving scalability and enables slicing for a service-oriented 5G network. In rural/ suburban sites where latency is higher and fiber isn’t available to support a less dense population, the operator may consider co-located deployment of RU/ DU or an integrated CU/DU into the same form factor. These extra fiber fronthaul/backhaul requirements, depending upon fiber availability, could offset Open RAN cost savings. More standardized hardware for both the edge cloud server (DU) and standardized data center hardware (CU) can lead to operational benefits overall for operators. Furthermore, there are clear operational cost savings including energy savings and cell site space savings when compared to a legacy RAN approach.
OPEN RAN NETWORK AUTOMATION Through automation, Open RAN can reduce human intervention in network operations. A traditional nationwide wireless network might require several thousand people to run while a fully virtualized network could require just hundreds. Network orchestration can reduce this effort. Automation and orchestration are different, but related concepts. In general, automation refers to automating a single task. Examples include: • Automatically restarting a software procedure or hardware card when a fault or exception occurs • Automatically sending via FTP a new software instance that needs to be loaded into a baseband processor Open RAN brings to the telecom concepts already ingrained in the IT industry. Open RAN introduced orchestration to the RAN. Orchestration is gaining momentum. In MPLS networks, for example, orchestration lets routers coordinate to direct traffic more efficiently along a hub and spoke framework, minimizing congestion and bottlenecks. Orchestration in the IT world is the automated configuration, management, coordination of computer systems, applications, and services. These are only some examples of automation that still require different levels of human supervision and intervention. By aggregating multiple automation routines across clustered applications, multiple data
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Figure 4. Open RAN radios can
support multiple frequency bands. centers, clouds, and applications with complex dependencies, we can orchestrate processes ensuring that single tasks occur in proper order and eliminate human intervention. Manually intensive legacy RAN functions include: • Lifecycle management of baseband hardware and software: upgrading, applying patches, monitoring and cleanup. • Auto integration: configuring and commissioning of new hardware added to the network. Orchestration can provision or deploy servers, assign storage capacity, create virtual machines and manage networking, among other tasks. • Self-healing: locating faults and executing troubleshooting routines to bring a service back to working state. • Capacity optimization: scaling capacity dynamically by spawning new CU/DU instances as capacity demands shift. This can be achieved by deploying spare servers located in a data center pool, assigning storage, creating virtual machines when capacity scales up and then decommissioning these logical entities when capacity recedes, freeing up server resources for other tasks. Automation also reduces human error. Orchestration supports a DevOps approach helping operations teams deploy applications more quickly, securely and accurately.
ACCELERATING 5G USE CASES The foundation of many 5G use cases is a concept called network slicing, where the user plane operates separately from the control plane traffic in 5 • 2021
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5G HANDBOOK Figure 5. A partially virtualized RAN deployment places non-real-time functions in a central location.
relevant network domains (i.e., RAN, core, transport) to offer multiple virtual services with different latency, performance and other characteristics. This de-coupling of user and control plane also happens to be one of the fundamental tenets in SDN and NFV. This has led to a natural adoption of end-to-end network slices based on data paths being already separated in a virtualized environment. It is possible to partition the same physical network to support different access types and service-level agreements in support of different use cases and this ability has existed for a long time. Network slicing is, however, required to meet all the demands placed on 5G networks, such as very low latency and drastically increased bandwidth. Network slicing must be designed on fully virtualized networks to accelerate and facilitate the adoption of innovative 5G use cases. Examples of software defined Network (SDN) concepts accelerating 5G use case adoption include:
• The application of SDN in wireless networks, and associated cost savings, could accelerate applications for 5G technology in industrial internet of things (IIoT) given that industrial IoT use cases (e.g., factory automation) do not have a pre-existing large number of consumers to provide an economic underpinning for data provision. Cost effective deployment through virtualized networks may help drive uptake. • At Altiostar, we believe that harnessing virtualization could improve the viability of IIoT. This use case, in a factory/automated driving setting, typically leverages 5G fast connectivity, allied to the collection of extremely large quantities of data from sensors and real-time data analysis. Given the proximity of the data to the processing location, applications reside in the same data center as the CU. For example, we believe this will improve latency/ speed required for use case quality of service.
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OPEN RAN TELECOM NETWORKS Figure 6. A fully Virtualized RAN deployment disaggregates the radio access network into radio units, distributed units, and centralized units.
• For use cases such as autonomous vehicles, where the volume of data collected and sent over the network could potentially be extremely large, the capability to scale capacity up or down dynamically via orchestration is advantageous. The task of identifying and applying use cases and network slicing belongs to the whole industry. Open RAN provides the foundation that will allow the strict latency and capacity targets to be achieved by providing a virtualized and scalable network via orchestration framework.
CONCLUSION Virtualization in enterprise data centers perfected the technology and demonstrated the value of a software-centric and cloud-native approach to networks. Now the mobile telecom industry is realizing the potential of this industry shift, taking an open and virtualized approach to the RAN. Incorporating SDN as an approach, Open RAN has reinvented how operators deploy and maintain their networks. This approach, which is associated with virtualization and open interfaces, offers new and more efficient ways to manage the mobile network. This in turn leads to reduced human intervention in maintaining the network, while enabling increased orchestration and automation, accelerating new 5G use cases.
André Biscoto is a Network Consultant for Altiostar’s Product Go-To-Market team. He has over 20 years of experience in mobile network engineering in the US and Latin America, being responsible for 5G and LTE systems engineering, product management, legacy network design and optimization, consulting and sales support. He assists operators in adoption of open virtualized radio access network (Open vRAN) networks.
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THE CHALLENGES OF BUILDING A 5G BASE STATION Paul A Moakes, CommAgility
To meet 3GPP specifications, a 5G New Radio (NR) implementation must meet demanding processing requirements and RF capabilities. Compared to LTE, this results in a need for higher performing, more flexible 5G NR hardware.
Looking at 5G’s technical challenges, we see the frequencies and spectrum supported now include a sub-6 GHz range, FR1, with bandwidths up to 100 MHz as well as a mmWave band, FR2, with bandwidths up to 400 MHz. This requires high bandwidth transceivers, resulting in interfaces such as JESD204 becoming faster and more expensive in power [Ref. 1]. A more integrated platform becomes preferable.
To meet the higher RF power demands and increased crest factor of 5G, IC designers are turning to more efficient technologies such as Gallium Arsenide for the RF power amplifiers (PAs). This amplifier class has highly non-linear characteristics with a memory effect, meaning that digital pre-distortion (DPD) techniques are required to maintain signal integrity. At higher bandwidths, the digital front end (DFE) that handles DPD requires a proportionate increase in processing performance. 5G NR introduces the cyclic-prefix orthogonal frequency-division multiplexing (CP-OFDM) and direct Fourier transform spread OFDM (DFT-s-OFDM) waveforms [Ref. 2]. These are important for increasing the spectral efficiency of 5G, but a consequence is that the physical-layer baseband processing is more complex than with LTE’s OFDM. Additionally, the forward error correction (FEC) standards of low-density paritycheck (LPDC) and polar codes for improving throughput on noisy
channels mean that there is a need for hardened processor accelerators to offload calculation. 5G also introduces disaggregated models to the network architecture. These enable such features as network virtualization in the core and low latency at the network edge. Architectures such as the O-RAN model, however, require flexibility to split the PHY and support of a high bandwidth fronthaul connection over time-synchronous Ethernet.
COMPONENTS OF A 5G BASE STATION Which components of a 5G base station can meet these technical challenges? How do we build a system with the software flexibility to enable vertical markets to address the features they require, while keeping up to date with specification enhancements? Figure 1 shows the basic functional components required to build an integrated gNodeB base station. Layer 2 and Layer 3 of the OSI model are responsible for the packet processing elements and have less strict timing requirements than the Layer 1 PHY. These can run on generalpurpose processors (GPPs), usually comprising multiple Arm or Intel cores — the number of cores scaling with the cell throughput required. Packet accelerators for connectivity and security on
Figure 1. An integrated gNodeB includes a 5G Core, PHY, DFE and RF front end, as well as Layer 2 and Layer 3 packet processing.
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5G BASE STATION DESIGN
Figure 2. Software components of an integrated gNodeB perform functions that can be distributed in different locations.
the backhaul help processor offload to reduce power and improve throughput. The baseband PHY (Layer 1) requires a time-deterministic architecture where the multiple signal-processing blocks are better suited to dedicated digital signal processor (DSP) units, which also improves efficiency. Again, throughput, bandwidth, carrier count, and number of antennas determine the number of DSP units required. In the high-PHY, offloading the Forward-Error Correction (FEC) significantly reduces the DSP resources required by the PHY. At the DFE, more DSP resources are required for digital filtering, up/down conversion, and RF transmit power improvement techniques such as Crest Factor Reduction (CFR) and Digital pre-distortion (DPD). Tight integration with the digital transceivers (ADCs and DACs) is
required, which can be on-chip or connected externally over an interface such as JESD. The RF analog front end comprises an RF transceiver to convert the analog baseband signal to the required over-the-air band, a power amplifier (PA) to provide the required gain for the cell coverage, a low-noise amplifier (LNA) to recover the mobile signal, and associated filtering to ensure emission mask compliance and prevent blocking signals affecting reception. Time-Division Duplex (TDD) requires a transmit/ receive switch activated on frame boundaries under the control of PHY software. The PAs and filters are band-specific, and it is common for equipment to have a personality module to address specific 3GPP bands.
Figure 3. An Open RAN network architecture disaggregates the functions within a gNodeB.
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5G HANDBOOK
SOFTWARE COMPONENTS Figure 2 shows in more detail the algorithms and functionality performed by the software components identified in the previous section. This architecture lends itself to various partitions across the dedicated processing elements, GPP and baseband SoC, to maximize efficiency and scalability. The stack and the PHY use the 5G Femto Application Platform Interface (5G FAPI) defined by the Small Cell Forum, which can also run over Ethernet. 5G FAPI physically separates the GPP and baseband SoC functionality. This lets integrators mix and match PHY and stack solutions or add baseband SoCs to increase the number of MIMO layers. Beyond the integrated small cell, a disaggregation model such as O-RAN, shown in Figure 3, lets network architects physically locate 5G network equipment in appropriate environments. For example, a centralized unit (CU) can perform Layer 3 processing with a general-purpose processor (GPP) can run closer to the core network, deployed in server type equipment. The CU can support multiple distributed units (DUs) that run Layer 2 on a GPP and high-PHY on a dedicated baseband SoC with FEC accelerators. These units can be deployed either centrally or close to the radio sites, which in turn determines whether the
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Figure 4. NXP’s LX2160A SoC includes 16 Arm Cortex-A72 cores, 8 MB L2 cache, and multiple interfaces. equipment is based on plug-in cards to server equipment or is a custom outdoor cabinet unit. A DU can support multiple radio units (RUs) comprising only the RF and low-PHY elements, which run on baseband SoCs or FPGAs. RUs minimize size and power, which supports installation at sites with limited access or reduces cell site rental costs. This allows more units to be deployed for densification, at reduced cost, while also supporting a mix of operating bands and throughput at different sites.
PICK THE PROCESSORS 5G requires higher processing performance than LTE. At CommAgility, we looked at the differing requirements for processors in a gNodeB. Doing so helps us to decide which tasks can run on a GPP and which need dedicated DSPs. Physically compact hardware meets the requirements of private networks, which have space and weight limitations. Minimizing power consumption avoids excessive heat dissipation. All such decisions come with cost constraints. What do these requirements mean for hardware designers? In practice, the best answer may well be a System on Chip (SoC), that offers GPP cores from Arm or Intel, specialized DSP capabilities, off-load accelerators, and on-chip memory and peripherals. 5 • 2021
We chose processors with Arm cores. Our gNodeB development platform uses NXP’s LX2160A Layerscape communications processor, which includes sixteen Arm Cortex-A72 cores optimized for L2/L3 packet processing. The LX2160A also provides 8 MB L2 cache, and high-performance data path accelerators and peripheral bus interfaces including PCIe Gen3 and Ethernet up to 100 Gb/sec. (see Figure 4). The development platform also includes an NXP LA1200 Layerscape Access baseband processor, which provides high performance from its vector-signal processing architecture (VSPA) vector engines, delivering more than 1 TFLOP. The LA1200 is specifically optimized for 5G NR, providing the DSP and hardware acceleration needed for compact, powerefficient designs. Working with such highly-integrated SoCs can make you feel that you’re introducing excessive complexity. To overcome this, choose a silicon manufacturer who provides suitable software tools, such as real-time monitoring and debug, virtualization, and software management. Such tools can help developers cut time to market and make sure they can take full advantage of the flexibility offered by the SoCs. 5Gtechnologyworld.com
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5G BASE STATION DESIGN SYSTEM-LEVEL OPTIONS
When looking at how to build your small cell, consider these starting points at various levels of integration: small cell suppliers, 5G modem suppliers, and 5G SoC suppliers. Each have advantages and disadvantages, as Figure 5 shows. The approach which works best will depend on the volumes of sales involved, and the nature of any specialization required. For example, small cell suppliers are unlikely to support all the features of every 3GPP release of the 5G specification. Ruggedization, to meet the demands of harsh environments, may also be impossible or limited. Alternatively, beginning with an SoC supplier allows designers to achieve a bespoke product with a high degree of specialization, but means that trusted third-parties must be found for specific components, particularly when supporting uncommon RF bands. This can be complicated and add risk and development time to a project. For many applications, the middle ground of 5G modem suppliers is a good compromise. However, this does mean that the features remain constrained by the modem manufacturer –- it’s only at high volumes that these can be modified. To minimize costs, risk, and development time, you can start with an off-the-shelf reference platform. Once a system has been fully specified and proven, you can optimize the hardware design before production.
5G REQUIREMENTS 5G NR, established in 3GPP Release 15 and expanded in Release 16, introduces key improvements over LTE, including more efficient modulation, waveform optimization and scalable numerology – thus offering advantages such as low latency, faster speeds, and high reliability. This means there is a wide range of end use cases for which 5G will be attractive. 3GPP standards address specialized application areas such as
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mission-critical applications. Support began in 3GPP Release 13, LTE, in 2016 with Mission Critical Push-to-Talk, continued through Release 14 in 2017 with Mission Critical Video and Data. This trend continues to evolve in 5G, with Release 15 supporting legacy system interworking, and Release 16 supporting Public Warning. Each application area, however, involves tens to hundreds of engineer-years of development effort, with varying degrees of complexity and commercial attractiveness. Not all equipment and integrated devices can or will support all features of every release. For many applications, engineers need to migrate products from 4G to 5G, and then to customize them beyond the 3GPP 5G specifications. Private networks and specialized applications are predicted by many to potentially overtake operator networks in the market — for example, analyst ABI Research predicts that the private wireless networking market could reach US$16.3 billion by 2025 [Ref. 3]. Systems, therefore, must facilitate the migration to 5G, ideally by software upgrade or at least re-use. They also need sufficient flexibility to enable vertical markets to cherry-pick appropriate specification features. All equipment doesn’t need to be a superset of features, which would require more expensive approaches than are strictly needed. Private networks or specialized applications may use only some parts of the standards or may require algorithmic and protocol adaptations to deal with issues such as higher latency, longer range, higher Doppler shift, specific interference, or multiple parallel channels. For example, connecting from an aircraft traveling at typical cruising speeds of over 900 km/h and altitudes above 10,000 meters, creates a specific set of technical challenges. In particular, the network Figure 5. Each of these three must support high speeds and large approaches to building a gNodeB cell sizes as well as flight certification has pros and cons, depending on for airborne equipment. the application requirements.
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5G HANDBOOK CONCLUSION
REFERENCES
Engineers designing and building a 5G gNodeB have several options. Picking the right design depends on your application — in particular, the functionality required, the financial constraints, and the expected volume. A gNodeB will typically need higher processing performance than for previous LTE systems, but without excessive cost, size, or power consumption. In many cases, applications may also require a customized gNodeB, which goes beyond the capabilities of the 5G 3GPP standard, and a system that is flexible enough to meet evolving requirements. To meet these needs, SoCs such as those with multiple Arm cores provide the necessary performance and capabilities. On the software side, the development effort required to build a customized implementation from the ground up is impractical and customizing an existing PHY and protocol stack is likely to deliver the optimum system in a reasonable timescale.
1. Lars-Peter Clausen, “Understanding JESD204B High-speed inter-device data transfers for SDR,” https://archive.fosdem.org/2017/schedule/event/ jesd204b/attachments/slides/1549/export/events/ attachments/jesd204b/slides/1549/jesd204_ fosdem2017_lpc.pdf 2. Bob Witte, “The basics of 5G’s modulation, OFDM,” https://www.5gtechnologyworld.com/ the-basics-of-5gs-modulation-ofdm/ 3. Press release, “Nokia, Elisa join forces in Finnish private mobile networks push,” https://www. nokia.com/about-us/news/releases/2021/02/17/ nokia-elisa-join-forces-in-finnish-private-mobilenetworks-push/
Paul Moakes, PhD CEng MIET, is Chief Technology Officer at CommAgility. He has previously held positions at Motorola and Blue Wave Systems. He is co-inventor of two patents in the field of MicroTCA and AdvancedMC. He holds a PhD in Electrical and Electronic Engineering from Sheffield University and a 1st Class Honours degree in Electronic Communications and Computer Systems Engineering from Bradford University.
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ANTENNA PLACEMENT
5 TIPS FOR DESIGNING WITH EMBEDDED ANTENNAS Geoff Schulteis, Antenova
Designing an antenna into a wireless embedded or IoT device requires special care to maximize performance.
When designing a wireless device, pay attention to the antenna placement on a PCB. Space on the PCB, position, clearance, the ground plane, and correct connection to other components all affect antenna performance. Getting these aspects correct from the initial design concept will help to achieve a successful launch with reliable wireless performance. An antenna that performs well in free space as outlined in the manufacturer’s datasheet may perform differently when installed in a device. In the worst case, an antenna squeezed into a space towards the end of the design phase without consideration of the manufacturer’s intended layout guidance will probably function poorly. The design will need a revision, which adds cost and lengthens time to market. Follow these five basic principles of designing an embedded antenna into an electronic device and get a design right the first time.
ANTENNA PLACEMENT
chambers [Ref. 1]. Such tests reveal how well the device operates close to a person or in a person’s hand (Figure 1). Similarly, metal objects close to an antenna can affect its performance. Take the case of a tracking device for bicycles. The tracker will probably be fixed to the bike’s metal frame. When a device is used against a metal object, you should place the antenna as far away from the metal structure as possible. Doing so lets the antenna radiate without interference from the metal object. A device’s outer casing can play a role in how the antenna performs. If the case is made of metal or glass-filled plastics, it can also inhibit the antenna’s radiated energy. Consider making the case from non-glass-filled plastic unless you use a special antenna. An antenna’s position on a PCB can affect RF performance. Antennas need to radiate in six directions. Figure 2 shows how placing an antenna can radiate energy in more directions when placed correctly on a PCB. Fig. 2 also shows how a position on the corner of the PCB is better because fewer of the antenna’s fields
Because an antenna is sensitive to its surroundings, the general position of the antenna within the device can help or hinder a design. Place the antenna in a location that protects it from objects that might cause electrical interference with the antenna. The human body absorbs RF energy. If you are designing a wearable device that will be used on or close to the body, you may need to place the antenna on the side of the device facing away from the body. This is one reason why RF design companies conduct tests with phantom heads, bodies, and hands in anechoic 5Gtechnologyworld.com
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Figure 1. A phantom head in an anechoic chamber lets you test antenna performance in a controlled environment.
Figure 2. For best performance, place the antenna on the end of the PCB, not along the side edge.
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5G HANDBOOK will be obstructed, and the antenna’s performance improves compared to other locations. Antennas are often designed to operate in a corner position, but some antennas work best on a PCB’s long or short edge. Some antennas have right and left variants. You have options and thus you can select an antenna that best suits the PCB’s design and layout. You may have some freedom to vary the exact position of the antenna but study the manufacturer’s datasheet and position the antenna as recommended. This lets the antenna radiate correctly and achieve optimum performance.
POSITION THE ANTENNA IN RELATION TO OTHER COMPONENTS Even a high-performing antenna with good efficiency will not operate properly when close to another component that causes interference. Such interference might come from something on the PCB or another object close to the antenna. Some components cause interference in the radiated signal coming from an antenna. Batteries, LCDs, motors, and other metal objects can create noise or reflections that interfere with the antenna’s performance. Thus, you should keep the antenna as far away from these objects as possible. Figure 3 shows the antenna’s footprint and clearance through all the layers of the PCB. Other nearby antennas can cause problems, especially if they operate at similar frequencies or harmonic resonances of the primary wavelengths. They may cause an antenna to detune. Ideally, antennas should be isolated from each other, say up to -10 dB at 1 GHz and -20 dB at 2 GHz. The antenna may also need clearance. In this area, only the antenna pads and connections to the feed are present, letting it function correctly in free space. Check the datasheet for manufacturer’s clearance specifications and measurements. The area of clearance may be a little larger than the actual antenna. You may need to maintain clearance through all PCB layers. Design the PCB for the recommended antenna footprint and clearance space.
CHECK GROUND PLANE LENGTH REQUIREMENTS Surface-mount device (SMD) antennas usually need a ground plane to radiate energy. The ground plane is a flat surface that works a little like a mirror to balance the antenna for reciprocity. The ground plane is typically longer than the antenna. The lowest frequency of operation will dictate its length. Datasheets will specify requirements for the ground plane, but it also means that you will need
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Figure 3. When designing a PCB for an antenna, remember to allow for clearance.
a keep-out area below the antenna. If space is tight, we recommend choosing an antenna that requires a very small ground plane to meet the efficiency requirements to pass carrier radiated specs. Figure 4 shows how the ground plane can be adjacent to, or below the antenna or both. The distance between the antenna and its ground is also important and should meet specifications published in manufacturer datasheets. Every antenna is different, and the ground plane requirement is likely a deciding factor in your antenna choice.
HOW TO DESIGN THE RF FEED TRACE LINE The trace line connects the antenna to the radio. A poorly designed transmission line could degrade the performance of an antenna by as much as 50%. This part of the design needs specific care. Keep in mind two important principles. First, keep the RF circuits close to the antenna to minimize losses during transmission. A welldesigned transmission line with vias spaced evenly along its edges will help to minimize the noise and signal losses, which can affect antenna performance. Second, remember to consider the layers in the PCB stackup; do not place wires or traces directly above or below the antenna. Design all transmission lines for a 50 Ω characteristic impedance. Commercial software packages help you design a co-planar transmission line, and the easiest way to design the transmission line will be to use one. The program will calculate the appropriate transmission line width and gaps on either side Figure 4. Ground planes under or adjacent to antennas of the trace, so enhance their ability to radiate and balance the antenna. the characteristic
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ANTENNA PLACEMENT
Figure 5. FPC and SMD antennas give designers options for mobile and IoT device designs.
impedance of the co-planar transmission line is 50 Ω. We offer a similar free tool.
CONSIDER AN ALTERNATIVE ANTENNA Surface mount device (SMD) antennas are useful in a design because they only take up a small amount of space, but there are alternatives. Antennas are available in flexible printed circuit (FPC) form, and these offer a useful design alternative for certain situations. Figure 5 shows both SMD and FPC antennas. The FPC antenna connects to the circuit by its own RF coax cable with various connection options. You can use it in a different position within the device and it will not take up any of the valuable space on the PCB. An FPC antenna can be bent or folded, which introduces many options for placing it. For example, it might be fixed to the inside of the device’s outer case. Another important advantage to the FPC: it does not require a ground plane, which eases integration. The coax cable, however, of the FPC becomes part of the antenna. Thus, the routing of the cable should be designed with care. We recommend keeping this part of the antenna away from other components that might create noise and interference. FPCs offer some useful benefits, but ultimately the decision between the FPC and the SMD antenna will depend upon the nature of the device, the manufacturing process for the device, and the quantity produced. The FPC antennas are fixed to a surface with adhesive, and each FPC antenna needs to be placed by hand. This makes FPCs suitable for smaller production runs with shorter timescales and hand assembly. SMD antennas can be placed on the PCB by a pick-and5Gtechnologyworld.com
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place machine. These antennas are the better choice for devices that will be manufactured at larger volumes to reduce labor costs.
CONCLUSION Generally, you will find designing easier if you use a low profile, high performance antenna. RF design is, however, complex and we recommend that you obtain advice from an RF specialist — from inside or outside your company — before the finalizing a design. In most cases, the design will be tested in an anechoic chamber to check for performance and spurious emissions. This is the first step towards gaining certification in your design.
REFERENCES 1. Keyvan Yasami, “5G radios increase emphasis on compliance testing,” 5G Technology World, March 12, 2021. https:// www.5gtechnologyworld.com/5g-radiosincrease-emphasis-on-compliance-testing/ 2. Free Calculator for GCPW Transmission Lines, Antenova, https://blog.antenova.com/ rf-transmission-line-calculator (registration required).
Geoff Schulteis leads technical support for Antenova’s North American customer designs. He has over 20 years’ experience designing, integrating, and testing antenna systems for consumer products from R&D through manufacturing & commercial deployment. Geoff has designed antennas for various IoT applications ranging from 2G through 5G, earning him multiple patents.
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5G RADIOS INCREASE EMPHASIS ON COMPLIANCE TESTING Compared to 4G and previous generations, 5G’s mmWave frequencies and tight integration increase the complexity of both performance and regulatory compliance testing. Keyvan Yasami, Anritsu Many 5G test processes parallel those of 4G LTE, but 5G’s specifications and need for time and cost efficiencies are much more stringent. Designers of user equipment (UE), systems, and networks must account for compliance testing to ensure 5G products meet established guidance. Critical compliance tests include RF exposure, radio testing, and electromagnetic compatibility (EMC). Testing requirements vary by country and region. Table 1 lists government and industry associations standards that establish regulatory specifications for 5G devices in relation to testing of the transmitter and receiver. These tests differ from established RF exposure measurements that gauge the unintentional generation, propagation, and reception of electromagnetic energy from 5G
devices and systems into human tissue. All UE manufacturers and mobile carriers must guarantee product operation that falls within safe parameters for both RF exposure and EMC. ANSI and IEEE have passed a series of test standards for regulatory compliance. To verify regulatory compliance for EMC and RF exposure, 5G UE manufacturers, system manufacturers, and mobile operators must meet these standards.
RF EXPOSURE MEASUREMENTS RF exposure testing includes absorption and power density. Specific Absorption Rate (SAR) measurements cover UE that utilizes Frequency Range 1 (FR1) sub-6 GHz frequencies. SAR is defined in watts per kilogram of human tissue and measures the rate of RF energy absorbed by the body from the source of RF energy. SAR is a straightforward means to measure the RF exposure characteristics of devices to ensure
Figure 1. A robotic arm system performs SAR measurements at numerous locations on a courtesy of SPEAG. device under test. Schmid & Partner Engineering AG)
TABLE 1. GLOBAL 5G COMPLIANCE STANDARDS Region
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Organization
Specifications
Time line
FCC: Federal Communications Commission
Code of Federal Regulations(CFR) Title 47 Part 2, Part 22, Part24, Part27 (FR1) Title 47 Part 2, Part30 (FR2)
Available
ARIB: Association of Radio Industries and Businesses
Technical Regulations Conformity Certification(TRCC) Article 2-1-11-30 (FR1) Article 2-1-11-32 (FR2)
Available
ETSI: European Telecommunications Standards Institute
Radio Equipment Directive(RED) ETSI EN 301 908-25 (FR1, FR2) (Publication Target Feb. 2021)
Publication Target Feb. 2021
CTA: China Type Approval Network Access License
Test requirements follow 3GPP TS 38.521-1/3GPP TS 38.521-3, but test with specific channel. Focus on “Transmitter” and Receiver” part.
Under approval (*1)
RRA: National Radio Research Agency
KS X 3270:2019) (FR1) KS X 3270:2019) (FR2)
Available (*1)
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5G COMPLIANCE TESTS
they fall within the Federal Communications Commission (FCC) and other global safety guidelines. The FCC’s RF exposure standard sets maximum SAR levels well below those used in laboratory testing. SAR levels are also lower than what medical and biological experts say can cause adverse health effects. The FCC regulates SAR under 47 CFR Part 2, section 2.1093 [Ref. 1]. Products intended for general use must meet a SAR limit of 1.6 mW/g averaged over one gram of tissue in any part of the head or body, and 4 mW/g averaged over 10 grams for hands, wrists, feet, and ankles. Table 2 lists the main parameters that can affect SAR.
TABLE 2. PARAMETERS THAT MAY AFFECT SAR Radio service types (i.e. cellular, PCS, LMR, WLAN) Modulation types (CDMA, GMSK, TDMA, AMPS, etc.) Physical orientation to person (held-to-ear, held-to-face, belt-clip, lap-held, etc.) RF power level (in Watts or milliWatts) Changes to transmitter, antenna (extracted/retracted) or accessories (i.e. clips, batteries)
REGULATORY COMPLIANCE TESTS SAR tests use standardized models of the human head and body filled with liquids that simulate the RF absorption characteristics of different human tissues. Precisely positioned probes measure the RF energy penetrating the models. To determine compliance, engineers measure emissions from each device while it operates at its highest power level in all operational frequency bands. The established procedure outlined in IEEE C95.1 and IEEE 1528 utilizes a robotic arm system (Figure 1) that performs a series of measurements of the electric field at specific pinpoint locations [Ref. 2, 3]. It can be a tedious process, due to the robotic movements and multiple measurement points. An alternative method for these tests gaining adoption is outlined in Technical Standard IEC 62209-3 2019 [Ref. 4]. This approach (Figure 2) utilizes a vector measurement-based system with an array of probes to create a 3D field reconstruction of the wave pattern. This test process takes less time than the robotic method. The method is currently under consideration by the FCC in the United States. 5Gtechnologyworld.com
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POWER DENSITY FOR MMWAVE For UEs that use millimeter wave (mmWave) frequencies — Frequency Range 2 (FR 2) — power-density measurements are the accepted practice. Power density is the amount of power (time rate of energy transfer) per unit volume. For 5G devices, the dosimetric quantity of the electromagnetic field (EMF) exposure is the incident power density (IPD). To prevent excessive temperature elevation at the body surface, IPD specifies the restrictions on human exposure to EMF at frequencies above 10 GHz. Power density of a transmitting antenna can only be accurately measured in the far-field — farther than two wavelengths from the source.
MMWAVE TEST CHALLENGES In a 4G device, you can connect the device to the test instrument using a coaxial cable. Thus, you can evaluate the transceiver and antenna separately. 5G radios, however, use tightly integrated transceivers and antennas due to the introduction of mmWave frequencies and Massive MIMO. Over-the-air (OTA) testing is necessary with the device in an anechoic chamber.
Figure 2. IEC 62209-3 2019 testing makes RF energy measurements using a phantom head.
OTA tests include: • Equivalent, Isotropically Radiated Power (EIRP): tests taken spherically around the device under test (DUT) to gauge the antenna’s effect on radiated power. Isotropic antennas may be used for FR1, but FR2 links require specialized antennas that have additional measurement requirements, adding complexity to the test process. • Total Radiated Power (TRP): the sum of all radiated power over a 3D sphere surrounding the antenna must be taken. • Effective Isotropic Sensitivity (EIS): A measurement of sensitivity in a given direction. • Total Isotropic Sensitivity (TIS): the total available receive performance of a UE is determined by the average sensitivity of an antenna/receiver over a 3D sphere. OTA testing uses Direct Far Field (DFF) and Indirect Far Field (IFF) measurements. With 5 • 2021
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5G HANDBOOK DFF, the distance between the UE and the antenna approximates the plane wave. The OTA equipment configuration is relatively simple. You can simulate the arrival of signals from certain directions using multiple antennas. Use DFF with known UE antenna sizes. Use IFF for unknown UE antenna sizes. With IFF, a reflective mirror generates the plane wave, which lets it generate the wave in a shorter distance than DFF. Because the compact antenna test range (CATR) needs a reflector, the configuration is more complex and the equipment more cumbersome than DFF. It’s also impossible to simulate arrival of signals from certain directions/angles.
SMART TRANSMIT CONSIDERATIONS An emerging 5G test requirement focuses on Smart Transmit technology now being implemented into chipsets, which gives devices the intelligence to calculate the power required while avoiding high RF exposure. Sometimes referred to as Dynamic Power Control, it also factors the average transmit time, which is an important consideration. Smart Transmit utilizes time-averaging technology and spatialaveraging of power to monitor and control RF transmit power across multiple antennas. It is especially important due to the number of antennas in a 5G device, as well their frequencies, which include mmWave and high, mid-, and low sub-6 GHz bands. New regulatory testing will address this type of dynamic power control. Corresponding standards will also establish testing requirements for legacy technologies used by the UE to confirm conformity.
EMC AND REGULATORY TEST SYSTEMS Given the evolving standards by multiple regulatory bodies, engineers need flexible test systems. Flexibility controls cost-of-test because it allows for an efficient upgrade path. Overall, test systems need the following features: • Advanced, intuitive, and graphical/numerical user interface (UI): Various test parameters and test cases are implemented easily and efficiently with an advanced UI. • 2D/3D graphing: Antenna characteristics for 5G NR FR2 need to be displayed in 2D/3D graphs. Such views allow results to be intuitively understood. • Automation test software: a simple GUI to set test conditions and automated Pass/Fail measurement results improve measurement efficiency, while giving engineers greater confidence that the products are in compliance. • RF test support: test systems must measure key items, such as those in Table 3.
RF exposure test processes need to be enacted by UE, system, and network designers, as well as mobile operators to ensure they meet stringent guidelines set forth by industry and government agencies. EMC compliance, while always a test consideration in wireless designs, has become more in focus with 5G’s commercialization. Test environments need to meet current test parameters while still providing a clear and efficient upgrade path as 5G evolves.
REFERENCES 1. FCC Policy on Human Exposure to Radiofrequency Electromagnetic Fields, https://www.fcc.gov/general/fcc-policy-human-exposure 2. IEEE C95.1-2019 - IEEE Standard for Safety Levels with Respect to Human Exposure to Electric, Magnetic, and Electromagnetic Fields, 0 Hz to 300 GHz, https://standards.ieee.org/standard/C95_1-2019.html 3. IEEE 1528-2013 - IEEE Recommended Practice for Determining the Peak Spatial-Average Specific Absorption Rate (SAR) in the Human Head from Wireless Communications Devices: Measurement Techniques, https://standards.ieee.org/standard/1528-2013.html 4. IEC/IEEE 62209-3:2019 Measurement procedure for the assessment of specific absorption rate of human exposure to radio frequency fields from hand-held and bodymounted wireless communication devices - Part 3: Vector measurementbased systems (Frequency range of 600 MHz to 6 GHz), https:// webstore.iec.ch/publication/30773
Keyvan Yasami is Market Development Manager for Anritsu. Keyvan has 10+ years in the wireless market and holds a Master of Science Degree in Electrical Engineering from the University of Maine.
TABLE 3. TYPICAL RF MEASUREMENTS FOR COMPLIANCE TESTING RF output power (peak power, conducted, and radiated)
Frequency stability
Peak and average power spectral density – PAPR limits
Tx adjacent channel leakage power ratio
Occupied Bandwidth (OBW)
Rx reference sensitivity level
Tx/Rx spurious emissions
Rx adjacent channel selectivity (ACS)
Tx spectrum emission mask
Rx blocking & intermodulation Characteristics
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MEET TIMING REQUIREMENTS IN 5G NETWORKS Jim Olsen, Microchip Technology
5G New Radio (NR) networks pose a variety of engineering challenges. They bring significant changes to every part of the network, from the core clocking function to the Radio Unit (RU) air interface. System designers will need to engineer 5G NR units to meet new timing and cost requirements. That includes re-engineering 5G advanced network and radio services, synchronization architectures, and both fronthaul and core transport. Meeting these and other challenges requires a new set of best practices for selecting RU components, and a thorough understanding of how these decisions will affect the entire network.
ENGINEERING THE 5G RADIO UNIT Mobile networks depend on synchronization between radios. Specifically, the time alignment error (TAE) between different frequencies at the transceiver array boundary (TAB) determines the synchronization, transport engineering, and components required for adjacent radios to connect to user equipment (UE) and operate without co-channel interference. This applies to both frequency division duplex (FDD) and time division duplex (TDD). NR will primarily operate on the latter, which is a new operating mode for most network operators. TDDs will imply re-engineering the timing network to meet the 5G requirements at the RU. NR has stringent TAE engineering requirements that carried over from LTE-Advanced (LTE-A), creating additional challenges for RU engineers. NR introduces, for example, a new power-management schema. LTE Evolved Node B (eNB) remains active in idle state, with continual transmission of idle mode signals such as Synchronization Signal Block (SSB) and Cell-Specific Reference (CSR). The longer idle periods in NR reduce the NR network’s heat, power consumption and UE paging while improving overall performance with lower energy consumption than LTE [Ref. 1]. 5G RU design engineers, therefore, need components with fast start-up, high-frequency on/ off cycles, and high MTBF that integrate hardened Digital Front End (DFE) application-specific blocks for maximum power saving and high performance-per-watt. These components also need to scale from small cells to macrocells while delivering carrier aggregation and multi-band 400 MHz, multimode, and instantaneous bandwidth 5Gtechnologyworld.com
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5G needs tighter timing requirements than do 4G networks. The timing must perpetuate from the radio throughout the telecom network core.
allocation over Frequency Range 1 (FR1) and FR2. In addition, they must support existing/emerging Gallium Nitride (GaN) power amplifiers (PAs) to future-proof the RU. mmWave (FR2) has well-known power challenges aggregating Multi-User-Multiple Input, Multiple Output (MU-MIMO) interfaces while managing beamforming, etc. Maintaining RF timing stable enough to meet the network time error (TE) requirement also impacts the on-board oscillator used to provide timing to the radio’s transimpedance amplifier (TIA) and PA DFE blocks. This timing must ensure a stable TAE at the RU. Traditionally, oscillators on the Baseband Unit (BBU) ensured clock holdover should the radio lose its timing signal. This is no longer feasible for two reasons. First, the BBU is no longer a timing interface or demarcation point in 5G networks because it’s disaggregated into the Centralized Unit (CU) and Distributed Unit (DU) functional blocks in an Open RAN architecture. Second, oscillator choice is also constrained by cost, heat, and power considerations, which is why Temperature Compensated Crystal Oscillator (TCXO) or Micro-ElectroMechanical Systems (MEMS) technology are replacing high-performance oscillators built with other technologies. When choosing oscillators, you should understand that 5G fronthaul transport cannot use Common Public Radio Interface/Open Base Station Architecture Initiative (CPRI/OBSAI) over fiber with proprietary timing and high-frequency timing pulses. Instead, 3GPP mandates IEEE 1588 Precision Time Protocol (PTP) over Ethernet (Figure 1). PTP has implications for the behavior of the selected oscillators. Low-cost MEMS oscillators introduce severe constraints, react poorly to physical-layer rearrangement, and typically cannot sustain the bandwidth used in PTP G.8275.2 profile [Ref. 2]. The result: they must be engineered with the lower-bandwidth G.8275.1 PTP
Figure 1. 5G fronthaul transport must use PTP over Ethernet, which impacts both the fronthaul and backhaul networks.
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Figure 2. Option 6/Option 8 small cells with the DU integrated in the RU with the TAA and DFE can more easily support both G.8275.1 and/or G.8275.2.networks.
profile on the fronthaul network. This has a concomitant impact on engineering both the fronthaul (DU to RU) and the backhaul network.
FRONTHAUL ENGINEERING In 4G LTE, CPRI fronthaul transport impacted the network’s capacity, synchronization, and cost. NR introduces the new Enhanced CPRI (eCPRI) over Ethernet fronthaul, which must be engineered such that time-sensitive radio control services operate effectively. 5G NR requires adjacent radios to adhere to both absolute and relative TE specifications [Ref. 3], which implies either Primary Reference Time Clock (PRTC), ITU standard G.8272, or Telecom Boundary Clock (T-BC) within 260 (nsec) of the RU. The removal of protocol constraints in NR compared to LTE, combined with moving timing to Ethernet, has added considerable flexibility to fronthaul engineering, with some caveats. The lack of operationally viable clock holdover on the BBU or RU requires the use of a highperformance T-BC on the fronthaul network’s switches. Component choices of RU design engineers drive the timing architecture on the 5G fronthaul network. Table 1, from the Telecom Boundary Clock ITU standard G.8273.2, shows the maximum cTE allocated to the various BC classifications. As we have seen, the combination of tight TAE at the RF interface, the use of
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zero-holdover oscillators and the mandatory use of G.8275.1 for synchronization impact fronthaul engineering considerations. As in LTE (TS.36.104), 5G NR mandates the use of ±1.5 µsec absolute TE at the air interface (TS 38.104). To let operators meet these TE requirements on the transport network, the T-BC (G.8273.2) recommendation has changed four times, taking the industry from Class A – constant TE (cTE) ±50 nsec to Class D cTE @ ±5 nsec. These rapid changes have forced T-BC component redesign in core switches and a succession of onerous network upgrades for mobile operators. In other words, the increasingly tight TE requirements at the edge of the network, along with the way the RU is being engineered, continues to impact the overall switch/router fabric of the transport network.
SMALL CELLS AND THE RETURN OF G.8275.2 While the development of power-efficient highbandwidth small RUs has created challenging network upgrades, it also facilitates highly distributed small cell Open RAN service architectures. These architectures will enable small cell-based 3GPP Release 17 applications such as integrated access and backhaul (IAB), cellular vehicle-to-everything (C-V2X), IoT, and new private networks with local PRTC (source clocks). These applications use 5G Core 5 • 2021
(5GC) or 5G-U unlicensed or leased spectrum, operator network slicing, and the UK Joint Operator Technical Specification (JOTS), [Ref. 4, 5] for a neutral host “gateway.” Small cell infrastructures of this type will rapidly replace proprietary distributed antenna system (DAS) which can’t compete with the TDD based sub 6 GHz C-band, or mmWave RU. 5G small cells will also allow deployment of IAB systems. IAB can be fixed or ephemeral, line of sight or meshed, and use FR1 or FR2 for both mobile termination (MT) and backhaul with configurable radio clusters. Moreover, small cells will have a more flexible deployment profile than Option 7.2 (Open RAN macro cells), which is tied to G.8275.1. Option 6/Option 8 small cells with the DU integrated in the RU with the TAA
T-TSC Class
Permissible range of constant phase/time error — cTE(nsec)
A
±50
B
±20
C
±10
D
±5
Table 1. Telecom Boundary Clock ITU standard G.8273.2 specifies the maximum cTE allocated to the various BC classifications.
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Figure 3. These calculations show an example of available time error (TE) in a fronthaul network.
and DFE can more easily support both G.8275.1 and/or G.8275.2 (Figure 2). The latter profile will be crucial in a non-engineered environment such as an existing LAN where the switches lack T-BC. Simultaneous with the changes in the radio network to meet phase requirements, planners and synchronization engineers have been deploying high-availability clocks in the core network, replacing time-division multiplexing (TDM)-based primary reference clocks (PRC) and synchronization supply units (SSU) with Ethernet-based clocking functions including Enhanced PRC (ePRC)/G.811.1, ePRTC /G.8272.1 @ ±30 nsec maximum absolute time error (maxTE), and PRTC-A or PRTCB/G.8272 @±100 nsec or ± 40 nsec maxTE respectively, to provide PTP and/or Synchronous Ethernet (syncE) timing. These functions must be comprehensively engineered into the core transport and timing networks.
ENGINEERING THE CORE TRANSPORT Two issues have forced a rethink about the most effective core transport layer for PTP timing: • The need to avoid being dependent on Global Navigation Satellite System (GNSS) satellite-based timing by meshing ePRTC to create “GNSS failure-resilient” networks using landbased time transfer. • The increasing need for stable high-performance extremely tight timing to the 5G RU. Until recently, Ethernet G.8275.1 (On Path Support) networks dominated timing deployments. Operators, however, now deploy PTP engineered on the optical layer. Carrying PTP on the lambda or optical timing channel using boundary clocks designed for deployment with dense wavelength division multiplexing (DWDM)/coarse wavelength division multiplexing CWDM systems has brings extremely low TE of less than ±3 nsec (better than G.8272.3 Class D) and extremely high stability. With this implementation, a network can be engineered to PRTC-A (= ±100 nsec) at all service points, also referred to as a “virtual PRTC.” vPRTC enables synchronization engineers to push ±100 nsec TE to the edge of the network, nearer to the DU where fronthaul begins. Such low TE in the core, coupled with equally low TE on the engineered fronthaul, provides the network timing and planning tools needed for significantly greater elasticity in engineering the synchronization network in the access/midhaul distribution network. Consider this simple calculation for a vPRTC with a fast fronthaul and a metropolitan area Ethernet network (Metro E) network between the vPRTC dropoff and the DU pool (Figure 3). The vPRTC could span several hundred kilometers over ten or more DWDM hops with less than ±100 nsec TE to the aggregation router. The metroE may have any Class T-BC. As both vPRTC and fronthaul have low TE, there is a huge timing budget available to appropriately engineer the access network (±1.24 µsec). This gives the network engineer tremendous flexibility in how to engineer the end-to-end network. 5Gtechnologyworld.com
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CONCLUSION
The logical and geo footprints of core LTE networks will stay relatively stable as operators migrate to 5G, but subtle engineering challenges and the solutions applied in deploying 5G RUs will have repercussions throughout the network. The ability to reduce cost and form factor in the 5G RU while maintaining TAE will require proper component selection which, in turn, will impact the fronthaul network and core network transport architectures. There also will be a corresponding evolution of synchronization engineering with the development of highperformance source clocks and optical-layer boundary clocks for the next-generation transport network. In short, there will be growing reliance on meeting timing requirements at the RU and the way this is accomplished will have a significant impact on how the entire mobile network is engineered.
REFERENCES 1. Pål Frenger and Richard Tano, More capacity and less power: How 5G NR can reduce network energy consumption, Ericsson, https://www. ericsson.com/en/reports-and-papers/research-papers/how-5g-nr-canreduce-network-energy-consumption 2. Tim Frost, The PTP Telecom Profiles for Frequency, Phase and Time Synchronization, May 2013. https://www.microsemi.com/documentportal/doc_download/133481-ptp-telecom-profiles-for-frequency-phaseand-time-synchronization 3. TS 38.104 Section 6.2 4. Neutral Hosts on JOTS NHIB, Small Call Forum document 250.01.01 https://www.smallcellforum.org/neutral-hosts-on-jots 5. Joint Operators Technical Specifications, Mobile UK, https://www. mobileuk.org/jots
Jim Olsen is a solutions architect in the frequency and time systems (FTS) business unit at Microchip Technology. He has extensive experience in designing and implementing network synchronization and timing architectures in more than 50 countries. He joined Microchip in 1984 and has since served in a wide range of service, sales and marketing roles. In 2000, he transitioned to an advanced technologies role, helping identify new technologies and investment opportunities, and is currently a solutions architect for the North America region. Olsen speaks regularly at industry seminars and events and his numerous articles and whitepapers on synchronization and timing have appeared in books and trade publications.
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HOW TIMING SOURCES SYNCHRONIZE OPEN RAN NETWORKS IEEE 1588 PTP and SyncE protocols keep radio units, switches, and distribution units in sync. Jeff Gao, SiTime
Deployment of 5G networks in cities around the world marks one of the largest and fastest evolutions in networking infrastructure. To inspire competition and innovation, the open radio access network, or Open RAN, is an industrywide initiative to enable interoperability among hardware suppliers. The standardization of Open RAN levels the playing field, driving lower costs with more competition while letting operators mix and match components for best performance. Network providers, and by extension consumers, will welcome decreased costs from interoperability. The growing number of components that need synchronization brought on by the densification of the network makes careful consideration of network timing functions critical.
Figure 1. The 5G Open RAN architecture is split into three base components: the RU, fronthaul switch, and DU.
The OPEN RAN architecture consists of three major components: the remote radio unit (RU), the fronthaul switch, and the distribution unit (DU), shown in Figure 1. The RU serves as the consumer’s access point to the network. The DU acts as the connection to the
Figure 2. TCXOs, OCXOs, MEMS oscillators, and network synchronizers compose the clock tree for RU and DU systems.
TCXO/OCXO/ MEMS
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NETWORK TIMING Figure 3. The frequency over temperature slope of a TCXO is a critical factor in Open RAN timing and independent of the frequency stability.
centralized unit (CU) and the mobile core. Finally, the fronthaul switch routes traffic between the RU and DU. The standardized interface that transmits information between components of the radio access network is known as enhanced Common Public Radio Interface (eCPRI). These components need precise synchronization to avoid data packet loss and network interruptions. Before we cover methods of synchronization, you should understand time division duplexing (TDD) and why it’s becoming the dominant form of duplexing for 5G OPEN RAN. Unlike frequency division duplexing (FDD), TDD separates outbound
and inbound signals by transmission time slots, not separate frequencies. TDD offers more flexibility as the ratio of outbound to inbound transmission can be adjusted based on the demand at any one time, resulting in a more efficient use of the channel. This flexibility is necessary in 5G Open RAN as upload and download demand change constantly. Because TDD depends on an understanding of timing, the network uses IEEE 1588 Precision Timing Protocol (PTP) to synchronize transmitted and received signals. While time-based synchronization is becoming more common in the network, frequency-based synchronous Ethernet
(SyncE) remains an underlying technology critical to system synchronization. Additionally, both PTP and SyncE are robust alternatives to GPS synchronization. While accurate, GPS reference timing can be affected by poor weather and is vulnerable to jamming or spoofing. Depending on the network architecture, Open RAN networks can use PTP, SyncE, or both for synchronization. Not only is PTP critical because it’s the gateway to nanosecond time error, it’s also an intelligent system that can adapt to the loss of a grandmaster and can selectively reassign the “highest ranking” timing packet. The growing amount of data passing through the network at faster speeds makes the advantages of PTP synchronization crucial for a reliable open RAN. Therefore, oscillators and jitter cleaners that enable the highest performance from the IEEE 1588 protocol are just as crucial. Of the three main components in the Open RAN architecture (Figure 2), the RU has the least stringent timing requirements, but it must be the most environmentally robust. RUs are often installed in dense, uncontrolled environments and must remain precisely synchronized to the rest of the network while subjected to heat, airflow, and vibration. The densification of radios in 5G Open RAN requires them to be placed in environmentally
Figure 4. The DU server usually requires the added stability of an OCXO to maintain accurate timing even without a reference clock.
TCXO/OCXO/ MEMS
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Figure 5. Two similarly specified TCXOs have dramatically different ADEV under airflow.
unforgiving locations: on rooftops, poles, and near roads and highways. RUs generally require the timing of high-performance temperature-compensated oscillators (TCXO) or MEMS oscillators. Similar in architecture to the RU, the fronthaul switch uses a reference oscillator and jitter cleaner to clock an FPGA accelerator, which also performs the IEEE 1588 processing.
WHICH TIMING SOURCE?
Figure 6. System architects must ensure the combination of the TCXO and network synchronizer doesn’t exceed the SyncE TDEV mask.
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Only the most precise TCXOs available today can maintain the performance required by the RU and fronthaul switch under environmental stresses. Frequency slope is one of the most critical oscillator parameters for Open RAN. This specification describes how the frequency will behave as ambient temperature changes. Any sudden changes in frequency can correlate to a high frequency slope. In applications dependent on PTP, having a low frequency-over-temperature slope, usually considered ±1 ppb/°C or less (Figure 3), lets the TCXO maintain an accurate reference between timing packets, even under fast temperature ramp. This enables a longer loop bandwidth, giving the IEEE 1588 algorithm more time to choose the packet from the highest-ranking clock available. The component of the 5G open RAN with the most demanding timing specification is the DU (Figure 4), usually requiring an oven controlled oscillator (OCXO) for added stability. As the gateway to the network core, it must maintain a precisely synchronized time reference which it passes down to the fronthaul switch and RU. In some networks, the fronthaul switch will also require an OCXO. In Open RAN systems, the need for an OCXO over a TCXO is generally driven by a time holdover requirement, which defines how long it can “free run” without a reference before accumulating a certain time error. If all timing references are lost, the DU must maintain an accurate output clock until connection to the reference is restored. Most DUs currently being designed will support four to six hours, and in some cases up to 12 hours of holdover before exceeding a 1.5 µsec time error. Increasing time holdover in the DU improves reliability across the system because accurate time pass down to the fronthaul switch and RU even when connection to the reference time is interrupted.
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NETWORK TIMING STABILITY COUNTS In addition to providing sufficient time holdover, the OCXO must remain stable under environmental stressors such as airflow and rapid temperature change. An OCXO must retain its accuracy even when placed near a fan or when the SoC emits heat under heavy load. Additionally, the push to minimize latency through edge computing is leading to more DUs being placed at the tower where they are exposed to similar stressors. Allan deviation (ADEV), a measure of oscillator stability in the time domain, is an important parameter for OXCOs used within Open RAN. Figure 5 demonstrates the difference in ADEV performance under airflow between high quality and low quality TCXOs. When the devices are subjected to airflow, the TCXO 1 has 38 times better ADEV at a 3 sec averaging time. A similar difference can be seen when comparing high and low quality OCXOs as well. When operating in a pole mounted RU or in a DU next to a fan, time error due to poor ADEV can cause delays in PTP packets and ultimately lead to data errors and loss of synchronization. While environmental resilience is needed to get the most out of a PTP synchronized system, PTP can be combined with SyncE for the best overall performance. To regulate these incoming timing references and operate the IEEE 1588 loop, an advanced type of PLL called a network synchronizer is needed. The ITU Telecommunication Standardization Sector (ITU-T) has defined the maximum time deviation (TDEV) allowable for SyncE and for best system performance, it is crucial to ensure the total TDEV is below this mask with considerable margin (Figure 6). Both IEEE 1588 PTP and SyncE are cornerstones of 5G OPEN RAN and using them together is essential to achieve the best system performance. As RUs are subjected to harsh outdoor conditions, better dynamic performance of the TCXO and OCXO under fluctuating temperature and airflow leads to less disruptions and service outages. The oscillators frequency over temperature slope must also be considered as lower a slope directly translates to more accurate PTP time stamps. Finally, network synchronizers are instrumental in managing the reference inputs, generating an array or clock outputs for various system, and functions facilitating the IEEE 1588 loop.
5Gtechnologyworld.com
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eeworldonline.com
Jeff Gao, Senior Director of Product Marketing at SiTime, has over 20 years of experience in the semiconductor and networking/communications industries in wireless systems, VoIP, biometrics, semiconductor timing, and embedded software. Prior to SiTime, Jeff held various product marketing and engineering positions of increasing responsibility with Atmel, Cisco, Vovida Networks and ArrayComm. His current technical interests include high precision timing and synchronization in 5G, data center, optical transports, and next-gen industrial applications. Jeff earned his MBA from the University of California, Berkeley and MSEE from the University of Wisconsin–Madison.
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