TERALIGHT
March 7, 2014 Author: Tom Wilson
CONVERGENCE OF LTE-FDD/TDD THE NEW FOUNDATION OF WIRELESS BROADBAND NETWORKS
Table of Contents Executive Summary....................................................................................................................................... 1 Background ...................................................................................................................................................... 2 Spectrum Allotment for LTE ...................................................................................................................... 2 FDD & TDD Duplexing .................................................................................................................................. 3 Half Duplex Operation ............................................................................................................................................... 3 Full Duplex Operation ................................................................................................................................................ 3 Difference between FDD & TDD ............................................................................................................................ 3
LTE v/s. LTE A ................................................................................................................................................. 4 Conclusion......................................................................................................................................................... 5
Executive Summary Spectrum is both a finite and limited resource. As subscribers demand faster and increased mobile broadband services capabilities, operators face stiff challenges via limited available spectrum. As per previous 3GPP standards, mobile operators deployed Frequency Division Duplex (FDD) technology and the expectation was the same for LTE. However in designing LTE, the 3GPP committed to the first truly global technology standard by ensuring it supported not only FDD, but also Time Division Duplex (TDD) spectrum use. This paper addresses how LTE-FDD sub 1Ghz (referring to either 700/800 MHz bands or both depending upon the region of use that is applicable) spectrum would best serve as the foundation for wireless broadband networks and LTE-TDD be utilized to serve special high density and high capacity requirements of operator networks. This paper also illustrates how FDD and TDD can be leveraged to produce a stronger broadband opportunity – where FDD utilizes paired spectrum with multiple operators desiring the same band(s), the MNOs potentially will need to share these bands and may receive actual allotments of spectrum, which will not provide adequate growth capacity. However LTE FDD coupled with higher capacity TDD utilizing unpaired higher frequency spectrum bands better serving urban network density requirements coupled with small cell potential, the network would be aided with greater broadband capacity.
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Background Mobile broadband demand is at an all-time high, with some operators reporting a doubling of data traffic during each of the last five years. With increasing adoption of sophisticated Smartphone’s and tablet devices, more and more users are turning to mobile broadband (MBB) as their primary means for Internet access, content, applications, communications and messaging. In particular, video streaming, content downloading, gaming and other high bandwidth, dataintensive multimedia applications are accelerating mobile data traffic growth at exponential rates. We are reaching a point of network saturation as more smart devices are penetrating the market, enabling users to satiate their hunger for advanced services and applications. As such, some predictions of the growth of mobile data traffic is expected to grow 11-fold over the next five years (Cisco VNI Report). Mobile technologies have evolved from 2G to 4G HSPA+ and LTE Advanced (Long Term Evolution-A as per the 3GPP) providing increased mobile broadband data capacity at a lower cost per bit, reduced latency and enhancing the customer’s online experience. In many countries, competitive and market forces have combined to accelerate deployment, aggressive marketing and rapid adoption of 4G LTE, even seeing recent LTE-A trials. LTE supports two duplexing modes: FDD (Frequency Division Duplexing) and TDD (Time Division Duplexing). As of October 2013, according to statistics published by the Global Mobile Supplier Association (GSA), 268 LTE networks have been commercially launched in 100 countries, reaching 126 million subscribers primarily concentrated in North America and the Asia/Pacific region. Most initial LTE deployments have used paired frequency-division duplex (FDD) spectrum in the 700MHz, 1800MHz, and 2600MHz bands. 23 LTE TDD networks have been identified in 18 countries, with nearly all of which using 2300GHz or 2600GHz spectrum.
Spectrum Allotment for LTE As a consequence of its propagation characteristics, spectrum in sub 1GHz frequencies is better suited for rural applications than higher frequencies. In comparison to the higher frequency bands sub 1GHz frequencies allow for much greater distances between base stations and improved penetration of obstructions such as building walls. For these reasons, sub 1GHz spectrum is much more efficient for deployment in regions of low population, and therefore subscriber density. However given the high desirability of these spectrum frequencies, in markets with multiple MNOs vying for such sub 1GHz spectrum bandwidth allocation and hence capacity to each operator is limited by narrow allotments. Contrary to sub 1GHz spectrum, higher frequency deployment for data based networks, such as LTE – TDD at other higher frequency bands (bands 38, 40, 41, 42 and even some higher frequencies or even much higher) works well for urban dense populations given the need for much larger bandwidth requirements, rather than the sparse density of rural applications. Whilst buildings and other obstructions can block higher frequency signals, the use of small cell technologies and other means help alleviate these issues. TDD unpaired spectrum at above 1880 MHz as a consistent allotment along with similar spectrum assignments for other markets would constitute a form of harmonization, thereby allowing much larger volumes for handset manufacturing of transceiver chip sets, thus lowering smartphone costs.
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FDD & TDD Duplexing The main difference between FDD and TDD is in how they divide the single channel to provide uplink and downlink paths. FDD does this by dividing the frequency band allotted into two discrete smaller channels. On the other hand, TDD uses the entire channel but alternates between uplink and downlink.i Due to the use of dedicated uplink and downlink channels, FDD is classified as a full duplex system. This means that both the upload and download are always available. However TDD is classified as half duplex because it allows for the dynamic allocation of frequency channels as either uplink or downlink channels, but not uplink or downlink in the same channel at the same time. This is not noticeable in voice applications that would normally require full duplex operation because the time divisions are extremely short (meaning ultra quick).
Half Duplex Operation In half duplex, the two communicating parties take turns transmitting over a shared channel. Two-way radios work this way. As one party talks, the other listens. In the distant past, speaking parties used to say “Over” to indicate they’re finished and it’s time for the other party to speak. In data networking, a single cable is shared as two computers communicating take turns sending and receiving data. Full Duplex Operation Full duplex refers to simultaneous two-way communications. The two communicating stations can send and receive at the same time. Landline telephones and cell phones function this way. Some forms of networking permit simultaneous transmit and receive operations to occur. This is the more desirable form of duplexing, but it is more complex and expensive than half duplexing. Primary Strengths and Advantages of FDD & TDD FDD and TDD LTE have their own strengths and weaknesses. FDD is generally better suited for applications like voice calls that have symmetric traffic profiles. LTE A further accentuates this point. This is because traffic in both directions is always constant. Therefore TDD would be wasting bandwidth in constantly switching from one to the other. TDD excels in applications that have asymmetric traffic profile, an example of which is online browsing. When browsing the web, it is typical that download speed is higher than upload; but when uploading videos, for example, the reverse is true. TDD can allocate more time for the part that requires more bandwidth, thereby balancing the load. With FDD, the bandwidth cannot be dynamically reallocated and the unused bandwidth is wasted. An additional advantage of FDD appears when planning sites for base stations. Due to FDD base stations using different 3|Page
frequencies for receiving and transmitting, they effectively do not “hear” each other and no special planning is needed. With TDD, special considerations need to be taken in order to prevent neighboring base stations from interfering with each other. Furthermore, prior assignment or use of Bands 7 and 38 for both TDD and FDD applications in the same 2600 MHz range may create more support for potential use of FDD at the lower sub 1GHz bands. According to the latest statisticsii currently 268 commercial LTE networks have been launched in 100 countries. Out of these, 240 are based on the LTE-FDD networks whereas only 28 of these are LTE-TDD Networks.
240 LTE FDD Networks
28 LTE TDD Networks
268 LTE Networks 44% LTE Networks deployed LTE 1800 Mhz
157.7 Million LTE Subscribers
Therefore it can be summarized that:
FDD LTE uses frequency division, while TDD LTE uses time division FDD LTE is full duplex, while TDD LTE is half duplex FDD LTE is better for symmetric traffic, while TDD is better for asymmetric traffic TDD LTE is better at reallocating traffic than FDD LTE FDD LTE allows for easier radio planning than TDD LTE
LTE v/s. LTE A LTE Advanced utilizing carrier aggregation is the next major step in the evolution of LTE Networks. It is an evolution network technology, which is expected to help in aiding the massive increase in mobile data demand as well as deliver higher data demands for all. Deployment of LTE-A will allow operators to claim 4G network performance in meeting technical ITU-R IMT Advanced specifications per ITU-R WG Party 5D. Advancements of LTE Advanced also include higher order of MIMO (multiple input and multiple output) where the number of transmit for downlink and uplink functions (antennae) is increased to eight and four respectively, SON (self optimizing networks), and stronger interference management provide for even pronounced performance impact than in the lower bandwidth standard ( usually either APT 700MHz Band 28 and 800MHz Bands 20 and others) associated with 1st generation LTE networks. LTE Advanced provides additional efficiency and performance motivation for LTE – Advanced FDD to be used at sub1GHz, predominantly APT 700 FDD Band.
Peak Rate Download rate (actual rate) 4|Page
LTE 300 Mbps 10 – 100Mbps
LTE Advanced 1 Gbps 100 – 300 Mbps
Upload Rate (actual rate)
5 – 50 Mbps
10 – 70 Mbps
The IMT-A concept outlined in the ITU IMT-2000 document states: “With the expectation that there will be a need for commercial services in multi-user environments targeting peak data rates approaching 100 Mbps for ‘highly mobile’ users, and up to 1 Gbps for nomadic (low mobility or stationery) users, the IMT-A concept requires mandatory backward compatibility with prior systems to match these high data rates.”
Conclusion In order to improve the efficiency of use of frequencies, operators should be assigned harmonized spectrum for convergence of LTE TDD / FDD where LTE – FDD in the sub-1G range such as APT – 700 Band 28, along with existing Bands 1 and 3 in the higher frequencies. The allotment of FDD at sub 1G spectrum, allowing for efficient network design (lower costs) and accordingly enables reach to rural subscribers at a more efficient cost quotient. MNOs can apply TDD unpaired spectrum at the higher bands such as Bands 38, 40, 41 and 42 to serve network capacity needs in dense urban areas. Spectrum bands 7 and 38 are a bit flexed, given mixed applications of both FDD and TDD are in the same 2600 range. The over arching issues are efficiency, international harmonization, stronger instrument/handset/tablet ecosystem and effective use of technologies that provide the greatest reach and capacity for real world broadband deployment, where operators are capable of handling all types of handsets for all types of users in all markets, developed, undeveloped and developing in a systemic and organized efficient manner. Bringing the opportunity of mobile broadband to every user is paramount. i ii
Ascom, “TD-LTE & FDD-LTE Basic Comparison”, Angel Ivanov , 12 Jan 2012 "LTE Deployment Map." Mapping LTE Deployments: LteMaps. N.p., n.d. Web. 17 Feb. 2014.
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