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Optimized Commissioning of Fiber-to-the-Antenna Cell Sites


Contents 1. Cable and Antenna System Verification................................................. 3

Coaxial Cable............................................................................................................... 7

Connectors................................................................................................................. 10

7-16 DIN connector......................................................................................... 10

Type N connector............................................................................................ 10

SMA connector.................................................................................................. 11

Base Station Antenna............................................................................................ 12

Tower Mounted Amplifiers (TMA).....................................................................13

Remote Radio Head (RRH)...................................................................................15

Diplexers and Duplexers........................................................................................17

Base Station Cabinet.............................................................................................. 18

2. Certification of Cable and Antenna Systems...................................... 19

Why Impedance Matching is Important....................................................... 20

Causes of Impedance Mismatches...................................................................22

Power Loss Through Cable and Antenna Systems......................................25

Sweeping of Cable and Antenna Systems.....................................................28

Return Loss in Coaxial Cable.............................................................................. 30

Voltage Standing Wave Radio (VSWR)...........................................................32

Insertion Loss............................................................................................................35

Distance-to-Fault (DTF)........................................................................................ 37

Calibration..................................................................................................................39

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3. Optical Fiber Cell Site Solution...............................................................41

Some Fiber Concepts............................................................................................ 44

Fiber-Optic Connector Standards.....................................................................52

4. Use Case 1: Measurement of Insertion Gain and Loss for TMA with Viavi CellAdvisor Cable and Antenna Analyzer......................... 54

CellAdvisor Cable and Antenna Analyzer (CAA) Overview......................56

TMA Measurement.................................................................................................58

Calibration.........................................................................................................58 Connectivity...................................................................................................... 61 Analysis.......................................................................................................................62 Reporting...................................................................................................................65 5. Use Case 2: Fiber Testing with CellAdvisor CAA................................. 67

1

Equipment Required............................................................................................. 68

Procedure.................................................................................................................. 69 6. Conclusion................................................................................................. 73 7. References................................................................................................. 75

Cable and Antenna System Verification Global mobile subscriptions are growing around 5 percent year-over-year. According to the 2015 Ericsson Mobility Report, by 2021 mobile subscription will cross 9.1B subscribers, from 7.3B today. With smartphone subscriptions taking the lions’ share, this continuous growth in subscriptions and data demand has forced many operators to accelerate the expansion and upgrade of their networks. At the same time, service providers are under a lot of pressure to keep costs in check while maximizing ROI.

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Cable and Antenna System Verification

Cable and Antenna System Verification

As one of the key CAPEX drivers for any service provider is the expansion

equipment—AKA remote radio head (RRH)—is installed at the top of the

of the RAN infrastructure, it is essential that every cell site, regardless of

tower. The RRH is connected to digital equipment, or base band unit (BBU),

type (macro, small, Pico, DAS network, etc.), is delivering the best possible

at the base of the tower, through a fiber cable using either CPRI or OBSAI

service. Failure to maintain quality standards during cell site construction

protocol. But regardless of the type of cell site, certifying all of the cables

or maintenance can significantly impact subscriber experience, causing

(coax or fiber), connectors, antennas, and other components is critical for

potential for churn. To prevent this from happening, all cell site deployments

optimal performance. These standards equally apply to tower mount or roof

and maintenance must follow a strict high quality of standard. All cables,

top mount installations.

whether coax, fiber, or twisted pair must be thoroughly certified, and every active and passive component in the antenna and cable system must be properly tested and performance validated.

In this paper we will discuss some of the key requirements and rationales of cell site cable and antenna system testing. Before we go into the details of test requirements for cable and antenna systems, it will be

Typical macro cell sites have evolved from coax-based infrastructure,

worthwhile to understand the different components of a cell site and

where generally radio units are installed close to digital equipment at the

their respective functions.

base of the tower, to fiber-to-the-antenna (FTTA) cell sites, in which radio

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Cable and Antenna System Verification

Cable and Antenna System Verification

Coaxial Cable

Coaxial cable is used as a transmission line for radio frequency signals. One of its applications includes feedlines connecting radio transmitters and receivers with their respective antennas. Coaxial cable conducts an electrical signal using an inner conductor (usually a solid copper, stranded copper, or copper plated steel wire) surrounded by an insulating layer and enclosed by a shield, typically one to four layers of woven metallic braid and metallic tape. The cable is protected by an outer insulating jacket. Normally, the shield is kept at ground potential and a voltage is applied to the center conductor to carry electrical signals. The advantage of coaxial design is that electric and magnetic fields are confined to the dielectric with little leakage outside the shield. Conversely, electric and magnetic fields outside the cable are largely kept from causing interference to signals inside the cable. Larger diameter cables and cables with multiple shields have less leakage. This property makes coaxial cable a good choice for carrying weak signals that cannot tolerate interference from the Figure 1: Common cell site structure and components

environment or for stronger electrical signals that must not be allowed to radiate or couple into adjacent structures or circuits.

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Cable and Antenna System Verification

Cable and Antenna System Verification

Figure 2: Coaxial cable

Coaxial cable attenuation depends on the following factors: length of the cable, frequency of use, the outside diameter of the inner conductor, and the inside diameter of the shield. At higher frequencies, due to skin effect,

Figure 3: Attenuation vs. frequency for different sizes

attenuation is much higher. Larger diameter cables offer less resistance, and the longer the cable, the higher the attenuation will be at the other end. Figure 3 gives a general view of attenuation with respect to frequency and cable diameter. Signal leakage is the passage of electromagnetic fields through the shield of a cable, and it occurs in both directions. Ingress is the passage of an outside signal into the cable which can result in noise and disruption of the desired signal. Egress is the passage of signal intended to remain within the cable into the outside world, which can result in a weaker signal at the end of the cable and radio frequency interference to nearby devices. It is important that the integrity of the cable shield and the insulation is not compromised; this will greatly impact the performance of the antenna system. Severe leakage usually results from improperly installed connectors or faults in the cable shield.

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Cable and Antenna System Verification

Connectors

Cable and Antenna System Verification

SMA connector Subminiature version A (SMA) connectors are semi-precision coaxial RF connectors developed in the 1960s as a minimal connector interface for coaxial cable with a screw type coupling mechanism. The connector has 50Ί impedance. SMA is designed for use from DC to 18 GHz.

The ends of coaxial cables usually terminate with connectors. Coaxial connectors are designed to maintain a coaxial form across the connection and have the same impedance as the attached cable. Connectors are usually plated with high-conductivity metals such as silver or tarnish-resistant gold. Due to the skin effect at higher frequencies, the RF signal is only carried by the plating and does not penetrate to the connector body.

SMA Male

Some of the key types of RF connectors used in the transmission system are

SMA Female

RP-SMA Male

RP-SMA Female

Figure 6: SMA connectors

outlined below. 7-16 DIN connector The 7-16 DIN connector or 7/16 (seven and sixteen millimeter DIN) is a 50 Ί threaded RF connector used to join coaxial cables. It is among the most widely used high power RF Figure 4: DIN connectors

connectors in cellular network antenna systems. Type N connector The

type

N

connector

is

a

threaded,

weatherproof, medium-sized RF connector used to join coaxial cables, ideal for RF performance Figure 5: N connectors

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from DC to 11 GHz.

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Cable and Antenna System Verification

Base Station Antenna

Cable and Antenna System Verification

Tower Mounted Amplifiers (TMA)

An antenna is an electrical device which converts electric power into radio

A Tower Mounted Amplifier or Mast Head Amplifier is a low-noise amplifier

waves, and vice versa. In radio transmission, a radio transmitter supplies

(LNA) mounted as close as practical to the antenna at base transceiver

an electric current oscillating at radio frequency (i.e., a high frequency

stations. A TMA reduces the base transceiver station noise figure (NF) and

alternating current [AC]) to the antenna’s terminals, and the antenna radiates

therefore improves its overall sensitivity; in other words, the receiver at the

the energy from the current as electromagnetic waves (radio waves). In

base station is able to receive weaker signals from the cell phone. Remember

reception, an antenna intercepts some of the power of an electromagnetic

that mobile transmit power is limited and cannot generally transmit more

wave in order to produce a tiny amount of voltage at its terminals that is

than 200 mW. Improving the uplink will also result in an overall cell coverage

applied to a receiver to be amplified.

improvement, as mobiles will not have to inject too much interference into the system plus battery drain will be lower. AWS band

Cellular band

Figure 7: Dual band antennas

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Figure 8: Tower mounted amplifiers

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Cable and Antenna System Verification

Cable and Antenna System Verification

The main components in the TMA are a band-pass filter which passes only signals at the receive bandwidth, an LNA which provides the signal gain, and a bypass switch which opens when the TMA is powered up and closes when there is no power. It may also have a duplex filter depending on the

Remote Radio Head (RRH)

configuration of the radio system and the TMA. Both filters and amplifiers affect the return loss measurement, which will be discussed later. The addition of the TMA complicates the testing of the antenna system, requiring that the TMA itself must be tested. An insertion loss/gain measurement is well-

The advancement in radio technology (improved power amplifier efficiency,

suited for this purpose.

better design, and longer mean-time before failure, has allowed service providers to install radios at the top of the tower close to the antenna. This has enabled service providers to improve capacity, coverage, and signal integrity, while at the same time reducing electrical, leasing, and cable costs. Now, instead of carrying bulky coaxial cables for every transceiver path (six or more per operator), service providers can use lighter and significantly fewer fiber and power cables between the remote radio unit at the top of the tower and the baseband unit. The baseband unit can be located at the base of the tower or remotely at a BBU hotel (think distributed RAN architecture).

Figure 9: Remote Radio Head (RRH)

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Cable and Antenna System Verification

Cable and Antenna System Verification

Having the radio next to the antenna has virtually eliminated the losses caused by coax cables. However, having the RRH at the top of the tower

Diplexers and Duplexers

also offers some challenges. Since all RF functions reside on the RRH, any RF maintenance or troubleshooting, such as interference analysis, requires reaching the top of the tower to get access to the RRH. This represents a higher operational expense and security concern.

Diplexers separate two different frequency bands in the receive path and combine them in the transmit path. These frequency bands usually will be wide apart in frequency domain for the diplexer to work satisfactorily. It is often referred as an RF power combiner/divider with the added functionality of filtering. Broadband filters are used to pass appropriate bands at the Tx and Rx path. It is very helpful in reducing the number of cables running up the cell-tower.

Figure 10: Diplexers Multiple bands connected to the antenna 6 cables, being reduced down to 2 cables using a diplexer.

A duplexer is a device that allows the use of a single antenna for both a transmitter and a receiver. In other words, a duplexer is a device which couples the transmitter and receiver to the antenna while producing isolation between transmitter and receiver.

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Certification of Cable and Antenna Systems

Base Station Cabinet

Generally located at the base of the tower or in some cases on a rooftop, the base station cabinet contains the hardware for the base station. Cabinets

2

are weather-tight and climate-controlled. Common equipment in a cabinet includes the power system, battery backup, transport network equipment, baseband units, radios, and fans for controlling temperature.

Fans to control temperature

Battery backup Digital unit (BBU) Backhaul

Certification of Cable and Antenna Systems

Radios

To understand why cable and antenna systems need to be certified, it is

Sectors are color coded

worthwhile to understand a few concepts explaining the role cables,

RF jumper cables

connectors, and antennas play in the transfer of electromagnetic and optical energy.

Figure 11: Base station cabinet

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Certification of Cable and Antenna Systems

Certification of Cable and Antenna Systems

If water is traveling from a larger high-flow hose to a smaller hose, there will

Why Impedance Matching is Important

be a backlog and water will leak from the weak point. Impedance mismatch causes a similar type of effect; as electro-magnetic waves travel through the different parts of the antenna system (radio, feed line, antenna, free space), they may encounter differences in impedance. At each interface, depending on the impedance match, some fraction of the wave’s energy will reflect back to the source, forming a standing wave (backlog) in the feed line. The

The flow of energy can be explained through a water flow system analogy.

better the impedance match, the lower the possibility of reflection, and in

Consider the diagram shown in Figure 12.

a real system there will always be some reflection. The ratio of maximum power to minimum power in the standing wave can be measured and is called the standing wave ratio (SWR). An SWR of 1:1 is ideal. An SWR of 1.5:1 is considered to be acceptable in low power applications where power loss is more critical, although an SWR as high as 6:1 may still be usable with the right equipment. Minimizing impedance differences at each interface (impedance matching) will reduce the SWR and maximize power transfer through each part of the antenna system.

Traditional Cell site RF Connectors

BBU

RF Jumper

Higher impedance hose

Figure 12: Flow of energy

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Backhaul

RRU

FTTA System BBU

High loss point

Antenna

Backhaul

RF

Coaxial Cable

Mismatch of impedance

Lower impedance hose

RF Jumper

Antenna

Backlog of water (Return Loss)

Fiber Fiber Distribution

Fiber Distribution

RF Jumper

Figure 13: Cable and antenna system

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Certification of Cable and Antenna Systems

Certification of Cable and Antenna Systems

Causes of Impedance Mismatches

Most of cable and antenna installation issues are related to the poor care of connectors, adapters, and cables. Cables can be deformed during manufacturing, shipment, or assembly, or during use from extreme temperatures and/or flexure. Coax cable, if bent too sharply, can crimp the dielectric filling, causing discontinuities in the impedance of the cable.

Figure 15: Dirty connector before and after it was cleaned

Too much pull (tension) or side pull can damage the

degradation. So the most important

cable-to-connector interface

way

(or boot) as well as the

torque to make safe, consistent

put a boot around the cable-

connections. Proper torquing of

to

Some even put bends that

connector

It is also a best practice to use proper

Most cable manufacturers

strengthen the connection.

the

with is to keep it clean.

connector can deform it.

interface

keep

operating well and easy to work

connector; pressure on the

to-connector

to

threaded RF coaxial connectors is Figure 14: Deformed outer shield of a cable

are thermo-formed in the boot section. This will prevent connector interface deformations and separation, which can certainly cause a discontinuity in the desired signal-conducting quality.

essential for reliable, maximumperformance interfaces. A properly installed

connector

with

the

Figure 16: Crimped connector

recommended torque value creates a low-resistance electrical path between the conductor and the connector.

Wear debris is made up of metal particles from the connector plating and

There are a lot of good quality torque wrenches available in the market for

body material wearing. This debris can cause poor turning, added wear

different types of connectors; it is highly recommended to use the proper

on the connector’s moving (rotating or sliding) parts, and RF performance

torque wrench per manufacturer’s recommendation.

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Certification of Cable and Antenna Systems

Certification of Cable and Antenna Systems

In summary, the following items should be considered for proper installation: • Coax shielding damage • Dielectric damage

Power Loss Through Cable and Antenna Systems

• Dirty connectors • Improper torque

When verifying and maintaining the operation of RF and transmission

Once installed, connectors and cables can malfunction. Some of the common reasons for malfunction noticed in the field include:

systems and antennas, measurements are often made along the coaxial cable connecting a transmitter (radio unit) to its antenna and/or between an antenna and its receiver (radio unit). This process, called line sweeping,

• Corrosion

measures signal attenuation or insertion loss and return loss as a function

• Loose connectors

of frequency. Line sweeping is also used to estimate the physical location of a fault or damage along the transmission line using the Distance-to-Fault

• Water intrusion

(DTF) measurement available on many RF and microwave signal analyzers.

• Coax clip fail • Tower climber stepping on cables • Bullet holes

When any cell site is designed, a link budget is assigned to both the uplink and the downlink. It is essential that the cable and antenna system supports that link budget; otherwise, the performance of the cell site will not be optimal. Non-optimal performance will negatively impact the coverage area,

To insure a cell site is performing per design, auditing and testing the cable and antenna system every three to six months, and fast issue resolution are highly recommended.

capacity, throughput, dropped calls, access failures, and end-user experience. A more simplistic view of the link budget can be explained as follows: • Pwr (DL) = Pwr (Radio) - losses due to transmission line (connectors, cables, jumpers, duplexers, diplexers) + antenna gain - path loss - fading loss • Pwr (UL) = Mobile TX Pwr + BTS antenna gain - path loss - fading loss + TMA (if present) - losses due to transmission line (connectors, cables, jumpers, duplexers, diplexers)

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Certification of Cable and Antenna Systems

Certification of Cable and Antenna Systems

Insertion Loss (dB)

10.0 8.0 6.0 4.0 2.0 0.0 0

3000

6000

9000

12000

15000

18000

Frequency (MHz) Figure 18: Insertion loss as a function of frequency for SMA cable assembly

Some connectors, such as 7/16 DIN connectors, are designed for low-distortion performance, in particular low passive inter-modulation distortion (PIM). High PIM can affect the performance of communications systems that rely on digital modulation formats, and these specially characterized connectors 3dB Additional Cable Loss Impact to Coverage

can ensure minimal levels of connector PIM. The PIM characterization is usually applied to any coaxial cables as well, with the connectors considered

Figure 17: Coverage difference between well-maintained site and site with line issues

as part of the system’s cable assemblies. No matter how well designed, a connector will eventually wear out

Just like cables, coaxial connectors are also evaluated by the same set of

and its performance will degrade with extended use. Connections and

parameters: maximum insertion loss (IL) and maximum voltage-standing-

disconnections result in wear and tear on any connector.

wave ratio (VSWR) as functions of frequency. The IL for a mated connector pair is simply 10log10 (PR/PT), where PT is the power transmitted (or applied to a connector) and PR is the power received from the connector after losses.

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Certification of Cable and Antenna Systems

Certification of Cable and Antenna Systems

NOTE: If the RL value is unusually good or low (say -35dB for example), this

Sweeping of Cable and Antenna Systems

may be an indication of excessively high-loss cable which could indicate a water ingress condition. Having a really good RL can also mean a loss of signal at the other end, which may be the reason returned energy was too low, causing an excellent RL value. Larger diameter, low-loss cable will generally sweep worse for return loss than high-loss, narrow diameter cable, because the high loss cable absorbs more of the reflected signal, resulting

As discussed earlier, line sweeping measures signal attenuation or insertion

in a smaller amplitude reflection received at the test equipment. Long cable

loss and return loss as a function of frequency. Once the measurements

runs will sweep better for RL than short rooftop applications using the same

indicate an issue with the line, then sweeping is also used to estimate the

logic.

physical location of a fault or damage along the transmission line using the distance-to-fault (DTF) measurement available on many RF and microwave signal analyzers.

Now it will be valuable to discuss the RL, IL, DTF, and VSWR, how they relate to each other, and the role they play in cable and antenna system health.

It is a best practice to certify every connector, cable, antenna, and other passive component before a cable and antenna system is put in service. Before an antenna is installed, always perform a return loss (RL) test on the antenna, make sure every port on the antenna is tested, and if the RL is worse than the vendor specification, never use that antenna. Similarly for the cable system, a return loss test should be performed first. A general rule of thumb is if the RL is better than -20dB (1% of the energy is lost), then run an insertion loss (IL) test for the cable system to see if this is within the design limits of the cell site in test (frequency of test, and length and size of the cable will play a major role in determining the insertion loss). If the RL value is above the -15 or -20dB limit (per service-provider-defined limit) anywhere across the sweep range, use the DTF test to troubleshoot and identify the questionable component and repair or replace as necessary. Repeat the return loss test, and verify the reading is below the –20 dB across the entire sweep range.

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Certification of Cable and Antenna Systems

Certification of Cable and Antenna Systems

Remember reflected power should be a very small fraction of the incident

Return Loss in Coaxial Cable

power, for example, a 1% {10log10 (1/100) =-20dB} return of power results in a -20dB RL. For a RL of -10dB, 10% of the power is returned. In general, -15dB (3% of power is reflected back) or better is considered a good starting limit for cable and antenna systems.

Return loss is the loss of power in the signal returned/reflected by a discontinuity in a transmission line or optical fiber. This discontinuity can be a mismatch with the terminating load or with a device inserted in the

RL’(dB) = 10log10

line; RL is a measure of how well the device or lines are matched. If the impedance match is good, all the energy will be transmitted through and

Pr Pi

Figure 19: Return loss around a cable connector

nothing should be reflected. Values for return loss range from infinity, for a perfectly matched system, to zero for open or short circuits. Some signal sources are sensitive to power being reflected back to them and require an overall system performance with lower return loss. Return loss is a negative number, but the correct definition of return loss is the difference in dB between the incident power sent towards the device under test (DUT) and the power reflected, resulting in a positive sign:

RL(dB) = 10log10

Pi Pr

However, taking the ratio of reflected to incident power results in a negative sign for return loss:

RL’(dB) = 10log10

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Pr Pi

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Figure 20: A sample reflection (return loss) measurement with a CellAdvisor JD720c

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Certification of Cable and Antenna Systems

Certification of Cable and Antenna Systems

To calculate VSWR for this scenario, simply take the ratio of the largest

Voltage Standing Wave Radio (VSWR)

standing wave to the smallest standing wave. In this case: VSWR= Vstanding1 / Vstanding2 = 1.2/0.8 = 1.5 Because VSWR and return loss are two ways to measure the same property,

Just like return loss, the VSWR test also indicates how well the cable and

you can use the following equations to convert between the two:

antenna system is matched. VSWR is also referred to as Standing Wave Ratio (SWR). It is also a measure of how efficiently RF power is transmitted from the power source, through the transmission line, and into the load. As its name implies, VSWR is calculated by taking the ratio of the largest to the smallest amplitude values of the standing wave created by the combination

VSWR =

1+10 1-10

-RL 20 -RL 20

RL = -20log10

VSWR-1 VSWR+1

of the incident and reflected waveforms. Values of VSWR range from one for a perfect impedance match to infinity for an open or short circuit. This scenario, in which both the incident and reflected waves are in phase (the standing wave has an amplitude that is the vector sum of the incident and reflected waves), shows the largest possible magnitude of the standing wave. Conversely, the smallest possible standing wave must then occur when the incident and reflected waves are 180 degrees out of phase.

Figure 21: Standing wave scenarios

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Figure 22: A sample VSWR measurement made with a CellAdvisor

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Certification of Cable and Antenna Systems

Certification of Cable and Antenna Systems

A higher VSWR ratio depicts a larger mismatch. We can summarize our discussion with the following table:

Insertion Loss

VSWR

Return Loss (in dB)

Notes

1:1

Infinity

A perfect match

1.1:1

26.44

1.2:1

20.83

1.3:1

17.69

1.4:1

15.56

1.5:1

13.98

1.6:1

12.74

1.7:1

11.73

1.8:1

10.88

1.9:1

10.16

2.0:1

9.54

3.0:1

6.02

4.0:1

4.44

5.0:1

3.52

6.0:1

2.92

10:01

1.71

Infinity:1

0

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Insertion loss is the loss of signal power resulting from the insertion of a device in a transmission line or optical fiber and is usually expressed in decibels (dB). If the power transmitted to the load before insertion is PT and the power received by the load after insertion is PR , then the insertion loss in dB is given by:

A good rule of thumb: 1.5:1 = 14 dB for most low power coax systems

IL(dB)

10log10

PT PR

Simply stated, loss is incurred by simply inserting the object (cable, connector, etc.) into the path of the RF signal between the source and the intentional radiator. There are three main causes of insertion loss: reflected losses, dielectric losses, and copper losses. • Reflected losses are those losses caused by the VSWR of the connector. • Dielectric losses are those losses caused by the power dissipated in the dielectric materials (Teflon, rexolite, delrin, etc.). • Copper losses are those losses caused by the power dissipated due to the conducting surfaces of the connector. It is a function of the material and plating used. In general, the insertion loss of a connector is on the order of a few Short or open circuit

Optimized Commissioning of Fiber-to-the-Antenna Cell Sites

hundredths to a few tenths of a db, e.g., LDF4-40A attenuation at 1 GHz is 0.022 dB/ft (0.073 dB/m).

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Certification of Cable and Antenna Systems

Certification of Cable and Antenna Systems

Distance-to-Fault (DTF)

Figure 23: Insertion loss

Distance-to-fault (DTF) measurements, typically expressed in units of Transmission line losses depend on cable type (larger diameter cables will

reflection coefficient, return loss, or VSWR as a function of distance, are

have smaller loss), operating frequency (the higher the frequency, the higher

used to find common faults in cable and antenna systems. DTF measures the

the loss) and the length of the cable run (the longer the length, the greater

distance-to-fault along the various system components of the transmission

the loss). A site with a longer antenna centerline will require a much larger

line in order to determine the locations of excessive measured reflections.

diameter coax cable to have a relatively low insertion loss. A damaged or

Basically, a built-in source sends signals through the antenna and cable

kinked cable can cause significant increase in the insertion loss, greatly

system and looks for reflection back to pinpoint the fault location caused

impacting network performance.

by poor connections, damaged cables, or faulty antennas. Once it has been determined that the cable insertion loss is higher than expected or the return loss and associated VSWR are out of spec, it becomes necessary to find the possible locations of the fault(s) along the transmission system. To run DTF, a few key things need to be considered:

1. Correct cable type is selected in the test instrument; this is essential

so the right loss / ft and velocity factor for that cable is used in determining the fault location.

2. End distance entered in the measurement tool is at least 20% and no more than 50% longer than the maximum line length being tested.

Figure 24: A sample IL measurement made with CellAdvisor

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Certification of Cable and Antenna Systems

Certification of Cable and Antenna Systems

Calibration

Accuracy and precision of measurements depends on proper calibration of the instrument. Before making any transmission line measurements, the instrument should always be calibrated. Errors introduced due to imperfections in measurement instruments and accessories used to perform a test, such as cables, adapters, etc., can be eliminated by proper calibration. As these errors are repeatable and predictable, when the calibration process is executed, they are accounted for in the overall measurements. However, if there are temperature differences between tests, the calibration can go off. Because of environmental changes, recalibrating the equipment is always recommended. Modern cable and antenna analyzers have electronic Figure 25: A sample DTF measurement made with CellAdvisor

calibration kits to reduce the time for calibration.

Figure 26: Calibrating CellAdvisor using Y-Cal kit or electronic EZ-Cal kit

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Certification of Cable and Antenna Systems

Once calibration is complete, the measurement should show no reflection or a VSWR value of 0 or return loss of -60dB, as shown in the following example.

3 Optical Fiber Cell Site Solution Figure 27: CellAdvisor calibration measurement

Optical cables transfer data at the speed of light in glass. Operators have found that fiber-to-the-antenna (FTTA) architectures offer a number of advantages over legacy coaxial systems, including improved signal integrity, capacity gains, smaller site footprint, and significantly lower energy consumption.

FTTA systems use fiber feeder cables for communication

between the ground-based controllers and the tower-mounted radios. Optical loss in these digital communication links is insignificant. The towermounted remote radio units (RRUs) connect to their antennas through short coaxial jumper cables. Signal loss is minimized because the RF signal only travels a short distance over the coax.

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Optical Fiber Cell Site Solution

Optical Fiber Cell Site Solution

Eliminating the need for coaxial cables and extending fiber simplifies installations because it eliminates the need to pull heavier coaxial cables (site loading factor), typically 1.5 to 2.25 inches thick depending on the height of the antenna centerline, to the top of the towers. Furthermore, typically each sector may require two cables, so an average deployment may have six for the three-sector cell site. When multiple operators share the same infrastructure, towers quickly become entangled with cables and cause power, wind, and weight load concerns. Figure 28: FTTA system, tower top view.

However, FTTA installation also presents new challenges. RF installers who are familiar with pulling thick, heavy cables up towers to the RRU must now learn to work with and handle more sensitive fiber cables to confidently install reliable, robust services. Fiber is more susceptible to damage from crimping, bending (even microbending), or straining caused while pulling or hauling the cable trunks or feeders. The fiber-optic industry is well aware that scratches, defects, and dirt on fiber-optic connector end faces negatively impact optical signal performance. Troubleshooting statistics show connector contamination as the number one cause of poor network performance, and mating contaminated fibers is the primary cause of permanent optical component damage. Dirty connectors can create disproportionate numbers of failures

Components of FTTA system:

such as increased attenuation, back reflection, and bit errors that can prevent

• FTTA trunking system: FTTA distribution enclosure, fiber jumper, fiber trunk

network delivery and impact quality of service. This failure mode extends

• Tower multi-fiber system, RDC jumper

troubleshooting times because often the contamination sources are difficult

• Discrete cabling: individual fiber feeder pairs

to isolate. Without systematically addressing dirty and damaged end faces,

• Hybrid cables (fiber and power)

the defects can degrade RF network performance and eventually take down

• Jumper cables, feeder cables, and trunk systems (multi-fiber)

the entire link.

• Fiber distribution units • Indoor equipment jumper cable • Accessories, e.g. cable clamps or rubber sleeves

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Optical Fiber Cell Site Solution

Optical Fiber Cell Site Solution

Light is transmitted and retained in the core of the optical fiber by total

Some Fiber Concepts

internal reflection. Any dust particles will impact the performance of the transmission of the signal through the fiber. Particles closer to the core will have more impact than those farther out.

Fiber optics communication technology, in which optical signals are sent down hair-thin strands of glass fiber, was developed in the 1970s. At the end of each fiber cable there are connectors. A fiber optic interface (connector) consists of the following four components:

Figure 30: Inspection image of LC type fiber connector

1. Body

Houses the ferrule that secures the fiber in place

2. Ferrule

Thin cylinder where the fiber is mounted and acts as the fiber alignment mechanism

3. Fiber Cladding

Glass layer surrounding the core, which prevents the signal in the core from escaping

4. Fiber Core

The 3 basic principles that are critical to achieving an efficient fiber optic connection are “the 3 Ps”: • Perfect core alignment

The critical center layer of the fiber; the conduit that light passes through

• Physical contact • Pristine connector interface Today’s connector design and production techniques have eliminated most of the challenges to achieving core alignment and physical contact.

Figure 29: LC type fiber connector

The key remaining challenge is maintaining a pristine end face. As a result, contamination is the number one reason for troubleshooting in optical networks. A single particle mated into the core of a fiber can cause significant back reflection, insertion loss, and even equipment damage. Each time connectors are mated, particles around the core become displaced,

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Optical Fiber Cell Site Solution

causing them to migrate and spread across the fiber surface. Particles larger

Optical Fiber Cell Site Solution

Two types of fiber are used: single mode (SM) and multimode (MM).

than 5 Âľm usually explode and multiply upon mating. These large particles can create barriers (air gaps) that prevent physical contact. Particles smaller than 5 Âľm tend to embed into the fiber surface creating pits and chips. For each successive mating, actual dB values increase as signal performance decreases. Dirt particles near or on the fiber core significantly affect signal performance.

Figure 33: Fiber types and their characteristics Figure 31: Fiber connector mating issues

Optical energy transmission in the fiber interface works on the same principle as in coax cable interface.

Figure 34: Loss of power through fiber Figure 32: Optical signal performance affected by connector contamination

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Optical Fiber Cell Site Solution

Optical Fiber Cell Site Solution

Optical power is lost as light travels along fibers, primarily from absorption

Scattering is the loss of a light signal from the fiber core caused by impurities

and scattering usually expressed in dB or dB/km.

or changes in the index of refraction of the fiber. Scattering accounts for about 95% of total loss.

Source

Receiver

Pin (Emitted power)

Pout Power variation

(Received power)

Figure 35: Absorption phenomenon in fiber

Figure 37: Scattering phenomenon in fiber

Light is absorbed due to chemical properties or natural impurities in the

Microbending losses are due to microscopic fiber deformations in the core-

glass. Absorption accounts for about 5% of the total loss.

cladding interface caused by induced pressure on the glass. Microbending occurs when the fiber core deviates from the axis. It can be caused by manufacturing defects, mechanical constraints during the fiber laying process, and environmental variations (temperature, humidity, or pressure) during the fiber’s lifetime, such as freezing water that causes external pressure or sharp objects that impede the fiber.

Figure 36: Impurities can impact optical signal transmission

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Figure 38: Microbending phenomenon in fiber

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Optical Fiber Cell Site Solution

Optical Fiber Cell Site Solution

Macrobending losses are due to physical bends in the fiber that are large

To ensure proper cell site performance now and into the future with new

in relation to fiber diameter; attenuation due to bending increases with

technologies and data rates, correct fiber installation techniques must be

wavelength (e.g. greater at 1550nm than at 1310nm); macrobends are

used. In addition, proper and thorough inspection, cleaning, and testing

typically found in RRU enclosures, junction boxes and fiber trays.

must be conducted and documented. Tests include: • Connector end face inspection and test • Circuit path verification using visual fault locator (VFL) • Insertion loss measurements (IL) • Optical return loss measurements (ORL) • OTDR testing (bidirectional)

Figure 39: Macrobending phenomenon in fiber deployments

Before connecting any fibers together, you must inspect and (if necessary) clean all connectors. Failure to inspect and clean prior to mating can cause

Loss through a fiber path can be represented as shown in Figure 40. Coupling Loss

Impurities Input

Injection Absorption loss loss

permanent damage to both connectors.

Junction Loss Output

Scattering loss

Scattering loss

Macro/micro bending loss

Figure 40: Power loss through a fiber path

The primary fiber problem is related to the optical connector interfaces, the most common issue being contamination due to poor inspection and cleaning practices. Other typical problems often include over-tightening of tower riser clamps (micro-bending losses) and cable routing exceeding the recommended bending tolerances (micro- or macro-bending losses). Other excessive losses can result from improper practices employed in managing

Figure 41: Inspect before you connect

strain relief at critical transition points or improper connector mating.

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Optical Fiber Cell Site Solution

Optical Fiber Cell Site Solution

Fiber-Optic Connector Standards

To guarantee a common connector performance level, the International Electrotechnical

Committee

(IEC)

created

standard

IEC-61300-3-35,

specifying pass/fail requirements for end face quality inspection before they are connected. Designed as a common reference for product quality

Figure 42: Mishandling can create cable kinks. A typical bend fault found on a macrocell using an OTDR SmartLink Mapper (right) during construction (loopback mode).

and performance, the IEC standard supports quality control throughout the entire fiber component life cycle, but only after testing compliance at every handling stage. This objective assessment eliminates varied results, allows installers to certify and record product quality, empowers installers of any skill level, ultimately improves installation quality, and reduces the potential for future performance issues. As FTTA pushes fiber into new and unfamiliar locations, connector contamination, bending, and poor practices become increasingly probable because of both environmental factors and mishandling. Proper fiber handling and a pre-service fiber acceptance testing process become essential

Figure 43: Inspect before you connect (left). A typical contaminated connector issue found on a macrocell link using an OTDR SmartLink Mapper (right) during construction (loopback mode).

and include, but are not limited to, visible fault location/continuity checks, connector inspection certification, and cleaning. It also includes measuring the optical power level, end-to-end link loss, return-loss, or testing optical time domain reflectometry (OTDR) to qualify fiber continuity and link element parameters.

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Use Case 1

4 Figure 44: TMA used at a cell site to improve uplink performance

Improving the uplink signal power translates into a combination of better

Use Case 1:

base station coverage while the mobile is transmitting at lower power, which in turn implies a lower demand from the mobile’s batteries.

Measurement of Insertion Gain and Loss for TMA with – Viavi CellAdvisor Cable and Antenna Analyzer A Tower Mounted Amplifier (TMA) is a low-noise amplifier (LNA) mounted as close as practical to the antenna in mobile base stations. A TMA reduces the base station noise figure, and therefore improves its overall sensitivity, allowing it to receive weaker signals from mobiles.

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Use Case 1

Use Case 1

CellAdvisor Cable and Antenna Analyzer (CAA) Overview The CellAdvisor CAA JD720C product line is the most cost-efficient cell site installation, commissioning, and maintenance solution on the market; it is the comprehensive market leading FTTA solution for both coax and fiber. The JD720C has all of the necessary measurement functions to perform performance verification such as insertion gain, insertion loss, and antenna isolation on passive or active RF devices, including cable, connectors, antennas, and amplifiers. Figure 46: Fiber Microscope P5000i

JD720C analyzers are capable of fiber inspection using the Viavi fiber microscope and optical power measurement using Viavi optical power meters. This single integrated solution with RF and fiber capabilities provides all the physical layer tests needed for the installation and maintenance of cell sites. The JD720C is an easy-to-use field instrument, equipped with an optional Bias Tee function providing DC power at 5 different levels through its RF-IN port, specifically designed for TMA testing.

Figure 45: CellAdvisor CAA720C series

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Use Case 1

TMA Measurement

Use Case 1

1. RF-OUT OSL Calibration Follow the procedure to calibrate the RF-OUT port of the instrument, which will

To perform TMA measurements, the following steps are required:

go through the Open-ShortLoad (O-S-L) sequence.

1. Calibration 2. Connectivity

2. RF-IN Load

3. Analysis

Immediately after the RF-OUT O-S-L calibration, connect the load on the

Calibration

RF-IN port in order to

The instrument must be calibrated in order to get reliable Insertion Gain/Loss

perform an isolation

measurement results. For best results, set the instrument to the frequency

calibration.

range that will be used for the test, and calibrate the instrument.

3. RF-OUT to RF-IN Immediately after the RF-IN Load, remove the load of the RF-IN port and connect a thru-cable in order to perform a through calibration.

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Use Case 1

Use Case 1

The measurement accuracy is achieved due to the calibration

TMA

procedure where the JD20C has compensated and normalized

Figure 47: JD725C/726C

any loss induced by the other

LNA

elements of the test setup such as cables and connectors,

ANTENNA

BTS

making a reliable performance test of the TMA. Connectivity The

performance

verification

on TMA is done by injecting a O-S-L Calibration Using Electronic EZ-Cal Kit The e-Calibration is an Open-Short-Load calibration using the electronic EZCal kit, which allows you to perform the O-S-L calibration quickly and easily with only one connection to the instrument. The O-S-L calibration must be

known

low

value

signal

The function of the TMA is to amplify the uplink signal

1. Connect an extension cable, if necessary, to the RF Out / Reflection port

be connected at the BTS Port

2. 3.

Connect the USB cable to the side of the EZ-Cal and then to the USB

the end of the extension cable if used.

RF IN

Port simulating an uplink signal.

and transmit it to the base

Connect the EZ-Cal to the instrument’s RF Out / Reflection port or at

RF OUT

(-30dBm) into the TMA Antenna

performed after setting the frequencies and connecting an extension cable.

of the instrument.

BIAS TEE

station. The instrument will of the TMA and perform an accurate measurement of the amplification factor of the TMA.

Figure 48: CellAdvisor CAA720C series used to test TMA

Host port to power the e-calibration kit.

4. Press the CAL hard key. On-screen instructions will appear to guide you through the e-calibration.

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Use Case 1

Use Case 1

Analysis

The JD720C has the flexibility required to properly characterize the performance of TMA. This flexibility is provided with the following set of analysis tools. Multiple Segment Limit (MSL) MSL allows setting multiple limit line segments at different frequency points, making a conformance mask for TMA.

Markers Six markers can be used simultaneously. Markers can be set on the trace(s) to indicate the specific power vs. frequency points on the trace. All of the marker information is presented in the marker table.

Marker Bands Marker Bands are user definable markers on frequency sub-bands enabling a visual identification of uplink and downlink frequencies performing compliance verification with a single trace.

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Use Case 1

Use Case 1

Reporting The JD720C provides the following options to transfer measurements into the instrument’s software (JDViewer) for further measurement analysis and reporting.

Multiple Traces Function This function is used to retrieve multiple measurement traces for comparison analysis.

USB Memory

USB Interface

Wireless/Wireline LAN Interface

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Use Case 1

JDViewer is the instrument’s application software that has all the necessary functionality to perform further analysis and report generation, including: • VSWR, Return-Loss, and Smith Chart conversion • Captures plots remotely from the JD725A

5

• Registers or edits user-definable RF bands or cables into custom lists • Edits measurement charts • Generates and prints reports

Use Case 2:

Fiber Testing with CellAdvisor CAA Inspection of fiber optic connections is essential for the optimal performance and longevity of fiber optic connectivity. Throughout their lives, fiber connectors must be inspected, analyzed, and cleaned to maintain an acceptable level of functionality. The JD720C series makes it fast and easy to troubleshoot and certify that every connection at a cell site is optimized for a lifetime of performance. The JD720C series supports the handheld video microscope P5000i that can capture video images from the sensor and analyzes the images for fiber end face defects and contamination with reliable PASS/FAIL results to guarantee the performance of your optical connections.

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Use Case 2

Use Case 2

Equipment Required

Procedure

• CellAdvisor JD720C

Connecting the fiber microscope P5000i is mandatory for fiber inspection

• Fiber microscope P5000i with tips, adapters and cleaning material.

and analysis.

1. Connect your fiber microscope P5000i to one of the USB Host ports of the JD720C series. The instrument detects the connected video

microscope automatically. The Fiber Inspection icon and name appears on the measurement mode screen.

2. Select measurement mode 3. Set testing parameters from the drop-down lists, such as analysis profile, tip, test button, and auto center.

Figure 49: P5000i fiber microscope kit

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Use Case 2

Use Case 2

4. Connect the fiber connector to be inspected to the tip of your microscope P5000i.

5. Touch the Live screen key in the screen menu bar. The live image is

displayed on the screen. Touch the Freeze screen key to capture the properly focused image.

9. To run another test on a different fiber, change the fiber connection

and tap the Next Fiber screen key. The instrument stores results of up to 10 fibers. Repeat the test process again.

6. Save the captured image as a file (.png). 7. Touch the test screen key to capture and analyze the image. 8. Once the test is completed, check the result displayed on the screen with the following information:

a. Image overlay of the zone locations, defects, and scratches b. PASS/FAIL result c. Specific test result for each Zone

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Use Case 2

10. Clean the fiber ends using the cleaning kit.

6 Figure 50: Fiber Cleaning

11. Re-test to make sure fiber test passes. 12. Save the fiber inspection results with specific details as a report in PDF or HTML file.

Conclusion As cell sites continue to evolve from a typical macro to a FTTA and CRAN architecture, service providers and their partners are being challenged to find efficient and cost-effective ways to install, commission, and optimize them. Installation teams need tools that can help them run a much wider spectrum of tests with fewer tools to purchase, train and carry.

Every

component in the cable and antenna system must be working as designed and should be swept properly to ensure that service providers are delivering the best quality of service to the subscribers and are getting the maximum return on their spectrum and network investments.

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Conclusion

As the market-leading FTTA solution, Viavi CellAdvisor Cable and Antenna Analyzer 720C Series helps save time and money by offering one instrument to test both coax and fiber. The TestWizard solution offered on the Cable and Antenna Analyzer significantly reduces field setup and testing time by allowing technicians to create required test cases in the back office. Viavi’s cloud based Stratsync™ solution enables technicians to centralize configuration and report results in real time. Remote control enables techs to troubleshoot site problems from a safe location, while enhanced

7

workflow reduces installation test time by up to 70%. CellAdvisor’s dual display feature means less toggling between screens, reducing test time by up to 50% without affecting sweep quality.

Viavi End-to-End Cell Site Deployment Solution

References Tessco Technologies Inc. https://www.tessco.com/knowledge-center/2013/articles/fiber-to-theantenna-revolutionizing-cellular-coverage-and-capacity My Broadband http://mybroadband.co.za/vb/

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Contact Us

+1 844 GO VIAVI (+1 844 468 4284)

To reach the Viavi office nearest you, visit viavisolutions.com/contacts.

Š 2016 Viavi Solutions Inc. Product specifications and descriptions in this document are subject to change without notice.

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