Multi-Carrier WCDMA Base Station Design Considerations

8/14/2019 Multi-Carrier WCDMA Base Station Design Considerations

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Multi-Carrier WCDMA BasestationDesign Considerations -

Amplifier Linearization and Crest Factor

Control

Technology White Paper

Andrew WrightDirector, Product Research

Oliver NesperDSP Design Engineer

Issue 1:August, 2002

PMC-2021396

2002 PMC-Sierra, Inc.
http://0.0.0.0/http://0.0.0.0/http://0.0.0.0/http://0.0.0.0/http://0.0.0.0/http://0.0.0.0/http://0.0.0.0/http://0.0.0.0/http://0.0.0.0/http://0.0.0.0/http://0.0.0.0/http://0.0.0.0/http://0.0.0.0/http://0.0.0.0/http://0.0.0.0/http://0.0.0.0/http://0.0.0.0/http://0.0.0.0/http://0.0.0.0/http://0.0.0.0/http://0.0.0.0/http://0.0.0.0/http://0.0.0.0/http://0.0.0.0/http://0.0.0.0/


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Multi-Carrier WCDMA Basestation Design Considerations -Amplifier Linearization and Crest Factor Control


PMC-2021396 (1) 1


Abstract

This paper presents issues to be considered when designing multi-carrier WCDMA basestations.

Two topics will be the main focus of this discussion; the power amplifier linearization and thepeak-to-average power reduction of a multi-carrier WCDMA signal, both of which are importantfor efficient operation of wideband power amplifiers and cost-effective design of the overall base

station. WCDMA signal characterization, technology selection, linearization, and peak reductionmethods are discussed.

About the Author

Andrew Wright is Director of Wireless and Signal Processing Product Research at PMC-Sierra.

Dr Wright is a former co-founder and CTO of Datum Telegraphic Inc. and holds a Ph.D. inMicrowave Engineering (meteorology). Since 1995, he has specialized in signal processingsolutions for third generation wireless systems.

Oliver Nesper is a DSP Design Engineer in the Access Product Division. He has worked on thedevelopment of the PALADIN (Predistortion) and PALADIN Waveshaper products. Prior to thathe was with Spectrum Signal Processing as a hardware development engineer working on the

design of Soft Radio Receiver Platforms.

Revision History

Issue No. Issue Date Details of Change

1 August, 2002 Document created
http://0.0.0.0/http://0.0.0.0/


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PMC-2021396 (1) 2


Contents

Abstract .............................................................................................................................. 1

About the Author...............................................................................................................1

Revision History................................................................................................................1

Contents............................................................................................................................. 2

List of Figures ................................................................................................................... 3

List of Tables .....................................................................................................................4

1 Introduction.................................................................................................................5

2 WCDMA Signal Characteristics.................................................................................6

2.1 WCDMA Signal Waveform Requirements.......................................................... 7

2.1.1 WCDMA Parameter Selection ...............................................................9

3 BTS Architecture Evolution..................................................................................... 10

4 Amplifier Linearization and Efficiency Enhancement via DigitalPredistortion .............................................................................................................13

4.1 Introduction....................................................................................................... 13

4.2 Amplifier Linearization via Digital Predistortion ................................................ 14

4.3 Amplifier Operating Point and Efficiency .......................................................... 17

5 Waveshaping: A Method for Signal Combining and Signal Crest factorReduction .................................................................................................................. 20

5.1 Introduction....................................................................................................... 20

5.2 Crest Factor Reduction:- The Basic Problem Statement..................................20

5.3 PAR / Crest Factor Reduction Methods............................................................ 22

5.4 OVSF Code Selection.......................................................................................23

5.5 Baseband Clipping............................................................................................ 23

5.6 Pulse Compensation and the PALADIN Waveshaper ...................................... 23

5.7 Final Clipping.................................................................................................... 24

5.8 Summary and Implementation Issues ..............................................................24

6 PALADIN - Waveshaper PM 7819:- Construction and Operation.........................26

7 Performance Results for the PALADIN Waveshaper PM7819.............................. 30

8 Summary ................................................................................................................... 33

9 References ................................................................................................................ 34


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List of Figures

Figure 1 Spreading for all downlink channels except SCH[4]......................................... 6

Figure 2 Combining of all Down-link Channels including SCH[4]................................... 7

Figure 3 CCDF Test Model 1, 64 Active Users, 4 Carriers............................................. 9

Figure 4 Comparison Between Single Carrier Multi Amplifier and MultiCarrierSingle Amplifier Basestation Architectures..................................................... 10

Figure 5 Basic Digital Multi-Carrier Single Amplifier Basestation Architectures........... 11

Figure 6 Waveshaped & Predistortion Digital Multi Carrier AmplifierBasestation Architectures...............................................................................12

Figure 7 Feed Forward Amplifier Topology................................................................... 13

Figure 8 Basic Principles of Predistortion .....................................................................14

Figure 9 Comparative Linearization Performance of 1x, 2x, 3x and 4x Carriersystems...........................................................................................................16

Figure 10 Amplifier Transfer Characteristics...................................................................18

Figure 11 Signal Aggregation and Expanding Crest Factors.......................................... 21

Figure 12 Crest Factor Inflation with Modem Aggregation ............................................. 22

Figure 13 Waveshaper Compensation Signal in the Complex Plane............................. 24

Figure 14 Signal Statistics............................................................................................... 26

Figure 15 Waveshaper Kernel ........................................................................................27

Figure 16 Basic Waveform Construction Process Time Domain Analysis .................. 28

Figure 17 Waveform Construction Process Frequency Domain Analysis ................... 29

Figure 18 Waveshaping vs. Baseband Clipping, PAR versus EVM...............................30

Figure 19 Waveshaping vs. Baseband Clipping, PAR versus PCDE.............................31

Figure 20 Waveshaping vs. Baseband Clipping, PAR versus ACLR1 ........................... 32


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List of Tables

Table 1 3GPP Requirements.........................................................................................7

Table 2 Test Signal for 4 Carrier TM1 Signal with 64 Active Users...............................8

Table 3 PARs for Three-Carrier WCDMA Signals, 32 Active User Channels...............9

Table 4 Summary of ACLR Performance ....................................................................15

Table 5 Comparison of PAR Reduction Methods........................................................ 25

Table 6 Summary of Base Band Clipping versus Pulse CompensationPerformance ...................................................................................................32


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2 WCDMA Signal Characteristics

The WCDMA down-link signal model for a single-carrier is shown in the following figures. Each

down-link signal consists of a number of control and pilot channels that are always required.Additionally, each user operating in the cell can utilize one or more traffic channels (DPCHs)with a variety of spreading factors. Figure 1 shows the spreading operation for all downlink

channels (DPCH and control channels), with the exception of the synchronization channel SCH.The incoming data streams are mapped to QPSK symbols and spread with the OVSF spreadingcode assigned to that channel. This operation provides the separation (orthogonality) between

channels/users. The complex spread symbols are then multiplied by a scrambling code specific tothe base station. This operation provides the signal separation between base stations.

Figure 1 Spreading for all downlink channels except SCH[4]

Figure 2 shows the combining of all physical channels with the primary (P-SCH) and secondary

(S-SCH) synchronization channel. The synchronization channel provides radio frame and timeslot synchronization. As WCDMA is an asynchronous system1, these sequences are needed to

simplify the fast timing acquisition by the mobile subscriber unit. At the output of this block, thebase band WCDMA signal samples are available. These are ordinarily pulse-shaped to form a

bandlimited waveform. This waveform, depending upon the number of users and type ofinformation being transferred, can cause very high peak to average (crest factor) waveforms to begenerated. Combining individual information carriers on separate 5 MHz frequency allocations toform a multi carrier 20 MHz system further expands the peak to average waveform. Withoutintervention or additional signal processing crest factors that exceed 16 dB are not uncommon.

Ordinarily this would lead to a very inefficient power amplifier design simply to ensure linearityis maintained.

1 unlike IS-95 or cdma2000 which are synchronized by GPS.


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The 3G system specification[3] specifies different test models that are to be used for specific teststo be performed. The first test model, TM1, employs a user population of 64 with 4 carriers. This

test case is used here to exemplify a typical traffic scenario that is well defined and so that resultscan be easily reproduced. Table 2 displays the settings that were employed for the test signalgeneration.

Table 2 Test Signal for 4 Carrier TM1 Signal with 64 Active Users

CarrierActive

ChannelsOVSFCodes

ScramblingCodes

Power Levelsof all

Channels

Time Offset/fraction of a time

slot duration

1 TM12

TM1 0 TM1 0

2 TM1 TM1 1 TM1 1/5

3 TM1 TM1 2 TM1 2/5

4 TM1 TM1 3 TM1 3/5

It is this high number of users, all with independent data streams that leads to very high crestfactor waveforms. Consider the following discussion. AssumeNindependent signal streams

(users) of equal power. It is well known that the peak power of such a signal is N2times the power

of one individual signal, in the event that all signals amplitudes phase align. On the other hand,

the average power is only identical toNtimes the power of the individual signal. The maximum

signal PAR that can occur in such a signal in dB is therefore 10 * log10(N).

Taking only the maximum PAR of a signal into account when sizing the PA would be overlypessimistic as nothing is yet said on the frequency of occurrence of that condition. Consider for

example a typical CCDF3

of a 4 carrier, TM1, WCDMA signal with 64 active user channels(Figure 3), which shows that peak-to-average power ratios for a specific signal span over a widerange depending on the probability of peak occurrences that one is interested in. Often of primary

concern is the 10-4 probability point of peak occurrence for the following reasons; peak eventsthat occur with a probability lower than 10-4 contribute very little to the actual intermodulation

distortion (IMD) performance of the amplifier or waveform quality parameter degradation andcan therefore be handled by driving the amplifier into saturation or by simple digital clipping. Inour case, choosing the amplifier to handle peak-to-average power ratios of 10 dB would besufficient. Compare that to a worst case consideration, which would result in choosing theamplifier to be able to handle peak-to-average power ratios of 24.3 dB4.

2 P-CCPCH +SCH, Primary CPICH, PICH, S-CCPCH containing PCH (SF=256) and 64 DPCH

(SF=128)

3 Note that the confidence level of the CCDF is up to 10-5

.

4 Assuming 4 equal power carriers with 64 equal-power DPCHs and 4 equal-power control overhead

channels each, this would result in a theoretical maximum PAR of 10 * log10(4 * 68) = 24.3 dB.


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Figure 3 CCDF Test Model 1, 64 Active Users, 4 Carriers

2.1.1 WCDMA Parameter Selection

The actual CCDFs of the multi-carrier WCDMA signals are highly dependent on the underlyingindividual carriers signal characteristics. ConsiderTable 3, which shows the dependency of the

signal PAR of three-carrier WCDMA signals on some of the signal parameters. The results wereobtained using Rhode & Schwarzs WinIQSim signal source and were also confirmed with our in-

house signal generator.

Comparing the results for TM1 with the minimum results obtainable shows a very close match inPARs. However, selecting the codes and power levels in an inefficient way yields a PAR increase

of 6.4 dB at the probability of 10-4.

Table 3 PARs for Three-Carrier WCDMA Signals, 32 Active User Channels

DPCH CodeSelection

Number ofChannels

Carrier Timeshift/ samples

ScramblingCode

PAR at 10-4

prob.

TM1 32 0/512/1024 0/1/2 9.8

TM3 32 0/512/1024 0/1/2 10.6

min. PAR 32 0/512/1024 0/1/2 9.5

max. PAR 32 0/512/1024 0/1/2 16.3

In this section, we discussed the WCDMA down-link signal model, certain WCDMA base station

requirements, WCDMA test modes, PARs and CCDFs and how to interpret them when selectingan appropriate amplifier. This concludes the background presentation; now we can continue our

discussion on amplifier linearization and PAR control.


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3 BTS Architecture Evolution

PCS and Cellular basestation designs have dramatically evolved since the analog first-generation

systems were originally introduced. Figure 4below illustrates the single carrier BTS architecture,where each information-bearing RF carrier was amplified and combined at RF prior to

propagating to the egress antenna. The ohmic power loss that occurred in the power combining

network was typically dismissed as immaterial due to the inherent 50% efficiency associated witheach class C amplifier that could be utilized with the constant envelope FM radio waveform. Asan alternative, expensive cavity combiners could be employed to mitigate a portion of the ohmic

combining loss.

Figure 4 Comparison Between Single Carrier Multi Amplifier and MultiCarrier SingleAmplifier Basestation Architectures

The evolution of second generation cellular communication systems was spurred by the need formore system capacity and a significant increase in the clarity of the voice communication link.

This caused digital modulation schemes, which offer a dramatic increase in spectral efficiency, tobe utilized. Unfortunately, such schemes do not offer constant RF amplitude envelopes. This

implies that highly efficient class C amplifier technologies could not be employed. This sparked achange in BTS architectures because employing linear class AB amplifiers in the same postamplification ohmic combining architecture rapidly caused basestation efficiencies to become

unmanageable. This forced the evolution of a multi carrier pre-amplification combiningarchitecture that forms a composite multi carrier signal that is fed to the amplifier assembly.Figure 4 above also illustrates this topology.

Unfortunately, the combination of multiple RF carriers with fluctuating envelopes causes the crestfactor or peak-to-average statistics of the composite waveform to expand. The amplification ofthe composite multi carrier signal cannot now be faithfully amplified and reproduced in adistortion-free manner by a simple class AB amplifier. To overcome this difficulty linear Feed

Forward amplifiers are employed to counter the distortion problems and provide sufficientlinearity that spectral regrowth does not pollute adjacent channels. This requirement, however,causes the efficiency of the amplifiers assembly to be further degraded to levels that are typicallyless than 10%.


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The next logical architectural step is to eliminate significant component costs by provisioning adigital baseband carrier combining technology that permits only a single radio up conversion card

to be employed. This is illustrated in Figure 5.

Figure 5 Basic Digital Multi-Carrier Single Amplifier Basestation Architectures

This new architecture permits evolutionary technologies such as Predistortion and Waveshapingto be employed which further reduce manufacturing costs, eliminate analog design complexity

and simultaneously permit significant increases in power amplifier efficiency to be achieved. Thisnew digital approach is portrayed in Figure 6. The Waveshaping element permits multipleinformation sources from a plethora of modems to be combined and shifted to baseband carrier

frequencies that when translated to RF will form specific RF carriers. Most importantly thecombination of random sources always causes very large crest factor waveforms to be generated.

Due to system linearity requirements this significantly impacts linearity because an amplifiersaverage power operating point needs to backed off to accommodates the signal peaks. Backingoff an amplifier significantly degrades efficiency. Waveshaping is a key process that occurs

during combining and permits a significant reduction in the crest factor of a multi carrierWCDMA Waveform.


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Figure 6 Waveshaped & Predistortion Digital Multi Carrier Amplifier BasestationArchitectures

Digital predistortion is an approach to amplifier linearization that permits the efficiency of themulti carrier amplifier to be dramatically increased. The principle of predistortion is intrinsicallyvery simple, requiring a non-linear distortion function to be built in the numerical digital

baseband signal processing domain that is commensurate (equal) but opposite to the distortionfunction exhibited by the amplifier. A highly linear distortion free system is achieved when the

cascade of these two non-linear distortion functions equates to a linear system. The beauty of thisapproach is that the analog power amplifier is permitted to become a simple class AB platform.This frees BTS vendors from the burden and complexity of manufacturing feed forwardamplifiers. Moreover, because the amplifier is not burdened with the need for error amplifierdistortion correction circuitry, the efficiency of the system is significantly enhanced.

Once this baseband signal processing has been completed a single digital stream is fed to a digital

to analog convertor and passed into a single RF up convertor. This in turn is fed to the amplifierand subsequently to the antenna. A desirable attribute of this architecture is the significantreduction in analog circuitry associated with a single radio up convertor system. The difficultiesof analog circuit design and manufacturing can not be underestimated and so any approach thatsignificantly reduces this requirement is readily adopted by BTS vendors.

The subsequent sections of this white paper address the technical details of Waveshaping andPredistortion technologies.


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4 Amplifier Linearization and Efficiency Enhancement viaDigital Predistortion

4.1 Introduction

Non-linearity is a fundamental property of high power RF semi-conductor transistors.Consequently, any amplifier design approach will be burdened by the management of non-linearspectral regrowth and the degradation of signal integrity (See EVM measurements, Section 2 on

page 6). Figure 7below illustrates the incumbent feed forward approach. Operation isintrinsically quite simple, with a second error amplifier and reference signal cancellation circuit

being utilized to extract and amplify only the distortion components created by the mainamplifier. The balanced output of the error amplifier is subtracted from the output of the mainamplifier to leave a near perfect signal. In practice this approach works very well, but it isencumbered with the utilization of a second amplifier which often consumes exactly the same

amount of power as the main amplifier. This significantly limits the efficiency of the assembly.Furthermore, to ensure that the circuit provides a significant reduction in distortion products themain and error loops have to be critically adjusted to ensure distortion cancellation occurs. This isa complex analog circuit design task and represents a major issue when cost reductions and

increased volume production is to be considered.

Figure 7 Feed Forward Amplifier Topology

In contrast, digital predistortion, as illustrated earlier in Figure 6 on page 12, is a baseband signalprocessing technique that eliminates the analog manufacturing complexity of feed forwardamplifiers. Furthermore, because the error amplifier is eliminated, the efficiency of the system isdramatically improved because a single class AB amplifier is required. Importantly, volume

production issues are eased because the digital manufacturing environment is significantly more

reliable than the integration and alignment of analog signal processing elements.


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PMC-2021396 (1) 14


4.2 Amplifier Linearization via Digital Predistortion

Figure 8 illustrates the principles of operation of a digital predistortion system. The objective is to

numerically generate, in the real time digital complex baseband signal processing domain, a non-linearity that has a complimentary characteristic to that exhibited by the amplifier. If the basebandnon-linearity is correctly constructed, then the overall system response to a signal that flows

serially through the cascade of the baseband non-linearity and the amplifier will be that of a lineargain response. A linear gain response is highly desirable because it implies that distortion andspectral regrowth will not occur.

Figure 8 Basic Principles of Predistortion

Figure 8 is utilized to explain the basic principles of digital predistortion. Unfortunately, thesimplified non-linear amplifier characteristic that is illustrated is not representative of a practical

class AB amplifiers. Ordinarily, radio engineers are predominately concerned with both AM-AMand AM-PM distortion. These distortion mechanisms are referred to as memoryless andcorrespond to the belief that the instantaneous distortion observed at the output of the amplifier

can be directly mapped to the instantaneous amplitude of the signal driving the amplifiers input.This distortion mechanism represents the bulk of the amplifiers distortion characteristic.

However, eliminating this bulk distortion mechanism is not sufficient to entirely eliminate allspectral regrowth generated by the amplifier because small, residual non-linear memory effectsare present.

The exact definition of a non-linear memory effect is often subject to debate. However, a practicalworking definition is that the current output of the amplifier is affected by current and previous

input stimuli. Moreover the relationship between the current output and the current and previousinput stimuli is not restricted to being linear. In practice power amplifiers exhibit several distinct

non-linear memory characteristics, which are distinguished by substitutionally different timeconstants.


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Following the basic principles of predistortion, if a linear system is to be constructed from acascade of non-linearities and the amplifier is classified as a weak Volterra kernel then the

complimentary non-linearity will also require the construction of a Volterra kernel. This is thevery essence of the PMC-Sierra PALADIN Predistortion product family. Figure 9 on page 16illustrates the amplification of a 1x, 2x, 3x and 4x WCDMA carrier systems occupying 5 MHz of

signal bandwidth per carrier by a system employing a raw class AB amplifier and a systemutilizing memoryless and enhanced memory based predistortion approaches. Clearly the spectral

regrowth performance as measured by the adjacent channel power ratio measurements, see Table4, indicates memory based predistortion provides a significant advantage over traditional basic

predistortion approaches. This is particularly advantageous when considering 20 MHz systems.

Table 4 Summary of ACLR Performance

Predistortion MethodALCR

-15MHz- dBc

ALCR-10MHz- dBc

ALCR-5MHz- dBc

ALCR+ 5MHz- dBc

ALCR+ 10MHz

- dBc

ALCR+15MHz

- dBcEVM

1x WCDMA @ 30 Watts

Raw (Green Trace) -46.7 -60.3 -72.2 -46.3 -60.7 -72.3 1.8%

Memoryless PD (RedTrace)

-64.2 -71.8 -73.8 -58.2 -71.7 -73.2 1.3%

PALADIN Memory PD(Black Trace)

-64.3 -73.1 -73.6 -64.6 -73.0 -73.1 1.2%

2x WCDMA @ 30 Watts

Raw (Green Trace) -44.5 -48.4 -54.2 -43.0 -48.9 -54.5 2.0%


-57.2 -61.1 -67.0 -53.4 -61.9 -67.6 1.6%


-60.2 -68.2 -69.9 -62.1 -69.1 -68.9 1.3%

3x WCDMA @ 30 Watts

Raw (Green Trace) -44.3 -46.3 -42.0 -44.9 2.5%


-52.9 -53.3 -50.6 -53.2 2.1%


-58.9 -65.1 -61.3 -65.6 1.8%

4x WCDMA @ 30 Watts

Raw (Green Trace) -44.9 -45.0 -46.4 -41.3 -42.9 -45.9 3.0%


-49.4 -49.1 -51.3 -49.5 -50.1 -52.6 2.8%

PALADIN Memory PD

(Black Trace)

-58.6 -63.1 -62.7 -61.0 -62.7 -62.1 2.0%


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Figure 9 Comparative Linearization Performance of 1x, 2x, 3x and 4x Carrier systems


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4.3 Amplifier Operating Point and Efficiency

The topic of amplifier efficiency often leads to much consternation when examined for the first

time. Referring to amplifier texts will often result in discussions comparing the merits of class A,class AB, class B and class C amplifiers in terms of gain, power utilization factor and efficiency.It is not uncommon for these to state that the theoretical efficiencies of a Class A amplifier is 50%

which rises to 70% as the conduction angle is reduced and class AB operation is invoked. Class Band Class C offer efficiencies that theoretically exceed 70% but with severe non-linearity anddiminishing gain. The efficiency numbers quoted are often at odds with the power addedefficiencies of 5% to 20% that are observed in practice. Further investigation readily resolves thisconundrum. The key issue is to realize that theoretical efficiencies are based upon the assumption

that the amplifier is required to amplify a RF sinusoid, whose peak to peak variation exercises theentire load line of the amplifier (active transistor or FET) from cut-off/turn-on to full powersaturation. Under these circumstances Class A amplifiers yield efficiencies of 50% while class AB

yields efficiencies of 70%. The lost energy is utilized to support the quiescent bias operatingcondition of the amplifier. These efficiency bench marks rapidly degrade when information-

bearing signals are amplified because the operating point of the amplifier and input drive levelsmust be set up to ensure that signal peaks just exercise the saturation or maximum output power

point of the amplifier. When these signal peaks (or crests) occur the amplifier does approach its

theoretical operating efficiencies because the waveform at RF does appear as a large amplitudesinusoid for a very short duration of time. However, for the majority of the time, the average

operating point excursion, defined by some stochastic mean or average excursion, issubstantially less than that of the peak handling capability of the amplifier. Under thesecircumstances the quiescent power consumption becomes a much bigger percentage of the

consumed power when compared to the actual power delivered to the load. An attempt to portraythis is provided in Figure 10.


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Section 4.2 on page 14 indicated that the topology of predistortion offered efficiencyenhancements because, unlike a feed forward amplifier, a second energy wasting error amplifier

was not required. This is indeed true, however, predistortion does offer further incrementalefficiency gains. These are extracted by remembering that the predistortion kernel develops a

baseband non-linearity that is complimentary to the entire amplifiers characteristic. This permits

the back-off requirement to be minimized because additional margin does not need to besacrificed to avoid unwanted distortion that is commensurate with operating near the saturation

and 1 dB compression point of the amplifier. Basically, predistortion provides correction thatpermits utilization of the amplifier right up to the saturation point. Aggressive efficiency gainscan also be achieved when it is realized that the maximum signal crests within a multi-carrier

system occur on a very rare and infrequent basis. Thus the amplifiers back-off or operating pointmay be adjusted, to a more efficient point, such that on these rare occurrences the amplifier is

actually over driven deep into saturation. This event can never be compensated for in apredistortion system because no amount of correction will enable the amplifier to deliver morepower than it is capable of generating. During the overdrive event the distortion that is generated

will result in very high instantaneous spectral regrowth, however because of its very infrequent

nature the energy contribution to the average power spectral density will remain negligible.Astute system operators will in fact deliberately overdrive predistortion systems to extractincreased efficiency, knowing that any signal crest that has a probability of occurrence that is lessthan 10-4 will not measurably degrade the systems average power spectral density. The PMC-

Sierra PALADIN predistortion system has been developed to permit these aggressive efficiencystrategies to be executed whilst maintaining absolute system stability. Using these approaches

efficiencies up to 20% can be readily achieved with WCDMA multi carrier systems.

The previous paragraphs have demonstrated that the crest factor of an information-bearing

waveform has a profound effect upon amplifier efficiency. Thus there is clear motivation toexplore techniques that dramatically reduce the crest factor of single and multi carrier WCDMAwaveforms. The following section on waveshaping provides details of a powerful approach that

permits significant crest factor reductions to be achieved. Naturally when combined with efficientpredistortion amplifier designs, the gains of both technologies are magnified.


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5 Waveshaping: A Method for Signal Combining andSignal Crest factor Reduction

5.1 Introduction

Figure 4 illustrates that multi-carrier signal amplification in the base station requires thecombining of independent information-bearing signal streams. Typically, this involves severalstages consisting of signal pulse-shaping, up-sampling, filtering, signal modulation andaggregation. Controlling the signal crest factor or PAR at the 10 -4 probability point of occurrenceis an important task since this determines the amplifiers peak power requirement and operating

efficiency. Reduction in the crest factor or peak to average ratio of a multi carrier information-bearing signal stream in an efficient way is not a trivial development effort and can consume aninordinate amount of resources. The PALADIN Waveshaper, introduced in Section 6, provides anoff-the-shelf solution to this problem and can significantly reduce the time-to-market in designs

that adopts the technology.

5.2 Crest Factor Reduction:- The Basic Problem Statement

Figure 11 illustrates the basic and debilitating property of increased crest factor signals when twoor more signals are linearly combined. The diagram illustrates three sinusoid signals of different

frequency but identical amplitude and their linear aggregation to form a single compositewaveform. Clearly, the composite signal exhibits a significant increase in the amplitude of signalcrests, yet visually the average power does not appear to increase by the same factor. In practicethis is found to be true especially in multi-bearer multi-carrier systems such as WCDMA andCDMA-2000. This is borne out if the following hypothetical argument is followed. The

information to be transported on a per user basis can be regarded as independent.


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Figure 11 Signal Aggregation and Expanding Crest Factors

This will translate into a radio or analog signal that has a given average and peak power andstatistical profile (see Figure 3 on page 9). Each of these user-defined signals may be regarded as

an independent random variable with an arbitrary but constrained probability density function.Furthermore, each of these user defined signals may be combined into a single carrier WCDMA

stream which in turn may be combined with additional WCDMA carriers to form a true multi-bearer multi-carrier waveform. An accurate description of this waveform may be computed if theindividual probability density functions were defined or known. In practice, this is of littleimportance because the number of random variables is sufficiently large that the central limittheorem may be readily invoked which permits each contributing user information-bearing signal

to be regarded as a Gaussian random variable. The summation of many Gaussian randomvariables is characterized by another Gaussian random variable with a mean that is equal to themean of the contributors and a variance or average power that is the sum of the contributingvariances or average power. Thus the average power grows by a factor of N when N contributorsof equal average power are combined. However, since all contributing component waveforms are

orthogonal sequences, the peak or crest voltage grows by a factor of N, but most importantly thepeak power grows by a factor of N

2. This reflects the fact that the waveforms are not true

Guassian random variables. Thus as the number of users or contributing signals in a compositesignal increases, the crest factor or peak to average of the resulting waveform expands by a N2/Nfactor, i.e., N. Naturally, the probability of this occurring, that is also signals exhibiting the same

amplitude and phase at the same time, is reduced but it is unfortunately finite and must beconsidered, as explained previously, when sizing the peak power capability of the RF poweramplifier. In a WCDMA applications multi-user source combining, pulse shaping and multi


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PMC-2021396 (1) 22


carrier combining are the three most important contributors to crest generation within thewaveform.

Figure 12 illustrates the peak to average expansion that occurs when a sequence of symbols from

a modem is aggregated to form a composite WCDMA carrier. Again the Peak to Average expandsas the number of users increase. The final far right plot also illustrates the instantaneous symbolstream signal trajectory in the complex baseband space and the post pulse shaping signaltrajectory that occurs as the transmission signal is formed. Clearly, the action of pulse shape filter

also expands the crest factor of the waveform. Typically, this additional expansion provides a 1 to2 dB increase in crest factor. The expansion is dependant upon the properties of the pulse shapingfilter. However, it is important to recognize that even a 1 dB expansion has an important effectupon the power amplifier sizing and efficiency of its operating point.

Figure 12 Crest Factor Inflation with Modem Aggregation

5.3 PAR / Crest Factor Reduction Methods

Signal PAR/crest factor reduction is constrained by the requirements shown in Section 2.1,namely the EVM, PCDE and ACLR requirements. When attempting to change the signals crestfactor sacrificing and trading-off these requirements is often inevitable as this operationfundamentally alters the signal and contributes to the degradation of these signal quality

measurements. However, the specifications leave sufficient margin to permit signal manipulationsthat trade reduced crest factor for slight degradations in EVM, PCDE and ACLR measurements.

Various methods for crest factor reduction of single and multi carrier WCDMA signals can be

considered and several key methods will be discussed with attention being drawn to specific

advantages and disadvantages. A key point to remember, however, is that many of these methodsare compatible and may be jointly utilized to form significant reduction in a particular signalscrest factor. Code selection, digital clipping and pulse injection are approaches that will beillustrated in the following sections, which provides a simple inventory of preferred techniques.


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5.4 OVSF Code Selection

Section 2.1.1 indicated the importance of selecting complimentary OVSF codes in order to

minimize the signal crest factor of the MC-WCDMA signal. Using optimal code selections hasthe advantage that the crest factor is reduced over a signal constructed with a set of non-complimentary OVSF codes with no impact on the EVM, PCDE or ACLRs. Recent work[2]

focussed on Walsh code selection for IS-95 and cdma2000 standards and reported that the runlength of the modulo-2 sum of each pair of Walsh codes determined the resulting signal crestfactors. The work also demonstrated a code selection scheme that the base station should employto minimize the transmitted signals crest factor. As OVSF codes used in WCDMA are simplynothing else than re-ordered Walsh codes, this work can be directly employed in WCDMA

systems.

Code selection is ordinarily done by the base band modem radio resource manager within a BTSdesign and as such represents an abstraction detail that has no context within the scope of the

waveform processing elements of the BTS design. Nevertheless, the approach is very powerfuland can be cascaded with all additional waveform crest control techniques that may be exploited

in the design of the radio and waveform processing subsystem(s).

5.5 Baseband Clipping

Baseband clipping is a widely used mechanism for PAR reduction in multi-carrier signals. Itconsists typically of a clipping function that is employed on individual symbol streams on the

base band signal. Since the processing is performed only on individual carriers, theimplementation complexity for MCPA and SCPA base station designs is equal.

In the simplest case the clipping function would provide a hard limit; however, other clippingfunctions with a softer clipping characteristic could be employed. One advantage of baseband

clipping is that it does not alter the spectral properties of the signal since the pulse shaping filteroperation is performed after the clipping is applied.

The use of base band clipping alone delivers, as we will see in Section 6, only limited

performance. First of all, the clipping is performed on each signal stream individually, and soonly an isolated decision can be made as to when to clip the signal. It is in fact quite possible thatthis operation actually increases the peak-to-average ratio when the signal streams are added uplater on in the processing chain, as it might remove destructive interference that would reduce

peaks. Furthermore, this operation is performed before the pulse shaping which represents a

major contributor to the increase in signal PAR.

5.6 Pulse Compensation and the PALADIN WaveshaperPulse compensation is a powerful PAR control mechanism. The principle is simple to understand;

consider a peak occurrence after the multi-carrier signal summation. All that needs to be done toeliminate the peak would be to apply a pulse of magnitude equal to the difference between thedesired magnitude and the actual uncompensated magnitude rotated by 180 degrees in phase.


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Clearly, as the multi-carrier signal at this point in the processing chain is band limited, thecompensation pulse should be band limited as well to avoid the transmission of out-of-band

power in adjacent channels. This implies that a sufficient pulse length and pulse shape is used forthe compensation pulses. Also, to spread the error introduced by this operation evenly thecompensation pulse is constructed by individual compensation pulses for each carrier that will

align to form the desired composite compensation.

Figure 13below shows a snapshot of signals in the complex plane at the peak instant with two

active carriers. Notice that the compensation impulses for each carrier are proportional to thesignal magnitude but are phase-reversed.

Pulse compensation is easier to implement in MCPA base station designs as this method can be

performed on the low-power signal stream, rather than on the combined high-power signal streamat the output of the amplifiers in the SCPA case.

Figure 13 Waveshaper Compensation Signal in the Complex Plane

5.7 Final Clipping

The final clipping is in the simplest case a hard clipping function that hard-limits the signal for

peak events that occur with a probability of less than 10-4. As we discussed before this PARreduction mechanism can only be employed for low probability peak events as it will otherwiseseverely degrade the signal ACLR.

5.8 Summary and Implementation Issues

Table 5 summarizes the performance of different PAR reduction methods.


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Table 5 Comparison of PAR Reduction Methods

PARReductionMethod

ImplementationComplexity in

MCPA

ImplementationComplexity in

SCPA

Effect onPAR at 10

-4

prob.Effect on

EVMEffect on

PCDEEffect on

ACLR

OVSF Codeselection

low low high none none none

[Dummy OVSFCodes

5]

unknown, possiblyhigh

unknown, possiblyhigh

unknown none none none

[CodingSchemes

6]

high high unknown none none none

BasebandClipping

low low medium tolow

yes yes none

PulseCompensation

medium high high yes yes yes

Final Clip low high none yes yes yes

As we mentioned before, off-the shelf solutions exist that can simplify the implementation ofPAR control in a system. One such solution is the PMC-Sierras PM7819 PALADIN Waveshaper,

which combines three mechanisms previously discussed7

and a proprietary prediction engine toreduce the signal PAR of up to four WCDMA signal inputs. All of Waveshapers features can be

independently controlled to match the best settings for any particular signal.

5 Note that the system capacity will be decreased by the number of dummy channels used for PAR

control.

6 Note that this method cannot be applied if non-centralized transmitters/receivers are used, as is the

case in the cellular system considered here.

7 Baseband Clipping, Pulse Compensation, Final Clipping


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6 PALADIN - Waveshaper PM 7819:- Construction andOperation

Figure 14 below illuminates the commercial requirements for a Waveshaper product. The graph

illustrates that the cumulative distribution function of the raw or intrinsic waveform is required tobe modified such that the cumulative probabilty density function provided is met. The key feature

is that waveshaping process must ensure that a particular amplitude threshold is not exceeded.This can be seen by the vertical descent of the second distribution function, which indicates thatfor this particular scenario amplitude excursions that exhibit crests greater than 5 dB above the

average power do not occur.

Figure 14 Signal Statistics

Figure 15 illustrates the construction of the PALADIN Waveshaper kernel. Four individualWCDMA signal streams are accepted as the chips input. Each of them enters a base band

preconditioning soft clipping stage, which is followed by a programmable pulse shaping and up-sample-by-two filter stage. The signals are then further up-sampled by 4 in two half-band filterstages and followed by a modulation stage which frequency-converts the signals to individuallyspecified carrier frequencies within a 20 MHz WCDMA frequency allocation. This chip supportsa 1 Hz raster which offers significantly more precision than the required 200 kHz specified raster.


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Figure 15 Waveshaper Kernel

All four up-sampled signal streams are then digitally combined and delayed prior to transmission.The delay stage is critical to successful operation for it permits the Predictive Decresting

Waveform Generator to examine the entire waveform construction process, that is raw input data,preconditioned data, pulse shaped and frequency shifted data stream and to assess the probabilityand magnitude of a potential signal crest. This permits a waveform to be constructed andcombined with the transmission stream that destructively interferes with the transmission signals

crest to reduce the signal crest to below the predetermined customer-set threshold. This process isillustrated in Figure 16. A key and important property of the corrective waveform is that, should

specific carrier allocations not be utilized, injection of energy into these allocations is notpermissible. Figure 17 illustrates this important frequency domain characteristic. An additionaland important property of the Predictive Decresting Waveform Generator is that it examines the

composite waveform and individual component carrier power levels and manipulates theproperties of the corrective waveform so that signal quality metrics for each individual carrier areequally modified. That is EVM, ACLR and Roe measurements for all channels will be equally

impacted. The peak controlled and combined four-carrier signal is then up-sampled again to thefinal output rate and followed by a final frequency translation or DQM stage that can be by-


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passed. After a sin(x)/x compensation stage the signal passes through the final clipping block thatclips extremely rare peak events.

Figure 16 Basic Waveform Construction Process Time Domain Analysis


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Figure 17 Waveform Construction Process Frequency Domain Analysis


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7 Performance Results for the PALADIN WaveshaperPM7819

This section will present some selected results that can be achieved using Waveshapers

technology. The results are derived by considering the 10-4 probability point of peak occurrenceand varying the desired maximum signal peak level. The results compare simulations for pure

base band clipping and waveform compensation only, operating on 4-carrier TM1 WCDMAsignals with 64 active DPCHs plus control channels. Results are shown for the 3GPPrequirements that were identified in Section 2.1.

Figure 18 shows results for the resulting PAR versus EVM and the large gap between using baseband clipping and pulse compensation is evident. Consider for example the 12% EVM point,

where the PAR for base band clipping is around 9.2 dB and for pulse compensation around 7.1dB, an improvement of more than 2 dB. Uncompensated signal PARs were around 10 dB.

Figure 18 Waveshaping vs. Baseband Clipping, PAR versus EVM

Results for the PCDE are shown in Figure 19. For the selected target of 12% EVM around -38dBPCDE is attainable for baseband clipping and pulse compensation. Again, the superior

performance of the pulse compensation method is evident.


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Figure 19 Waveshaping vs. Baseband Clipping, PAR versus PCDE

Considering the ALCR18 (Figure 20) of the processed signal, we can see the trade-offs that need

to be made when using the pulse compensation. Base band clipping, as was noted before, isperformed on the baseband signals before the pulse shaping filtering is applied. The amount ofclipping applied has, therefore, no effect on the adjacent channel power leakage ratio. Using the

pulse compensation we note that the method is not ideal as a certain amount of power leaks intoadjacent bands. The limit set forth in the 3GPP specification is 45 dB and for our chosen

operating point of 12% EVM we end up with 71dB ACLR1. Note that the compensation pulseshape in Waveshaper is programmable, which would permit trading off better ACLR1

performance for other signal measurements, if desired.

8 ACLR2 was not affected by the signal processing performed.


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Figure 20 Waveshaping vs. Baseband Clipping, PAR versus ACLR1

The results we presented were generated using TM1 with 64 users and four carriers. Results forother test signals with a different numbers of carriers and different OVSF codes showed different

PAR levels, but an overall similar characteristic between baseband and pulse compensation andan overall consistent performance gap as well.

Table 6 compares the improvement in signal PAR obtainable for the different compensationmethods. As a reference point the uncompensated case is shown as well. We can see from theseresults that, depending on the parameters we can allow to trade-off, improvements of almost 3 dB

can be obtained with leaving enough margin to the specification. This translates into significantsavings in the base stations power amplifier.

Table 6 Summary of Base Band Clipping versus Pulse Compensation Performance

PAR Control

Method

PAR at 10-4

prob. EVM (%) PCDE (dB) ACLR1 (dB)

Base band Clipping 9.2 12 -38 >82

Pulse Compensation 7.1 12 -38 71

Uncompensated 10.0 0 -76 >82


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8 Summary

Designing WCDMA basestations is a complex task that requires finding a good solution, while

constrained by several parameters, including cost. The PA is a major contributing factor effectingthe cost of the overall design. This paper has illustrated how amplifier linearization techniquesand PAR control schemes can lower the peak power requirements placed on the PA and in turn

reduce costs through sophisticated signal processing methods. However, implementing thesemethods is expensive and results in significantly elevated development budgets. PMC can assistin this regard; two solutions, PALADIN (Predistortion) and PALADIN Waveshaper are available

off-the-shelf products that reduce costs and development effort. Future products on the receiverside will complement our product line and will assist the base station designers task further.


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Head Office:PMC Sierra Inc

To order documentation,send email to:

All product documentation isavailable on our web site at:

CopyrightPMC Sierra Inc 2002

9 References

[1] Design Considerations for Multicarrier CDMA Basestation Power Amplifiers, J.S. Kenney, A. Leke,

Spectrian, Inc., Sunnyvale, CA, November 9th, 1998.

[2] Peak-to-Average Reduction Via Optimal Walsh Code Allocation in 3rd Generation CDMA Systems,A. G. Shanbhag, E.G. Tiedemann, IEEE 6th Int. Symp. On Spread Spectrum Tech & Application,

NJIT, New Jersey, Sep 6-8, 2000.

[3] 3GPP Specification TS 25.141, Base Station Conformance Testing (FDD).

[4] 3GPP Specification TS 25.213, Spreading and Modulation (FDD).

[5] 3GPP Specification TS 25.211, Physical Channels and Mapping of Transport Channels ontoPhysical Channels (FDD).

Documents

Multi-Carrier WCDMA Base Station Design Considerations