Multi-carrier Wcdma Base Station Design Considerations

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

Technology White Paper Andrew Wright Director, Product Research Oliver Nesper DSP Design Engineer Issue 1: August, 2002 PMC-2021396 © 2002 PMC-Sierra, Inc.

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

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 the peak-to-average power reduction of a multi-carrier WCDMA signal, both of which are important for efficient operation of wideband power amplifiers and cost-effective design of the overall base station. WCDMA signal characterization, technology selection, linearization, and peak reduction methods 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. in Microwave Engineering (meteorology). Since 1995, he has specialized in signal processing solutions for third generation wireless systems. Oliver Nesper is a DSP Design Engineer in the Access Product Division. He has worked on the development of the PALADIN (Predistortion) and PALADIN Waveshaper products. Prior to that he 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

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

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 Digital Predistortion ............................................................................................................. 13

5

4.1

Introduction ....................................................................................................... 13

4.2

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

4.3

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

Waveshaping: A Method for Signal Combining and Signal Crest factor Reduction .................................................................................................................. 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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Multi-Carrier WCDMA Basestation Design Considerations Amplifier Linearization and Crest Factor Control Technology White Paper

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 MultiCarrier Single Amplifier Basestation Architectures ..................................................... 10

Figure 5

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

Figure 6

Waveshaped & Predistortion Digital Multi Carrier Amplifier Basestation 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 Carrier systems........................................................................................................... 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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Multi-Carrier WCDMA Basestation Design Considerations Amplifier Linearization and Crest Factor Control Technology White Paper

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 Compensation Performance ................................................................................................... 32

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

1

Introduction The promise of third generation WCDMA cellular technology with its attendant broadband voice and data services is still yet to be fulfilled. The delay in availability is predominately influenced by the macro economic conditions faced by many of the world’s service providers that have been forced to significantly constrain capital expenditure. This delay has caused manufacturers to reevaluate BTS product designs and consider exploitation of emerging disruptive technologies which provide significant cost reduction opportunities. Open IP radio networks are yielding basestation designs that are becoming little more than a network router with specialized radio interface cards. Within the design of a radio card, the microwave power amplifier is a significant contributor in the overall cellular basestation cost and operating power consumption. The incumbent feed-forward power amplifier technology is generally regarded as a low efficiency device (typically <8%) that is difficult to manufacture in volume in a cost-effective manner. The combination of the recent commercially available digital baseband predistortion and waveshaping signal processing engines is providing a disruptive technology that is dramatically changing BTS vendors’ approach to BTS design practice. Utilizing these technologies multi carrier amplifier assemblies can be constructed that achieve efficiencies that reach 20% and offer the ease of a predominately digital manufacturing environment. The purpose of this paper is to illuminate these technologies and how they can be exploited in WCDMA systems. Prior to this discussion, however, a brief introduction and review of the composition of a WCDMA information-bearing signal is provided. This information is essential when comparing different RF and baseband signal processing technologies.

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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 spreading code 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 to the 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 time slot 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, the base 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 of information being transferred, can cause very high peak to average (crest factor) waveforms to be generated. Combining individual information carriers on separate 5 MHz frequency allocations to form a multi carrier 20 MHz system further expands the peak to average waveform. Without intervention 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 linearity is maintained.

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

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

Figure 2

2.1

Combining of all Down-link Channels including SCH[4]

WCDMA Signal Waveform Requirements Table 1 defines the system requirements that need to be met by the base station transmitter [3] and that are of interest in the scope of this discussion. Error vector magnitude or EVM is a measure of the difference between the ideal transmitted waveform and the waveform actually transmitted. Peak code domain error or PCDE is a measure of how much cross-talk exists between different channels within the WCDMA signal. In other words, it is an indication of how well the orthogonality between different code channels is maintained. Adjacent channel leakage ratio or ACLR is the ratio of the transmitted power to the power measured in an adjacent channel. [3] The most important point to realize that each of these significant signal quality metrics is undermined as an microwave amplifier is driven close to saturation, which unfortunately is the most efficient operating point. Typically, a compromise between the efficient and distortion-free operating points is found. This results in amplifiers that are operated in a mode where the average operating point is set such that the signal crests are just less than the maximum saturated output power that the amplifier can deliver. Depending upon amplifier technology and circuit topology, this can result in very inefficient operation because the signal quality metrics defined in Table 1 can not be violated. Table 1

3GPP Requirements 3GPP Requirement

Limits

EVM

17.5%

PCDE

-33 dB

ACLR1

45 dB

ACLR2

50 dB

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

The 3G system specification[3] specifies different test models that are to be used for specific tests to 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 results can be easily reproduced. Table 2 displays the settings that were employed for the test signal generation. Table 2

Test Signal for 4 Carrier TM1 Signal with 64 Active Users OVSF Codes

Scrambling Codes

Power Levels of all Channels

Time Offset/ fraction of a time slot duration

TM1

0

TM1

0

Carrier

Active Channels

1

TM1

2

TM1

TM1

1

TM1

1/5

3

TM1

TM1

2

TM1

2/5

4

TM1

TM1

3

TM1

3/5

2

It is this high number of users, all with independent data streams that leads to very high crest factor waveforms. Consider the following discussion. Assume N independent signal streams (users) of equal power. It is well known that the peak power of such a signal is N2 times 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 to N times 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 overly pessimistic 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 wide range 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 events that 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 and can therefore be handled by driving the amplifier into saturation or by simple digital clipping. In our case, choosing the amplifier to handle peak-to-average power ratios of 10 dB would be sufficient. Compare that to a worst case consideration, which would result in choosing the amplifier 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) -5

3 Note that the confidence level of the CCDF is up to 10 . 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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Multi-Carrier WCDMA Basestation Design Considerations Amplifier Linearization and Crest Factor Control Technology White Paper

Figure 3

2.1.1

CCDF Test Model 1, 64 Active Users, 4 Carriers

WCDMA Parameter Selection The actual CCDFs of the multi-carrier WCDMA signals are highly dependent on the underlying individual carrier’s signal characteristics. Consider Table 3, which shows the dependency of the signal PAR of three-carrier WCDMA signals on some of the signal parameters. The results were obtained using Rhode & Schwarz’s WinIQSim signal source and were also confirmed with our inhouse signal generator. Comparing the results for TM1 with the minimum results obtainable shows a very close match in PARs. 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 -4

DPCH Code Selection

Number of Channels

Carrier Time shift/ samples

Scrambling Code

PAR at 10 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 selecting an 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 4 below 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 with each class C amplifier that could be utilized with the constant envelope FM radio waveform. As an 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 Single Amplifier Basestation Architectures

The evolution of second generation cellular communication systems was spurred by the need for more 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, to be 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 a change in BTS architectures because employing linear class AB amplifiers in the same post amplification ohmic combining architecture rapidly caused basestation efficiencies to become unmanageable. This forced the evolution of a multi carrier pre-amplification combining architecture 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 crest factor or peak-to-average statistics of the composite waveform to expand. The amplification of the composite multi carrier signal cannot now be faithfully amplified and reproduced in a distortion-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 sufficient linearity 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 typically less than 10%.

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

The next logical architectural step is to eliminate significant component costs by provisioning a digital 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 Waveshaping to be employed which further reduce manufacturing costs, eliminate analog design complexity and simultaneously permit significant increases in power amplifier efficiency to be achieved. This new digital approach is portrayed in Figure 6. The Waveshaping element permits multiple information 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 the combination of random sources always causes very large crest factor waveforms to be generated. Due to system linearity requirements this significantly impacts linearity because an amplifiers average power operating point needs to backed off to accommodates the signal peaks. Backing off 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 carrier WCDMA Waveform.

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

Figure 6

Waveshaped & Predistortion Digital Multi Carrier Amplifier Basestation Architectures

Digital predistortion is an approach to amplifier linearization that permits the efficiency of the multi carrier amplifier to be dramatically increased. The principle of predistortion is intrinsically very 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 distortion function 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 this approach 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 forward amplifiers. Moreover, because the amplifier is not burdened with the need for error amplifier distortion 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 amplifier and subsequently to the antenna. A desirable attribute of this architecture is the significant reduction in analog circuitry associated with a single radio up convertor system. The difficulties of analog circuit design and manufacturing can not be underestimated and so any approach that significantly reduces this requirement is readily adopted by BTS vendors. The subsequent sections of this white paper address the technical details of Waveshaping and Predistortion technologies.

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

4 4.1

Amplifier Linearization and Efficiency Enhancement via Digital Predistortion 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-linear spectral regrowth and the degradation of signal integrity (See EVM measurements, Section 2 on page 6). Figure 7 below illustrates the incumbent feed forward approach. Operation is intrinsically 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 main amplifier. The balanced output of the error amplifier is subtracted from the output of the main amplifier to leave a near perfect signal. In practice this approach works very well, but it is encumbered 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 the main and error loops have to be critically adjusted to ensure distortion cancellation occurs. This is a 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 signal processing technique that eliminates the analog manufacturing complexity of feed forward amplifiers. Furthermore, because the error amplifier is eliminated, the efficiency of the system is dramatically 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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Multi-Carrier WCDMA Basestation Design Considerations Amplifier Linearization and Crest Factor Control Technology White Paper

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 nonlinearity that has a complimentary characteristic to that exhibited by the amplifier. If the baseband non-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 linear gain response. A linear gain response is highly desirable because it implies that distortion and spectral regrowth will not occur. Figure 8

Basic Principles of Predistortion

Figure 8 is utilized to explain the basic principles of digital predistortion. Unfortunately, the simplified 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-AM and AM-PM distortion. These distortion mechanisms are referred to as memoryless and correspond 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 amplifier’s distortion characteristic. However, eliminating this bulk distortion mechanism is not sufficient to entirely eliminate all spectral regrowth generated by the amplifier because small, residual non-linear memory effects are present. The exact definition of a non-linear memory effect is often subject to debate. However, a practical working 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 previous input stimuli is not restricted to being linear. In practice power amplifiers exhibit several distinct non-linear memory characteristics, which are distinguished by substitutionally different time constants.

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

Following the basic principles of predistortion, if a linear system is to be constructed from a cascade 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 the very essence of the PMC-Sierra PALADIN Predistortion product family. Figure 9 on page 16 illustrates the amplification of a 1x, 2x, 3x and 4x WCDMA carrier system’s occupying 5 MHz of signal bandwidth per carrier by a system employing a raw class AB amplifier and a system utilizing memoryless and enhanced memory based predistortion approaches. Clearly the spectral regrowth performance as measured by the adjacent channel power ratio measurements, see Table 4, 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 ALCR -15MHz - dBc

ALCR -10MHz - dBc

ALCR -5MHz - dBc

ALCR + 5MHz - dBc

ALCR + 10MHz - dBc

ALCR +15MHz - dBc

EVM

Raw (Green Trace)

-46.7

-60.3

-72.2

-46.3

-60.7

-72.3

1.8%

Memoryless PD (Red Trace)

-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%

Raw (Green Trace)

-44.5

-48.4

-54.2

-43.0

-48.9

-54.5

2.0%

Memoryless PD (Red Trace)

-57.2

-61.1

-67.0

-53.4

-61.9

-67.6

1.6%

PALADIN Memory PD (Black Trace)

-60.2

-68.2

-69.9

-62.1

-69.1

-68.9

1.3%

Raw (Green Trace)

-44.3

-46.3

-42.0

-44.9

2.5%

Memoryless PD (Red Trace)

-52.9

-53.3

-50.6

-53.2

2.1%

PALADIN Memory PD (Black Trace)

-58.9

-65.1

-61.3

-65.6

1.8%

Raw (Green Trace)

-44.9

-45.0

-46.4

-41.3

-42.9

-45.9

3.0%

Memoryless PD (Red Trace)

-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%

Predistortion Method 1x WCDMA @ 30 Watts

2x WCDMA @ 30 Watts

3x WCDMA @ 30 Watts

4x WCDMA @ 30 Watts

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

Figure 9

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

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

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 B and Class C offer efficiencies that theoretically exceed 70% but with severe non-linearity and diminishing gain. The efficiency numbers quoted are often at odds with the power added efficiencies of 5% to 20% that are observed in practice. Further investigation readily resolves this conundrum. 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 the entire load line of the amplifier (active transistor or FET) from cut-off/turn-on to full power saturation. 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 operating condition of the amplifier. These efficiency bench marks rapidly degrade when informationbearing signals are amplified because the operating point of the amplifier and input drive levels must 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 amplitude sinusoid 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, is substantially less than that of the peak handling capability of the amplifier. Under these circumstances 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 portray this is provided in Figure 10.

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

Figure 10 Amplifier Transfer Characteristics

The concept of “power average back off” arises as a means to describe the practical operating point that permits linear distortion-free transmission. Typically, additional back off is also required to ensure the transistor is not driven into saturation where it becomes very non-linear creating substantial spectral regrowth. Thus further loss of efficiency is incurred. Historically this has not been a big concern in OQPSK or QPSK satellite systems in which the modulation exhibits a modest 3dB crest factor. This, in turn, yields efficiencies of some 30%, which were often deemed acceptable. Unfortunately, the advent of WCDMA and multi carrier WCDMA has lead to information-bearing waveforms that often exhibit crest factors that exceed 10dB. Backing off a Class A or AB amplifier to operate within its linear operating region with this kind of waveform rapidly forces highly inefficient operation. Typically, efficiencies of less than 10% will be observed when operating under these conditions.

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

Section 4.2 on page 14 indicated that the topology of predistortion offered efficiency enhancements because, unlike a feed forward amplifier, a second energy wasting error amplifier was not required. This is indeed true, however, predistortion does offer further incremental efficiency gains. These are extracted by remembering that the predistortion kernel develops a baseband non-linearity that is complimentary to the entire amplifier’s characteristic. This permits the back-off requirement to be minimized because additional margin does not need to be sacrificed to avoid unwanted distortion that is commensurate with operating near the saturation and 1 dB compression point of the amplifier. Basically, predistortion provides correction that permits utilization of the amplifier right up to the saturation point. Aggressive efficiency gains can 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 amplifier’s back-off or operating point may 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 a predistortion system because no amount of correction will enable the amplifier to deliver more power 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 extract increased efficiency, knowing that any signal crest that has a probability of occurrence that is less than 10-4 will not measurably degrade the systems average power spectral density. The PMCSierra PALADIN predistortion system has been developed to permit these aggressive efficiency strategies 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 to explore techniques that dramatically reduce the crest factor of single and multi carrier WCDMA waveforms. The following section on waveshaping provides details of a powerful approach that permits significant crest factor reductions to be achieved. Naturally when combined with efficient predistortion amplifier designs, the gains of both technologies are magnified.

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

5 5.1

Waveshaping: A Method for Signal Combining and Signal Crest factor Reduction Introduction Figure 4 illustrates that multi-carrier signal amplification in the base station requires the combining of independent information-bearing signal streams. Typically, this involves several stages consisting of signal pulse-shaping, up-sampling, filtering, signal modulation and aggregation. Controlling the signal crest factor or PAR at the 10-4 probability point of occurrence is an important task since this determines the amplifier’s peak power requirement and operating efficiency. Reduction in the crest factor or peak to average ratio of a multi carrier informationbearing signal stream in an efficient way is not a trivial development effort and can consume an inordinate amount of resources. The PALADIN Waveshaper, introduced in Section 6, provides an off-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 two or 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 composite waveform. Clearly, the composite signal exhibits a significant increase in the amplitude of signal crests, yet visually the average power does not appear to increase by the same factor. In practice this is found to be true especially in multi-bearer multi-carrier systems such as WCDMA and CDMA-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 and statistical 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 multibearer multi-carrier waveform. An accurate description of this waveform may be computed if the individual probability density functions were defined or known. In practice, this is of little importance because the number of random variables is sufficiently large that the central limit theorem 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 random variables is characterized by another Gaussian random variable with a mean that is equal to the mean of the contributors and a variance or average power that is the sum of the contributing variances or average power. Thus the average power grows by a factor of N when N contributors of 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 the peak power grows by a factor of N2. This reflects the fact that the waveforms are not true Guassian random variables. Thus as the number of users or contributing signals in a composite signal increases, the crest factor or peak to average of the resulting waveform expands by a N2/N factor, 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 be considered, as explained previously, when sizing the peak power capability of the RF power amplifier. In a WCDMA applications multi-user source combining, pulse shaping and multi

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

carrier combining are the three most important contributors to crest generation within the waveform. 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 expands as the number of users increase. The final far right plot also illustrates the instantaneous symbol stream signal trajectory in the complex baseband space and the post pulse shaping signal trajectory 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 to 2 dB increase in crest factor. The expansion is dependant upon the properties of the pulse shaping filter. However, it is important to recognize that even a 1 dB expansion has an important effect upon 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 signal’s crest factor sacrificing and trading-off these requirements is often inevitable as this operation fundamentally alters the signal and contributes to the degradation of these signal quality measurements. However, the specifications leave sufficient margin to permit signal manipulations that 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 methods are compatible and may be jointly utilized to form significant reduction in a particular signals crest factor. Code selection, digital clipping and pulse injection are approaches that will be illustrated 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 has the advantage that the crest factor is reduced over a signal constructed with a set of noncomplimentary 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 run length of the modulo-2 sum of each pair of Walsh codes determined the resulting signal crest factors. The work also demonstrated a code selection scheme that the base station should employ to minimize the transmitted signals crest factor. As OVSF codes used in WCDMA are simply nothing 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 BTS design 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 powerful and 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. It consists 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, the implementation complexity for MCPA and SCPA base station designs is equal. In the simplest case the clipping function would provide a hard limit; however, other clipping functions 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 filter operation 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 so only an isolated decision can be made as to when to clip the signal. It is in fact quite possible that this operation actually increases the peak-to-average ratio when the signal streams are added up later 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 Waveshaper Pulse 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 to eliminate the peak would be to apply a pulse of magnitude equal to the difference between the desired 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, the compensation 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 for the compensation pulses. Also, to spread the error introduced by this operation evenly the compensation pulse is constructed by individual compensation pulses for each carrier that will align to form the desired composite compensation. Figure 13 below 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 the signal 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 stream at 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 PAR reduction mechanism can only be employed for low probability peak events as it will otherwise severely 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

PAR Reduction Method

Implementation Complexity in MCPA

Implementation Complexity in SCPA

Effect on -4 PAR at 10 prob.

Effect on EVM

Effect on PCDE

Effect on ACLR

OVSF Code selection

low

low

high

none

none

none

unknown

none

none

none

[Dummy OVSF unknown, possibly unknown, possibly 5 high high Codes ] [Coding 6 Schemes ]

high

high

unknown

none

none

none

Baseband Clipping

low

low

medium to low

yes

yes

none

medium

high

high

yes

yes

yes

low

high

none

yes

yes

yes

Pulse Compensation Final Clip

As we mentioned before, off-the shelf solutions exist that can simplify the implementation of PAR control in a system. One such solution is the PMC-Sierra’s PM7819 PALADIN Waveshaper, which combines three mechanisms previously discussed7 and a proprietary prediction engine to reduce the signal PAR of up to four WCDMA signal inputs. All of Waveshaper’s 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 and Operation 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 to be 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 that for 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 individual WCDMA signal streams are accepted as the chip’s input. Each of them enters a base band preconditioning soft clipping stage, which is followed by a programmable pulse shaping and upsample-by-two filter stage. The signals are then further up-sampled by 4 in two half-band filter stages and followed by a modulation stage which frequency-converts the signals to individually specified carrier frequencies within a 20 MHz WCDMA frequency allocation. This chip supports a 1 Hz raster which offers significantly more precision than the required 200 kHz specified raster.

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

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 probability and magnitude of a potential signal crest. This permits a waveform to be constructed and combined with the transmission stream that destructively interferes with the transmission signal’s crest to reduce the signal crest to below the predetermined customer-set threshold. This process is illustrated 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 not permissible. Figure 17 illustrates this important frequency domain characteristic. An additional and important property of the Predictive Decresting Waveform Generator is that it examines the composite waveform and individual component carrier power levels and manipulates the properties of the corrective waveform so that signal quality metrics for each individual carrier are equally 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 the final 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 that clips 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 Waveshaper PM7819 This section will present some selected results that can be achieved using Waveshaper’s technology. The results are derived by considering the 10-4 probability point of peak occurrence and 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 WCDMA signals with 64 active DPCHs plus control channels. Results are shown for the 3GPP requirements that were identified in Section 2.1. Figure 18 shows results for the resulting PAR versus EVM and the large gap between using base band 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 7.1 dB, 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 -38dB PCDE is attainable for baseband clipping and pulse compensation. Again, the superior performance of the pulse compensation method is evident.

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

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, is performed on the baseband signals before the pulse shaping filtering is applied. The amount of clipping 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 into adjacent 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 pulse shape 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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Multi-Carrier WCDMA Basestation Design Considerations Amplifier Linearization and Crest Factor Control Technology White Paper

Figure 20 Waveshaping vs. Baseband Clipping, PAR versus ACLR1

The results we presented were generated using TM1 with 64 users and four carriers. Results for other 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 and an overall consistent performance gap as well. Table 6 compares the improvement in signal PAR obtainable for the different compensation methods. As a reference point the uncompensated case is shown as well. We can see from these results 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 significant savings in the base station’s power amplifier. Table 6

Summary of Base Band Clipping versus Pulse Compensation Performance

PAR Control Method

-4

PAR at 10 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 effecting the cost of the overall design. This paper has illustrated how amplifier linearization techniques and 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 these methods is expensive and results in significantly elevated development budgets. PMC can assist in 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 receiver side will complement our product line and will assist the base station designer’s task further.

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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 onto Physical Channels (FDD).

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