WE4C-2

A Crest Factor Reduction Technique for

W-CDMA Polar TransmittersJau-Horng Chen and J. Stevenson Kenney, Senior Member, IEEE

Abstract-This paper presents a crest factor reduction (CFR)technique compatible with polar-modulated signals for use in highefficiency polar transmitters. This technique processes only theamplitude signal of the polar-modulated signal, which reducespower consumption. In this implementation, the addition of theCFR technique to a polar transmitter consumes only 1% of thepeak output power while increasing the maximum output powerby 20 %.

Index Terms-Power amplifiers, Crest Factor Reduction,Wideband Code Division Multiple Access

I. INTRODUCTION

Crest factor reduction (CFR) reduces the peak-to-averagepower ratio (PAR) of a signal, while maintaining the powerspectrum to meet stringent guidelines. By reducing the crestfactor, the amount of back-off needed to meet linearityrequirements can be reduced. Crest factor reduction technique isa cost-effective way to increase the peak output power and peakefficiency of a power amplifier with minimum addedcomponents. Various methods of CFR technique forconventional I/Q modulation are discussed in [1-3]. In thispaper we investigate the use of CFR in conjunction with highefficiency polar transmitters.

Polar transmitters provide an attractive way to achieve highefficiency power amplification for today's wireless applications.The authors have demonstrated an open-loop implementation ofa polar transmitter using a dual-phase PWM supply circuit in [4].An FIR filter is used for gain compensation to achieve widebandoperation for W-CDMA applications. The block diagram isshown in Figure 1. This paper demonstrates a way to implementa polar transmitter compatible CFR technique.

II. CREST FACTOR REDUCTION USING SOFT-CLIPPING

Pulse shaping is used to limit the occupied bandwidth ofdigital data while transmitting and thus increasing spectralefficiency. However, the trade-off of limiting bandwidth isincreased amplitude variation, as shown in Figure 2, where adigital pulse train is shaped by a raised cosine filter. Figure 3shows the I/Q constellation of a QPSK signal pulse-shaped witha raised cosine filter. After pulse shaping, the peak amplitude isgreatly increased, and it is the main source of high PAR or crestfactor in digitally modulated signals. The high PAR poses amajor difficulty in designing PAs by requiring the PAs to bebacked-off by the PAR, which leads to lower efficiency. It canbe seen that local peaks in Figure 2 and Figure 3 often do notoccur in the original data point. It is therefore desirable toreduce amplitude of the local peaks while preserving theamplitude of the original data point and bandwidth of the signal.

1 .5

0.5

-0.5

-1.5

0 1 2 3 4 5 6 7 8

Figure 2. Binary data pulse-shaped with raised cosine filter.

Phase Path

Figure 1. Block diagram of a polar transmitter using a dual-phase PWMsupply circuit.

1-4244-0445-2/07/$20.00 ©2007 IEEE345

Peak

- Original DataPulse-Shaped DataRaised Cosine Pulse

2

(}L 4-v v v v v v v v v

1.5

0.5s

0

-0.5

-1.5

-2 -1.5 -1 -0.5 0 0.5 1 1.5

performance of CFR using various filter lengths are comparedand shown in Figure 6.

4.0 3 65

3.5 2.5 603.0 2

a 2.5- 2

22.0 - 1'.52IL1.5 -1 W

1.0 '

0.5 -1.92 MHz 0.5-3.84 MHz

0.0 080 85 90 95 100

Clipping Threshold (%)

55

2.50~ ~ ~ ~ 'Ia 50 - 0..4 .

-J 45 -

40

35 -

301.5 2.0 2.5

PAR (dB)

- 1.92 MHz- 3.84 MHz

3.0 3.5

Figure 5. System-level simulation of CFR using subtraction ofKaiser-windowed sinc function with various cut-off frequenciesfor W-CDMA.

Figure 3. Constellation of QPSK signal pulse-shaped with raised cosinefilter.

Various methods of CFR are discussed in [1-3]. However,those methods have focused on quadrature modulation. ForEER PAs and polar transmitters, where the signals are in polarformat, it is unattractive to convert the signals to I/Q format forCFR. This paper focuses on CFR using polar format signals.

For a signal in polar format, the signal is composed ofseparate amplitude and phase signals. The high crest factor is aresult of the peaks in the amplitude data. By clipping theamplitude data, the crest factor can be reduced. The clipping ofthe amplitude data increases the occupied bandwidth of theoriginal band-limited signal. Since it is not easy to performdigital filtering on polar signals, such implementation seemsunattractive.

In this work, soft-clipping by adding or subtractingband-limited signal is used, as in [1]. A Kaiser-windowed sincfunction is used for the implementation. Compared tohard-clipping, soft-clipping does not require actual filtering forthe processed signal, which results in lower power consumption.Since soft-clipping is only performed on the envelope signalinstead of both I and Q, the power consumed by the algorithm isreduced by half. The basic block diagram of the implementedscheme is shown in Figure 4.

Threshold BandlimitedFunction

I(t)

0(t)

Env'(t)

Figure 4. Basic block diagram of implemented CFR scheme.

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3.5

3.0

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c: 2.0

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-N=41 0

80 85 90 95 100Clipping Threshold (%)

Figure 6.

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a 454~~~ ~ 11

c 0 A&. * -Nl

40 N21< 45 4 "I~~~~ 4

351.5 2.0 2.5 3.0 3.5

PAR (dB)

System-level simulation of CFR using subtraction ofKaiser-windowed sinc function with various filter lengths forW-CDMA.

The envelope of a W-CDMA voice signal before and afterCFR is shown in Figure 7. The clipping threshold is 84% of thepeak magnitude and the filter length used is 21 with a filterbandwidth of 1.92 MHz. The CCDF of the signal before andafter CFR are compared in Figure 8.

0.35

0.3

0.25-

0.2

0.15

0.1

0.05-

1000 1100 1200 1300 1400 1500

Figure 7. Envelope of single voice channel W-CDMA signal before andafter CFR using subtraction of Kaiser-windowed sinc function.

Using a filter length of 21 and bandwidths of 1.92 MHz and3.84 MHz, performance of CFR is shown in Figure 5. Theresults show degraded error vector magnitude (EVM) from CFR.However, it is substantially lower than the 17.5% W-CDMAspecification. Using a filter bandwidth of 1.92 MHz,

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EVM=2.07%, Filter length=21, Clip=84%

45

S 42m

cc 39-J0 36

33 ACLR1ACLR2

3016 18 20 22

Output Power (dBm)24

Figure 10. Measurement results of ACLR with VDD = 3.5 V using aW-CDMA voice signal with CFR at 836.5 MHz.

1 2 3 4 5PAR (dB)

Figure 8. CCDF of single voice channel W-CDMA signal before and afterCFR using subtraction of Kaiser-windowed sinc function.

To evaluate the performance of CFR with a polar transmitter,measurements of efficiency and ACLR were performed. TheCFR algorithm is first applied to the envelope data using a PC,and loaded into an Agilent 4432B signal generator to drive thedual-phase PWM supply circuit. The phase information isloaded into an Agilent 4438C signal generator to drive the gateof a Sirenza SHF-0289 power amplifier. The drain efficiencyand PAE are compared in Figure 9. The results of the ACLRmeasurements are shown in Figure 10. The polar transmitterpossesses a dynamic range of over 8 dB passing both ACLRrequirements for W-CDMA. The maximum measured power

that passes the W-CDMA specification is 24.3 dBm, which is0.8 dB higher than the polar transmitter without using CFR.

60

50

o 40

r 30

20

10

0

16 18 20 22Output Power (dBm)

-Drain Eff- PAE

24

Figure 9. Drain efficiency and PAE of the polar transmitter with VDD = 3.5V using a W-CDMA signal with CFR at 836.5 MHz (PAR =

2.12 dB).

Comparing the peak output power level for the polartransmitter with CFR and without CFR, the difference is only0.8 dB, which is lower than the reduced PAR, 1.26 dB. Suchdifference is the result of hard clipping at the peak output powerlevel without using CFR. Slight hard clipping does not createenough distortion for the polar transmitters to fail the ACLRspecifications.

The efficiency of the polar transmitters with and withoutCFR is compared for various output power levels and shown inFigure 11. The efficiency curves of both methods match fairlywell form 14 dBm to 23.5 dBm. From the measurement results,CFR does not improve the efficiency for a specific power level.However, CFR increases the peak output power and peakefficiency. The performance of the polar transmitter with andwithout CFR are summarized and compared with otherrecent-reported wide-band polar transmitters in Table I.

55

50

45

40

C35

Q 30wU"0~/O

20

15

WWWithout CFR--oWith CFR

14 16 18 20 22 24Output Power (dBm)

Figure 11. Efficiency comparison of polar transmitter with and withoutCFR

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Table I Performance comparison of polar transmittersThis This

[5] [6] work workw/o withCFR CFR

Modulated 200 20 3.84 3.84Carrier BW kHz MHz MHz MHz

Supply Voltage 3.3 V 5V 3.5 V 3.5 V23.8 19 23.5 24.3

Maximum P0ut dBm dBm dBm dBm

PAE 22% 28 % 48.1 % 51.1 %

Gain 25 dB 6.5 dB 12 dB 12.8 dB

Predistortion Yes Yes No No

Integrated Yes No Partial Partial

III. CONCLUSION

This paper demonstrates the use of CFR in conjunction withpolar transmitters. Assuming InJ of energy consumed per

multiply and accumulation (MAC) operation, the power

consumed by CFR is only 1% of the maximum output power.

Furthermore, CFR only needs to be preformed at peak outputpower levels where it is beneficiary. By adding CFR to theenvelope path of a polar transmitter, the maximum output poweris increased by 20%, while the peak PAE is increased from48.1% to 51.1%.

REFERENCES

[1] R. Sperlich, Y. Park, G. Copeland, and J. S. Kenney, "Power amplifierlinearization with digital predistortion and crest factor reduction," IEEEMTT-S Int. Microwave Symp. Dig., vol. 2, pp. 669-672, 2004.

[2] N. Chen and G.T. Zhou, "Distortionless crest factor reduction for forwardlink CDMA," IEEE Workshop on Signal Processing Advances inWireless Comm., pp. 294-297.

[3] K. Yadavelli, D. Efstathiou, and M. Manglani, " Crest factor reductionengine for multi-carrier WCDMA transmitted signals," IEEE Int. Symp.on Personal, Indoor and Mobile Radio Comm., vol. 3, pp. 2207-2211,2005.

[4] J.-H. Chen, P. Fedorenko, and J.S. Kenney, "A low-voltage polartransmitter with digital envelope path gain compensation," IEEEMicrowave and Wireless Component Letters, pp. 428-30, July 2006.

[5] P. Reynaert and M. S. J. Steyaert, "A 1.75-GHz polar modulated CMOSRF power amplifier for GSM-EDGE," IEEE J. Solid-State Circuits, vol.40, pp. 2598-2608, Dec. 2005.

[6] F. Wang, D. Kimball, J. Popp, A. Yang, D. Lie, P. Asbeck, and L. Larson,"Wideband envelope elimination and restoration power amplifier withhigh efficiency wideband envelope amplifier for WLAN 802.1 I gapplications," IEEE MTT-S Int. Microwave Symp. Digest, vol. 2, pp.645-648, 2005.

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