Ericsson WCDMA MCPA

RF power amplifiertechniquesThe composite RF signal is amplified andfed to the antenna via a transmitter band-pass filter (Figure 1). The coverage and ca-pacity of the base station define the requiredamount of output power. Several transistorstages connected in series constitute thegain, and the final stage delivers the outputpower. High-power amplifiers often havemultiple transistors connected in parallel atthe output. Consequently, the output stageconsumes large amounts of power. Silicon(Si) or gallium-arsenide (GaAs) field-effectdevices are adequate choices at an operatingfrequency of 2 GHz.

The low-power stages at the input side arebiased in a linear mode called class A. Thehigh-power transistors operate in a more ef-

ficient but less linear mode called class AB.As seen from the input-output characteris-tics in Figure 2, class A mode is more linearfor small signals.

The complex modulation schemes inWCDMA systems have a high peak-to-average power relationship, which producesamplitude and phase distortion in non-linear amplifiers. This distortion is evenmore pronounced as the output power levelof the amplifier approaches saturation. Someamplifiers also experience a memory ef-fect—that is, the output signal at a givenmoment is affected not only by the instan-taneous input signal, but also by the previ-ous signal history. Distortion causes signalquality to deteriorate and adjacent channelpower (ACP) to increase; it also gives rise tospurious emissions.

Amplifier efficiency is dependent on thecharacteristics of the power transistors andthe bias scheme. Efficiency improves withhigher output power, but distortion in-creases rapidly as the power level approach-es saturation. The high peak-to-averagepower relationship is thus fraught withcompromise.

Figure 3 shows a simulated example ofhow a two-carrier WCDMA signal is dis-torted by the gain transfer characteristic ofthe class AB amplifier. The figure shows thefrequency spectrum before and after ampli-fication. The output signal does not satisfysystem requirements for spurious emissions.

The best solution for multicarrier ampli-fiers is to operate the power transistors in ef-ficient but slightly non-linear class AB

184 Ericsson Review No. 4, 2001

RF multicarrier amplifier for third-generationsystemsBo Berglund, Thorsten Nygren and Karl-Gösta Sahlman

Multicarrier transmitters traditionally have one power amplifier per carrierand a combiner circuit at the output, or they combine the carriers at lowpower level and then utilize a common multicarrier power amplifier(MCPA) for power generation. This latter solution seems to be simpler, butthe need for high linearity—to avoid intermodulation distortion—makesthe multicarrier amplifier more complex.

The modulation scheme in third-generation (WCDMA) mobile systemsinvolves a high degree of amplitude modulation, which requires a certainamount of linearization even for single-carrier amplifiers. Notwithstanding,the high capacity and flexibility of the multicarrier amplifier makes it a log-ical choice for third-generation systems.

The authors describe the technical challenges and Ericsson’s solutionfor building the MCPA for WCDMA.

3GPP Third-generation Partnership Pro-ject

ACLR Adjacent channel level ratioACP Adjacent channel powerADC Analog-to-digital signal converterDAC Digital-to-analog signal converterDC Direct currentDSP Digital signal processorEPA Error power amplifierFF Feed-forwardGaAs Gallium-arsenideLD-MOS Lateral-diffusion metal-oxide semi-

conductorMCPA Multicarrier power amplifierMPA Main power amplifierMTBF Mean time between failuresRBS Radio base stationRF Radio frequencySi SiliconTDMA Time-division multiple accessWCDMA Wideband code-division multiple

access

BOX A, TERMS AND ABBREVIATIONS

Amplifier

DAC and up- converter Baseband User data

Bandpass filter

Antenna

Figure 1Transmitter power amplifier in radio basestation (RBS).

Figure 3A simulated two-carrier WCDMA signalspectrum at output and input of a non-linear power amplifier.

Ericsson Review No. 4, 2001 185

mode, which will need a linearizing tech-nique that complies with system require-ments for spurious emissions and adjacentchannel power.

First generation MCPA forWCDMA

Technical challengesThe MCPA is part of the RBS 3000 forWCDMA systems.1 The primary technicalgoals in developing MCPAs for WCDMAhave been to achieve • high linearity—to satisfy requirements

set by the Third-generation PartnershipProject (3GPP); and

• optimum efficiency—to reduce the powerconsumption of the base station.

Another goal was automated, large-volume

production of MCPAs at different facilitiesaround the world.

Stringent 3GPP requirements for adja-cent channel power, the spectrum mask, andspurious emissions make it necessary to lin-earize the power amplifier. A minimum re-quirement of four-carrier operation(WCDMA) stipulates a bandwidth of atleast 20 MHz. For detailed WCDMA radiorequirements, see 3GPP TS 25.104.2

MCPA technology for WCDMA

Selection of technology

Feed-forward technology was selected as themain linearization method for the first-generation MCPA for WCDMA, since it canyield the necessary linearity and bandwidth.It is a mature linearization method that canachieve good linearity over fairly large band-

Mhz

dB0

–30 –20 –10 0 10 20 30–100

–50

Output spectrum

Mhz

dB0

–30 –20 –10 0 10 20 30

–100

–50

Input spectrum

Figure 2Input-output characteristics for transistoramplifiers.Input

Output

Class A Class AB

Input

Output

widths. Feed-forward MCPAs for TDMAsystems, which have been on the market forsome years, have bandwidths of up to 20 MHz and a spurious emission level ofaround –60 dBc. A drawback to the feed-forward technology has been low efficien-cy—approximately 6-7%. Therefore, toyield greater efficiency, and to be suitablefor high-volume production, the technolo-gy had to be improved.

Feed-forward concept

Figure 4 shows a block diagram of the feed-forward architecture. The input signal is

split into two paths. The signal in the toppath is amplified by the main power ampli-fier (MPA), which operates in class AB. Thenon-linearities in the MPA result in inter-modulation distortion, which adds to theoriginal signal. A sample of the MPA out-put signal is fed to the subtracter—a direc-tional coupler—where the signal is sub-tracted from a delayed portion of the origi-nal signal (present in the lower part). Thisresults in an error signal that contains thedistortion signal. Ideally none of the origi-nal signal energy should remain. But in re-ality it is possible to suppress the carriers


Pre- distorter

Carrier

Four-carrier input

RF in RF out

Amplitude and phase adjust

Amplitude and phase adjust

Main amplifier

Error amplifier

Error detect 1

Loop 1 Loop 2

2

1

Figure 4Block diagram of a feed-forward MCPAfor WCDMA.

Gain 52 dB Gain flatness +/- 0.5 dBOutput power 46 dBm (40 W) Linearity

– First adjacent channel ACLR 1 < -51 dBc– Second adjacent channel ACLR 2 < -60 dBc

Efficiency > 9%, typically 10% including DC/DC converterTemperature range +5° to +45°CHot-swapping AllowedVolume 7 litersWeight 7 kg Bandwidth Any 20 MHz band within 2110 – 2170 MHz, field-adjustable

BOX B, MCPA TECHNICAL DATA


with 25–30 dB. In Figure 4, the carrier-cancellation operation is marked Loop 1. Thefeed-forward cancellation loop is markedLoop 2. The error signal is amplified linear-ly in an error power amplifier (EPA)—to alevel needed to cancel the distortion in themain path—and is then fed to the outputcoupler. The MPA output signal is delayedto match delay through the error amplifierpath. The contributed distortion from thetwo paths is added in opposite phase and ide-ally only the amplified original signal re-mains at the MCPA output.

Analog RF predistortion

The main power amplifier can be improvedby employing a predistorter whose transfercharacteristic complements that of the mainpower amplifier. A configuration in whichthe predistorter and main power amplifierare cascaded ensures that the resulting sys-tem has low distortion (Figure 5). The non-linear predistorting element operates at thefinal carrier frequency. This method has theadvantage of linearizing the entire band-width of an amplifier.

Predistortion does not add losses at the car-rier output, since distortion is compensatedfor at the input. Thus amplifier efficiency isnot affected. In fact, the efficiency can be in-creased by driving the MPA closer to com-pression with the same intermodulationlevel.

Product technologySeveral design challenges had to be over-come in order to achieve the function as de-scribed:

• Because the MPA must have inherentlygood linearity and bandwidth as well ascorresponding efficiency, a class AB am-plifier was designed using lateral-diffusion metal-oxide semiconductor(LD-MOS) transistors. With this tech-nology, at adjacent channel level ratio(ACLR) values of approximately –40 dBc,the MPA efficiency is around 20%.

• Loops 1 and 2 must have very good gainand phase flatness to ensure good cancel-lation in each loop. Flatness is tuned elec-tronically during production, whichmeans that manual trimming is not re-quired.

• Adaptive loop control ensures good loopstability. A digital signal processor (DSP)is used for this control.

• The delay (τ2) in Loop 2 must have lowloss to reduce the amount of output powerlost as heat. Excessive loss reduces MCPAefficiency. To minimize loss, the delay el-ement has been implemented as a cavitybandpass filter.

• To cancel distortion, the EPA must havea bandwidth three to five times greaterthan the MCPA signal bandwidth inorder.

A combination of good linearity and goodefficiency is needed for the EPA, which mustamplify distortion from the MPA withoutintroducing further distortion. Since goodefficiency is one of the primary design goals,power consumption in the EPA must beminimized.

In classical MCPA designs, the EPA is avery linear class A amplifier (to achieve su-perior linearity). Nonetheless, advances in

Predistorter

+ =

Predistorter PA

Amplifier Linearized amplifierV1

V1

F (V1)

V2

V2

V0

V0

V1

V0

V2

Figure 5The predistortion concept.

LD-MOS transistor technology have made itpossible to design a class AB amplifier withconsiderably lower power consumption.

For optimum efficiency, an analog pre-distorter has been added in front of theMPA. The predistorter improves MPA lin-earity by 5 to 10 dB. The linearity en-

hancement allows for more output powerfrom the MPA with improved efficiency.

The final MCPA design (Figure 6)—in-cluding the DC/DC converter—yields an ef-ficiency of 10%.

MeasurementsFigure 7 shows a typical measurement withtwo WCDMA carriers at total average out-put power of 40W/46 dBm. The distancebetween the center frequencies of the twocarriers is 10 MHz.

The adjacent channel level ratio and spu-rious emission levels are far below the max-imum values allowed.

Future MCPAtechnologiesPresent-day RF power linearization tech-niques employ the feed-forward techniqueand variants thereof. To further enhance per-formance, attempts are being made to im-prove amplifier linearity by predistortingthe signal to a power amplifier. Predistort-ed signals can be generated in either analogor digital techniques.

Digital linearization techniquesGiven that semiconductor technology hasimproved DSP, ADC and DAC techniques,steps have been taken to design an MCPAbased solely on predistortion in the digitaldomain. This approach makes use of mod-ern signal-processing techniques andpromises to be cost-effective. A down-con-verted sample of the RF output signal iscompared to the digital input signal. The


Figure 6Ericsson’s MCPA for WCDMA.

Figure 7Measured output power spectrum of theMCPA.


difference is minimized by predistorting theinput signal in a digital ASIC controlled bya DSP for adaptive update. Figure 8 showsthe basics of digital predistortion. Greaterefficiency is feasible, since distortion is com-pensated for at the input of the power am-plifier.

Ericsson is conducting intensive researchin the area of digital predistortion, and theapplication is expected to become a maturesolution for future WCDMA systems.

Ericsson is also investigating power am-plifier concepts (efficiency-enhancementtechnologies) that will yield greater effi-ciency in the power amplifier itself.

ConclusionThe growing market for third-generationsystems requires a base station RF poweramplifier designed for large-volume pro-duction. Ericsson has chosen the feed-forward technique with analog predistor-tion, which gives excellent spurious emis-sion values and high production yield. A de-sign for efficiency and cooling guaranteesthe best MTBF in a small-size amplifier.

Progress in semiconductor technology andimproved power amplifier designs have set thestage for a next-generation MCPA with bet-ter efficiency, smaller size, and high MTBF.

Digital ASIC

DAC

RF up- converter

DSP

Memory

PA

RF down- converter

ADCDigital input

RF output

Figure 8Digital predistortion MCPA for WCDMA.

1 Zune, P.: Family of RBS 3000 products forWCDMA systems. Ericsson Review3/2000, pp. 170-177.

2 Third-generation Partnership Project;Technical Specification Group RadioAccess Networks; UTRA (BS) FDD; RadioTransmission and Reception.

3 GPP TS 25.104.

REFERENCES

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Ericsson WCDMA MCPA