RF multicarrier amplifier for third-generation

RF multicarrier amplifier for third-generation
RF multicarrier amplifier for third-generation
systems
Bo Berglund, Thorsten Nygren and Karl-Gösta Sahlman
Multicarrier transmitters traditionally have one power amplifier per carrier
and a combiner circuit at the output, or they combine the carriers at low
power level and then utilize a common multicarrier power amplifier
(MCPA) for power generation. This latter solution seems to be simpler, but
the need for high linearity—to avoid intermodulation distortion—makes
the multicarrier amplifier more complex.
The modulation scheme in third-generation (WCDMA) mobile systems
involves a high degree of amplitude modulation, which requires a certain
amount of linearization even for single-carrier amplifiers. Notwithstanding,
the high capacity and flexibility of the multicarrier amplifier makes it a logical choice for third-generation systems.
The authors describe the technical challenges and Ericsson’s solution
for building the MCPA for WCDMA.
BOX A, TERMS AND ABBREVIATIONS
3GPP
Third-generation Partnership Project
ACLR
Adjacent channel level ratio
ACP
Adjacent channel power
ADC
Analog-to-digital signal converter
DAC
Digital-to-analog signal converter
DC
Direct current
DSP
Digital signal processor
EPA
Error power amplifier
FF
Feed-forward
GaAs
Gallium-arsenide
LD-MOS Lateral-diffusion metal-oxide semiconductor
MCPA
Multicarrier power amplifier
MPA
Main power amplifier
MTBF
Mean time between failures
RBS
Radio base station
RF
Radio frequency
Si
Silicon
TDMA
Time-division multiple access
WCDMA Wideband code-division multiple
access
RF power amplifier
techniques
The composite RF signal is amplified and
fed to the antenna via a transmitter bandpass filter (Figure 1). The coverage and capacity of the base station define the required
amount of output power. Several transistor
stages connected in series constitute the
gain, and the final stage delivers the output
power. High-power amplifiers often have
multiple transistors connected in parallel at
the output. Consequently, the output stage
consumes large amounts of power. Silicon
(Si) or gallium-arsenide (GaAs) field-effect
devices are adequate choices at an operating
frequency of 2 GHz.
The low-power stages at the input side are
biased in a linear mode called class A. The
high-power transistors operate in a more ef-
ficient but less linear mode called class AB.
As seen from the input-output characteristics in Figure 2, class A mode is more linear
for small signals.
The complex modulation schemes in
WCDMA systems have a high peak-toaverage power relationship, which produces
amplitude and phase distortion in nonlinear amplifiers. This distortion is even
more pronounced as the output power level
of the amplifier approaches saturation. Some
amplifiers also experience a memory effect—that is, the output signal at a given
moment is affected not only by the instantaneous input signal, but also by the previous signal history. Distortion causes signal
quality to deteriorate and adjacent channel
power (ACP) to increase; it also gives rise to
spurious emissions.
Amplifier efficiency is dependent on the
characteristics of the power transistors and
the bias scheme. Efficiency improves with
higher output power, but distortion increases rapidly as the power level approaches saturation. The high peak-to-average
power relationship is thus fraught with
compromise.
Figure 3 shows a simulated example of
how a two-carrier WCDMA signal is distorted by the gain transfer characteristic of
the class AB amplifier. The figure shows the
frequency spectrum before and after amplification. The output signal does not satisfy
system requirements for spurious emissions.
The best solution for multicarrier amplifiers is to operate the power transistors in efficient but slightly non-linear class AB
Figure 1
Transmitter power amplifier in radio base
station (RBS).
Antenna
Amplifier
User data
184
Baseband
Bandpass
filter
DAC and up-
converter
Ericsson Review No. 4, 2001
Output
Output
Class A
Class AB
Input
mode, which will need a linearizing technique that complies with system requirements for spurious emissions and adjacent
channel power.
First generation MCPA for
WCDMA
Technical challenges
The MCPA is part of the RBS 3000 for
WCDMA systems.1 The primary technical
goals in developing MCPAs for WCDMA
have been to achieve
• high linearity—to satisfy requirements
set by the Third-generation Partnership
Project (3GPP); and
• optimum efficiency—to reduce the power
consumption of the base station.
Another goal was automated, large-volume
Output spectrum
dB
0
Input
Figure 2
Input-output characteristics for transistor
amplifiers.
production of MCPAs at different facilities
around the world.
Stringent 3GPP requirements for adjacent channel power, the spectrum mask, and
spurious emissions make it necessary to linearize the power amplifier. A minimum requirement of four-carrier operation
(WCDMA) stipulates a bandwidth of at
least 20 MHz. For detailed WCDMA radio
requirements, see 3GPP TS 25.104.2
MCPA technology for WCDMA
Selection of technology
Feed-forward technology was selected as the
main linearization method for the firstgeneration MCPA for WCDMA, since it can
yield the necessary linearity and bandwidth.
It is a mature linearization method that can
achieve good linearity over fairly large band-
Figure 3
A simulated two-carrier WCDMA signal
spectrum at output and input of a nonlinear power amplifier.
Input spectrum
dB
0
–50
–50
–100
–100
–30
–20
–10
0
10
20
30
Mhz
Ericsson Review No. 4, 2001
–30
–20
–10
0
10
20
30
Mhz
185
Four-carrier input
RF in
RF out
2
Pre-
distorter
Amplitude
and phase
adjust
Main
amplifier
Loop 1
Loop 2
Error
detect 1
1
Figure 4
Block diagram of a feed-forward MCPA
for WCDMA.
Amplitude
and phase
adjust
Carrier
widths. Feed-forward MCPAs for TDMA
systems, which have been on the market for
some years, have bandwidths of up to
20 MHz and a spurious emission level of
around –60 dBc. A drawback to the feedforward technology has been low efficiency—approximately 6-7%. Therefore, to
yield greater efficiency, and to be suitable
for high-volume production, the technology had to be improved.
Feed-forward concept
Figure 4 shows a block diagram of the feedforward architecture. The input signal is
Error
amplifier
split into two paths. The signal in the top
path is amplified by the main power amplifier (MPA), which operates in class AB. The
non-linearities in the MPA result in intermodulation distortion, which adds to the
original signal. A sample of the MPA output signal is fed to the subtracter—a directional coupler—where the signal is subtracted from a delayed portion of the original signal (present in the lower part). This
results in an error signal that contains the
distortion signal. Ideally none of the original signal energy should remain. But in reality it is possible to suppress the carriers
BOX B, MCPA TECHNICAL DATA
Gain
Gain flatness
Output power
Linearity
Efficiency
Temperature range
Hot-swapping
Volume
Weight
Bandwidth
186
52 dB
+/- 0.5 dB
46 dBm (40 W)
– First adjacent channel
ACLR 1 < -51 dBc
– Second adjacent channel ACLR 2 < -60 dBc
> 9%, typically 10% including DC/DC converter
+5° to +45°C
Allowed
7 liters
7 kg
Any 20 MHz band within 2110 – 2170 MHz, field-adjustable
Ericsson Review No. 4, 2001
with 25–30 dB. In Figure 4, the carriercancellation operation is marked Loop 1. The
feed-forward cancellation loop is marked
Loop 2. The error signal is amplified linearly in an error power amplifier (EPA)—to a
level needed to cancel the distortion in the
main path—and is then fed to the output
coupler. The MPA output signal is delayed
to match delay through the error amplifier
path. The contributed distortion from the
two paths is added in opposite phase and ideally only the amplified original signal remains at the MCPA output.
Analog RF predistortion
The main power amplifier can be improved
by employing a predistorter whose transfer
characteristic complements that of the main
power amplifier. A configuration in which
the predistorter and main power amplifier
are cascaded ensures that the resulting system has low distortion (Figure 5). The nonlinear predistorting element operates at the
final carrier frequency. This method has the
advantage of linearizing the entire bandwidth of an amplifier.
Predistortion does not add losses at the carrier output, since distortion is compensated
for at the input. Thus amplifier efficiency is
not affected. In fact, the efficiency can be increased by driving the MPA closer to compression with the same intermodulation
level.
Product technology
Several design challenges had to be overcome in order to achieve the function as described:
V1
Predistorter
F (V1)
• Because the MPA must have inherently
good linearity and bandwidth as well as
corresponding efficiency, a class AB amplifier was designed using lateraldiffusion metal-oxide semiconductor
(LD-MOS) transistors. With this technology, 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 gain
and phase flatness to ensure good cancellation in each loop. Flatness is tuned electronically during production, which
means that manual trimming is not required.
• Adaptive loop control ensures good loop
stability. A digital signal processor (DSP)
is used for this control.
• The delay (τ2) in Loop 2 must have low
loss to reduce the amount of output power
lost as heat. Excessive loss reduces MCPA
efficiency. To minimize loss, the delay element has been implemented as a cavity
bandpass filter.
• To cancel distortion, the EPA must have
a bandwidth three to five times greater
than the MCPA signal bandwidth in
order.
A combination of good linearity and good
efficiency is needed for the EPA, which must
amplify distortion from the MPA without
introducing further distortion. Since good
efficiency is one of the primary design goals,
power consumption in the EPA must be
minimized.
In classical MCPA designs, the EPA is a
very linear class A amplifier (to achieve superior linearity). Nonetheless, advances in
V2
V0
+
Predistorter
Ericsson Review No. 4, 2001
V1
V0
PA
V0
V2
Figure 5
The predistortion concept.
=
Amplifier
V2
Linearized amplifier
V1
187
hancement allows for more output power
from the MPA with improved efficiency.
The final MCPA design (Figure 6)—including the DC/DC converter—yields an efficiency of 10%.
Measurements
Figure 7 shows a typical measurement with
two WCDMA carriers at total average output power of 40W/46 dBm. The distance
between the center frequencies of the two
carriers is 10 MHz.
The adjacent channel level ratio and spurious emission levels are far below the maximum values allowed.
Future MCPA
technologies
Present-day RF power linearization techniques employ the feed-forward technique
and variants thereof. To further enhance performance, attempts are being made to improve amplifier linearity by predistorting
the signal to a power amplifier. Predistorted signals can be generated in either analog
or digital techniques.
Figure 6
Ericsson’s MCPA for WCDMA.
Digital linearization techniques
LD-MOS transistor technology have made it
possible to design a class AB amplifier with
considerably lower power consumption.
For optimum efficiency, an analog predistorter has been added in front of the
MPA. The predistorter improves MPA linearity by 5 to 10 dB. The linearity en-
Given that semiconductor technology has
improved DSP, ADC and DAC techniques,
steps have been taken to design an MCPA
based solely on predistortion in the digital
domain. This approach makes use of modern signal-processing techniques and
promises to be cost-effective. A down-converted sample of the RF output signal is
compared to the digital input signal. The
Figure 7
Measured output power spectrum of the
MCPA.
188
Ericsson Review No. 4, 2001
RF
output
PA
RF up-
converter
DAC
Digital
input
RF down-
converter
Digital ASIC
ADC
DSP
Memory
Figure 8
Digital predistortion MCPA for WCDMA.
difference is minimized by predistorting the
input signal in a digital ASIC controlled by
a DSP for adaptive update. Figure 8 shows
the basics of digital predistortion. Greater
efficiency is feasible, since distortion is compensated for at the input of the power amplifier.
Ericsson is conducting intensive research
in the area of digital predistortion, and the
application is expected to become a mature
solution for future WCDMA systems.
Ericsson is also investigating power amplifier concepts (efficiency-enhancement
technologies) that will yield greater efficiency in the power amplifier itself.
Conclusion
The growing market for third-generation
systems requires a base station RF power
amplifier designed for large-volume production. Ericsson has chosen the feedforward technique with analog predistortion, which gives excellent spurious emission values and high production yield. A design for efficiency and cooling guarantees
the best MTBF in a small-size amplifier.
Progress in semiconductor technology and
improved power amplifier designs have set the
stage for a next-generation MCPA with better efficiency, smaller size, and high MTBF.
REFERENCES
1 Zune, P.: Family of RBS 3000 products for
WCDMA systems. Ericsson Review
3/2000, pp. 170-177.
2 Third-generation Partnership Project;
Technical Specification Group Radio
Access Networks; UTRA (BS) FDD; Radio
Transmission and Reception.
3 GPP TS 25.104.
Ericsson Review No. 4, 2001
189
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