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Green Power Amplification Systems for 3G+ Wireless
Communication Infrastructure
Oualid Hammi(1), Andrew Kwan(2), Mohamed Helaoui(2), and Fadhel M. Ghannouchi(2)
(1) Department of Electrical Engineering, King Fahd University of Petroleum and Minerals, Dhahran
31261, Saudi Arabia
(2) iRadio Lab., Dept. of Electrical and Computer Engineering, Schulich School of Engineering,
University of Calgary, Calgary, AB, T2N 1N4, Canada
Abstract — This paper presents an overview of system-
level solutions for green wireless communication
infrastructure. It illustrates that a combination of high
efficiency amplification system with a linearity restoration
technique is unavoidable. Currently, Doherty power
amplifiers along with digital predistortion technique are able
to achieve high power efficiency. However, due to the limited
bandwidth of digital predistortion systems (typically
20MHz), feedforward based linearizers which result in
significantly lower power efficiency are still considered for
wideband solutions (up to 60MHz). An experimental
approach for extending the bandwidth of digital
predistortion systems is reported. This setup is then applied
for the linearization, using digital predistortion, of a Doherty
power amplifier driven by an 8-carrier WCDMA signal
having a total bandwidth of 40MHz.
Index Terms — Digital predistortion, Doherty amplifier,
drain modulation, efficiency, feedforward, green radio,
LINC, load modulation, nonlinearity, power amplifier,
wideband.
I. INTRODUCTION
Third generation (3G) and beyond (3G+) wireless
communication systems offer a wide range of data
transmission capabilities including voice, data and
services. This is achieved by using spectrum-efficient
modulation and access techniques. At the hardware level,
this sets stringent requirements especially on the design of
the radio frequency (RF) power amplification stage.
Indeed, first, modulating the signals in compact
constellations makes it very sensitive to the nonlinearity of
the transmitter which is mainly generated by the power
amplifier (PA). Second, the resulting signals which end up
having relatively high peak to average power ratios
(PAPR) require that the power amplifier operates at a
maximum average output power that is lower than its
saturation power by at least the PAPR of the signal. For
example, a 100-Watt peak power amplifier transmitting a
signal with a 10 dB PAPR should operate at an average
power that does not exceed 10-Watt to avoid saturation
and thus unlinearizable distortions. Linearity performance
of transmitters must meet the regulatory requirements in
terms of spectrum emission mask, error vector magnitude
(EVM), etc.
Besides this need for linearity, power efficiency is
becoming an increasingly important concern. With the
trend toward more efficient and greener wireless
communication systems, wireless operators target a 50%
reduction in their power consumption by 2020. Improving
the power efficiency of wireless communication systems
can be achieved by working at different levels such as:
- Improving the quality of service (QoS) to the RF
power ratio: This consists in minimizing the
transmitted RF power while maintaining the same
QoS.
- Scaling the energy needs with the traffic: This can
be done by switching the base station to sleep mode
at low-traffic time windows for example.
- Increasing the overall base station efficiency: This
consists mainly in improving the efficiency of the
power amplification stage since it is the sub-system
that has the greatest impact on the base station
power consumption.
This paper focuses on the RF front-end section of base
stations and discusses the different approach that are
considered as potential solutions for implementing high
power-efficiency green wireless communications
infrastructure. The considerations in designing such
systems are linearity and efficiency. Yet, both the linearity
and the efficiency of the transmitter are heavily impacted
by those of the of the power amplification stage. Here, it is
important to recall that in power amplifiers, linearity is
high at low power drive where the efficiency is low, and
that amplifiers get more and more nonlinear and also
efficient as they are driven closer to saturation. Since
linearity is a must, the design criterion of transmitters in
general and RF power amplifiers in particular is to meet
the linearity requirements with the highest possible
efficiency. This can be achieved by designing relatively
nonlinear power amplification systems with high
efficiency and then restoring the linearity performance by
using appropriate system level architectures and by means
of linearity enhancement techniques.
In this paper, an overview of potential system-level
architectures suitable for green wireless communication
infrastructure is presented. Then, an experimental
approach is proposed for extending the bandwidth of
digital predistortion system from 20MHz to 40MHz.
978-1-4244-3574-6/10/$25.00 ©2010 IEEE
Experimental results are reported for the linearization of a
Doherty power amplifier driven by an 8-carrier 40-MHz
wide WCDMA signal. In Section II, high efficiency power
amplification architectures suitable for wireless
communication infrastructures are discussed. In Section
III, the wideband experimental setup for digital
predistortion is presented, and used for the linearization of
a Doherty power amplifier prototype. The conclusions are
presented in Section IV.
II. HIGH EFFICIENCY POWER AMPLIFICATION SYSTEMS
Power amplification systems are designed to meet the
linearity requirements with the highest possible efficiency.
Such process requires two major steps: designing a high
efficiency power amplification system and, to meet the
linearity specifications, compensating for the PA
distortions using a linearization technique.
Due to the nature of the signals being transmitted and
their statistics (probability density function), the efficiency
of the amplification stage is mainly impacted by the
average efficiency (that is the efficiency at the operating
average power) rather than the peak efficiency (that is the
efficiency at the peak power). Thus, single-ended power
amplifiers operating at a fixed bias are not a valuable
solution since they do not offer any mean for efficiency
enhancement at the back-off region. Thus, more advanced
power amplification systems such as Doherty amplifiers,
linear amplification using nonlinear components (LINC),
and drain modulation system are being considered.
A. Doherty Power Amplifiers:
Doherty power amplifiers were introduced by W. H.
Doherty in 1936 [1]. The block diagram of a conventional
two-branch Doherty power amplifier is presented in
Figure 1. Typically, the input signal is split equally to feed
the two single-ended amplifiers, namely the carrier
amplifier in the upper branch and the peaking amplifier in
the lower branch. The carrier amplifier is biased in class
AB, while the peaking amplifier is biased in class C. The
gate bias of the class C PA is chosen to control its turn ON
region.
At low input power drive, the peaking amplifier is OFF
and the carrier amplifier sees a load higher than the output
load ( L
R
). This causes the carrier amplifier to saturate in
the back-off region with respect to the Doherty PA peak
power. This power level is commonly referred to as the
Doherty transition point, and can be controlled through the
choice of the design parameters. Theoretically, the
Doherty PA has two efficiency peaks that occur at the
Doherty transition point and at the peak power. Beyond
the Doherty transition point, the peaking amplifier is
turned ON and starts contributing to the output power of
the Doherty amplifier. This causes the load seen by both
amplifiers to change as a function of the input drive. This
mechanism maintains relatively high efficiency over the
power range spanning from the Doherty transition point to
the peak power. More detailed analysis of the Doherty
amplifier concept are provided in References [2]-[4].
0
Z
α
⋅
Fig. 1. Block diagram of a conventional Doherty amplifier.
Accordingly, the main advantage of the Doherty
amplifier is that it achieves high power efficiency at back-
off levels. The conventional two-way Doherty amplifier
(0.5
α
=) has a peak efficiency at 6 dB output power
back-off (OPBO). To further improve the efficiency of
Doherty amplifiers at various OPBO levels, N-way
Doherty PAs have been proposed [5]. The use of N-way
Doherty amplifiers, theoretically, creates 1N− efficiency
peaks with significantly flatter efficiency plateaus.
Doherty amplifiers designed for base station
applications are able to achieve high power efficiency
[6][7]. Even though they are inherently highly nonlinear,
Doherty amplifiers are still linearizable [6][7]. This
combination of high power efficiency and linearizability
has made the Doherty amplifier the standard choice for
current base stations. However, many issues can be
addressed to further improve the performances of Doherty
amplifiers. Indeed, the gain of the peaking amplifier is
unequal to that of the carrier amplifier, due to the
difference in the bias conditions. Accordingly, unequal
power splitting is required at the input of the Doherty
amplifier. This solves the problem of the difference in the
small-signal gain between the two branches of the Doherty
amplifier. At the transistor level, device suppliers tend to
customize and tweak the transistors’ characteristics to
optimize their performance in the Doherty PA
configuration.
B. Linear Amplification Using Nonlinear Components:
The linear amplification using nonlinear components
(LINC) concept was introduced in 1936 [8]. It is based on
the use of two amplification branches as presented in
Figure 2. First, the input signal is preprocessed within the
signal components separator. This transforms the
amplitude-modulated signal into two constant envelope
signals that are insensitive to the nonlinearity of the PA.
Since the signals feeding each amplifier are phase-
modulated, it is possible to use power efficient saturated
or switching mode nonlinear PAs. The amplified signals
are then combined to retrieve the original amplitude-
modulated input signal; therefore, the signal combiner is a
critical component in LINC systems. Indeed, the average
efficiency of LINC systems (
η
LINC) is given by:
LINC PA C omb
ηηη
=⋅ (1)
where PA
η
and Comb
η
are the average efficiency of the
amplifiers and the combiner, respectively.
Signal
Component
Separator
PA
S
1
S
2
S
out
Combiner
S
in
PA
Fig. 2. Block diagram of a LINC based amplification system.
The signal decomposition makes it possible to keep the
average efficiency of the amplifiers high and independent
of the input signal’s PAPR. However, maintaining a high
efficiency in the combiner is a more critical task for LINC
systems, especially when high PAPR signals are involved.
Indeed, in the case of isolated combiners, such as
Wilkinson combiners, the average combining efficiency is
decreased as the PAPR of the signal increases. This is
mainly due to the power dissipated in the isolation
resistance when low power levels are generated from out-
of-phase high power signals.
To improve the combining efficiency, lossless non-
isolated combiners can be used [8][9]. These combiners
induce time-varying amplitude-dependant load
impedances presented to the amplifiers. This degrades the
amplifier’s DC consumption and efficiency. It also alters
the linearity of the LINC system. Digital predistortion
have been proposed to linearize LINC transmitters;
however, the reported results, for both the efficiency and
the linearity, are far below the performance that can be
achieved using Doherty techniques along with digital
predistortion.
In addition to the performances that can be achieved by
LINC and LINC based amplification architectures, the
gain and phase imbalance between the two branches and
the bandwidth of the constant-envelope signals feeding the
amplifiers are critical issues that need to be tackled, in
order to make the LINC technique attractive for base
station infrastructure. In fact, LINC systems performance
is highly sensitive to the gain and phase imbalance
between the two branches [10][11]. Moreover, the
bandwidths of the phase-modulated signals at the output
of the signal components separator are typically five times
wider than that of the input signal.
C. Drain Modulation Architectures:
Linear amplification with high efficiency can also be
achieved through drain, or more generally bias,
modulation techniques. This consists in varying the bias
conditions of the transistor to maximize the amplifier’s
power efficiency, while maintaining high linearity levels.
Two drain modulation techniques have been proposed:
Kahn or envelope elimination and restoration (EER) and
envelope tracking (ET) [3]. The block diagram of EER
based amplification stages is presented in Figure 3. The
amplitude- and phase-modulated input signal is applied to
a highly nonlinear but power efficient amplifier, typically
operating in switching mode, through the envelope and
phase paths. In the envelope path, the amplitude
information is extracted using an envelope detector. This
information is then used to modulate the supply voltage of
the power amplifier via the amplitude amplifier / bias
modulator. In the phase path, a limiter is applied to strip
the signal from its amplitude. This envelope elimination
generates the constant amplitude phase-modulated signal
that is applied to the RF PA. At the output of the power
amplifier, the envelope information is restored through the
bias modulation.
PA
Delay
line
Limiter
Amplitude
detector
Amplitude
amplifier
input output
(Switching
mode PA)
Fig. 3. Block diagram of EER based amplification systems.
The EER concept was introduced by Kahn in 1952 [12];
however, its implementation for modern communication
systems is still facing serious challenges, which are
mainly related to the bandwidth of the signals to be
amplified. Indeed, the performance of EER systems is
significantly affected by the differential delay between the
envelope and phase paths. Practically, a delay alignment
resolution that is in the range of 1
10
B
W×
(where BW is
the bandwidth of the input signal) is needed. Furthermore,
the bandwidths of the phase and envelope signals are five
times wider than that of the input signal. Accordingly,
both the amplitude and RF amplifiers need to operate over
a wide bandwidth, which significantly decreases their
efficiency. To overcome this major limitation, hybrid /
wideband EER systems have been proposed [13]-[15].
This architecture is a variant of EER in which the
envelope limiter is removed. Thus, the bandwidth of the
RF signal feeding the switching mode PA is reduced and
kept equal to that of the original input signal. Power-added
efficiency performances reported for such amplification
systems are in the range of 30% with 20 MHz orthogonal
frequency division multiplexing (OFDM) signals around
2.4 GHz [13][14], and 35%-40% with single-carrier 5
MHz WCDMA signals around 1 GHz [15].
Another approach to implement drain modulation is the
envelope tracking technique presented in Figure 4. In the
RF path, the signal is fed to a continuously driven “linear”
power amplifier. In the envelope path, the amplitude
amplifier dynamically adjusts the bias of the RF amplifier
in order to maximize its efficiency. Indeed, the basic
conceptual difference between EER and ET systems is in
the envelope path, which is used to restore the system
linearity in EER and to maximize the system efficiency in
ET. Envelope tracking can be performed either following
the average power of the signal (average envelope
tracking – AET) or the instantaneous envelope of the
signal (wideband envelope tracking – WET).
PA
Delay
line
Amplitude
detector
Amplitude
amplifier
input output
(Linear mode PA)
Fig. 4. Block diagram of ET based amplification systems.
WET systems require high-speed bias modulators. This
limits their bandwidth capabilities to a few MHz. For base
station applications, state-of-the-art power-added
efficiencies are in the range of 50% [16] and 40% [17] for
GaN based and LDMOS based WET systems,
respectively, with single-carrier WCDMA signals having a
7.5 dB PAPR. Conversely, AET requires slower bias
modulators and, thus, can be applied for wideband and
multi-carrier amplification systems. It can be considered
as an efficiency enhancement technique that can be
utilized for any type of fixed bias based amplification
system to further improve efficiency. However,
introducing an extra variable to the system behavior will
certainly affect its linearity performance.
Practical implementations of EER and ET systems lead
to poor linearity performances that are unable to meet
regulatory specifications. Therefore, digital predistortion
has been proposed for EER and ET systems to improve
the overall linearity [13]-[17].
III. WIDEBAND DIGITAL PREDISTORTION OF DOHERTY PAS
Doherty power amplifiers are currently the preferred
architecture for base station PAs due to their advantages
discussed in the previous section. To meet linearity
requirements, Doherty architecture is either combined
with a feedforward or a predistortion based linearizer.
Feedforward linearizers are wideband but introduce a drop
in the system efficiency mainly due to the linear amplifier
required in the distortions cancellation loop of the
feedforward system. Conversely, digital predistortion lead
to higher power efficiency but has a bandwidth limited to
around 20MHz. A typical experimental setup for digital
predistortion of power amplifiers is reported in Figure 5.
The nonlinearity of the power amplifier is characterized
by processing the input and output signals, and the
corresponding predistortion function is synthesized by
means of digital signal processing.
In such systems, the receiver path commonly consists of
a vector signal analyzer. The output signal is down-
converted into an intermediate frequency and then
digitized. Such approach has a limited observation
bandwidth of 80MHz. This limits the bandwidth of the
input signal to around 20MHz. Indeed, the observation
bandwidth should be 5 times greater than that of the input
signal to capture third and fifth order intermodulation
distortions. To alleviate this limitation and cope with
current requirements for wideband predistortion, this work
proposes the digitization of the signal around the RF
frequency using a high speed oscilloscope and then
extracting the baseband information from the digitized
signal. The acquired output signal is then compared to the
input signal to generate the predistortion function.
The device under test used to validate this approach is a
GaN based Doherty power amplifier operating around
2140 MHz. The digital predistorter was constructed using
the parallel twin-nonlinear two-box models proposed in
[18]. The DUT was driven by an eight-carrier WCDMA
signal having a total bandwidth of 40 MHz. The measured
spectra at the output of the device under test before and
after linearization are reported in Figure 6. This clearly
illustrates the effectiveness of the proposed approach in
cancelling the distortions generated at the output of the
device under test. Moreover, it illustrates the capabilities
of the used digital predistorter to compensate for strong
memory effects due to the wideband input signal.
Fig. 5. Block diagram of wideband DPD experimental setup.
-50
-40
-30
-20
-10
0
10
2080 2100 2120 2140 2160 2180 2200
DPD ON
DPD OFF
Power Spectrum Density (dBm/Hz)
Frequency (MHz)
Fig. 6. Measured spectra at the output of the DUT.
IV. CONCLUSION
In this paper, an overview of recent advances in system
level architectures of power amplification stages was
presented. This included Doherty, LINC and drain
modulation based PAs. Doherty technique seems better
positioned to enable high efficiency green base station
transmitters especially with the continuously evolving
transistors technologies in which the transistors are
specifically optimized for such application. An
experimental approach for wideband digital predistortion
of Doherty amplifiers was then presented. This method
extends the bandwidth of the digital predistortion from
20MHz to 40MHz. Experimental validation was carried
out on a Doherty PA driven by 40-MHz WCDMA signal.
ACKNOWLEDGEMENT
O. Hammi would like to acknowledge the support
provided by the Deanship of Scientific Research at King
Fahd University of Petroleum & Minerals (KFUPM).
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