WiMAX - Revolutionizing the Art of Linear HPA`s

WiMAX - Revolutionizing the Art of Linear HPA`s
PAGE • OCTOBER 2007
FEATURE ARTICLE
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WiMAX - Revolutionizing the Art of Linear HPA’s
by Aethercomm
Introduction
he
emergence
of
WiMAX has enabled
opportunities to transform personal communication
and multimedia in the commercial marketplace. At the heart
of this technology is a scalable orthogonal frequency division multiple access (OFDMA)
modulation scheme (IEEE
802.16e-2005) that is capable
of transmitting and receiving information over various
sub-carriers within a scalable
bandwidth. This modulation
topology provides an efficient
way to securely transmit large
amounts of data with very
high spectral efficiency while
remaining resistant to multipath fading and inter-symbol
interference.
T
Table 1: SSPA 2.30-2.40-400 Typical Performance at 40 Watts Average
Output Power
Freq
(GHz)
P1dB
(dBm)
Small Signal
Gain (dB)
Noise
Figure
(dB)
2nd
Harmonic
(dBc)
3rd
Harmonic
(dBc)
OIP3
(dBm)
Lower
ACPR
Upper
ACPR
2.300
56.2
53.5
4.8
-54.0
<-60.0
73.0
-44.3
-45.0
2.335
56.3
53.7
4.9
<-60.0
<-60.0
73.0
-44.5
-45.9
2.370
56.2
53.5
4.9
<-60.0
<-60.0
72.0
-44.0
-44.3
2.400
56.1
53.4
4.9
<-60.0
<-60.0
71.0
-43.5
-43.5
Figure 1: Power Amplifier Transfer Function
To capture the potential
commercial advantages of
WiMAX, it is necessary to
transmit high power levels
with minimum distortion keeping the error vector magnitude
(EVM) low and minimizing the
bit error rate (BER). These high
power levels must maintain a
low adjacent channel power
ratio (ACPR) when transmitted to ensure adjacent channels
are not swamped with spectral
energy. WiMAX signals contain
high crest factors of 9.8 dB on
the complementary cumulative
distribution function (CCDF)
that contain complex modulation that is very sensitive to
amplitude and phase integrity.
These stringent system demands
require ultra-linear high power
amplification with minimal signal degradation.
With these goals in mind,
Aethercomm has developed an
ultra linear solid state power
amplifier module called the
SSPA 2.30-2.40-400. The RF
amplifier does not contain
any linearization. Operating
through the 2.30 to 2.40 GHz
frequency range, it fully supports the WiMAX profiles 1A
and 2B. The amplifier provides
high gain, an extremely flat
gain response, high OIP3 and
high peak power capabilities.
It maintains excellent ACPR
over the entire operating band
with only slight deviation when
subjected to extreme operating
temperatures.
This power amplifier is
designed to operate from a
+36.0VDC power source. Each
power transistor incorporates
a DC-DC regulation subcircuit
which provides excellent isolation from external noise and
transients. The power amplifier
includes proprietary switching
circuitry allowing the unit to
settle to within 95% of its final
output power within 1.5 uS.
This allows the unit to be well
suited for basestation applications requiring Tx and Rx
operation. Additional features
include protection against input
RF overdrive faults, voltage
standing wave ratio (VSWR)
faults and power supply voltage faults that could degrade
the performance the amplifier.
Table 1 summarizes the recorded performance results at an
average output power of 40
Watts.
To test the frequency capabilities of the power ampli-
fier, the frequency range was
expanded between 2.30 to 2.50
GHz. Under these experimental
conditions, the power amplifier demonstrated it could fully
support the WiMAX profile
1A, 1B and 2B bands.
Design Methodology
This RF amplifier incorporates
GaAs and LDMOS devices to
achieve the high system linearity.
Historically, GaAs and LDMOS
technologies have been ideal
choices for linear applications
due to their low inter-modulation distortion (IMD) products,
consistent gain and consistent
phase properties over various
output power levels. In fact,
these technologies exhibit better linearity characteristics than
newer transistor technologies
such as Gallium Nitride (GaN)
and Silicon Carbide (SiC).
In WiMAX applications, an
OFDMA signal is comprised of
data sub-carriers that contain
parts of a data stream that are
modulated with an Amplitude
Modulation (ie: 256QAM).
Since this modulation scheme
is sensitive to the amplitude
and phase properties of each
sub-carrier, an amplified signal
must have a low EVM in order
to maintain the signal integrity.
This minimizes the BER when
the signal is received and the
data package is reconstructed.
The amplitude errors associated with the EVM are addressed
by the power amplifier’s transfer function which yields a linear gain response up to a high
P1dB power level.
Since spectral efficiency is
very important for WiMAX,
it is necessary to minimize the
ACPR while maximizing the
available power to be transmitted. This ensures that adjacent channels are not swamped
by the spectral energies that
result from non-linear effects
of the power transistors. This
RF amplifier achieves excellent
ACPR due to the enhanced
linear performance and consistent PM-AM characteristics
throughout the power range.
In WiMAX applications, it
is very important to ensure
that the modulated signal is
minimally distorted during
amplification. This can be
accomplished by ensuring the
peak-to-average ratio (PAR) of
the modulated signal is not
reduced after amplification.
The RF amplifier accomplishes
this task by the balanced final
amplification stage that maintains sufficient margin between
the peak and the average power
levels of the modulated signal.
Amplifier Performance
The Aethercomm SSPA 2.302.40-400 is characterized with
a WiMAX signal modulated
with 256QAM that exhibits
a PAR of 9.8 dB at 0.01%
CCDF. The power amplifier
module produces a minimum
average output power of +46.0
dBm (40 Watts) and does not
exhibit a reduction of the PAR
on the CCDF. All data presented is characterized with these
Aethercomm, Con’t on pg X
PAGE • OCTOBER 2007
Aethercomm, Con’t from pg X
signal parameters unless otherwise stated.
Since WiMAX signals have a
specific peak power response, it
is important to characterize the
peak power capability of the
power amplifier to ensure that
the PAR of the amplified signal
is preserved. From the transfer
function detailed in Figure 1,
the power amplifier exhibits
1dB gain compression P1dB
at an output power level of
+56.0dBm (400W). This reveals
that the optimal average output power is +46.0dBm (40W),
which is predictable based on
the input signals PAR.
To determine the ACPR performance of this RF amplifier, the power spectral density
(PSD) is measured 1MHz above
and below the main channel
bandwidth. The PSD measured
from the power amplifier module is displayed graphically over
different average output power
levels in Figure 2.
As can be seen from the
figure, a noticeable “knee”
occurs at the point where the
average power is backed off
9.8 dB from the P1dB of the
amplifier. This coincides with
a nominal performance average output power of +46.0
dBm and results in an ACPR of
-43.5 dBc over the entire operating band. As output power
increases beyond this point,
signal peaks are not amplified
linearly and become distorted
while the ACPR degrades at
a constant rate of 3.8 dBc per
each 1 dB increase in output
power. Thus depending on the
desired ACPR and EVM, average output power can be scaled
based on the requirements of
the system.
Since ACPR is closely linked
to the IMD products of a power
amplifier, it is important to
reduce these in-band distortion
effects to optimize the ACPR.
This was accomplished by biasing the power transistors in a
Class AB mode and resulted in
an OIP3 of +73.0dBm. This bias
condition allows the amplifier
to take advantage an improved
IMD3 product at higher output
power levels. Figure 3 demonstrates the IMD characteristics
over various tone power levels.
Since WiMAX is targeted for
commercial markets, it is necessary to ensure thermal stability of the power amplifier over
various temperature ranges. As
can be seen from Figure 4,
the power amplifier module
FEATURE ARTICLE
Figure 2: ACPR Performance Over Average Output Power
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demonstrates high peak power
capabilities, low distortion
effects and good bandwidth
making it well suited for basestation applications. The result
is an ultra-linear high power
amplifier that can maintain signal integrity and maximize system performance.
With the superior performance characteristics of the
amplifier, Aethercomm is developing a variant of this amplifier; the SSPA 2.496-2.700400. This power amplifier will
expand the operating frequency
range between 2.5 to 2.7 GHz
to fully support the WiMAX
profile 3A with similar performance characteristics as is
delivered to the lower WiMAX
profile bands.
References:
1. Kensington P. B., High
Linearity RF Amplifier Design,
Artech House, Massachusetts,
USA, 2000
2. “Mobile WiMAX From
OFDM-256 to S-OFDMA”,
ATDI, January 2007
3. “Mobile WiMAX - Part1:
A Technical Overview and
Performance
Evaluation”,
WiMAX Forum, August 2006
Figure 3: Class AB bias IMD products
Figure 4: ACPR Performance over 40 Watts Average
Output Power
exhibits excellent ACPR stability with a variation of +/-1.0dB
over the entire temperature
range tested. Since LDMOS
FET’s have a negative temperature coefficient and are generally thermally stable, ACPR
stability is easy accomplished
using passive temperature compensation techniques that continuously monitor and adjust
the bias levels for each power
transistor. Additional thermal management provisions
include an integrated heat-sink
that helps maintain consistent
performance through various
temperatures at altitudes up to
10,000 feet.
Conclusion
Aethercomm has successfully
manufactured a 2.30 to 2.40
GHz solid state power amplifier module that can be easily expanded to cover the 2.30
to 2.50 GHz frequency range.
This power amplifier module
AETHERCOMM
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