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PALO ALTO, Calif., Jan. 13, 2003
The digital modulation schemes employed in
TDMA, CDMA, and now
W-CDMA services require an entirely new set of tests as compared
to analog FM transmission. One of the most critical tests is
measuring the linearity of modern amplifiers employed in the
latest base stations.
Unlike their predecessors, these amplifiers
-- multichannel power amplifiers or MCPAs -- now pass multiple
signals simultaneously, rather than handling a single carrier
at a time. This makes it absolutely essential that none of the
content from one signal leaks into the area occupied by neighboring
signals. Amplifier distortion must be maintained at extremely
low levels, which requires the amplifiers to operate linearly
and employ one of two popular techniques (feedforward cancellation
or predistortion) to ensure this linearity is maintained.
MCPAs dramatically reduce the number of power
combiners and filters that are required when many more single-channel
amplifiers are used. The amplifiers also require no guard band,
so duplexers may no longer be needed. They allow dynamic channel
allocation and more efficient use of spectrum resources, handle
both wideband and narrowband modulation formats, and make it
much easier to add more carriers, a characteristic that eases
the transition from 2G to 3G systems.
To keep MCPAs linear, the feedforward cancellation
technique was first employed. This technique samples the amplifier's
output signal (which contains distortion products) and compares
it with the signal at the input of the amplifier. The difference
between the two signals, which contains the undesired spurious
products, is amplified and subtracted from the desired carrier
signal, which produces signal conditions in which the spurious
levels are much lower than the desired signal. While this technique
is effective, it is not terribly efficient, and gets more difficult
to implement as bandwidth increases.
In the other technique, called predistortion,
a signal is sent through a predistortion circuit (a complex
gain adjuster) before it reaches the amplifier. This circuit
distorts the signal with the inverse of the distortion characteristics
of the amplifier. Once the signal is sent through the amplifier,
these characteristics cancel out, leaving an undistorted signal.
This technique is generally more efficient, and provides the
best linearization performance, but is expensive because it
measures signal characteristics and compensates for those characteristics
in real time. However, it is growing in popularity as signal
bandwidths increase.
Obviously, this type of linearization technique
requires data that defines the distortion characteristics of
the amplifier as closely as possible, so that the predistortion
circuit can be precisely "curve fit." To gather this
information, a stimulus signal must be used, preferably one
with characteristics similar (or identical) to those that the
amplifier will see in service. CW tones have been used for this,
which require multiple sources and are better suited to narrowband
signals. Additive White Gaussian Noise (AWGN), which has "random"
magnitude characteristics and bandwidth closer to those of digital
signals, was used in the tests of early digital base station
amplifiers. However, neither of these sources has characteristics
that are exactly the same as those of today's complex digital
modulation schemes. Hence, using these signals as stimuli for
testing MCPAs can produce results that do not accurately depict
how the amplifier will perform when "real-world" signals
are input. Distortion can then occur, which leaks into adjacent
channels, degrading their performance.
The Agilent 89604A Distortion Test Suite relies
on signals that can come from an operating base station or from
signals generated by digital signal generators that can precisely
mimic the signal environment in which the MCPA will operate.
When these signals are used to stimulate the MCPA under test,
the results are more indicative of the amplifier's true characteristics.
The data can then be sent to the advanced curve fitters used
by the predistortion circuitry to speed the process of optimizing
MCPA linearity.
The technique also produces additional useful
information, including time-aligned, corrected data, which would
otherwise have to be compiled by the person doing the test.
# # #
Contact:
Janet Smith, Agilent
+1 970 679 5397
janet_smith@agilent.com
Heather Van Schoiack
Weber Shandwick, for Agilent
+1 425 452 5457
hvanschoiack@webershandwick.com
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