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CHAPTER 11
Applications
in Adaptive
Filtering
In Chapters 7 and 8 we described methods for designing FIR and IIR digital
filters to satisfy some desired specifications. Our goal was to determine
the coefficients of the digital filter that met the desired specifications.
In contrast to the filter design techniques considered in those two
chapters, there are many digital signal processing applications in which
the filter coefficients cannot be specified a priori. For example, let us consider
a high-speed modem that is designed to transmit data over telephone
channels. Such a modem employs a filter called a channel equalizer to compensate
for the channel distortion. The modem must effectively transmit
data through communication channels that have different frequency response
characteristics and hence result in different distortion effects. The
only way in which this is possible is if the channel equalizer has adjustable
coefficients that can be optimized to minimize some measure of the distortion,
on the basis of measurements performed on the characteristics of
the channel. Such a filter with adjustable parameters is called an adaptive
filter, inthis case an adaptive equalizer.
Numerous applications of adaptive filters have been described in the
literature. Some of the more noteworthy applications include (1) adaptive
antenna systems, in which adaptive filters are used for beam steering and
for providing nulls in the beam pattern to remove undesired interference
[29]; (2) digital communication receivers, in which adaptive filters are
used to provide equalization of intersymbol interference and for channel
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