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604 Chapter 11 APPLICATIONS IN ADAPTIVE FILTERING
FIGURE 11.5
Application of adaptive filtering to adaptive channel equalization
transmission. In telephone channels, filters are used throughout the system
to separate signals in different frequency bands. These filters cause
amplitude and phase distortion. The adaptive equalizer is basically an
adaptive FIR filter with coefficients that are adjusted by means of the
LMS algorithm to correct for the channel distortion.
A block diagram showing the basic elements of a modem transmitting
data over a channel is given in Figure 11.5. Initially, the equalizer
coefficients are adjusted by transmitting a short training sequence, usually
less than one second in duration. After the short training period,
the transmitter begins to transmit the data sequence {a(n)}. Totrack
the possible slow time variations in the channel, the equalizer coefficients
must continue to be adjusted in an adaptive manner while receiving data.
This is usually accomplished, as illustrated in Figure 11.5, by treating
the decisions at the output of the decision device as correct and by using
the decisions in place of the reference {d(n)} to generate the error signal.
This approach works quite well when decision errors occur infrequently,
such as less than one error in 100 data symbols. The occasional decision
errors cause only a small misadjustment in the equalizer coefficients.
11.5.1 PROJECT 11.4: ADAPTIVE CHANNEL EQUALIZATION
The objective of this project is to investigate the performance of an adaptive
equalizer for data transmission over a channel that causes intersymbol
interference. The basic configuration of the system to be simulated
is shown in Figure 11.6. As we observe, five basic modules are required.
Note that we have avoided carrier modulation and demodulation, which
is required in a telephone channel modem. This is done to simplify the
simulation program. However, all processing involves complex arithmetic
operations.
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