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Adaptive Channel Equalization 603
The project may be generalized by adding a second sinusoid of a
different frequency. Then H(e jω ) should exhibit two resonant peaks, provided
the frequencies are sufficiently separated. Investigate the effect of
the filter length N on the resolution of two closely spaced sinusoids.
11.4 ADAPTIVE LINE ENHANCEMENT
In the preceding section we described a method for suppressing a strong
narrowband interference from a wideband signal. An adaptive line enhancer
(ALE) has the same configuration as the interference suppression
filter in Figure 11.3, except that the objective is different.
In the adaptive line enhancer, {s(n)} is the desired signal and {w(n)}
represents a wideband noise component that masks {s(n)}. The desired
signal {s(n)} may be a spectral line (a pure sinusoid) or a relatively
narrowband signal. Usually, the power in the wideband signal is greater
than that in the narrowband signal—that is, P w >P s .Itisapparent that
the ALE is a self-tuning filter that has a peak in its frequency response
at the frequency of the input sinusoid or in the frequency band occupied
by the narrowband signal. By having a narrow bandwidth FIR filter, the
noise outside the frequency band of the signal is suppressed, and thus
the spectral line is enhanced in amplitude relative to the noise power in
{w(n)}.
11.4.1 PROJECT 11.3: ADAPTIVE LINE ENHANCEMENT
This project requires the same software modules as those used in the
project on interference suppression. Hence the description given in
the preceding section applies directly. One change is that in the ALE, the
condition is that P w >P s . Second, the output signal from the ALE is
{s(n)}. Repeat the project described in the previous section under these
conditions.
11.5 ADAPTIVE CHANNEL EQUALIZATION
The speed of data transmission over telephone channels is usually limited
by channel distortion that causes intersymbol interference (ISI). At data
rates below 2400 bits the ISI is relatively small and is usually not a problem
in the operation of a modem. However, at data rates above 2400 bits,
an adaptive equalizer is employed in the modem to compensate for the
channel distortion and thus to allow for highly reliable high-speed data
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