Digital Signal Processing Reference
In-Depth Information
Example 7.8: Adaptive Predictor for Cancellation of Narrowband
Interference Added to a Desired Wideband Signal Using External Inputs
(
adaptpredict_2IN
)
This example extends the previous one, which implements an adaptive FIR predic-
tor for the cancellation of narrowband interference in the presence of a wideband
signal. The program
adaptpredict_2IN.c
, shown in Figure 7.26, implements the
adaptive predictor using two external signals as inputs: a desired wideband signal
and an undesired narrowband sinusoidal signal. The desired wideband signal with
an additive narrowband interference is delayed and becomes the input to a 60-
coefficient adaptive FIR filter. See also Example 7.7, where the desired wideband
signal is software-generated.
The desired wideband signal is obtained from an HP signal analyzer. The band-
width of the undesired interference is narrow compared with the bandwidth of
the desired random noise signal. As a result, the samples of the interference are
highly correlated. By contrast, the samples of the wideband signal are relatively
uncorrelated.
The characteristics of the narrowband interference permit the estimation of the
narrowband interference from past samples of
splusn
in the program. The signal
splusn
, which represents the desired wideband signal with an additive narrowband
interference, is delayed before becoming the input to the adaptive FIR filter. The
delay is sufficiently long so that the delayed wideband signal is uncorrelated with
the undelayed sample. The output of the adaptive FIR filter is an estimate of the
correlated narrowband interference. As a result, the error signal
E
is an estimate of
the wideband signal desired.
Build and run this project as
adaptpredict_2IN
.
The adapter introduced in
Example 2.3 (
loop_stereo
) in Chapter 2 and used again in Example 7.3 is required to
implement this example. Apply random noise into the left input channel, represent-
ing the desired wideband signal. Apply a sinusoidal signal with a frequency of 2 kHz
into the right input channel, representing the undesired narrowband interference. See
also Example 7.3. Run the program and verify that the output spectrum of the error
signal
E
adapts (converges) to the desired wideband signal, showing the 2-kHz input
narrowband sinusoidal interference being gradually canceled.
Change slightly the frequency of the input sinusoidal external interference and
observe the adaptation process repeated to cancel the undesirable external inter-
ference. A faster rate of convergence can be observed by increasing
beta
by 10.
The wideband signal with the additive interference can be observed using
output_sample(
splusn[0]
).
Figure 7.27
a
shows the output spectrum of
splusn[0],
displaying both the desired wideband noise signal and the undesired nar-
rowband interference. Figure 7.27
b
shows the output spectrum of the error signal
E
after the adaptation process converged to the desired wideband signal. Note that
the desired wideband input signal can be observed by outputting
wb_signal
.
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