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Sec. 7–4 Noncoherent Detection of Bandpass Binary Signals 511
reference is often obtained from the noisy input signal so that the reference itself is also noisy.
This, of course, increases P e over those values given by the preceding formulas. The circuitry
that extracts the carrier reference is usually complex and expensive. Often, one is willing to
accept the poorer performance of a noncoherent system to simplify the circuitry and reduce
the cost.
Example 7–5 BER FOR FSK SIGNALING WITH COHERENT DETECTION
Evaluate and plot the BER for FSK signaling in the presence of AWGN with coherent detection.
Obtain results for both a matched-filter receiver and an LPF receiver, where B = 2/T. See
Example7_05.m for the solution.
7–4 NONCOHERENT DETECTION OF BANDPASS
BINARY SIGNALS
The derivation of the equations for the BER of noncoherent receivers is considerably more
difficult than the derivation of the BER for coherent receivers. On the other hand, the circuitry
for noncoherent receivers is relatively simple when compared with that used in coherent
receivers. For example, OOK with noncoherent reception is the most popular signaling technique
used in fiber-optic communication systems.
In this section, the BER will be computed for two noncoherent receivers—one for the
reception of OOK signals and the other for the reception of FSK signals. As indicated in
Chapter 5, BPSK cannot be detected noncoherently. However, as we shall see, DPSK signals
may be demodulated by using a partially (quasi-) coherent technique.
On–Off Keying
A noncoherent receiver for detection of OOK signals is shown in Fig. 7–9. Assume that an
OOK signal plus white Gaussian noise is present at the receiver input. Then the noise at the
filter output n(t) will be bandlimited Gaussian noise and the total filter output, consisting of
signal plus noise, is
r(t) = e r 1(t), 0 6 t … T, binary 1 sent
r 2 (t), 0 6 t … T, binary 0 sent
(7–48)
Signal
plus
noise in
Receiver
Bandpass filter
(B p = equivalent bandwidth)
r(t)=s(t)+n(t)
Envelope
detector
r 0 (t) Sample r 0 (t 0 )
and
hold
Analog output
Threshold
device
~ m m ~
0
V T
r 0
Digital
output
Figure 7–9
Noncoherent detection of OOK.