B. P. Lathi, Zhi Ding - Modern Digital and Analog Communication Systems-Oxford University Press (2009)

378 PRINCIPLES OF DIGITAL DATA TRANSMISSION
Fi g ure 7.33
(a) Noncoherent
detection of FSK.
(b) Coherent
detection of FSK.
H0(m)
Envelope
detector
t= T 1,
Decision
H1(D
Envelope
detector
(a)
Low-pass
filter
t= Tb
Decision
COS (J)Co
Low-pass
filter
COS ffic/
(b)
Fi g ure 7.34
Coherent binary
PSK detector
(similar to a
DSB-SC
demodulator).
detector output following the Wei filter, and the receiver decides that a O was transmitted. In
the case of a 1, the opposite happens.
Of course, FSK can also be detected coherently by generating two references of frequencies
Wc o and Wei , for the two demodulators, to demodulate the signal received and then comparing
the outputs of the two demodulators as shown in Fig. 7.33b. Thus, coherent FSK detector
must generate two carriers in synchronization with the modulation carriers. Once again, this
complex demodulator defeats the purpose of FSK, which is designed primarily for simpler;
noncoherent detection. In practice, coherent FSK detection is not in use.
PSK Detection
In binary PSK, a 1 is transmitted by a pulse A cos W e t and a O is transmitted by a pulse
-A cos w e t (Fig. 7.31b). The information in PSK signals therefore resides in the carrier phase.
Just as in DSB-SC, these signals cannot be demodulated via envelope detection because the
envelope stays constant for both 1 and O (Fig. 7 .31 b ). The coherent detector of the binary PSK
modulation is shown in Fig. 7.34. The coherent detection is similar to that used for analog
signals. Methods of carrier acquisition have been discussed in Sec. 4.8.
Differential PSK
Although envelope detection cannot be used for PSK detection, it is still possible to exploit the
finite number of modulation phase values for noncoherent detection. Indeed, PSK signals may
be demodulated noncoherently by means of an ingenious method known as differential PSK,