0195136047.pdf

15.3 DIGITAL COMMUNICATION SYSTEMS 725

PHASE-SHIFTKEYING(PSK)

In PSK, the phase angle of a carrier is keyed between two values. When the values are separated
byπradians, it is known asphase-reversal keying(PRK). The PSK waveform in the bit interval
is given by

sPSK(t)=

{

Accos(ωct+φc), 0 <t<Tb, for 1

−Accos(ωct+φc), 0 <t<Tb, for 0

(15.3.19)

Equation (15.3.18) can be viewed as a carrierAc cos(ωct+φc)modulated in amplitude by
a digital signald(t) with amplitudes of 1 when the binary digit in the bit interval is 1 and 0,
respectively. Figure 15.3.16 illustrates the functions of a coherent PSK communication system,
which is similar to the ASK system of Figure 15.3.14 with the following differences:

• The received signal is now given by

sR(t)=

{

Accos(ωct+φc), 0 <t<Tb, for 1

−Accos(ωct+φc), 0 <t<Tb, for 0

(15.3.20)

• A gain of 2 is assigned to the integrator.


• A threshold is absent when binary digits 0 and 1 occur with equal probability.
  No truly noncoherent PSK version is possible because the PSK signal carries its information
  in the carrier’s phase, whereas a noncoherent system purposely disregards phase and operates
  only on signal amplitude.

DIFFERENTIALPHASE-SHIFTKEYING(DPSK)

In order to eliminate the need of a local carrier, DPSK has been developed in which the receiver
uses the received signal to act as its own carrier. Figure 15.3.17 shows the functions of a DPSK
system in which the leftmost product operation along with 1-bit delay is the differential encoder.
The digital signald(t) is a polar waveform of levels±1, corresponding to binary digits 1 and
0, respectively. The output signala(t) from the differential encoder PSK-modulates a carrier to
produce the DPSK signalsDPSK(t). The product device followed by the wide-band low-pass filter
acts as the coherent detector. The two inputs to the product device are obtained from the output
of a filter matched to the input pulse in a single bit interval. Note that the 1-bit delayed input to
the product device serves the purpose of the local oscillator for the coherent detector; that is to

Sampler

Sample at time Tb for bit interval 0 < t ≤ Tb

Waveform formatterChannel Demodulator and code generator of receiver

and modulator

A cos (ωct + φc)

(local oscillator)

Ac cos (ωct + φc)

Broad-band system noise

Digital

signal d(t) sPSK(t) sR(t)

R ≥ 0 choose 1

< 0 choose 0

Σ

+

+

2 (.) dt D

Tb

0

Figure 15.3.16Functions of a coherent PSK communication system.