Page 94 - DCAP406_DCAP_207_Computer Networks

Unit 6: Multiplexing

This complicates the receiver’s design. The advantages of PSK are that it is immune to noise and Notes
is not band limited.
Differential Phase Modulation: A sub method of the phase modulation is differential phase
modulation. In this method, the modem shifts the phase of each succeeding signal in a certain
number of degrees for example, a 0 for 90 degrees and 1 for 270 degrees as illustrated in Figure
6.10.
Figure 6.10: Phase Shift Keying

p p- k

00 00 10 00 10 00

PSK is a technique, which shifts the period of a wave. The wave shown in Figure 6.10 (a) has a
period of p starting from 0. The wave shown in Figure 6.10 (b) is the same wave as shown in
Figure 6.10 (a), but its phase has been shifted. Notice that the period starts at the wave’s highest
point 1 on the vertical axis. It just so happens that we have shifted this wave by one quarter of the
wave’s full period. We can shift it another quarter, if we want to, so the original wave would be
shifted by half of its period. And we could do it one more time, so that it would be shifted three
quarters of its original period.
This means there exist 4 separate waves and therefore each wave is provided for some binary
value. Since there are 4, 2 bits are provided to each wave which is represented below:

Bit value Amount of Shift
00 None
01 ¼
10 ½
11 ¾

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