Question

Research on the Web the types of PSK modulation and answer the following questions:

a) What is differential phase shift keying (DPSK) modulation?

b) What are its advantages and disadvantages over the non-differential PSK modulation?

c) Why is DPSK used in power line communications instead of plain PSK?

Answer 1

Differential Phase Shift Modulation - It may view as a noncoherent version of PSK. It eliminates the need for a coherence reference signal by combining two operations at the transmitter:

1- Differential encoding of the input binary wave.

2-Phase shift keying, hence the name differential phase shift keying.

In effect of send symbol 0,we phase advance the current signal waveform by 180 degrees,and to send symbol 1, we leave the phase of the current signal unchanged.

You can observe here, when the symbol is zero , there is no phase reversal. When the symbol is 1 , It's phase is reversed by 180.

Advantages and Disadvantages over non-differential PSK modulation:

Differential phase shift modulation facilitates non coherent modulation, whereas non-differential PSK does not support this.Coherent demodulation means that the carrier sinusoid which is modulated by the message signal to get the passband signal has to somehow be reproduced at the receiver end and thereby used for demodulation of the passband signal to obtain the message.This can become difficult as recieved signal's carrier frequency has deviated from original because of doppler and other effects.So using the same carrier frequency as the transmitted signal will give a noisy demodulated message signal.

As coherent system provides better noise immunity over the non coherent one. So a high SNR required to achieve a bit error equivalent to non-differential phase shift keying.

Because DPSK is non coherent Scheme ,It is simpler to implement than non-differntial PSK, i.e there is no need for the demodulator to have a copy of the reference signal to determine the exact phase of the received signal, a trade-off is that it has more demodulation errors.