The differential encoding block and PSK modulator is common to both DPSK
and DEPSK, but the receiver operates by comparing the phase of the current incoming
carrier symbol with that of the previous carrier symbol. In this process, it rolls
'coherent detection' and 'differential decoding' into one operation.
Clearly, this detection process is much simpler than that required for true coherent PSK
and consequently DPSK is widely used in wired and radio modems for medium-rate signalling
(up to 4800 bps). DPSK, however, has a slightly poorer noise immunity than PSK since the
phase reference for DPSK is now a noisy delayed version of the input signal rather
than potentially a well-filtered, virtually noiseless reference from a carrier recovery
process. |