Differential phase shift keying is a non coherent form of phase shift keying which avoids the need for a coherent reference signal at the receiver. Further PSK needs coherent detection due to its phase shift with a refrence signal. you should keep in mind that no coherent receivers are easy and cheap to build.
the reason behind non-coherent detection of DPSK is because its phase changes with its own delayed signal,nd we generally use this technnique.
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