The use of trellis coded modulation leads to the requirement of a 2-fold increase in the channel alphabet size,e.g.from 4-PSK to 8-PSK.It is shown that multi-dimensional 6-PSK trellis codes can be designed which achieve the same spectral efficiency at a reduced channel symbol alphabet size.This allows for easier phase synchronization over 8-PSK.Examples of rotationally invariant as well as non-invariant codes are presented.
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