Cutoff rate for the M-ary PSK modulation channel with optimal quantum detection

The cutoff rate R/sub 0/ of an optical channel transmitting M-ary phase-shift-keyed coherent states is calculated for a receiver using the optimum quantum detector. This is shown to be nearly equivalent, when M>>1, to a system that measures the phase of the received signal. A closed-form expression yielding a good approximation to R/sub 0/ is derived.<
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