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In coherent optical communication schemes, the FM noises of the transmitter and/or the local oscillator seriously deteriorate the receiver sensitivity. A theory is developed describing the effect of the FM noise on the bit-error rate (BER) performance of various heterodyne and homodyne schemes. To verify the theory, the BER of a phase-shift keying (PSK) heterodyne system is measured. The feasibility of the PSK scheme using semiconductor lasers is also discussed on the basis of analysis and experimentation.