Crosstalk and phase-noise effects in multi-channel coherent optical CP-FSK systems with tight IF filtering

Reported is a rigorous bit error rate analysis appropriate to continuous phase frequency shift keying (CP-FSK) coherent optical receivers with delay demodulation and an integrating IF filter with integration time less than or equal to the delay time. For an integration time equal to the delay time the model is extended to two optical channels. An important result of the analysis is that IF filtering for CP-FSK modulation reduces the phase swing due to modulation at the detector and this leads to a degradation in system performance with regard to phase noise, receiver noise, and crosstalk influence. For a modulation index of 0.5, for a single optical channel a minimum 2.4-dB penalty relative to matched filter detection is found. The required channel separation for CP-FSK systems appears to be significantly larger than for differential phase shift keying systems and phase noise enhances this effect