Performance of narrow deviation coherent optical CPFSK-DD with tight IF filtering and an amplified interfering echo

An exact probability of error expression that includes the impact of tight IF filtering and an amplified interfering echo is presented for a narrow deviation binary CPFSK-DD coherent optical receiver. The result is given in terms of the Marcum Q-function and accommodates the phase memory in the CPFSK signaling format; the impact of IF filtering on the signal, echo, and additive noise; the impact of reflections on optical amplifier noise; the non-Gaussian noise statistics at the decision moment; and the influence of laser phase noise. Sensitivity penalty results are presented for three modulation indices, both with and without intersymbol interference and phase noise, thus providing the first detailed insight into the impact of an interfering echo on CPFSK-DD receiver performance. Interferometric interference effects are observed independent of the accumulated laser phase noise between the main and echo signals, and it is concluded that a relative echo magnitude of 10% or less represents a good design criterion to aim for in a practical communication system. In all cases, the analytical results have been confirmed through extensive Monte-Carlo simulation