Performance of coherent optical CPFSK-DD with intersymbol interference, noise correlation, and laser phase noise

An exact probability of error expression is presented for a narrow-deviation binary CPFSK coherent optical receiver utilizing differential detection (CPFSK-DD). The result is given in terms of the Marcum Q-function and takes into account the non-Gaussian noise statistics at the decision moment, intersymbol interference, noise correlation, and laser phase noise. Numerical results indicate a local oscillator shot-noise-limited receiver sensitivity of 23.4 photons/b when using a modulation index of 0.67 in combination with an IF filter having a sixth-order Butterworth magnitude response and a normalized 3-dB bandwidth of 1.09. For a given IF filter and IF linewidth, it is found that the modulation index and the IF filter bandwidth should be jointly optimized in order to achieve the best overall receiver sensitivity. When the IF linewidth times the differential delay product is 0.34 %, modulation index and the optimum normalized 3-dB bandwidth are found to be 0.72 and 1.1, respectively. By using Monte Carlo simulation, it is demonstrated that adding laser phase noise at the demodulation stage of the analysis is a valid simplifying assumption for a wide range of practical design parameters