Influence of local oscillator-to-signal power ratio on the gaussianity of CP-QPSK signal statistics

The statistics of quadrature phase-shift keying with coherent-detection and polarization multiplexing (CP-QPSK) signal at the decision circuit input is assessed by numerical simulation for different local oscillator-to-signal (LO/S) power ratios. It is shown that the signal statistics at the decision circuit input is approximately Gaussian distributed for high LO/S power ratios. However, the mean and variance of the Gaussian distribution that approximates the statistics of each symbol depend on the transmitted symbol sequence, especially for low LO/S power ratios, demonstrating the impact of the waveform on the statistics of the detected signal. Moreover, the signal statistics starts to deviate from Gaussian for LO/S power ratios below 17 dB. A semi-analytical simulation method (SASM) based on the Gaussian approximation for the signal statistics at the decision circuit input is used for performance evaluation of CP-QPSK system. The accuracy of the SASM is assessed for different LO/S power ratios by comparison with Monte-Carlo (MC) simulation results. The SASM shows good agreement with MC simulation results for LO/S power ratios between 6 and 30 dB.