Decision-directed PLL used for coherent optical pulse CDMA systems in the presence of multiuser interference, laser phase noise and shot noise

Abstract

A decision-directed phase-locked loop (PLL) is proposed for use in coherent optical pulse CDMA systems via coherent correlation demodulation. The effect of multiuser interference, laser phase noise and shot noise on the PLL performance is evaluated based on a nonlinear model (the Fokker-Planck method) since a linear analysis yields large deviations between the analytical results and actual performances for the case of low SNR. After describing the implementation of the PLL, the steady-state probability density function (PDF) of the phase estimator is derived. Numerical evaluations of the variance of the phase estimator are given for Gold code sequences. The laser linewidth requirements for a required variance and the value of the optimal loop bandwidth minimizing the impact of an noise on the PLL are discussed.