W. Huang et al., DECISION-DIRECTED PLL FOR COHERENT OPTICAL PULSE CDMA SYSTEMS IN THE PRESENCE OF MULTIUSER INTERFERENCE, LASER PHASE NOISE, AND SHOT-NOISE, Journal of lightwave technology, 16(10), 1998, pp. 1786-1794
A digital decision-directed phase-locked loop (PLL) for use in optical
pulse code-division multiple-access (CDMA) systems based on coherent
correlation demodulation is proposed. PLL performance is affected by m
ultiuser interference, laser phase noise and optical shot noise. The e
ffect of these sources of interference and noise on PLL performance is
evaluated based on a nonlinear model (the Fokker-Planck method) since
a linear analysis yields large deviations between the analytical resu
lts and actual performance at low signal-to-noise ratios (SNR's), Afte
r describing the implementation of the PLL, the steady-state probabili
ty density function (pdf) of the phase estimator is derived, Numerical
evaluation of the variance of the phase estimator is given for Gold c
odes. The linewidth requirements of the laser for an acceptable phase
estimator variance and the value of the optimal loop bandwidth minimiz
ing the impact of the interference and noise on the PLL are discussed.