Performance of QAM signals in optical fiber links in the presence of nonlinear phase noise

The amplified spontaneous emission (ASE) noise of optical amplifiers and nonlinear phase (NLP) noise generated due to interaction of ASE noise and Kerr nonlinearity are the two main sources of noise in optical fiber communications systems. This paper presents an analysis of the effects of ASE noise and NLP noise on the symbol error rate performance of M-ary quadrature amplitude modulated (QAM) signals with rectangular signal constellations. The analysis is carried out assuming that ASE noise and NLP noise are statistically independent, and approximating the probability density function (pdf) of NLP noise by a Gaussian pdf. An expression for the symbol error probability conditional on the NLP noise is derived and then averaging over NLP noise is carried out using numerical integration. Numerical results for the average symbols error probability are presented for M=4, 16, 64, and 256. The comparison of error probabilities with the simulated results shows that Gaussian approximation is sufficiently accurate to predict the error probabilities for QAM signals, when NLP noise is treated as an independent process.