Bit-Error-Rate of LDPC coded QAM in the presence of a residual phase noise process and a non-linear distortion

In this paper we present an analysis of the Bit Error Rate (BER) of LDPC-coded-QAM systems in an Additive-White-Gaussian-Noise (AWGN) channel impaired by a residual Phase-Noise (PN) process and a Nonlinear (NL) distortion. We start by deriving expressions for the Mean-Square-Error (MSE) caused by these common impairments. The BER is calculated separately for the coded bits and the uncoded bits, in a coded modulation scheme. In the first case, the MSE is added to the channel noise variance. As a consequence, the BER curve, which is assumed to be known, is shifted and an error floor is generated. The BER of the uncoded bits is calculated directly from the phase noise variance, the channel SNR and the nonlinear distortion which affects the distance between constellation points. Simulation results using an LDPC code validate the derivations.