Performance comparison between two soft demodulation methods for differentially encoded QPSK modulation in the presence of phase noise

The performance of coded modulation schemes based on the serial concatenation of a forward error correction (FEC) code and a differentially encoded (DE) quadrature phase shift keying (QPSK) in presence of phase noise is considered. The QPSK symbols are transmitted over an additive white Gaussian noise channel where the random-walk (Wiener) process is adopted as a model for the unknown carrier phase. Two different soft demodulation methods are considered. In the first, the log-likelihood ratios, required to perform soft decoding, are computed assuming that the phase is perfectly known to the receiver. In the second, the statistical characterization of the received carrier phase is taken into account to improve the carrier phase estimation process through a soft iterative demodulation/decoding process. Simulation results of the post-FEC bit-error rate are presented for a near-Shannon limit low-density parity check (LDPC) code with an overhead of 20%. The performance loss, compared to ideal coherent demodulation, is evaluated for two different linewidths of the phase noise.