Low-density parity-check-Feher quadrature phase shift keying signalling with frequency-offset compensated iterative demodulation and decoding algorithm

The Feher quadrature phase shift keying (FQPSK) modulation is significantly susceptible to frequency and phase offsets under low signal-to-noise ratios. In this study, the authors proposed a serially concatenated signalling scheme with FQPSK modulation and low-density parity-check coding, which could efficiently resist residual frequency offset by employing an intended compensation algorithm. The designed maximum-likelihood estimation-enabled compensation algorithm is incorporated into the iterative concatenated demodulation-decoding process by using soft-input-soft-output-based maximum-a-posteriori-probability criterion. On the other side, the codeword sequence to be transmitted at the sender is re-arranged in a pre-configured order different from original codeword, in order to help the compensation algorithm diminish the impacts of frequency offsets. Simulation results show that the bit error rate of the proposed scheme can be improved efficiently up to three orders of magnitude with the frequency offsets from 100 to 700 ppm.