Iterative carrier recovery in an LDPC coded QPSK system at low SNRs

This paper presents an iterative carrier recovery (ICR) via soft decision metrics (SDMs) of low-density parity-check (LDPC) decoding in an LDPC coded quadrature phase shift keying (QPSK) system. It is crucial for wireless communication systems to work effectively, especially at low signal-to-noise ratios (SNRs). By maximizing the sum of the square of the SDMs of LDPC decoding with gradient oriented optimization of the objective function, it adaptively updates the carrier phase and frequency parameter accurately. The structure of the proposed scheme is also given, along with the phase ambiguity solution. Meanwhile, it is combined with the Costas loop tracking and the LDPC decoding feedback to eliminate residual carrier offsets. Simulation results indicate that the proposed ICR algorithm achieves good performance in an LDPC coded QPSK system under rather large carrier phase offsets, which is just within 0.1 dB of the ideal code performance at the cost of some moderate complexity. By the proposed scheme, a rate-1/2 LDPC coded QPSK system can even work at low bit SNR (E_b/N₀) about 1-2 dB, which is useful in energy-limited wireless communications.