Pilotless Iterative Carrier Synchronization With LDPC Partial Decision Metrics in Satellite Communications at Low SNRs

This article mainly proposes a pilotless iterative carrier synchronization (ICS) by using the partial decision metrics (PDMs) from low-density parity-check (LDPC) decoding in quadrature phase shift keying (QPSK) satellite communications. It is effective for use in low power satellite or wireless communication systems at low signal-to-noise ratios (SNRs). By maximizing the squared sum of the PDMs of just a few LDPC decoding iterations with approximate stochastic gradient algorithm, it converges jointly the carrier phase and frequency parameters for much more synchronization accuracy. Besides, a two-stage hybrid ICS based carrier synchronization scheme is also supplemented to solve the phase ambiguity problem. In addition, the Costas loop tracking and the LDPC decoding feedback are united together to further cut residual carrier offsets. Simulation results show that the proposed ICS scheme obtains rather perfect performance in an LDPC coded QPSK satellite communication system at rather large carrier phase offsets. It obtains good performance,i.e., just within 0.1 dB away from the perfect symmetrical performance with a moderate computational complexity increment. A typical rate-1/2 LDPC coded QPSK scheme performs well even at low bit SNR (Eb/N0) of 1 ~ 2 dB. Therefore, it is significantly pragmatic in low-power satellite and wireless communications for better battery-powered transmission efficiency.