Research On Coded Modulation And Synchronization At Low SNRs

At low signal-to-noise ratio(SNR),the reliable data transmission has higher demands on the performance of the coded modulation scheme,and the premise of achieving the expected error performance is the accurate synchronization.This dissertation mainly researches on the coded modulation and synchronization suitable for the situation of low SNR.For the satellite communication system at low SNR,the coded modulation schemes combined with high-order modulations are necessary to improve its spectral efficiency.However,due to the non-linear characteristics of satellite channels,the signals mapped to the high-order constellations will be non-linear distorted,resulting in the decrease of error performance.This dissertation proposes a new 7-point constellation,which is expected to be effectively applied to the satellite communication system,and combines the new constellation with non-binary low-density parity-check(NB-LDPC)codes over GF(7)as the coded modulation scheme to guarantee the reliability of data transmission.The expression for the average symbol error probability(SEP)of the new constellation over the additive white Gaussian noise(AWGN)channel is derived,and its sensitivity to the non-linear effects is also analyzed.Simulation results show that,compared with the NB-LDPC coded 8-order phase-shift keying(8-PSK)scheme,the proposed coded modulation scheme has better error performance over both the AWGN channel and the non-linear satellite channel.Moreover,achieving accurate signal acquisition and synchronization is difficult for the receivers working at low SNR.Burst communication often requires a longer frame header or training sequence for the synchronization detection,which results in the reduction of data transmission efficiency.This dissertation proposes a sparsely coded transmission scheme,which can achieve accurate burst detection and frame synchronization through the cross-correlation detector with the same length as the frame length.Simulation results show that,the proposed scheme can guarantee the accuracy of frame synchronization detection at low SNR,and it can obtain certain performance gain compared with the traditional transmission method using the same coded modulation scheme.