Abstract: To tackle the issues of frame synchronization and symbol timing synchronization in wireless burst communication system, a novel synchronization method based on soft decision feedback in fading channel is proposed. Firstly, this paper samples time-based sliding windows to obtain the received sequence of the preamble and data symbols, so the signal-to-noise ratio and fading are estimated on the received preamble sequence. Then a new likelihood function is proposed by using the soft decisions on the unknown PSK or QAM data symbols, and the new likelihood value is calculated by the received signal. Frame synchronization completed when the likelihood value exceeds the synchronization threshold. Finally, after a peak seeking to the likelihood function, a fine synchronization is implemented using the Gaussian interpolation. When conducted on the frame of 80 symbols within 16 symbols preamble, the simulation results demonstrate that, for unknown PSK symbols, the proposed method outperforms the Cramer-Rao Bound（CRB（16）） with the length of 16 known symbols at low SNR and approaches CRB（80） at high SNR. For unknown QAM symbols, the proposed method outperforms CRB（16） at both low SNR and high SNR.

Key words: fading channel, preamble-aided, log-likelihood function, soft decision, Gaussion interpolation

摘要： 针对无线猝发通信中存在的帧同步与符号定时同步问题，提出了一种适用于衰落信道的基于软判决反馈的同步算法。算法利用滑动时间窗采样得到接收符号序列，其中同步头用来估计衰落和信噪比，重点研究利用未知PSK或QAM数据符号进行软判决来构建新的似然函数，依此进行似然函数计算，当似然函数值超过同步门限时完成信号帧同步，并确定搜索区间进行似然函数峰值搜索，最后对峰值及相邻两个值进行高斯插值以获取更精确的结果。仿真结果表明：在同步头长度为16，帧长度为80时，该算法在未知数据符号为PSK符号时，算法的MSE性能在低信噪比条件下即可超过长度为16的克拉美罗界理论线（CRB（16）），并在高信噪比条件下，逼近CRB（80）；在未知数据为QAM调制时，算法性能在低信噪比与高信噪比条件下均优于CRB（16）。

关键词: 衰落信道, 同步头辅助, 对数似然函数, 软判决, 高斯插值