中文摘要：

LDPC编码系统,接收端在译码前需要对频偏和相差进行估计,使残留频偏和相差在译码器收敛的允许范围之内,但在低信噪比条件下,即使在收敛范围内,较大的残留频偏和相差也会对LDPC编码系统性能有明显的恶化,所以必须结合迭代译码系统,对信号的残留频偏和相差进行进一步的估计。基于EM算法推导了频偏联合相差的迭代估计算法,给出了一种简单有效的残留频偏估计方法,并以此为基础,结合LDPC迭代译码输出软信息的统计特性,提出了一种码辅助的迭代载波同步算法,仿真结果表明,只要残留频偏和相差在迭代估计器收敛范围内,提出的算法可以使LDPC编码系统的误比特率接近理想同步条件下的译码性能。

英文摘要：

In LDPC-coded systems,a carrier frequency and phase estimator in front of the decoder is necessary,which must meet the decoder convergence condition.But at very low SNR,residual frequency and phase offset will make obvious degradations on the LDPC decoding performance,so,a carrier refiner is required.A code-aided iterative carrier estimation method was proposed based on the expectation-maximization（EM）algorithm,as well as a simple but effective frequency offset estimation algorithm was given.Simulations show that as long as meeting the decoder convergence condition,with the method proposed,little performance degradation is observed than the ideal synchronization system.