基于MAP-EM算法的COFDM频偏跟踪和信号检测技术

结合了串行卷积级联码编码（SCCC）的OFDM系统（COFDM）在时域上对信息段进行交织,在对抗突发干扰、提高系统性能上具有明显优势。但是实际通信系统中,频偏的存在严重地影响了COFDM系统的可靠性。由于频偏与收发两端振荡频率差和多普勒频移有关且时变,大大增加了问题的复杂性。文章将SCCC的MAP译码方法与频偏寻优的期望最大化（EM）算法相结合（MAP-EM）,采用反馈迭代技术实现了频偏跟踪与字符的联合估计。由于该算法是基于最大似然准则的寻优方法,且交织使每个OFDM帧之间的信息能够进行相互补偿,因此能够改善系统性能。计算机仿真结果表明,随着迭代次数增多,频偏跟踪的精度和整个系统的误码率性能都有所提高。

Frequency Offset Track and Signal Detective Technique in COFDM Systems

The OFDM systems with the serial concatenated convolutional codes（SCCC） interleave the information random in the time field（COFDM）.It is able to overcome the abrupt noises and enhance the performance of the whole system obviously.But in the actual communication systems,the frequency offset deteriorates the performance of COFDM systems.And this problem becomes more complex,because the frequency offset is relative to the difference of frequency between transmitter and receiver and the Doppler shifts of channel those are mutative.This paper combines the MAP decoding algorithm of SCCC with the expectation-maximization（EM） algorithm of tracking frequency offset（MAP-EM）,and uses the feedback iterative technique to implement the estimate of both the frequency offset and single.This technique can enhance the performance of systems,because it bases on the ML and the information could be helped each other between the OFDM single.Computer simulations show that the more iterative the better performance of the tracking precision and the BER of whole systems.