OFDM同步跟踪阶段的频偏估计算法

Estimation of OFDM Carrier Frequency Offset for Tracking

ＯＦＤＭ对栽频偏差是很敏感的，因此不仅在同步的捕获阶段，而且在同步的跟踪阶段，精确的频偏估计是很重要的。文献［３］提出了一种简化的相干判决反馈的迭代算法来估计同步跟踪阶段的载频偏差，对于１２８个子载波、ＱＰＳＫ调制的ＯＦＤＭ系统，在多径衰落信道下，当频偏小于子载波间隔的１５％时，频偏估计的标准方差小于１％，但是这个结果是在假设信道频率响应、定时偏差、载波相位等参数已知的条件下取得的，而当这些参数的估计存在误差时，由仿真结果可看出，频偏估计性能恶化了，因此，提出了采用差分判决反馈的频偏估计算法，虽然只能估计小于子载波间隔 ８％的频偏，但差分判决反馈算法不需要知道信道的参数，由于在同步跟踪阶段，频偏一般都较小，因此采用差分判决反馈算法可以很好地估计出同步跟踪阶段的频率偏差。

OFDM is sensitive to carrier frequency offset, so accurate frequency offset estimation is essential to OFDM systems, not only in acquisition mode , also in tracking mode a simple iterative algorithm based on coherent demodulation decisions feedback is proposed for frequency offset estimation in tracking mode[3], for a OFDM system with 128 sub - carriers, QPSK modulation, and a multi - path fading channel, the standard deviation of frequency offset estimation less than 1% can be achieved when the frequency offset is less than 15%, but the channel frequency response, timing offset and carrier phase must be known ,when these parameters are not estimated accurately, the performance of frequency offset estimation degrades rapidly according the simulation results in this paper, so a new algorithm based on differentially demodulation decisions is proposed in this paper, though only frequency offset less than 0. 08 can be estimated ,the frequency offset estimation is independent of channel frequency response ,timing offset and carrter phase estimation, because the frequency offset is small in tracking mode, so the new algorithm can be used for frequency offset estimation in tracking mode in OFDM systems.