基于星座点特征的OFDM残留频偏跟踪算法

残留频偏会导致星座点发生旋转，而且这种旋转与时间有关，时间越长，旋转越大。若残留频偏长时间得不到校正，将会使星座点旋转到其他象限从而出现判决错误，使系统的性能急剧恶化。针对这个问题，提出了一种基于星座点特征的残留频偏跟踪算法。该算法要求在信道估计之后完成，并假定一个OFDM帧内的信道频率响应保持不变。该算法包括三个步骤，首先将每个OFDM符号的星座点划分为四个子集，然后对每个子集的均值求相位角获得相位误差，最后经环路滤波器输出一个校正量补偿时域频偏。由于残留频偏估计值的提取充分利用了所有数据子载波信息，并对每个子集进行了求均值处理，所以有效的改善了环路的输入信噪比。它适用于子载波为QPSK或QAM调制的OFDM系统，不同的子载波调制方式跟踪范围不同，最大跟踪范围为±0.05个子载波间隔。仿真表明:新算法性能要优于基于循环前缀的算法，与基于导频的跟踪算法性能相当。 

A Residual Carrier Frequency Offset Tracking Algorithm Based on Constellation Characteristics for OFDM systems

The residual frequency offset(RFO) may lead to constellations rotation,and this rotation is related to the time.The longer the time is,the larger the rotation will be.If the RFO cannot be corrected for long time,the decision error may occur because of constellations rotating to other quadrant,which will severely degrade the performance of OFDM systems. In order to correct the RFO,this paper proposes a RFO tracking algorithm based on constellation characteristics for OFDM systems.Based on the assumption that the channel frequency response has been estimated and keeps constant within one frame,the proposed algorithm is accomplished by using the properties of constellations rotation.At first,constellations of every OFDM symbol are divided into four subsets,and then phase error is extracted by solving phase angle of every subset’ s average.Finally,the frequency offset compensation in the domain is achieved by using a two-tap loop filter. The input SNR of the loop may be improved effectively,because the CFO estimation is performed by using all data subcarriers and averaging every subset.With proper loop parameters,a CFO less than 5%of the subcarrier spacing may be well tracked for OFDM systems with QPSK modulation.While for OFDM systems using QAM modulation,the CFO tracking range may be reduced to±0.01 of subcarrier spacing.The simulation results show that the tracking performance of the proposed algorithm is better than the one of ML estimator based on cyclic prefix,and is very close to that of estimator based on pilot tones.