地空信道下基于OFDM/OQAM系统的时频同步算法

针对正交频分复用/交错正交幅度调制（OFDM/OQAM）系统无循环前缀而对时偏误差敏感，以及在具有大多普勒频移的快时变地空信道下对频偏估计要求较高的问题，提出了一种适用于地空信道OFDM/OQAM系统的自相关估计（ACE）时频同步算法。该算法中符号定时采用较少的辅助序列实现快速捕获和定时，频偏估计通过优选同步自相关序列进行两次自相关运算，将两次运算估计的频偏值进行加权平均，获得最终频偏估计值。在计算机上进行仿真，其中与改进最小二乘（MLS）算法和训练序列（TR2）算法相比，ACE算法符号定时相关峰值对比度提高至原来的3倍；在飞行状态下系统误码率（BER）为10^-2时有10dB信噪比增益，在地空信道到达状态下BER为10^-3时有3dB信噪比增益。仿真结果表明，ACE算法进一步提升了时频同步精度和误码性能。

Time and frequency synchronization for OFDM/OQAM in ground air channel

For the Orthogonal Frequency Division Multiplexing/Offset Quadrature Amplitude Modulation (OFDM/OQAM) system has no cyclic prefix, it is sensitive to time error and has a high requirement for frequency offset estimation in fast time-varying ground-air channel with large Doppler frequency offset, an AutoCorrelation Estimation (ACE) time frequency synchronization algorithm for OFDM/OQAM system in ground-air channel was proposed. In the algorithm, the symbol timing was used to achieve fast acquisition and timing with fewer auxiliary sequences. The frequency offset estimation was carried out by optimizing the autocorrelation sequence and performing two autocorrelation operations. The final frequency offset was obtained by weighting and averaging the estimated frequency offset of the two operations. The simulation results showed that symbol timing correlation peak of the ACE increased 3 compared with the Modified Linear Square (MLS) and Training Sequence 2 (TR2) algorithm. There was a 10dB SNR (Signal-to-Noise Ratio) gain when BER (Bit Error Rate) was 10^-2 at the en-route state of ground-air channel, and there was a 3dB SNR gain when the system BER was 10^-3 at the arrival state of ground-air channel. The simulation results show that the ACE algorithm further enhances time frequency synchronization accuracy and bit error performance.