利用空载波的OFDM系统的IQ误差盲估计方法

Blind estimation of IQ imbalance with null subcarriers for OFDM systems

针对正交频分复用(orthogonal frequency division multiplexing,OFDM)系统对由直接变频结构引入的IQ误差十分敏感这一问题,提出利用OFDM符号中的空载波对IQ误差进行补偿的方案。算法不需要训练符号,频谱利用率较高。分析了IQ误差对OFDM系统性能的影响,在此基础上利用一个序列的FFT变换结果与其共轭的FFT变换结果服从镜像关系的特性,估计出IQ误差并加以补偿。利用空时编码,算法同样适用于多输入多输出(multiple input multipleoutput,MIMO)系统。理论分析证明算法是无偏估计;估计的均方差与OFDM符号数、空载波数、信道频率响应的幅值以及信噪比成反比,与IQ误差的幅值成正比。仿真结果表明,算法能够获得几乎最优的误比特率(bit error rate,BER)。

Orthogonal frequency division multiplexing （OFDM） is sensitive to the in-phase and quadraturephase （IQ） imbalance induced by the direct conversion architectures. A compensation scheme of IQ imbalance exploiting null subcarriers in OFDM symbols is presented. The proposed algorithm is with high bandwidth utilization because the training symbols are not required. The effect of IQ imbalance on OFDM systems is studied, and the mirror relationship between the FFT of a sequence and that of its conjugate is employed to estimate and compensate the IQ imbalance. This method can be applied to multiple input multiple output （MIMO） OFDM systems with space time block code （STBC）. Performance analysis shows that the proposed algorithm is asymptotically unbiased. The variance is inversely proportional to the number of OFDM symbols, null subcarriers, the SNR and the channel frequency response, and proportional to the IQ imbalance. Simulation results demonstrate that this scheme can obtain almost the optimal bit error rate （BER）.