Zhang Yan, Wang Ping, Guo Lixin, Wang Wei, Tian Hongxin
Opt Express. 2017 Aug 21;25(17):19995-20011. doi: 10.1364/OE.25.019995.
The average bit error rate (ABER) performance of an orbital angular momentum (OAM) multiplexing-based free-space optical (FSO) system with multiple-input multiple-output (MIMO) architecture has been investigated over atmospheric turbulence considering channel estimation and space-time coding. The impact of different types of space-time coding, modulation orders, turbulence strengths, receive antenna numbers on the transmission performance of this OAM-FSO system is also taken into account. On the basis of the proposed system model, the analytical expressions of the received signals carried by the k-th OAM mode of the n-th receive antenna for the vertical bell labs layered space-time (V-Blast) and space-time block codes (STBC) are derived, respectively. With the help of channel estimator carrying out with least square (LS) algorithm, the zero-forcing criterion with ordered successive interference cancellation criterion (ZF-OSIC) equalizer of V-Blast scheme and Alamouti decoder of STBC scheme are adopted to mitigate the performance degradation induced by the atmospheric turbulence. The results show that the ABERs obtained by channel estimation have excellent agreement with those of turbulence phase screen simulations. The ABERs of this OAM multiplexing-based MIMO system deteriorate with the increase of turbulence strengths. And both V-Blast and STBC schemes can significantly improve the system performance by mitigating the distortions of atmospheric turbulence as well as additive white Gaussian noise (AWGN). In addition, the ABER performances of both space-time coding schemes can be further enhanced by increasing the number of receive antennas for the diversity gain and STBC outperforms V-Blast in this system for data recovery. This work is beneficial to the OAM FSO system design.
基于轨道角动量（OAM）复用的多输入多输出（MIMO）架构自由空间光（FSO）系统在考虑信道估计和空时编码的情况下，针对大气湍流的平均误码率（ABER）性能进行了研究。还考虑了不同类型的空时编码、调制阶数、湍流强度、接收天线数量对该OAM-FSO系统传输性能的影响。在所提出的系统模型基础上，分别推导了第n个接收天线的第k个OAM模式所携带接收信号对于垂直贝尔实验室分层空时（V-Blast）和空时分组码（STBC）的解析表达式。借助采用最小二乘（LS）算法的信道估计器，采用V-Blast方案的迫零准则与有序连续干扰消除准则（ZF-OSIC）均衡器以及STBC方案的Alamouti解码器来减轻大气湍流引起的性能下降。结果表明，通过信道估计得到的ABER与湍流相位屏模拟结果具有很好的一致性。该基于OAM复用的MIMO系统的ABER随着湍流强度的增加而恶化。并且V-Blast和STBC方案都可以通过减轻大气湍流以及加性高斯白噪声（AWGN）的失真来显著提高系统性能。此外，通过增加接收天线数量以获得分集增益，两种空时编码方案的ABER性能都可以进一步提高，并且在该系统中用于数据恢复时，STBC的性能优于V-Blast。这项工作对OAM FSO系统设计有益。
