基于OFDM子载波的大气湍流信道综合补偿关键技术研究

The coherent optical orthogonal frequency-division multiplexing (CO-OFDM) atmosphere laser communication (lasercom) systems have advantages of high transmission rate, high spectral efficiency, high sensitivity and so on. However, random optical intensity and phase fluctuation on the coherently received OFDM signal are caused by atmospheric turbulence, which has been the primary obstacle for reliable CO-OFDM atmosphere lasercom. So it is important for CO-OFDM atmosphere lasercom to study atmospheric turbulence channel compensation. This project focuses on the research of key technologies for compensating atmospheric turbulence channel based on OFDM subcarriers. First, according to the features of optical intensity fluctuation, random optical intensity signal is estimated and compensated using pilot subcarriers. Then based on the features of laser phase noise and optical wave-front fluctuation caused by turbulence, random phase noise is estimated and compensated using phase-conjugated subcarriers. Second, joint compensation method is proposed using both pilot and phase-conjugated subcarriers, thus distorted OFDM signal is corrected effectively. At last, simulation and experiments are carried out to verify the proposed methods, and some modification may be made after verification. This research can evidently increase the transmission distance for CO-OFDM atmosphere lasercom systems, lower the requirement of laser linewidth, and can be referred by channel compensation in single-carrier coherent atmosphere lasercom.

相干光学OFDM大气激光通信系统具有高传输速率、高频谱利用率和高接收灵敏度等优势，但大气湍流使相干接收的OFDM信号产生随机强度起伏和相位噪声，已然成为稳定相干光学OFDM大气激光通信的主要障碍。因此，研究大气湍流信道补偿技术对相干光学OFDM大气激光通信具有重要意义。本项目拟对基于OFDM子载波的大气湍流信道补偿关键技术展开研究，首先根据湍流信道中光强起伏特点，采用导频子载波对随机光强信号进行估计和补偿；根据激光器相位噪声和湍流信道中光场的波前相位起伏噪声特点，采用相位共轭子载波对随机相位噪声进行估计和补偿。然后提出基于导频子载波和相位共轭子载波的大气湍流信道综合补偿方法，实现对畸变OFDM信号的有效补偿。最后通过仿真和实验对提出的方法进行验证及修正。本项研究可显著扩展相干光学OFDM大气激光通信系统的传输距离，降低系统对激光器线宽的要求，同时为单载波相干大气激光通信的信道补偿提供思路。

自由空间光通信技术在民用和军用领域都具有广阔的应用前景。随着日益增加的通信带宽需求，目前广泛使用的强度调制和直接探测（IM/DD）自由空间光通信系统已无法满足远距离下高速率和高频谱效率通信的要求，而结合相干检测技术和正交频分复用（OFDM）技术的相干光学OFDM自由空间光通信系统具有在晴朗天气情况下满足这一要求的潜力，但受大气湍流引起的光场强度和波前相位起伏的影响，相干接收的OFDM 信号的强度和相位也产生随机起伏，这使相干光学OFDM 大气激光通信系统的潜力被大大压制。因此，如何抑制大气湍流的影响是相干光学OFDM 自由空间光通信系统亟待解决的关键技术。本项目通过在m-QAM-OFDM传输系统中引入相位共轭子载波，提出了具有相位共轭特性的OFDM数据帧结构，并提出并行传输具有相位共轭特性的两路OFDM信号，有效克服了OFDM的子载波间干扰和传输系统中的激光器相位噪声，以及大气湍流在相干系统中引起的相位噪声。通过搭建Matlab和VPI联合仿真平台，对所提出方法进行了仿真验证，并进一步通过实验进行了验证，仿真和实验结果都表明，利用相位共轭子载波的m-QAM-OFDM传输机制，可显著提高传输系统的误码率性能，有效提高通信系统的传输距离。因此本项研究可显著扩展相干光学OFDM大气激光通信系统的传输距离，降低系统对激光器线宽的要求，同时为单载波相干大气激光通信的信道补偿提供了思路。