采用分段阈值的自由空间光通信性能优化

激光在大气信道中传输时易受到大气湍流等影响，导致接收机对开关键控(On-Off Keying, OOK)信号发生错误判决，进而产生误码率门限，严重恶化了自由空间光通信(Fee Space Optical Communication, FSOC)的传输性能。采用自适应阈值进行判决虽然能抑制误码率门限，但是需已知比特级(ns级)的信道状态信息(Channel State Information, CSI),在高速激光通信中难以实现。基于大气信道的缓慢变化特征，对接收信号按照亚毫秒级时隙(0.01 ms级)进行分段检测，降低了高速信号的检测难度，并通过优化误码率求解出了最佳的分段阈值，从而抑制了误码率门限现象。与此同时，提出了正交偏振态导频光与信号光同传的系统方案，以估计大气信道模型与接收信号的分段阈值。仿真结果表明，在Log-Normal大气信道模型下，该方法虽然引入了3～6 dB的光功率损耗，但是有效抑制了误码率门限，且仅需估计毫秒级的CSI,降低了阈值判决的实现难度。

Performance optimization of free space optical communication by segmented threshold

When the laser beam is transmitted through the atmospheric channel,it is easily influenced by the atmospheric turbulence,resulting in errors in the decision of the On-Off Keying(OOK) signal for the receiver.In this way,the bit error rate(BER) is floored and the transmission performance of free space optical communication(FSOC) is severely deteriorated.Even the noise floor can be reduced by adaptive threshold,the channel state information(CSI) should be estimated bit by bit(ns),resulting in difficulties in realization of high-speed laser communication.According to the slow variation of the atmospheric turbulence,the received signal is detected by segment with a time slot of submillisecond(0.01 ms),reducing difficulties in detecting high-speed signals.Also,the floor phenomenon of BER is reduced by an optimal threshold value for each segment,which is derived by optimizing the BER.Furthermore,a system scheme of co-propagation of a pilot carrier and the signal light in different orthogonal polarization states is proposed,within which the estimation of atmospheric turbulence channel model and the segmented threshold for received signal are jointly realized.Through numerical simulation in Log-Normal channel model,the BER floor is reduced only at a cost of 3~6 dB power loss.Furthermore,the estimation of CSI is only around milliseconds,resulting in a much lower complexity for threshold and decision.