基于光子探测器阵列接收的PPM调制信号的开环同步技术研究

Laser communication based on PPM modulation and photon counting detectors arrays receiver offers a competitive method for deep space information transmission. PPM slot synchronization and combining the arrays signals are the key technologies. The traditional closed-loop synchronization methods are complex. Recently, an open-loop PPM slot synchronization method appears which based on asynchronous sampling the PPM slot signal and the sample frequency equals to the slot frequency. This slot synchronization method deserves further research. This project focuses on this kind of open-loop synchronous method, improving it to apply in photon counting detectors arrays. In this project, a blind sampling clock offset estimation method without pilot symbol is proposed. The multichannel binary level signals come from photon counting detectors arrays are asynchronous sampled at the PPM slot frequency. The offsets of sampling clock are estimated by using the density change rule of ‘1’ in the sample data, then the PPM slot synchronization and slot signal recovery can be realized. And this method can be improved to apply in multichannel arrays signals and the sampling clock offset of every subchannels can be estimated coordinately. Due to this slot frequency sampled signal is impaired easily by the time delay jitter, when the sampling clock offset is close to half of the slot duration time, the jitter liable to cause PPM pulse shift error. A method of array diversity receiving is proposed to suppress this error by making the adjacent elements in an array complementary sampled. Arrays signals combining based on likelihood ratio criterion will also be studied. Some experiments using two photon counting detectors will also be done to validate the feasibility of the open-loop synchronization and signal combining schemes.

采用PPM调制及光子探测器阵列接收的激光通信在深空通信中极具潜力。PPM时隙同步及阵列信号合并是其关键。传统的闭环时钟同步实现复杂。最近出现了一种以一倍PPM时隙频率对信号进行异步采样的开环时隙同步方式，值得深入研究。本项目重点研究这种开环同步方法并应用于阵列信号中。提出一种无需训练序列的定时误差估计方案：用一倍PPM时隙频率时钟异步采样光子探测器阵列输出的多路双电平信号，根据各路采样数据中‘1’密度的周期性变化规律估计定时误差，进而实现PPM时隙同步及数据恢复；并将这种方法应用于多路阵列信号协同进行定时误差估计；由于这种一倍时隙频率采样容易受时延抖动的影响，在定时误差接近半个时隙的位置，抖动容易引起PPM脉冲移位错误，提出用相邻阵列单元互补采样的阵列分集接收方法抑制这种错误；研究基于似然比准则的阵列信号合并方法；并用双路光子探测实验验证本项目的开环同步及阵列信号合成方法的可行性。

基于单光子探测阵列接收的PPM激光通信在深空通信及水下通信中极具应用前景。本项目主要研究了基于数字信号处理的光子探测PPM接收机的开环同步及相关的性能优化技术。主要研究内容及成果为：(1).对于光子探测器阵列接收的PPM通信系统，考虑探测器死时间的影响，提出了一种更准确的PPM时隙似然比计算模型，改进模型在单光子探测单元数目较少，死时间较长，背景光较严重等应用场景下可显著提高接收性能。(2).提出了一种以脉冲展宽波形为权系数的PPM时隙数据的插值恢复算法，同时这种方法对时延抖动引起的码间干扰具有明显抑制作用，在抖动标准差0.2个PPM时隙时，性能比经典的拉格朗日插值提升0.2dB，而运算量基本没有增加。(3).研究了SCPPM、LDPC、Turbo纠错码在光PPM系统中的应用及性能，考虑PPM调制及泊松信道特性，构造了一些1/2码率的纠错性能优良的不等保护LDPC码。提出了一种结构，解决了SCPPM码在基于EM算法的码辅助PPM时钟同步中不能直接应用的问题。(4).研究了在时域通过统计各个光子到达时间的分布形状，进行PPM时隙同步的方法，在脉冲占空比0.5，采样频率为2倍PPM时隙频率时，同步性能损失约有0.2dB。(5).研究了以PPM时隙频率异步时钟采样的PPM系统性能，一倍时钟频率采样相比传统的两倍频率采样，性能约有0.4dB劣化。提出了基于保护时隙及光子到达时间分布形状的PPM时隙同步及符号同步方法。(6).完成了基于多像素光子计数器(MPPC)接收的光PPM通信实验系统，用计算机离线处理的方式实现了PPM信号的时隙同步、帧同步功能，提出了一种基于相关检测的由MPPC输出信号恢复各时隙光子数的数据恢复算法。对于64PPM调制，2倍PPM时隙频率的采样信号，在BER为1.0e-4时，平均每个PPM信号脉冲实际探测到的光子数为1.28个。