相干光脉冲位置调制信道容量及传信率最大化研究

Study of Capacity of Coherent Pulse-Position Modulation Channel and Maximization of Information Transmitting Rate

分析了相干光脉冲位置调制的信道容量及传信率最大化条件。通过分析光外差接收的脉冲位置调制信号具有的概率分布特性,推导出采用最大似然检测的相干脉冲位置调制信道具有的转移概率矩阵和信道容量,证明在检测器散粒噪声极限假设下,信道容量只取决于接收脉冲能量和脉冲位置调制阶数。利用Jensen不等式化简信道容量得到其下限,该下限能够近似实际通信链路的信道容量,基于此估计出达到最高传信率时的脉冲位置调制阶数同粒子数反转的重建时间之间的关系,给出了二者在一定范围内的一般映射。数值仿真表明,在接收能量使信道误码率优于10-2时,估算出的脉冲位置调制阶数能够使传信率接近最大值。

The channel capacity of coherent pulse-position modulation （PPM） and maximization conditions of information transmitting rate were analyzed. After the probability distribution of the PPM signal detected by heterodyne method is analyzed, the transition probabilities matrix and capacity of optical pulse-position modulated channel with coherent receiver and maximum likelihood detection were deduced. It is proved that under the circumstance of shot-noise limit detection, the capacity of the channel is only related to number of received photons and the PPM order. Jensen＇s inequality is used to simplify the capacity of the channel and obtain its low limit, which is close to the capacity of the channel of practical communication link. The relation of the optimum order of coherent- PPM corresponding to the dead time for the greatest transmitting rate is estimated. General relationship between the optimum PPM order and the dead time was given. Using the proposed method, the estimated PPM-order can realize the maximum capacity of channel when received photons can promise the symbol error rate lower than 10^-2.