Characteristics Of Channel And Likelihood Ratio Combination For Photon-counting Detector Arrays Receiver Using Pulse Position Modulation

PPM combined with photon-counting detector arrays for direct-detection laser communication is an effective mean to solve the massive data transmission problem,which has great potential for deep space optical communication.However,the laser beam will be impaired by the channel characteristics when transmitted in the communication link.As a result,the characteristics of received signal have greatly changed compared to the original signal.So the performance analysis for the arrays receiver has great significance to system design,optimization and decoding process.In this thesis,we consider a variety of factors that may influence on initial signal in optical channel,including background,a random jitter of photon arrival times,the recovery time of detectors and so on.According to its features,we simulate the distribution of the received signal and develop a channel mode.In this mode the performance of the arrays receiver is evaluated in terms of various parameters in communication link.So far,the bit error rate mathematical mode of PPM has been developed in the absence of detector recovery time and ignoring the limitation of number detectors.Therefore,it cannot accurately analyze the performance for the PPM communication system when photon-counting detector arrays are used as receivers.Firstly,we consider a situation that the detector recovery time is less than a PPM slot and assume the photon-counting statistics of the same detector between the adjacent slots are independent.And then a bit error rate mathematical mode is obtained in the present of one detector unit one photon-detected characteristic.This mode can be used to design and optimize the parameters of the arrays receiver.Then in the present of the situation that the recovery time is more than a slot and during the period of recovery time photon-count statistics of one detector are dependent,we develop another bit error rate mode which can be as an approximate expression to analyze the performance of the arrays receiver.The simulation results show that there is a certain error in the result obtained by this expression,but it is still superior to the result of expression analysis under the ideal Poisson channel.In the Poisson channel,the PPM symbol likelihood ratio calculation does not take into account the detector recovery time,so it can not accurately represent the soft information for channel decoding.In the present of the recovery time,thephoton-counting probability model is analyzed,and the improved arrays receiver likelihood ratio synthesis formula is deduced.The simulation results indicate that this modified likelihood ratio synthesis method can improve the performance of the receiver obviously.And then according to the obtained likelihood ratio synthesis method of arrays receiver and considering both the random jitter characteristic and the recovery time characteristic,a more generalized likelihood ratio improved method is obtained.It is shown that the proposed method can still achieve a better performance.