Synthesis And Open-loop Synchronization Technology Of Photon Detection Array Signals In Deep-space Optical Communication

With the advantages of high energy efficiency,strong anti-interference ability and high detection frequency,pulse position modulation(PPM)combined with photon detection array is widely applied in deep-space optical communication.Under deep-space conditions,its communication link is extremely far and the detection signal is easily affected by the channel characteristics such as background noise and delay variation,along with very low output SNR,and there is a certain delay difference in the output signals among the branches.All of these explain why receiver synthesis and synchronization performance are not ideal.In deep-space optical communications,array signal synthesis and synchronization are critical.Firstly,as to the problem of delay difference in output signals among branches,a sequential correlation delay alignment method was proposed in the second chapter of this thesis.The branches are ranked according to the signal strength,and the sequence alignment of the sequence branches is performed by the correlation.Simulation results show that the delay difference can be estimated correctly through the method.In addition,the existence of the delay variation was taken into consideration,and the weight coefficient was used for the weightiness and combination and of the likelihood ratios.The simulation results show that the improved likelihood ratio can obtain a certain performance gain.Secondly,as to for the problem of complex system of closed-loop clock synchronization,an open-loop synchronization method for joint search of frequency offset and initial phase offset was proposed in the third chapter of the dissertation.Photon distribution with different frequency offsets and initial phase deviations is obtained by measuring the photon arrival time.the photon distribution will be flatter in the presence of the frequency offset;the peak of the photon distribution will deviate from the center position in the presence of initial phase offset.With the error of mean square or the secondary moment as a measurement of synchronization degree under different timing errors,the slot synchronization is achieved by using search method.The simulation results show that this method can achieve slot synchronization when the count frequency is twice or more than the slot frequency.Finally,in view of the high complexity of the synchronization method for joint search of frequency offset and initial phase offset,a slot synchronization method for direct prediction of frequency offset and initial phase offset was put forward in the fourth chapter.The frequency offset is estimated based on the cumulative deviation of the first and the second half of the frame,and the initial phase deviation is estimated based on the peak offset of the photon distribution.The simulation results show that this method can achieve slot synchronization when the count frequency is greater than or equal to four times the slot frequency at the time of transmission.The higher the count frequency,the larger the number of detectors,and the weaker the background interference,the better the synchronization performance.