Research On Carrier Frequency Offset Estimation And Phase Estimation Algorithms For MSK Signal In Coherent Detection System

Minimum shift keying(MSK)is a special continuous phase frequency shift keying(CPFSK).It is widely used in wireless digital communication system for its constant envelope,energy concentration,high spectrum efficiency,and continuous phase.With the renewal and development of optical devices,people have done a lot of research on MSK in coherent optical communication system.Due to the use of the coherent detection optical communication system,MSK signal will be affected by frequency offset and linewidth between lasers.The amplitude and phase of the received signal will be distortion caused by these damages.Therefore,the transmitted data sequence can not be recovered correctly.And the performance of receiver is degraded.In this paper,it mainly focuses on the frequency offset and phase offset of the MSK signal in coherent optical communication system.And the frequency offset and phase offset can be estimated and compensated by the proposed digital signal processing(DSP)algorithms.The main achievements are as follows:1.A high-precision frequency offset estimation algorithm is proposed.It is based on fast Fourier transform(FFT)and chirp Z-Transform(CZT).Numerical simulation was carried out in the coherent detection optical communication system based on the MSK modulation format.The algorithm is divided into two steps:firstly,a coarse step is introduced to calculate the range of the frequency offset by FFT.Secondly,a fine step employs chirp Z-Transform(CZT)to obtain the ideal frequency offset and eliminate the influence of frequency offset.The simulation results show that:compared with the conventional algorithm,the receiver sensitivity of the proposed algorithm has been improved about 1.7 dB when the EVM is 20%.This algorithm take into account the estimation accuracy and estimation range at the same time.It is suitable for transmitting over long single mode fiber.2.A better phase ambiguity solution is proposed.In coherent detection optical communication system,another important factor to cause signal distortion is phase noise.There is a phase ambiguity during the phase estimation.In order to solve this problem,a better phase ambiguity solution based on M-th power phase estimation algorithm is deduced.And numerical simulation results verify the effectiveness of the proposed algorithm.The simulation results show that:the improved M-th power phase estimation algorithm has better linewidth tolerance.It can effectively compensate the damage caused by frequency selective attenuation.Therefore,we can reduce the bit error rate of the system,and improve the performance of receiver.