Research And FPGA Implementation Of Blind Carrier Recovery Technology Suitable For LEO Satellite Communication System

The low-orbit satellite constellation system is a hot spot in the development of satellite communications because of its wide coverage,relatively strong multicast and broadcast functions,small signal transmission delay and low propagation loss.It is well known that there is a carrier frequency offset in the satellite communication receiver.Low-orbit satellites are also affected by Doppler shift due to their relative displacement with the ground receiver.The blind carrier recovery algorithm based on phase-locked loop decision feedback has strong versatility and high spectral efficiency,and has a certain ability to track Doppler frequency shift,which is very suitable for low-orbit satellite communication systems.However,the loop delay,loop parameter selection,cycle slip,etc.in the implementation will seriously limit its carrier recovery performance.The specific research results are as follows:(1)The influence of loop delay in FPGA implementation on NDA-DD system is analyzed for the first time.The loop delay has an impact on the frequency offset capture range of the system,the loop convergence speed,the accuracy after convergence,and the probability of cycle slips.(2)For broadband continuous systems,a blind carrier recovery algorithm is proposed which is based on NDA-DD algorithm with low loop delay and adaptive loop parameter parallel implementation.The algorithm supports QPSK,8PSK,16 APSK modulation modes in ACM.Especially for the 8PSK and 16 APSK modulation methods commonly used in satellite communication,a simplified decision method is designed.At the same time,the recommended values of the optimal loop parameters are given,which increases the range of carrier frequency offset correction.Thus,a good compromise between performance and tracking range is achieved.Compared with the original algorithm,the new algorithm improves the NMSE performance by 3% within the range of convergent signal-to-noise ratio.The simulation under 15 d B shows that the normalized frequency offset convergence range is 1.5 times that of the original algorithm.The probability of cycle slips at the same signalto-noise ratio decreases.Simultaneously,it is proved by simulation that the algorithm can work normally in the presence of typical satellite IMUX,OMUX distortion and HPA nonlinear distortion.The tracking ability of the system to Doppler frequency shift is verified by FPGA.(3)In the short burst system suitable for Io T communication,a method that can complete the fast,accurate and stable convergence of short sequences is designed based on the NDADD algorithm.Firstly,the method processes the data in a reverse order,so that the carrier recovery loop can complete the convergence within a limited symbol.Secondly,the problem of insufficient processing time caused by the short burst interval is solved by the ping-pong structure.Finally,the problem of poor convergence of the tail signal caused by the reverse order is solved by means of replacement.The algorithm can achieve full-symbol convergence of short burst sequences with the shortest 256 symbols.