Research And Implementation Of Key Technologies For Spaceborne AS

VHF Data Exchange System(VDES)is a new maritime communication system,supporting maritime information security and electronic chart maritime services,which can provide better data transmission and location information-sharing services for ships and ship traffic managers.It is an important equipment to realize ship collision avoidance,navigation security,and maritime control.As a communication service in VDES,Application Specific Message(ASM)of satellite links has some problems,such as short burst communication,high Doppler frequency shift,and long transmission delay.In this thesis,the baseband processing process of satellite-borne ASM signals is studied,and the software simulation,as well as the hardware implementations are carried out.The main research is as follows:(1)According to the structural characteristics of ASM-SAT data frames,a method of differential correlation combined with constant false alarm dynamic threshold detection is proposed to detect signal frame heads due to the problem of burst transmission and carrier frequency skew.When Eb/N0=0dB,the probability of successful detection can reach more than 98%.(2)Aiming at the problem that the satellite-borne ASM signal is greatly affected by Doppler frequency offset in the process of satellite link channel transmission,a two-step carrier frequency offset removal algorithm is proposed.Firstly,the FFT algorithm is used to estimate the coarse frequency offset of open-loop feed-forward with a wide estimation range,and the remaining frequency offset is controlled within a small range.Then,the fine frequency offset with high estimation accuracy is estimated by using the decision feedback loop.The simulation results show that when Eb/N0>4dB,the performance of demodulation error is consistent with the theoretical result.(3)Given errors in channel transmission of ASM signals on board,Reed-Muller code coding and decoding algorithms based on large number logic decision criteria as well as Turbo code coding and decoding algorithms are studied,and the coherent demodulation algorithm proposed in this paper is used for verification.The simulation results show that when Es/N0>1dB,both the bit error rate and the packet error rate of Reed-Muller decoding are close to 10-4,moreover,Turbo decoding can bring about 4dB gain to the whole demodulation system.(4)Based on the Zynq-7000 chip platform,the demodulation process of the overall scheduling and cross-clock domain data processing is designed.The overall scheduling of each module can greatly save hardware resources and control the data flow in an orderly manner.At the same time,the use of different clock domains can reduce the demodulation time and avoid data overwriting.