Research And Implementation Of ASM And VDE Signal Demodulation And Decoding Technology

The VHF Data Exchange System(VDES),which is composed of coastal base stations and satellites,is a new generation system for maritime data transmission.It can guarantee maritime communication quality and safety better.Application Specific Messages(ASM)is an important part of VDES.This paper studies the baseband demodulation and decoding algorithm of ASM signal and implements it on the hardware platform.The main work includes:(1)Designing ASM signal detection algorithm and ASM signal baseband demodulation algorithm,including two-step frequency offset estimation algorithm,matched filtering algorithm,symbol timing algorithm and two-step phase offset estimation algorithm,which can be used in the case of large frequency offset.(2)The channel encoding and decoding algorithms for ASM signals are studied,including the encoding and decoding algorithms of Reed-Muller codes,the encoding and decoding algorithms of Turbo codes.First,designing an optimization algorithm for Turbo code decoding for a specific code rate to ensure that the calculation accuracy does not decrease.This algorithm reduces the time required for decoding and saves storage space.At the same time,the Turbo code decoding algorithm using high-order modulation in VDE signal is also studied.In addition,an algorithm that can shorten the CRC calculation time is researched and designed—the fast CRC check algorithm.(3)Designing the hardware scheme of the ASM signal demodulation and decoding system.The hardware adopts the FPGA+DSP mode.The detection algorithm is implemented on the FPGA and the rest is implemented on the DSP.Dynamic memory management mechanism,which can reuse memory space,is implemented on the DSP side.The algorithm is optimized according to the characteristics of the DSP platform to save DSP memory space and speed up operating efficiency.(4)Designed the communication method between FPGA and DSP.The combination of EMIF+Rapid IO interface is used for data transmission.The EMIF interface is used as the control signal of the communication interface,and the Rapid IO interface is responsible for data transmission.