Research On Navigation Message Broadcasting Strategy Based On LT Codes And Decoding Algorithm

User groups of satellite navigation system are huge and very diverse,how to meet the needs of heterogeneous users has become a major problem for satellite navigation system.To solve this problem,this paper focuses on studying the application of LT codes in navigation message encoding.And according to the characteristic that navigation message is short,the decoding algorithm of LT codes for short block lengths is studied.The content of this paper includes the following two aspects:Since EBP(Enhanced Belief Propagation)decoding algorithm can not fully exploit the information in the encoding packets,this paper proposes EBPML decoding algorithm by making some adaptive changes to original EBP decoding algorithm and combining it with OFG(On the Fly Gaussian Elimination)decoding algorithm.The proposed algorithm,like OFG algorithm,can dig all information in the encoding packets,but its decoding complexity is significantly lower than OFG algorithm.The simulation results show that,when block lengths is short,the decoding complexity of EBPML algorithm and BP(Belief Propagation)algorithm are at the same order of magnitude,but the decoding overhead of EBPML algorithm is much lower than that of BP algorithm.In order to meet the needs of heterogeneous users of satellite navigation system,this paper proposes a broadcasting strategy of navigation message based on LT codes,and takes Galileo F/NAV navigation message as an example to conduct theoretical analyze and simulation.This strategy includes two aspects,one is satellite almanac broadcasting strategy based on LT codes;the second is a method for shortening Time to First Fix.The simulation results show that,F/NAV message broadcasting scheme using this strategy(abbreviated as new scheme)can effectively shorten the time to retrieve almanac.For example,when receiving navigation message from four satellites,the packet loss rate is0.25,compared with F/NAV message original broadcasting scheme,the new scheme shortens the average time to retrieve almanac by 64.6%.When almanac is valid,compared with F/NAV message original broadcasting scheme,the new scheme shortens the average time to retrieve the First-Fix-Data by more than 12%,thus effectively shortening Time to First Fix.