Simulation Based On Multi-element LDPC Code Underwater Acoustic Spread Spectrum Communication System

With the expansion of population increase and the land resources available for exploitation are gradually depleted.Due to the 70%of the earth's surface is ocean,it is rich in reserves of ocean resources,so many of country have to accelerate the pace of exploration and development of the ocean.Set ocean discipline and information science as one of the water sound technology and also as one of the leading technologies of marine development more and more attention by all countries,but the underwater channel is an extremely complex time-space-frequency variable parameter multipath fading channel,it is greatly restricts the underwater acoustic signal at high speed,long distance,low error transmission.At present,the channel error correction coding technology for both coherent modulation or non-coherent modulation of the underwater acoustic communication system are indispensable key technology.For the characteristics of underwater acoustic channel,the application of LDPC code to the spread spectrum acoustic communication system can improve the frequency utilization and reliability of the system.Low density parity(LDPC)code has the advantages of simple structure?high coding gain?close to Shannon's limit in Gaussian channel and strong error correction capability;Anti-burst error characteristics,don't need to join the interleaver,it is not only avoid the possible delay,but also has a good performance in the frequency selective fading channel;Decoding is parallel,the delay is much smaller than the traditional Turbo code serial iterative decoding algorithm.LDPC code can reduce the signal distortion in the underwater acoustic channel transmission,showing a good application prospects.The multi-path interference of the underwater acoustic channel can cause serious fading of the signal,which is one of the bottlenecks in the development of modern underwater acoustic communication.Spread spectrum technology?equalization technology?diversity technology is the development of more mature anti-multipath interference technology.Spread spectrum technology is both effective anti-multipath interference and a good multi-site performance,but also is build the underwater communication network key technology.The article first introduces the basic characteristics of underwater acoustic channels including propagation loss?channel noise?multipath interference?Doppler effect and also the frequency selective fading?time selective fading?Rayleigh channel fading characteristics of a quantitative description.A coherent and noncoherent acoustic channel model was established,Secondly,a method of constructing QC-LDPC code check matrix for ?-type structure is proposed,Linear encoding is achieved by iterative coding to reducing the complexity of coding,and the LLR BP decoding algorithm is improved to provide accurate decoding information,on the basis of spread spectrum,LDPC code channel error correction coding is added,the simulation parameters are set up to verify the error correction performance of LDPC codes in different order domains and the advantages of LDPC codes combined with spread spectrum technology in underwater acoustic communication;Finally,it is introduce the multi-carrier spread spectrum theory including MC-CDMA?MC-DS-CDMA and MT-CDMA,compare its advantages and disadvantages,the LDPC code MT-CDMA multi-carrier spread spectrum underwater acoustic communication system is proposed,set the simulation parameters,under different code length and different modulation modes,compare with the bit error rate of RS system which proves that the system optimization proposed in this article has certain rationality and practicability.