Performance Analysis And Design Of Digital Fountain Codes For Underwater Acoustic Channel

Underwater acoustic communication is vital to both military and civilian research. Low frequency sound wave itself and the underwater acoustic channel, however, are extremely complex, which make it reasonable for the underwater acoustic communication to have the features of narrow available bandwidth, large transmission delay, serious noise, presents serious selective in the time domain and frequency domain at the same time etc. It is extremely difficult to realize high speed and reliable communication in underwater acoustic channel, and transmission error caused by transmitting information in the underwater acoustic channel is inevitable and usually occurs at a very high probability, good performance of error control scheme is needed to improve the quality of underwater acoustic communication. Due to the long propagation delay of underwater acoustic communication ARQ(automatic feedback retransmission) is not feasible; FEC is a more feasible channel error correction scheme.As a channel error correction coding technology, remove fountain code is a new type of low density low bit error rate of channel coding code word, with the feature of automatically link rate matching without having to know the conditions of sender channel. The decoding algorithm of this code is quite complex in linear, while the rectifying delete effect is remarkable, especially its adaptive link rate matching(Rateless code) received wide attention of scholars both at home and abroad. Considering OFDM can overcome frequency selective fading, it is a hotspot of underwater acoustic communication, this paper will use delete fountain code as underwater acoustic OFDM communication system.This paper firstly introduces the basic principle and key technology of orthogonal frequency division multiplexing technology, explains the details of the LDPC code and digital fountain code decoding theories; Then this paper will design LDPC- OFDM, LT- OFDM and Raptor- OFDM underwater acoustic communication systems, and combining the existing underwater acoustic channel model that based on the theory of the sound field calculation a MATALB simulation model is established. The performance of LDPC code, LT code and Raptor code in underwater acoustic channel are analyzed, the simulation results show that the LDPC code has superior performance in the underwater acoustic channel, but the fast time-varying multi-path underwater acoustic channel has obvious influence on it; It is feasible to use Fountain code as the channel coding in underwater acoustic communication, though its ber performance is slightly inferior to the LDPC code, but it can use its feature of rateless to adjust rate automatically to ensure reliable transmission under the condition of low SNR in the underwater acoustic channel, besides the underwater acoustic channel adaptive characteristics.