| The ocean has attracted more and more attention because of its rich resource reserves and high strategic position in national security.Underwater acoustic communication technology plays a vital role in ocean development and maritime military activities.Because of the complex,changeable and harsh underwater communication environment,channel coding technology is used in underwater communication systems to improve their reliability.Polar codes,as a channel coding scheme for the control channel in the enhanced mobile broadband scenario of the fifth-generation(5G)mobile communication system,have the advantage of a clear coding structure and up to Shannon limit.However,there are few pieces of research on the application of polar codes in underwater acoustic communication.This thesis mainly studies the performance of the Orthogonal Frequency Division Multiplexing(OFDM)system with polar codes as a channel coding scheme on various underwater acoustic channels.The related work is as follows:1.The basic characteristics of the underwater acoustic channel are analyzed and two types of underwater acoustic channel models are studied.This thesis comprehensively analyzes the physical characteristics of sound propagation in the ocean from the aspects of propagation delay,background noise,path loss,multipath effect,and Doppler effect.Based on these characteristics,two types of channel models are studied.The first type is based on the ray acoustic theory,and three models of time-invariant,quasi-stationary,and timevarying are established according to the variation of fading coefficient and time delay of each path.The second type is based on the stochastic statistical theory.Jakes spectrum is used to describe the Doppler frequency shift,and the slow time-varying model and fast timevarying model are established according to the rate of variation of Doppler frequency shift and time delay of each path.2.An underwater acoustic communication system based on OFDM is established,and polar codes are used as the channel coding scheme of the system.To offset the influence of the strong multipath of the underwater acoustic channel on the communication,OFDM modulation technology is introduced and the length of the cyclic prefix of each OFDM symbol is guaranteed to be greater than the maximum delay extension of the channel.According to the transition probability of the underwater acoustic channel mentioned in the first content,the Monte Carlo algorithm is used to accomplish the construction of polar codes on the five underwater acoustic channels.3.The Intercarrier Interference(ICI)suppression algorithm is combined with polar codes to realize reliable communication on fast time-varying channels.For the high-intensity ICI caused by the Doppler effect and time-varying characteristics of the underwater acoustic channel,it cannot only be treated as background noise with polar codes.Therefore,an ICI suppression algorithm is implemented and combined with polar codes to reduce the bit error rate of the system to below 10-4.4.On the two types of channel models in this thesis,the performance simulation of the underwater acoustic OFDM system is completed based on polar codes.The performance of three kinds of underwater acoustic channels based on ray acoustic theory is compared with that of Low-Density Parity Check(LDPC)codes.Under the condition of the same code length and the same code rate,polar codes have a coding gain of 0.5 dB～0.8 dB relative to LDPC codes. |
