Research On Diversity Technique Of Multi-carrier Underwater Acoustic Communications

With the development of human society,the shortage of resources and energy is more and more highlighted,the 70% of the area on the earth is marine,but human's exploitation to marine resources is not mature enough.Marine also has important value in military strategy.Under such preconditions,underwater acoustic communication technology has become a hot topic in current research.The research shows that multi-carrier scheme is suitable for underwater acoustic communication.The multi-carrier underwater acoustic communication technology has gained great development.But due to the particularity and complexity of this double selection fading underwater acoustic channel,the OFDM scheme which is mature in wireless communication also has certain limitations applied in underwater acoustic communication,mainly reflected in following two aspects:1.The current research is not perfect for the solution of frequency selective decline caused by multipath channel,and the optimal channel equalizer needs further study.2.Underwater acoustic channel is a typical double selection channel.Large multi-path effect and Doppler extension can cause serious frequency selective and time selective decline.At present,the processed plan for the time selective decline of underwater acoustic communication has yet to be improved.According to the sparse multipath,fast time-varying characteristics of the underwater acoustic channel,this article will put forward stable and reliable acoustic data communication scheme based on OFDM and diversity technique,solving the problem of underwater high-speed information transmission in middle and long distance.The following three aspects are mainly studied:1.Aiming at the frequency selective problem of the underwater acoustic multipath fading channel,we take a careful study on the proposed unitary OFDM transformation scheme based on frequency diversity,analyze the limitation of the existing zero forced detection and minimum mean square error detection algorithm,and then the improved detection scheme is put forward.We set a threshold against the channel fading energy on the basis of zero forced detection scheme to get rid of the serious declined subcarriers,prevent the noise amplification and diffusion phenomenon,thus improve the bit error rate performance.Detailed theoretical analysis and simulation of the scheme is given.2.Based on the time-varying characteristics of underwater acoustic channel,we study the BEM model of time-varying channel,and then research the time invariant processing method of time-varying channel on the basis of BEM model.Then we convert the time-varying channel into a diagonal channel on frequency domain,making it suitable for more efficient OFDM detection method.3.For the double selection characteristic of underwater acoustic channel,we propose an unitary transform OFDM scheme under this channel.On the basis of the BEM model and the processing of the time-varying channel,this scheme add the unitary transformation procession in sending and receiving nodes,aiming to search the time-frequency diversity to combat double selection decline of the channel.This scheme can improve the transmission performance further.