Research Of Space-Time Coding Techniques In MIMO Underwater Acoustic Communication

Underwater acoustic channel is a typical time-varying multi-path channel, can cause signals fading in underwater acoustic(UWA) communication, which seriously affected the reliability of the receiving system. Space-time coding techniques is a channel coding scheme based on diversity technique and MIMO system, and has been applicated in wireless communication. This paper applies space-time coding techniques to UWA communication, considering the characteristics of underwater acoustic channel, we choose space-time block coding(STBC) as the main study object and construct a UWA space-time-frequency coding communication system to achieve reliable communication.Firstly, we introduce the transmission characteristics of underwater acoustic channel and its impact on UWA communication system. Analyse and build a theoretical model of underwater acoustic channel.Secondly, we analysed encoding and decoding methods, performance of three space-time coding techniques. Analysis shows that STBC is most suitable for UEA communication, then we simulated its performance and selectd it as the encoding technique used in following underwater acoustic communication system.Thirdly, we combined STBC and OFDM technology to construct a STBC-OFDM UWA communication system. Simulated and analysed the channel capacity and bit error rate of the UWA system, results show that the BER performance of STBC-OFDM system is better than the traditional OFDM systems. But as the transmission rate increased, system performance reduced. To reduce system bit error rate while ensuring transmission rate, we studied a quasi-orthogonal space-time coding scheme(QOSTBC-OFDM) which can achieve full diversity and full rate, simulation show that the scheme outperforms the STBC-OFDM systems.Finally, under the premise of OFDM systems, we studied a new space-time-frequency coding(STBFC-OFDM). The simulation results show that this scheme outperforms the STBC-OFDM system in time-varying underwater acoustic channels. On this basis, we studied a quasi-orthogonal space-time-frequency coding scheme (QOSTBFC-OFDM). Simulation results show that, at the same transmission rate, bit error rate of this system is lower than QOSTBC-OFDM and STBFC-OFDM system. To improve system performance, we combined QOSTBFC-OFDM with convolutional codes. Simulation shows that in low SNR this system can achieve a lower bit error rate and can achieve reliable transmission.To conclude, system researched in this paper can reduce the bit error rate and improve system performance effectively, and achieve reliable communication in low SNR underwater acoustic environment.