A Study On High Speed Underwater Acoustic Communication System Based On OFDM

The high-speed underwater acoustic communication system based on OFDM has a higher bandwidth efficiency and system capacity which can be against the frequency-selective fading in underwater acoustic multi-path channel effectively. And also can using the DSP technology to reduce the complexity of implementation and development costs in systerm. That is a very popular research in the field of underwater acoustic communications in recent years. In the past, the research on the OFDM underwater acoustic communication system is on the assumption that the length of the cyclic prefix can be against the multi-path interference in the channel. As a result of the severe multi-path interference in underwater acoustic channel when the maximum delay spread of the channel beyond the length of cyclic prefix there will be inter-symbol interference and inter-carrier interference in the receiver which can affect the performance of system seriously. In this paper, the main purpose is ensure the communication system has good performance when the maximum delay spread of the channel exceeds the cyclic prefix length without increasing the redundancy of system.First of all, the basic transmission characteristics of the underwater acoustic channel are introduced and the factors which can affect the performance of underwater acoustic communication systern are simulated and analysised using the deterministic model. Secondly, the basic principle and the main technique of the OFDM technology are studied deeply, designed the parameters of the OFDM system aimed at the characteristics of the underwater acoustic channel, analysised some of the main technique which can affect the performance of underwater acoustic OFDM system through simulation method. Finally, based on the detailed analysis of the impact that the multi-path interference of underwater acoustic channel has on the received signal, the structure of the basic OFDM underwater acoustic communication system was improved. A shorter time-domain equalizer is added before DFT in receiver to limit the length of the channel impulse response to the range of the cyclic prefix in system which can minimize the impact of multi-path interference without increasing the length of the cyclic prefix in system. And two kinds of training algorithm of equalizer in time domain which are used widely in multi-carrier modulation technique were compared in the underwater acoustic environment.The theoretical analysis and the simulation results show that the improved OFDM underwater acoustic system can guarantees the performance of communication when the channel with severe delay spread. And the time domain equalization algorithm based on MMSE criteria is more suitable for the underwater acoustic channel with lower complexity and robustness to noise.