Research Of TRM-OFDM Underwater Acoustic Voice Communication Under Time-varying Channels

The21st century witnesses rapid development and exploitation of ocean. Accordingly, underwater acoustic voice communication is getting more and more attention, as the requirement of transmitting imformation through underwater channel is greatly increasing. However, the radio voice communication techniques used on the land can not be applied to underwater acostic voice communication for the reason that sound is the only energy form for long-distance transmission, while the variable underwater acoustic channel with narrow-band, high-noise, multipath effect and Doppler shift turns out to be a great challenge to the high-speed, robust underwater acoustic voice communication.Orthogonal frequency division multiplexing (OFDM) technology is a multi-carrier modulation technique and it becomes a world focus because of its high spectral efficiency and strong anti-multipath capacilty. However, the communication performance of OFDM decreases under the underwater channel with long multipath spread and sevrse Doppler shift as the orthogonality of its sub-carriers is broken. With the time-space focus feature, time reversal mirror (TRM) can concentrate multipath and improve signal-to-noise ratio (SNR) without knowing channel character in advance. But the performance of TRM also decreases under time-varying channel. Time-frequency differential coding scheme has strong anti-interference ability and can resist the impact of the time-varying channel without the need of carrier recovery.Taking into account the above points, in order to resist the effects of the time—varying channel, combing the advantages of OFDM, TRM and time-frequency differential coding, this paper research the OFDM underwater acoustic voice communication technology.The main contributions of this paper are as follows:1. Introduce the principle of OFDM and time reversal mirror; analysis the influence of time-varying channel on the OFDM communication and time reversal mirror. Propose to apply the time reversal mirror with probe updating to OFDM communication.2. Introduce the principle of time-frequency differential modulation; investigate its application in OFDM underwater acoustic voice communication. Describe the basic model of TRM time-frequency differential OFDM underwater acoustic voice communication; analyze its performance of resisting multipath and time-variation. Simulation experiments were taken out in time-varying channel which show that the probe updating TRM time-frequency differential OFDM scheme can resist the impact on underwater communication of the time-varying channel.3. Sea trial results demonstrated the superiority of TRM time-frequency differential OFDM underwater acoustic voice communication under time-varying channel. Based on the above investigation, hardware implementation of the OFDM modulation and demodulation was carried out on DSP, with which the performance of hardware implementation was verified.