Passive time reversal communication with cyclic shift keying over underwater acoustic channels

Many advanced channel estimation and equalization algorithms have been proposed for high-rate coherent underwater acoustic communications. However, these algorithms are usually computationally demanding or ineffective under low signal-to-noise ratio (SNR). Energy-efficient or reliable communications over complex underwater multipath channels are required in many applications. In this paper, passive time reversal with cyclic shift keying (CSK) spread spectrum using hyperbolic frequency modulated (HFM) waveform is proposed for reliable point-to-point underwater acoustic communication. At the transmitter, multiple bits information is represented by the cyclically shifted version of the basic HFM waveform. Therefore, the CSK scheme provides a higher data rate than conventional direct sequence spread spectrum. At the receiver, the peak position of the cyclic correlator output is estimated to recovery information. A modified passive time reversal receiver for the proposed HFM based CSK is designed to: (1) focus the multipath signal without estimating the channel impulse response to increase the received SNR; (2) and alleviate the requirement for accurate symbol synchronization. Simulation and lake experimental results verify the proposed method and show its potential for low cost and reliable underwater acoustic communication systems.