Time-reversed OFDM communication in underwater channels

Time reversal is a feedback wave focusing technique that can be used to transparently compensate for multipath distortion in digital communications over several types of physical propagation media, such as radio or acoustic channels. While much of the work on this topic has focused on coherent communication through single-carrier modulation, time reversal can in fact be applied to any signaling scheme. This work discusses issues related to protocol design and data modulation/demodulation when multicarrier signals are used, with an emphasis on underwater communication. It is shown that focusing information can be derived at the transmitter array by prefiltering a single observed broadband channel probe, thus streamlining the design of communication protocols. Conventional prefix-based demodulation can be achieved for sufficiently well focused OFDM signals, but this may be somewhat demanding in terms of transmitter hardware. A receiver architecture based on multiple-input/multiple-output decision-feedback equalization is proposed when few transmit elements are used, creating significant residual intersymbol interference (ISI).