Underwater communications using virtual Time Reversal in a variable geometry channel

Abstract

Abstract-Field experiments using Time-Reversal Mirror(TRM) techniques have shown that the underwater acoustic channel presents a longer stability than it was previously anticipated. Applying such techniques to underwater communications requires that, either the emitted signals are previously filtered by time-reversed replicas of the channel impulse response or that a probe-signal should be transmitted ahead of the data-signal for post channel filtering. In the former case, the time-reversed filtered message is expected to be undone by the actual acoustic channel between the array and the receiver, while in the later, the undoing of the multipath is performed electronically inside the computer and is therefore termed as virtual Time Reversal Mirror(vTRM). The main issues being addressed in recent literature deal with channel stability, focus width and the required array aperture for obtaining reasonable temporal and/or spatial focusing. This paper focus essentially in two practical aspects, generally not addressed: one is to demonstrate the potential application of the vTRM approach to undo the severe intersymbol interference in a real variable geometry channel scenario and, the other, is the importance of optimization of the probe-signal time window length in a real application.

Acoustic Communication Using Time-Reversal Signal Processing: Spatial and Frequency Diversity

“…Acoustic communications represents a plausible application of time-reversal processing in the ocean. Both active [4][5][6] and passive [7][8][9] versions of the processing have been tested in experiments.…”

Probe timing optimization for time-reversal underwater communications

“…During the INTIFANTE'00 sea trial the pTR based data communications system was similar to that of Figure 1, with the p 2 (t) narrowband filter of the IR estimation operation (path above in Figure 1) distributed between the transmitter and the receiver, i.e., the transmitted probe-signal was a fourth-root raised-cosine pulse, p 1 (t), and IR estimates were obtained by pulse compression at the receiver side (see [7] for details). An example of estimated arrival pattern can be seen in Figure 3 that shows a number of arrival paths that are not as well defined as in the simulations due to noise corruption.…”

“…Like aTR, passive Time Reversal (pTR) relies on mode orthogonality but instead of the reciprocity property, uses an estimate of the underwater channel Impulse Responses (IRs) to perform a virtual ocean response match inside the computer. Despite its simplicity, a-pTR applied to high frequency underwater communications presents a lower performance than multichannel equalization [7]- [9]. That is due to the Time Reversal Mirror (TRM) requirement for a long and dense array [10], without which residual Inter-Symbol Interference (ISI) is always present due to poor sampling of the highorder modes and subsequent orthogonality property violation.…”

“…The start time and the duration of such timewindow should depend on the time dispersion of the acoustic channel. Heuristic reasoning would suggest that if a short timewindow fails to include all significant multipath it will result in an imperfect retrofocusing, while a too long time-window will reduce the efficiency of the communication system and introduce additional noise in the pTR operation [5], [7], [11].…”

“…The a-pTR output SNR have been addressed by several authors [9], [12], including heuristic characterizations of time-window dependence [5], [7], [11] though optimization was not attempted.…”