In December 2011 a long-range acoustic communication experiment was conducted in deep water, west of Izu-Ogasawara Islands, Japan. The experiment involved a stationary source (450–550 Hz) and an 18-element vertical array (102-m aperture), both deployed at around the sound channel axis. Initial analysis of data demonstrates that a data rate of 400 bits/s can be achieved over ∼600-km range in deep water using 16 quadrature amplitude modulation and passive time reversal equalization.

In a recent shallow water experiment, acoustic communication transmissions were carried out over the 10 to 32 kHz band in similar to 100m deep water over a 3 km range. A natural question is how best to utilize that bandwidth. In one multiband approach discussed previously, the band was divided into four smaller subbands that were processed independently using time reversal decision-feedback equalizers (TR-DFEs). This letter presents a complementary wideband approach using data from the same experiment achieving a data rate of up to 60 kbits/s with 32 quadrature amplitude modulation. These results suggest that a wideband approach can be beneficial in terms of spectral efficiency with modest computational complexity using a TR-DFE. (C) 2013 Acoustical Society of America.

Recently time reversal (TR) communications has been extended to time-varying channels. The basic idea is to implement it on a block-by-block basis such that within each block the channel remains time-invariant and subsequently is updated using detected symbols (decision-directed mode). Using experimental data (12–20 kHz) collected in shallow water, this letter investigates three different block-based TR approaches: (1) without explicit phase tracking, (2) with phase tracking, and (3) exploiting channel sparsity. The TR approaches then are compared to a conventional adaptive multichannel equalizer. It is found that approach (3) generally provides the best performance and robustness.

The array invariant proposed for robust source localization in shallow water exploits the beam-time migration of broadband signals. The approach requires minimal knowledge of the environment, but involves plane wave beamforming using a short-aperture vertical array in stratified acoustic waveguides. In this paper, the array invariant approach is extended to a large-aperture vertical array that is sparse with significant variation of the sound speed across the aperture for conventional beamforming. The extension is feasible because the array invariant in shallow water utilizes surface/bottom-reflected arrivals that behave like those in ideal waveguides. Robust source-range estimation is demonstrated using data from a 16-element, 56-m long vertical array at various ranges (1.5–3.5 km) from a broadband source (0.5–2 kHz) in approximately 100-m deep shallow water.

A recent communications experiment was conducted in a shallow water environment at high-frequency permitting the use of a large bandwidth (11-19 kHz). This paper investigates the communication performance versus various symbol rates (or bandwidths) in terms of output signal-to-noise ratio with, an assortment of constellations, illustrating a trade-off between performance and data rate. A high bandwidth efficiency of 4 bits/s Hz is demonstrated using 32 quadrature amplitude modulation with a data rate of 31.25 kbits/s over a 2.2-km range. (C) 2009 Acoustical Society of America. [DOI: 10.1121/1.3160284]

Passive multiuser communications in shallow water previously was demonstrated in the 3–4 kHz band using a time reversal approach. This paper extends those experimental results in three respects. First, a larger bandwidth at higher frequency (11–19 kHz) is employed allowing for the use of various symbol rates (or bandwidths). Second, two different shaping pulses are examined: a raised cosine filter and LFM (linear frequency modulation) chirp. Third, the adaptive time reversal approach with spatial nulling is applied to suppress the crosstalk among users. It is shown that the use of a larger bandwidth is beneficial along with the time reversal receiver which can handle significant intersymbol interference with minimal computational complexity. In addition, adding each user degrades the performance by about 4 dB for the benefit of linear increase in data rate. It is demonstrated that an aggregate data rate of 60 kbits/s can be achieved with a 7.5 kHz bandwidth (a spectral efficiency of 8 bits/s Hz) by three users distributed over 4.2-m depth at a 2.2 km range in shallow water using 16-QAM (quadrature amplitude modulation).