An at-sea experiment of multiuser communication in deep water using adaptive time reversal was carried out. In the experiment, two sources and a 20-channel receiver array were deployed at the range of 30 km in an area of 1500-2000 m water depth. One of the sources was moored and the other was suspended at various depths. For processing signals actually transmitted from two sources, it was demonstrated that adaptive time reversal could cancel multiuser interference independently of relative source positions. Additionally, for a more detailed investigation, the analysis of multiuser test signals by synthesizing signals from different depths was performed. As a result, especially in the case of adjacent sources, adaptive time reversal significantly suppressed crosstalk.

In this study, the performance of passive time reversal (PTR) communication techniques in multipath rich underwater acoustic environments is investigated. It is recognized empirically and qualitatively that a large number of multipath arrivals could generally raise the demodulation result of PTR. However, the relationship between multipath and the demodulation result is hardly evaluated quantitatively. In this study, the efficiency of the PTR acoustic communication techniques for multipath interference cancelation was investigated quantitatively by applying a PTR-DFE (decision feed-back filter) scheme to a synthetic dataset of a horizontal underwater acoustic channel. Mainly, in this study, we focused on the relationship between the signal-to-interference ratio (SIR) of datasets and the output signal-to-noise ratio (OSNR) of demodulation results by a parametric study approach. As a result, a proportional relation between SIR and OSNR is confirmed in low-SNR datasets. It was also found that PTR has a performance limitation, that is OSNR converges to a typical value depending on the number of receivers. In conclusion, results indicate that PTR could utilize the multipath efficiently and also withstand the negative effects of multipath interference at a given limitation.

The relationship between the performance of time-reversal-based multiple-input/multiple-output (MIMO) communication technique and the changes in the multipath environment is investigated quantitatively. In this study, the performance of adaptive time reversal (ATR)-MIMO is investigated by a parametric study approach. The ATR-MIMO communication technique is applied to the multipath-rich synthetic shallow sea acoustic dataset. We focused on discussing about the relationship among the changes in the multipath environment, the number of transmissions, the capability of ATR-MIMO to remove cochannel interference (CCI), and the efficiency of utilization of the energy of the multipath. The results reveal which characteristic of an acousticwavefield affects the performance of ATR-MIMO and how the performance saturates when the multipath environment changes. It is also demonstrated that ATR-MIMO can utilize the energy of the multipath as effectively as the passive-time-reversal-based single-input/multiple-output communication technique in a Gaussian-noise-dominant environment.

In this research, effects of a multipath signal with a nonuniform Doppler shift on underwater acoustic communication between surface and underwater vehicles are investigated by simulation. Single-carrier modulation (SCM) and orthogonal frequency domain multiplexing (OFDM) are utilized and the Doppler shift is caused by the roll motion of the surface vehicle. In SCM communication, it is revealed that a decision feedback equalizer suppresses effects of the phase shift of the multipath signal caused by the nonuniform Doppler shift. On the other hand, a resampling method is proposed and utilized to evaluate the performance of OFDM demodulation after removing the effects of the nonuniform Doppler shift of the direct signal. Consequently, it is shown that the Doppler shift of the multipath signal causes intercarrier interferences even with the method. Furthermore, it is demonstrated that OFDM communication is more sensitive to the Doppler shift of the multipath signal than SCM communication.

Multiple-input/multiple-output (MIMO) communication using adaptive time reversal is examined by comparing orthogonal frequency division multiplexing (OFDM) with simulated MIMO test signals by synthesizing experimental data in deep ocean. The experiment was carried out in a 1,100-m-depth area at a range of 10 km with a bandwidth of 500 ± 50 Hz. Although time variance is not included in analysis of OFDM, it is impossible to increase the numbers of MIMO channels with OFDM. On the other hand, with adaptive time reversal, it is possible to achieve 8 × 20 and 6 × 20 MIMO communication with binary phase shift keying (BPSK) and quadrature phase shift keying (QPSK), respectively, in spite of time variance and input signal-to-noise ratio (SNR) degradation due to synthesized signals. Thus, it is demonstrated that adaptive time reversal shows a much better performance than OFDM in MIMO underwater acoustic communication.