Passive time reversal has aroused considerable interest in underwater communications as a computationally inexpensive means of mitigating the intersymbol interference introduced by the channel using a receiver array. In this paper the basic technique is extended by adaptively weighting sensor contributions to partially compensate for degraded focusing due to mismatch between the assumed and actual medium impulse responses. Two algorithms are proposed, one of which restores constructive interference between sensors, and the other one minimizes the output residual as in widely used equalization schemes. These are compared with plain time reversal and variants that employ postequalization and channel tracking. They are shown to improve the residual error and temporal stability of basic time reversal with very little added complexity. Results are presented for data collected in a passive time-reversal experiment that was conducted during the MREA'04 sea trial. In that experiment a single acoustic projector generated a 2 / 4-PSK ͑phase-shift keyed͒ stream at 200/ 400 baud, modulated at 3.6 kHz, and received at a range of about 2 km on a sparse vertical array with eight hydrophones. The data were found to exhibit significant Doppler scaling, and a resampling-based preprocessing method is also proposed here to compensate for that scaling.

Abstract-Underwater acoustic networks (UANs) are an emerging technology for a number of oceanic applications, ranging from oceanographic data collection to surveillance applications. However, their reliable usage in the field is still an open research problem, due to the challenges posed by the oceanic environment. The UAN project, a European-Union-funded initiative, moved along these lines, and it was one of the first cases of successful deployment of a mobile underwater sensor network integrated within a wide-area network, which included above water and underwater sensors. This contribution, together with a description of the underwater network, aims at evaluating the communication performance, and correlating the variation of the acoustic channel to the behavior of the entire network stack. Results are given based on the data collected during the UAN11 (May 2011, Trondheim Fjord area, Norway) sea trial. During the experimental activities, the network was in operation for five continuous days and was composed of up to four Fixed NOdes (FNOs), two autonomous underwater vehicles (AUVs), and one mobile node mounted on the supporting research vessel. Results from the experimentation at sea are reported in terms of channel impulse response (CIR) and signal-to-interference-plus-noise ratio (SINR) as measured by the acoustic modems during the sea tests. The performance of the upper network levels is measured in terms of round trip time (RTT) and probability of packet loss (PL). The analysis shows how the communication performance was dominated by variations in signal-to-noise ratio, and how this impacted the behavior of the whole network. Qualitative explanation of communication performance variations can be accounted, at least in the UAN11 experiment, by standard computation of the CIR and transmission loss estimate.

This paper presents an environmental-based equalization algorithm for underwater communications. This algorithm is based on the passive time-reversal (pTR) and waveguide invariant properties of ocean channels. Passive time-reversal allows for the implementation of a simple communications system, but it loses performance in the presence of geometric mismatch between the probe-signal and the actual data symbols transmission. The waveguide invariant properties state that geometric mismatches, both in depth and range, can be partially compensated by applying an appropriate frequency shift in the passive time-reversal operator. Results with binary PSK data at a carrier frequency of 3.6 kHz, collected during the MREA'04 sea trial, show that the Mean Square Error (MSE) between the transmitted and the received data symbols remains stable at least to a range mismatch of about 37.5 m in the presence of source depth and an array depth oscillations of approximately 0.7 m. In such conditions, when comparing the proposed pTR equalizer with plain pTR, an overall gain of approximately 4.11 dB in output MSE is achieved. I. INTRODUCTIONIn recent years Time Reversal (TR) has received particular attention from the scientic community. After practical demonstration of its spatial-temporal focusing capabilities in the ocean [1] several applications of active TR (aTR), from tomography to communications, were suggested [1], [2]. Passive TR uses a receive-only array, and a probe-signal is transmitted ahead of the data for channel Impulse Response (IR) estimation. The IR estimate is then used as a synthetic channel for temporal focusing of the data signal, which is equivalent to the deconvolution of the multipath generated by the real channel.When applied to underwater coherent communications the achieved TR focus is not perfect due to errors on the IR estimate and the time variability of the channel, resulting in uncompensated intersymbol interference (ISI) [3]. That problem is even more relevant in communications with a moving source and/or receiver. In that case it is intuitive that a rapid degradation of passive TR temporal focusing will occur due to the increasing mismatch between the assumed and actual channels. In order to guarantee a longer stability of the focal spot, three solutions are usually proposed: one is to transmit probe-signals more frequently; another is to use an adaptive algorithm to track the IR from the initial probe signal IR estimation; and nally a third alternative is to use a low-complexity equalizer with only one coefcient per channel. A performance comparison between those adaptive pTR variants is presented in [4]. The major inconvenient of the rst method is that frequent transmission of probe-signals reduces the overall transmission rate. In the second the channel estimates are represented by a large number of coefcients that have to be adapted. The third case seems to be the best compromise between complexity and efciency. In this paper a different approach is proposed by considering that the ...

Abstract-This work addresses the problem of OFDM transmission in dispersive underwater channels where impulse responses lasting tens of miliseconds cannot be reliably handled by recently proposed methods due to limitations of channel estimation algorithms. The proposed approach relies on passive time reversal for multichannel combining of observed waveforms at an array of sensors prior to OFDM processing, which produces an equivalent channel with a shorter impulse response that can be handled much more easily. A method for tracking the narrowband residual phase variations of the channel after Doppler preprocessing is proposed. This is a variation of an existing technique that can improve the spectral efficiency of OFDM by reducing the need for pilot symbols. This work also examines techniques to handle sparse impulse responses and proposes a channel estimation method where an l1 norm is added to the standard least-squares cost function to transparently induce sparseness in the vector of channel coefficients. Algorithms are assessed using data collected during the UAB'07 experiment, which was conducted in Trondheim fjord, Norway, in September 2007. Data were transmitted with bandwidths of 1.5 and 4.5 kHz, and recorded at a range of about 800 m in a 16-hydrophone array. Significant multipath was observed over a period of at least 30 ms.

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