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- Linking acoustic communications and network performance. Integration and experimentation of an underwater acoustic networkPublication . Caiti, A.; Grythe, K.; Hovem, J. M.; Jesus, S. M.; Lie, A.; Munafò, Andrea; Reinen, Tor Arne; Silva, A.; Zabel, F.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.
- Underwater acoustic communication using a time-reversal mirror approachPublication . Silva, A.; Jesus, S. M.; Gomes, J.; Barroso, V.This work presents a technique for reducing the intersymbol interference (ISI) in underwater coherent communications using time-reversal acoustics. The paper introduces a “virtual" time-reversal mirror (TRM) that is implemented electronically at the receiver array and simulates the kind of processing that would be done by an actual TRM during the reciprocal propagation stage. In both cases, a probe pulse sent by the transmitter/receiver located at the (physical or virtual) focal point and received at the array provides a template impulse response for undoing the effects of multipath by straightforward linear filtering. Very simple equalization algorithms may subsequently be used to decode the message. Channel variations between transmission of the probe and the actual message lead to mismatch that can impact the coherence of TRMs, and hence degrade the focusing power of the array. Computer simulations using a normal-mode propagation model in a reallistic shallow water scenario show that, even with high uncertainty in the transmitter and receiver relative positions, the virtual mirror can strongly reduce the effects of multipath. Although a multichannel equalizer attains a lower mean-square error, the "virtual" TRM can provide comparable results under low mismatch with much smaller complexity.
- Using normal mode channel structure for narrow band underwater communications in shallow waterPublication . Silva, A.; Jesus, S. M.Multipath and high temporal and spatial variability of the propagation environment causes severe signal degradation in shallow water acoustic digital communications. Among the many solutions that have been proposed the most known is adaptive equalisation where cyclic training signals are used to adapt the equaliser to the variability of the acoustic channel. When the channel is rapidly changing, equaliser coefficients are frequently adapting and the effective transmitting rate rapidly decreases. Another approach consists in using a priori information obtained from acoustic propagation models. These models can give a deterministic estimate of the true channel impulse response that can be used to detect the transmitted signals. In practice, the use of deterministic acoustic models is mainly dependent of the accuracy of the input environmental parameters. As a first step, this paper presents an exhaustive study of the signal detection sensitivity to model parameters mismatch. The scenario used is composed of a 100 m depth water column with range dependent characteristics. The water column is located over a 10 m thick sediment layer with variable properties. Source-receiver communication is made over a variable distance between 500 and 600 m with the source near the bottom and the receiver near the surface. The communication signals are narrow band (1.5 kHz) pulse amplitude modulated with a carrier frequency of 15 kHz, and the detector is based on the maximum-likelihood sequence detector (MLSD)