In this paper, we present an analysis of the space-time correlation and power-delay profile (PDP) properties of the underwater acoustic channel in the nearabouts of the Pianosa Island, off the north-western coast of Italy. Our data has been collected during sea trials which took place from May to September 2009. Using the results from this evaluation, we compare the measured bit error rates affecting the transmission of Frequency-Hopping Binary Frequency Shift Keying (FH-BFSK) against those obtained by simulating the same digital modulation scheme over synthesized channels whose spread in time has the same statistical properties as those found in the trials, and whose channel taps are Rayleigh-distributed. The results show a generally good accordance of the simulated performance with the outcomes of the experiments. Moreover, given the absence of a widely agreed upon underwater channel model, and the recent interest in incorporating more accurate propagation simulators into network simulators, we compare the measured channel impulse responses against those obtained through the ray tracing tool Bellhop, and give some observations about the suitability of the tool for the purpose of reproducing realistic channel traces.

In this paper, we present an analysis of the performance of a variable-rate adaptive modulation system based on instantaneous SNR information. The SNR traces we consider are part of the SubNet'09 experimental dataset, and have been derived by processing an hyperbolic frequency-modulated signal in the 9-14 kHz band. We start by deriving the high level behavior of the channel in terms of the statistics of channel fading effects, which are found to be well modeled by a Nakagami-m distribution, where the parameter m is estimated over a whole experiment or over smaller time windows throughout the experiment, depending on the variability of the SNR. The statistics of the channel behavior are then used to derive the performance of a variable-rate modulation scheme switching between five different constellations; the cases of both instantaneous and outdated channel knowledge are considered. Analytical results are compared to simulations to show that the Nakagami-m distribution can satisfactorily capture the statistics of the channel, provided that the estimation of the m parameter, as well as of the correlation of the SNR process, is repeated in case of macroscopic channel variations.

The recent development of underwater acoustic modems has enabled multihop networking capabilities that can be used in important military and civilian applications. For this reason, routing protocols for underwater acoustic networks (UANs) have recently been proposed and evaluated. However, the interactions between channel dynamics and networking performance are not well understood. In this paper, we investigate and quantify the effect of the time-varying (TV) link quality on routing protocols in static UANs. In order to do so, we simulate the considered routing protocols in several network scenarios, obtained by changing the network density, the number of packet retransmissions, the packet length, the modulation type, and the power level with both TV and time-invariant (TI) channel conditions. Results confirm that, when evaluating the performance of routing protocols, it is important to understand the TV behavior of the channel quality over intervals of time sufficiently long to accommodate multihop communications. Finally, we also present experimental results, confirming the outcome of the simulations. The experiments have been conducted in collaboration with the NATO Centre for Maritime Research and Experimentation (CMRE) during the CommsNet12 sea trial.

In this paper, we present a comparative evaluation among two MAC protocols specifically proposed for underwater acoustic networks, namely Slotted FAMA and Tone-Lohi. Both protocols are based on random access and regulate transmissions by either four-way handshaking (Slotted FAMA) or contention resolution procedures (Tone-Lohi). Our comparison is carried out by means of analytical models, which are used to capture the behavior of a node that acts according to either protocol. We focus on throughput, delay and energy performance, studying the pros and cons of each approach. Our results highlight relevant design tradeoffs that can be exploited in order to properly tune protocol performance.

In this paper, we investigate how to represent the packet error process in a shallow water acoustic channel by means of Markov as well as hidden Markov models. To train the models, we employ experimental traces taken from transmissions performed during the SubNet'09 sea trials, off the coast of Pi-anosa island, Italy. In particular, signal-to-noise ratio (SNR) time series show significant transitions of the average SNR on a large time scale, which motivates the use of hidden Markov models. The discussion on which model best fits this experimental data is carried out considering relevant metrics for networking, i.e., packet error rate (PER), length of error bursts and correlation of errors after a given number of packet transmissions. Results show that hidden Markov models yield accurate reproduction of both first and second-order metrics.