Beatrice Tomasi

Experimental study of the space-time properties of acoustic channels for underwater communications

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.

A study of incremental redundancy hybrid ARQ over Markov channel models derived from experimental data

In this paper, we process channel Signal-to-Noise-Ratio time series gathered in the proximity of the Pianosa island, Italy, in Summer 2009. These traces are used to model the performance of capacity-achieving code ensembles as employed in an Incremental Redundancy (IR) Hybrid Automatic Repeat reQuest (HARQ) error control scheme. We apply a code-matched channel state quantization technique aimed at representing channel evolution over time with low quantization error; the evolution of the channel among the quantized states is then represented using a Markov model, over which we base the analytical evaluation of IR-HARQ performance. Results confirm that IR-HARQ consistently improves link performance with respect to Type I HARQ. In addition, we observe that the different channel statistics due to different transmitter and receiver placements, as well as to the acoustic propagation conditions considered in our scenario, have an impact on HARQ performance. This impact is correctly captured by our Markov model, suggesting good adherence of the model to actual channel behaviors. The validation of the models (by simulating over different traces than those used to train the models) suggests that they are robust to moderate non-stationarity, making them good candidates to give a compact representation of the channel behavior, e.g., in network simulators.

Impact of Time-Varying Underwater Acoustic Channels on the Performance of Routing Protocols

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.

A study on the SPIHT image coding technique for underwater acoustic communications

In this paper, we consider the transmission of progressively encoded images over underwater acoustic links, where transmitted symbols are protected by Forward Error Correction (FEC). The allocation of redundancy is performed according to both a Basic and a Multiple Description (MD)-like technique. The performance of this system is analyzed in terms of the resulting Peak Signal-to-Noise Ratio (PSNR) as image packets are transmitted over the measured realizations of acoustic channel impulse responses. These measurements were taken near the coast of Marthas Vineyard during October 2008, in different environmental conditions. In the results, we quantify the performance improvement of the multiple description (MD) technique compared to the Basic allocation, thus suggesting its suitability for the transmission of images in the underwater acoustic scenario.

A comparison between the Tone-Lohi and Slotted FAMA MAC protocols for underwater networks

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.

Cross-layer analysis via Markov models of incremental redundancy hybrid ARQ over underwater acoustic channels

Underwater acoustic networks make it possible to wirelessly convey information, e.g., coming from measurements and sensing applications from under water to the surface. However, underwater communications are characterized by long delays, small available bandwidths and high error rates. These aspects may significantly affect the design of a reliable data-link layer for such systems. In this paper, we assess the performance of hybrid automatic repeat request error control schemes and we evaluate their application to improve the reliability of time-varying underwater acoustic links. We employ a parametric Markov model, which has been trained over channel traces collected during at-sea experiments. The results, based on both experimental data and analysis, suggest that parametric probabilistic representations, such as the considered Markov model, are good candidates for describing the correlated underwater acoustic channel dynamics, and may be employed to achieve a realistic evaluation of the data-link layer performance for underwater acoustic scenarios. Analytical and simulation results confirm that incremental redundancy improves the throughput of underwater acoustic links, even when real channel conditions, such as those encountered in the considered experiments, have wide dynamics over time. Finally, this kind of evaluations, beyond the data-link design, can also be employed at the network level for routing and network deployment considerations.

On the predictability of underwater acoustic communications performance: the KAM11 data set as a case study

In this paper, we analyze the predictability of the communications performance of an adaptive modulation system for underwater acoustic communications. As a case study, we consider the KAM11 data set, which has been collected off the coast of Kauai Island during July 2011. We measure the signal to noise ratio at the output of the equalizer and we observe slow fluctuations over time intervals of up to six minutes. Based on an analytical model and using the estimated time correlation coefficient of subsequent values of the signal to noise ratio (SNR), we compute the communications performance as a function of the feedback delay for an adaptive modulation system, and we evaluate its predictability. We show that it is possible to know in advance the trend of the performance over intervals of three to four minutes.

Energy-Efficient Transmission Strategies for Delay Constrained Traffic With Limited Feedback

In this paper, we developed transmission strategies able to deliver a prescribed number of packets by a deadline T while minimizing transmission attempts. All the packets have a single common transmission deadline. The three systems analyzed here differ from each other on the way channel state information (CSI) is provided by the receiver. In the first system, the receiver sends back CSI only when packets fail; in the second system, the receiver provides CSI only if this minimizes a given cost function; and in the third system, feedback is sent whenever a given threshold condition is verified. The communication channel is modeled as a time-varying correlated fading channel, represented by a Markov chain. For the first two systems, the optimal schedules are obtained through dynamic programming (DP) . The third scheduler is based on a heuristic strategy, whose performance is evaluated through numerical simulations, and compared with that of the two optimal schedulers obtained through DP. Results showed that the heuristic scheduler can reach performance similar to that of DP schedulers.

Real-time redundancy allocation for time-varying underwater acoustic channels

In this paper, we jointly address reliability and spectral efficiency in underwater acoustic (UWA) communications by proposing optimal redundancy allocation over time-varying conditions in a communication link. To do so, we use recent results on tight approximation of codeword error probability in the finite block-length regime. First, we propose and evaluate an optimization framework. Then, we design a real-time algorithm, able to allocate the redundancy in a point-to-point link, without adding control messages. We evaluate the performance of the proposed algorithm by considering both rapidly and slowly time-varying experimental channel conditions. Finally, we compare the performance of our algorithm with that obtained with constant length codewords. Results show that the proposed redundancy allocation scheme is more efficient than constant allocation schemes under different channel conditions.

A study on channel dynamics representation and its effects on the performance of routing in underwater networks

We consider an underwater networking scenario, and test the performance of two multihop routing paradigms, source routing and hop-by-hop relay selection, in the presence of different representations of the channel dynamics. We focus on a static channel case (obtained via empirical equations for path-loss), and on a sequence of channel realizations obtained using ray tracing, that vary both slowly and rapidly over time with respect to the expected reaction time of routing protocols; the two latter cases are also explored in the presence both of a flat bottom and of a rough bottom with several seamounts, to yield a total of five different channel models. Our results show that channel variations induced by environmental changes over time have an impact on routing performance metrics in connected topologies. A sea bottom with a rough shape adds a further impact to the routing performance, which is shown to be larger for source routing. We conclude that while empirical channel models yield a good first-order approximation, the time-variability of the channel and the shape of the network area boundaries are to be taken into account in order to achieve more realistic network performance estimates.