Roee Diamant

Underwater Localization with Time-Synchronization and Propagation Speed Uncertainties

Underwater acoustic localization (UWAL) is a key element in most underwater communication applications. The absence of GPS as well as the signal propagation environment makes UWAL similar to indoor localization. However, UWAL poses additional challenges. The propagation speed varies with depth, temperature, and salinity, anchor and unlocalized (UL) nodes cannot be assumed time-synchronized, and nodes are constantly moving due to ocean currents or self-motion. Taking these specific features of UWAL into account, in this paper, we describe a new sequential algorithm for joint time-synchronization and localization for underwater networks. The algorithm is based on packet exchanges between anchor and UL nodes, makes use of directional navigation systems employed in nodes to obtain accurate short-term motion estimates, and exploits the permanent motion of nodes. Our solution also allows self-evaluation of the localization accuracy. Using simulations, we compare our algorithm to two benchmark localization methods as well as to the Cramer-Rao bound (CBR). The results demonstrate that our algorithm achieves accurate localization using only two anchor nodes and outperforms the benchmark schemes when node synchronization and knowledge of propagation speed are not available. Moreover, we report results of a sea trial where we validated our algorithm in open sea.

Spatial Reuse Time-Division Multiple Access for Broadcast Ad Hoc Underwater Acoustic Communication Networks

Underwater acoustic communication (UWAC) is often the only viable solution to establish an ad hoc underwater communication network. The specific features of UWAC, arising from the physics of underwater acoustics, make the design of resource-efficient media access control (MAC) protocols important as well as challenging. In this paper, we tackle this task considering ad hoc UWAC networks that support high-traffic broadcast communication. To this end, we propose the application of the spatial reuse concept and the exploitation of direct sequence spread spectrum used at the UWAC physical layer to obtain a new hybrid spatial reuse time-division multiple-access (HSR-TDMA) protocol. By tracking the time-varying network topology, our protocol adaptively optimizes the set of active communication nodes and overcomes problems of UWAC networks such as the near-far problem, flickering, and formation of islands. Pertinent performance parameters, namely network availability, message reliability, and transmission rate, are analyzed for the proposed protocol. Evaluation of these analytical performance expressions demonstrates the significant advantages of HSR-TDMA over commonly used conventional TDMA for broadcast UWAC networks. We also report performance results for both the HSR-TDMA and the conventional TDMA protocol from a sea trial at the Haifa harbor, which corroborate the results obtained from the analysis.

LOS and NLOS Classification for Underwater Acoustic Localization

The low sound speed in water makes propagation delay (PD)-based range estimation attractive for underwater acoustic localization (UWAL). However, due to the long channel impulse response and the existence of reflectors, PD-based UWAL suffers from significant degradation when PD measurements of nonline-of-sight (NLOS) communication links are falsely identified as line-of-sight (LOS). In this paper, we utilize expected variation of PD measurements due to mobility of nodes and present an algorithm to classify the former into LOS and NLOS links. First, by comparing signal strength-based and PD-based range measurements, we identify object-related NLOS (ONLOS) links, where signals are reflected from objects with high reflection loss, for example, ships hull, docks, rocks and so on. In the second step, excluding PD measurements related to ONLOS links, we use a constrained expectation-maximization algorithm to classify PD measurements into two classes: LOS and sea-related NLOS (SNLOS), and to estimate the statistical parameters of each class. Since our classifier relies on models for the underwater acoustic channel, which are often simplified, alongside simulation results, we validate the performance of our classifier based on measurements from three sea trials. Both our simulation and sea trial results demonstrate a high detection rate of ONLOS links, and accurate classification of PD measurements into LOS and SNLOS.

Choosing the right signal: Doppler shift estimation for underwater acoustic signals

In this paper, we consider the problem of estimating the coarse Doppler shift ratio for underwater acoustic communication (UWAC). Since underwater the constant motion of nodes results in Doppler shifts that significantly distort received signals, estimating the Doppler shift and compensating for it is required for all UWAC applications. Different than for terrestrial radio-frequency where the Doppler effect is modeled by a frequency shift, due to the slow sound speed in water, the effect of transceiver motion on the duration of the symbol cannot be neglected. Furthermore, since the carrier frequency and the signal bandwidth are of the same order, UWAC signals are considered wideband and Doppler-induced frequency shifts cannot be assumed fixed throughout the signal bandwidth. Considering these challenges, we present a method for Doppler-shift estimation based on comparing the arrival times of two chirp signals and approximating the relation between this time difference and the Doppler shift ratio. This analysis also provides an interesting insight about the resilience of chirp signals to Doppler shift. Our simulation results demonstrate improvement compared to commonly used benchmark methods in terms of accuracy of the Doppler shift estimation at near-Nyquist baseband sampling rates.

A Hybrid Spatial Reuse MAC Protocol for Ad-Hoc Underwater Acoustic Communication Networks

The most widely used medium access control (MAC) scheme for underwater acoustic communication (UWAC) networks is conventional time-division multiple access (TDMA), in which only a single node transmits at each time. Since this TDMA is the bottleneck in high traffic networks, in this paper we present a new MAC protocol for UWAC ad-hoc networks that applies spatial reuse to improve network throughput. More specifically, in the proposed protocol selected additional nodes can transmit simultaneously to the active TDMA node, thus improving the efficiency of the MAC protocol. By tracking the time-varying network topology, our protocol adaptively optimizes the set of active nodes and overcomes problems of UWAC networks such as the near-far problem, flickering, and formation of islands. We report performance results for both the conventional TDMA protocol and the proposed protocol from a sea trial at the Haifa harbor. The results show that the new protocol greatly increases the availability of nodes to transmit messages, which leads to an improved overall network throughput in high traffic networks.

Robust Spatial Reuse Scheduling in Underwater Acoustic Communication Networks

Resource assignment in underwater acoustic communication (UWAC) networks has recently drawn much attention in the research community. Although in most applications the number of nodes in the UWAC network is relatively small, the long propagation delay of acoustic signals underwater motivates the application of spatial reuse in channel access protocols for throughput enhancement. In this paper, we address the problem of spatial-reuse scheduling in UWAC networks that support frequent transmission of broadcast packets and require robustness to inaccurate topology information. Taking the possibility of outdated network topology information into account is of great importance for UWAC applications due to time-varying topologies in the underwater environment. Our main contribution is the derivation of a broadcast scheduling algorithm that combines topology-transparent and topology-dependent spatial-reuse scheduling methodologies to achieve high throughput in static and dynamic topology scenarios. Simulation results demonstrate that our protocol provides a favorable tradeoff between network throughput and robustness to outdated topology information due to topology changes, and that it also achieves fairness in terms of per-node throughput.

NLOS identification using a hybrid ToA-signal strength algorithm for underwater acoustic localization

The existence of obstacles in harbors or near shore environments leads to Non-Line-Of-Sight (NLOS) links between two underwater acoustic communication (UWAC) nodes. That is, only echoes of the transmitted signal arrive at the receiving node. Mistaking the first (strong) echo as a Line-Of-sight (LOS) measurement and using it for ranging causes significant degradation of the accuracy level of UWAC-based localization. In this paper, we propose a solution for the NLOS identification problem in underwater acoustic localization. Results from both extensive simulations and sea trial experiments confirm our approach and demonstrate a high detection rate of NLOS measurements.

Adaptive Error-Correction Coding Scheme for Underwater Acoustic Communication Networks

Underwater acoustic communication networks (UWANs) have recently attracted much attention in the research community. Two properties that set UWANs apart from most radio-frequency wireless communication networks are the long propagation delay and the possible sparsity of the network topology. This in turn offers opportunities to optimize throughput through time and spatial reuse. In this paper, we propose a new adaptive coding method to realize the former. We consider time-slotted scheduling protocols, which are a popular solution for contention-free and interference-free access in small-scale UWANs, and exploit the surplus guard time that occurs for individual links for improving transmission reliability. In particular, using link distances as side information, transmitters utilize the available portion of the time slot to adapt their code rate and increase reliability. Since increased reliability trades off with energy consumption per transmission, we optimize the code rate for best tradeoff, considering both single and multiple packet transmission using the incremental redundancy hybrid automatic repeat request (IR-HARQ) protocol. For practical implementation of this adaptive coding scheme, we consider punctured and rateless codes. Simulation results demonstrate the gains achieved by our coding scheme over fixed-rate error-correction codes in terms of both throughput and consumption of transmitted energy per successfully delivered packet. We also report results from a sea trial conducted at the Haifa harbor, which corroborate the simulations.

Joint Time and Spatial Reuse Handshake Protocol for Underwater Acoustic Communication Networks

In most existing handshake-based collision avoidance (CA) protocols, nodes in the communication range of the transmitter or the receiver are kept silent during an ongoing communication session (CS). In underwater acoustic communication (UWAC), this restriction results in low throughput and long transmission delay. In this paper, we utilize the long propagation delay in the underwater acoustic channel and the (possible) sparsity of the network topology, and formalize conditions for which a node can transmit even when it is located within the communication range of a node participating in a CS. We consider these conditions as design constraints and present a distributed CA handshake-based protocol, which, by jointly applying spatial and time reuse techniques, greatly improves channel utilization. Our simulation results show that our protocol outperforms existing handshake-based protocols in terms of throughput and transmission delay. These gains come at the price of some reduction in fairness in resource allocation.

Underwater localization with time-synchronization and propagation speed uncertainties

Underwater acoustic localization (UWAL) is a key element in most underwater communication applications. The absence of GPS as well as the signal propagation environment makes UWAL similar to indoor localization. However, UWAL poses additional challenges. The propagation speed varies with depth, temperature, and salinity, anchor and unlocalized (UL) nodes cannot be assumed time-synchronized, and nodes are constantly moving due to ocean currents or self-motion. Taking these specific features of UWAL into account, in this paper, we describe a new sequential algorithm for joint time-synchronization and localization for underwater networks. The algorithm is based on packet exchanges between anchor and UL nodes, makes use of directional navigation systems employed in nodes to obtain accurate short-term motion estimates, and exploits the permanent motion of nodes. Our solution also allows self-evaluation of the localization accuracy. Using simulations, we compare our algorithm to two benchmark localization methods as well as to the Cramér-Rao bound (CBR). The results demonstrate that our algorithm achieves accurate localization using only two anchor nodes and outperforms the benchmark schemes when node synchronization and knowledge of propagation speed are not available. Moreover, we report results of a sea trial where we validated our algorithm in open sea.