Uncooperative localization improves attack performance in Underwater Acoustic Networks

In the last ten years several simulation studies on Autonomous Underwater Vehicle (AUV) swarm fleet formation have been performed, and some preliminary sea demonstrations of proof-of-concept prototypes were carried out. However, their actual realization is hindered by the challenges imposed by the underwater acoustic channel and the difficulties of keeping track of the vehicles’ positions due to the long latency required by traditional Two-Way Travel-Time (TWTT) ranging measurements, that require a specific signaling, hence limiting the throughput of the underwater network. Although One-Way Travel-Time (OWTT) halves the latency, it requires a high precision oscillator, such as an atomic clock or an oven controlled crystal oscillator (OCXO), to be installed in each modem processing unit: while atomic clocks are still very expensive, OCXO are very power demanding, making their application to underwater acoustic networks not always possible, especially in the case of low cost vehicle swarms.In this paper we present a network protocol stack able to perform ranging and localization within the communication task in underwater acoustic networks, limiting the network overhead. Specifically, new layers have been added to the preexisting DESERT Underwater protocol stack to perform the ranging tasks without compromising the correct operation of the communication network. A ranging layer is placed on top of the Medium Access Control (MAC) scheme, allowing the latter to be changed according to the network topology and requirements. This MAC-agnostic ranging layer is further optimized by adapting the amount of data transmitted according to the channel state, and the ranging entries inserted in the data packets according to the least recent information transmitted, hence minimizing the Age of Information. Simulation results obtained with the DESERT Underwater Framework show how this layer allows all AUVs in the swarm to know their distance from every other node in the network, thus limiting the probability of vehicle collisions and allowing better mission coordination.

As a kind of small size mobile platform, autonomous underwater vehicles (AUV) are playing more and more important role in different marine fields such as underwater monitoring and construction. While acoustic communication offer an effective communication link for AUV, under the confined spaces in which small AUVs usually work, the structured obstacles and closure or semi-closure barriers pose serious challenges to communication link due to acoustic sheltering and significant reverberation. The underwater acoustic network is capable of improving acoustic connectivity of AUV via the distributed communication link of different nodes. In this paper, an acoustic network is designed and evaluated for communication support of AUV in a small lake with dense piers. While the presence of piers lead to obvious performance degradation of point-to-point acoustic communication, the experimental results verified the effectiveness of acoustic network for improving connectivity of AUV in confined space.

Due to the frequency-selective nature of the underwater acoustic channel, OFDM has been proposed for use in underwater acoustic communication networks. However, because of the relatively low speed of sound in water in comparison to the speed of electromagnetic waves in air, the underwater acoustic channel is prone to a significant amount of Doppler distortion, which OFDM is particularly sensitive to. MISO architectures have several potential applications in wireless networks, either in increasing signal diversity and thus increasing the effective SNR, or in increasing spectral efficiency of transmissions. However, the degradation effect due to Doppler shifting is exacerbated when a receiver detects signals from multiple transmitters which may see different Doppler distortions. In this thesis, we look at existing proposed OFDM receiver designs for use in underwater acoustic networks, and compare their performance in a MISO network composed of two transmitters and one receiver. These performance comparisons are analyzed first through the use of MATLAB simulations and then through physical experiments in a controlled laboratory environment. Two receiver algorithms in particular are explored in detail. The first is a block-by-block approach in which pilot tones are transmitted in every OFDM block and used to estimate the channel. The second is an adaptive receiver which uses previous channel estimates and a data-directed approach to further estimate the channel response. These two approaches achieve near equal performance, but the block-by-block receiver has a much lower spectral efficiency due to the overhead needed to carry the pilot tones. We thus conclude that in environments which can handle potential reductions in performance, the adaptive receiver is clearly the optimal choice. However, in fast-changing or low SNR environments, the block-by-block approach may be preferred.

Time synchronization is a supporting technology and necessary precondition for underwater acoustic (UWA) networks. Sensation and actuation in UWA networks need coordination across nodes. Under normal circumstances, the deployment of UWA networks is large, and the energy resources are limited. The existing protocols can’t achieve an accuracy high enough and consume overmuch energy, so they could not well adapt to UWA networks yet. Although many protocols are dedicated to improving accuracy, they often work on the perspective of ameliorating the exchanged mechanism of information and ignore the propagation delay with the greatest impact on accuracy that affected by channel quality. In this paper, we propose a fault tolerant time synchronization protocol. It dedicated to enhancing accuracy by improving the processing method of exchanged synchronization messages and enables the synchronization process adapt to unstable channel quality. The simulation results show that the protocol achieves a high accuracy with low energy cost, and as channel quality deteriorates, compared with existing protocols, its accuracy declines flatter and fluctuates in a smaller range.

Given a middle-scale underwater acoustic network, to collect data from the multiple underwater nodes, the surface destination node need to be working on mobile data collection mechanism. In this paper, compared to traditional automatic-repeat-request (ARQ) protocol, we proposed node cooperation (NC) to increase the data collection efficiency for the surface node, utilizing the fact that underwater nodes can overhear the transmission of the others. However, different data collection paths can lead to different data collection efficiency for the middle-scale network when employing NC protocol. For one specific node cooperative underwater acoustic network, the underwater nodes are distributed at random within a circle planar region. As the restriction of underwater transmission power, the planar region is divided into several sub-zones according to different mobile collection paths. And then we present several different data collection path examples and show their data collection efficiency. The numerical results show that the optimal mobile collection path should consider both the cooperative zone and the distribution of the underwater nodes.

Underwater Acoustic Networks and related energy based lifetime enhancement is one of the key domain of research in marine based engineering. A number of algorithms, protocols and approaches are developed so far for effective optimization of acoustic resources still a huge scope of research is there. In this research work, an effective and novel algorithm for energy harvesting is proposed using which the effective lifetime of the network can be enhanced. Using this approach, the solar based energy harvesting and optimization can be done with overall cost reduction.

The potential of exploiting large propagation delays in underwater acoustic (UWA) networks to maximize the network throughput is established in the recent past. Transmission scheduling strategies have been proposed to take advantage of large propagation delay. Super-TDMA is one among such Medium Access Control (MAC) strategies proposed. It is a form of Time Division Multiple Access (TDMA) protocol in which multiple transmissions are allowed in the same time slot and hence concurrently propagate in the medium. We present the implementation challenges of Super-TDMA on underwater acoustic modems and the experimental results demonstrating interference alignment, the crossing of simultaneously transmitted packets in water, and the time synchronization among the deployed nodes in the network.

Underwater acoustic (UWA) networks are a key form of communications for human exploration and activities in the oceanographic space of the earth. A fundamental issue of UWA communications is large propagation delays due to water medium, which has posed a grand challenge in UWA network protocol design. Conventional wisdom of addressing this issue is to live with this disadvantage by inserting a guard interval to introduce immunity to propagation delays. Recent advances in interference alignment (IA) open up a new direction to address this issue and promise a great potential to improve network throughput by exploiting large propagation delays. In this paper, we investigate propagation delay-based IA (PD-IA) in multi-hop UWA networks. We first develop a set of simple constraints to characterize PD-IA feasible region at the physical layer. Based on the set of PD-IA constraints, we develop a distributed PD-IA scheduling algorithm to greedily maximize interference overlapping possibilities in a multi-hop UWA network. Simulation results show that the proposed PD-IA algorithm yields higher throughput than an idealized benchmark algorithm without propagation delays, indicating that large propagation delays are not adversarial but beneficial for network throughput performance.

Many marine scientific, industrial, and military applications may require the deployment of underwater acoustic sensor networks for sensing and monitoring. A grid topology with multihop relaying is useful for wide area coverage as well as long distance data transmission. We investigate network architectures where data originate at one end of the grid, and are forwarded along multiple lines. We are particularly interested in transmission schedules that maximize network throughput by exploiting propagation delay to allow multiple simultaneous transmissions. We show that an optimal schedule is necessarily per-node fair, and derive the upper bound on throughput. Furthermore, we present a low-complexity algorithm to find schedules achieving the upper bound, regardless of the size of the network.

Multiple access with collision avoidance (MACA) is a popular medium-access control (MAC) protocol for terrestrial networks (e.g., 802.11). Underwater acoustic networks (UANs) differ from terrestrial networks as they are characterized by long propagation delays and higher data loss. Most of the published analysis for MACA in terrestrial networks ignores these factors, and, therefore, cannot be directly applied to UANs. As a result of the high data loss in UANs, it is common to implement reliability at a link level, rather than end-to-end acknowledgements. Keeping this in mind, we present a Markov chain analysis for a reliable variant of the MACA protocol for ad hoc UANs and derive closed-form expressions for mean service time and throughput. We show that the network performance is vastly improved with a few changes to the protocol, and propose a novel MACA-based protocol for use in UANs. For best performance, the protocol parameters such as batch size and backoff window have to be optimally chosen. We show that an optimum batch size minimizes the total waiting time. Finally, we compare our analysis results with experimental results obtained from a deployment of a small UAN.

Time synchronization is an important part of any distributed networked embedded system. It is essentially the process of achieving and maintaining common time base among all network nodes of the system. This task is quite challenging for the systems or applications like sensor networks, since these systems are highly resource constrained, yet need to process time-sensitive data in collaborative manner. Though many protocols have been suggested for terrestrial sensor networks (RSB, TPSN, FTSP and LTS) and they perform reasonably well, very few protocols (THSL, Tri-message) have been suggested for the high-latency underwater acoustic networks, since achieving time-synchronization for high-latency networks is even more challenging issue. Here we describe a very simple extension and implementation of Tri-message time synchronization protocol for multi-hop Underwater Acoustic Sensor Network (UASN) on the miniature test-bed setup.

In this chapter, we focus on a novel network architecture, UASN-MG, which has multiple surface gateways for underwater acoustic sensor networks. The surface gateways communicate with each other using surface radio and form a radio network on the water surface. Thus, two networks, an underwater acoustic network formed by underwater sensor nodes and a surface radio network formed by the surface gateways, are involved. Such a network architecture can bring about many benefits such as high throughput and energy efficiency. However, integrating multiple surface gateways into underwater acoustic sensor networks also incurs many new design challenges. In this chapter, we describe this new network architecture and its benefits in detail. Then we discuss the design challenges in this new architecture. Afterwards, three design examples are given, from different perspectives, to illustrate the techniques that cope with these challenges and optimize the network’s performance. Finally, we conclude the chapter with discussions on possible future research directions.

Underwater acoustic sensor networks (UWASNs) is an emerging technology, comprising of sensor nodes and unattended automated vehicles (AUVs), all working in a collaboration to sense various phenomenon, process digital information, store processed data and communicate among each other and base stations. Acoustic propagation in water is characterized by high and variable delays, fading effect, Doppler spread and multi path which in turn lead to a limited bandwidth and high error rates. Also, battery life and storage capacity of nodes is limited. So there is a need of suitable routing protocol that takes all these limitations into consideration and makes communication in underwater networks viable. This is the first attempt to analyze the performance of ad-hoc routing protocols in underwater acoustic network environment. We used performance metrics like packet delivery ratio, average end-to-end delay, throughput, routing overhead and energy consumption of the sensor nodes. AODV, DSDV, DSR and OLSR are compared for their performance at different traffic conditions, number of nodes and depths. By analyzing our simulation results, we found that among the discussed protocols, AODV may be used for denser networks but with less traffic and DSDV is suitable for higher traffic conditions with optimal number of nodes.

Distortion performance of underwater acoustic mobile networks

Localization of autonomous underwater vehicles (AUVs) is a very important and challenging task for the AUVs applications. In long baseline underwater acoustic localization networks, the accuracy of single-way range measurements is the key factor for the precision of localization of AUVs, whether it is based on the way of time of arrival (TOA), time difference of arrival (TDOA), or angle of arrival (AOA). The single-way range measurements do not depend on water quality and can be taken from long distances; however, there are some limitations which exist in these measurements, such as the disturbance of the unknown current velocity and the outliers caused by sensors and errors of algorithm. To deal with these problems, an AUV self-localization algorithm based on particle swarm optimization (PSO) of outliers elimination is proposed, which improves the performance of angle of arrival (AOA) localization algorithm by taking account of effects of the current on the positioning accuracy and eliminating possible outliers during the localization process. Some simulation experiments are carried out to illustrate the performance of the proposed method compared with another localization algorithm.