A Distributed Scheduling Algorithm for Underwater Acoustic Networks With Large Propagation Delays

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 networks can have large propagation delays as compared to typical packet durations, as a result of the low speed of sound in water. The ill effects of large propagation delay on medium access control (MAC) are well known. Conventional MAC protocol design for such networks focuses on mitigation of the impact of propagation delay. Most proposed protocols to date achieve, at best, a throughput similar to that of the zero propagation delay scenario. We have explored the possibility that propagation delays can be exploited to make throughput far exceed that of networks without propagation delay and shown that the throughput of a N-node wireless network with propagation delay is upper bounded by N/2. In a small set of illustrative network geometries, we can manually determine transmission schedules that allow us to achieve this N/2 bound. However, for a given network, the problem of determining transmission schedules that maximize throughput is as yet unsolved. In this paper, we put forward an algorithm that generates transmission schedules with high throughput for arbitrary network geometries.

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.

Optimizing the number of relays for energy efficient multi-hop covert underwater acoustic cooperative networks

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 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.

The capacity analysis of the existing underwater acoustic network mainly refers to the analysis method of a terrestrial wireless network. Network capacity is affected by many factors, such as node distribution, transmission delay, transmission strategy, and link layer characteristics. Aiming at the challenges of the capacity of the underwater acoustic network, a closed solution for one-hop transmission of any node based on the 3-D stochastic underwater acoustic network under the protocol model is derived, the closed solution takes into account the fact that the vertical transmission efficiency of the underwater acoustic signal is more effective than the horizontal transmission efficiency, it is in line with the channel characteristics of underwater acoustic communication, and with different network parameters, there is an optimal communication radiation range of nodes to maximize the throughput of the network. Aiming at the problem that some relay nodes in the underwater acoustic network run out of energy prematurely due to overuse, a model predictive control method based on maximum algebra is proposed, which can predict the information generation rate of nodes periodically and estimate the life value of nodes at different routing time points according to the current residual energy of nodes.

The large propagation delay of underwater acoustic signals significantly affects the throughput performance of underwater communication networks. While past research focused on mitigating the impact of large propagation delays, recent work has suggested exploiting large delays. In this paper, we consider an underwater linear unicast network which employs a time- division based scheduling strategy to exploit large propagation delays to improve network throughput. We assume the protocol model in a network with partially overlapping collision domains, where the transmission range is normalized as 1 and the interference range is an integer k. We systematically discuss the throughput of the linear networks with single traffic flow, showing that the average throughput of an N-node linear network with single traffic flow cannot exceed (N-1)/k. We then propose a general transmission scheduling strategy that can achieve the throughput upper bound and also give some examples of the optimal schedules.

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.

The underwater acoustic (UWA) channel causes large propagation delays and reduces the bit error rate (BER) of wireless communication systems. The t-distribution is the optimal distribution to perform UWA noise. In this study, polar-coded differential chaos shift keying (DCSK) and quadrature chaos shift keying (QCSK) communication with UWA noise are considered. First, we have proposed a PDF for the UWA noise channel, and based on this PDF, the theoretical BER is derived. Second, polar coding’s performance is determined to demonstrate the improvement in the BER performance compared to the uncoded UWA system by means of Monte Carlo simulations. The experimental results prove that the nearest model that is applicable to the UWA channel is a t-distribution with five and six degrees of freedom. The BER formulas of the proposed systems are derived and compared with the simulation results. The results confirm the performance improvement of the polar-coded chaotic modulation systems over uncoded systems in UWA channels.

In the design of media access control (MAC) mechanism in underwater acoustic sensor networks (UASNs), due to the propagation characteristic of low-speed underwater acoustic signals, it is necessary to solve the spatial–temporal uncertainty problem. In order to avoid the multi-user access conflict in underwater networks, reduce the energy cost and improve the throughput and fairness across the network, a dynamic channel negotiation MAC mechanism based on spatial–temporal mapping of receiving queue (DCN-MAC) was proposed. DCN-MAC uses a duty cycle mechanism and implements a network management based on dynamic single node wake-up. The awakening node collects the request to send (RTS) and network status information in the network to solve the problem of space-temporal uncertainty and the highly dynamic needs of network access nodes and access services. The simulation results show that in different network scenarios, especially in those featuring high density and heavy network load, compared with the traditional underwater acoustic network MAC protocols, this protocol can effectively improve the network throughput and reduce the packet loss probability caused by multi-node conflict.

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.

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.

Underwa ter Acoustic Networks (UANs) have become a focus of interest for emerging scientific research and military applications. Recent work has shown that performance of existing security attacks are sensitive to network topology. In this paper, we utilize the mobility of Autonomous Underwater Vehicles (AUVs) to discover the topology of UANs by monitoring the broadcast patterns of geographic routing protocols. In this way, a mobile attacker can take advantage ofthe geographic information used in UANs to improve attack performance. We evaluate our approach in Aqua-Sim and results show that attack performance of jamming is significantly improved. Index Terms-Underwater Acoustic Networks, Localization, Security, Network Discovery