Data Gathering in UWA Sensor Networks : Practical Considerations and Lessons from Sea Trials

Underwater acoustic (UWA) network protocol design is a challenging task due to several factors, such as slow propagation of acoustic waves, low frequency bandwidth and high bit error and frame error rates often encountered in real UWA environments. In this paper, we consider the design of a robust and scalable data gathering protocol for UWA sensor networks (UASNs), focusing on practical considerations and lessons learnt from multiple lake and sea trials. A cross-layer protocol is presented that integrates a network discovery process, intelligent routing, scheduling via Transmit Delay Allocation MAC (TDA-MAC) and multi-node Automatic Repeat Request (ARQ), to facilitate reliable data gathering in practical UASN deployments. Furthermore, this paper presents the details of a novel experimental testbed and underwater sensor node prototype that were used for the trials reported in this study. Based on the results of the trials, important conclusions are drawn on the protocol features required to achieve reliable networked communication in realistic UWA environments. The insights gained from the trials are valuable both for further development of the proposed data gathering protocol, and for the wider UWA networking research community concerned with developing practical solutions for real-world UASN deployments

LTDA-MAC v2.0 : Topology-Aware Unsynchronized Scheduling in Linear Multi-Hop UWA Networks

This paper investigates the use of underwater acoustic sensor networks (UASNs) for subsea asset monitoring. In particular, we focus on the use cases involving the deployment of networks with line topologies, e.g., for monitoring oil and gas pipelines. The Linear Transmit Delay Allocation MAC (LTDA-MAC) protocol facilitates efficient packet scheduling in linear UASNs without clock synchronization at the sensor nodes. It is based on the real-time optimization of a packet schedule for a given network deployment. In this paper, we present a novel greedy algorithm for real-time optimization of LTDA-MAC schedules. It produces collision-free schedules with significantly shorter frame duration, and is 2–3 orders of magnitude more computationally efficient than our previously proposed solution. Simulations of a subsea pipeline monitoring scenario show that, despite no clock synchronization, LTDA-MAC equipped with the proposed schedule optimization algorithm significantly outperforms Spatial TDMA

Target Detection Using Underwater Acoustic Networking

This paper presents a feasibility study for simultaneous underwater acoustic communication (UAC) and target detection using a network of underwater nodes. It can be achieved via anomaly detection in the estimated channel impulse response (CIR) of regular packet transmissions in the network. Such a network could serve as the first step in detecting and localising possible targets, which could then be followed up by the deployment of a sonar-equipped AUV to scan the identified area in more detail. The MAC layer based on Spatial Reuse TDMA (STDMA) fits the traffic requirements of such a network significantly better than contention-based MAC protocols. An enhancement of STDMA packet scheduling that utilises interference cancellation (IC) capabilities at the receivers can further increase the network throughput and, thus, the target detection performance. The simulation study shows that such an approach is feasible from the point of view of network throughput and the probability of the target ``crossing" an active acoustic path. Further work includes the integration of a more detailed acoustic environment model, and the development of a Network and Application Layer to deliver the detection information through the network and to enable target tracking

Environmental monitoring of coastal waters with a collaborative underwater acoustic and above water LoRaWAN sensor network

Climate changes are transforming the world as we know it and have a devastating impact on frail areas, such as coasts, afflicted by catastrophic events (rise in seawater temperature, floods) deteriorating the local biodiversity. Between the strategies undertaken to mitigate these effects, the EU Biodiversity Strategy for 2030 is one of the most ambitious. In particular, a relevant point is the inclusion of new solutions to monitor the conditions of the water, measuring specific parameters and polluting agents. However, up to today, there is no common ground when dealing with low-cost and low-power devices to collect data related to the quality of the water in coastal areas: a dense deployment of sensors would be the best option, but the technology used for long-range underwater acoustic communication is indeed extremely expensive. Nonetheless, in the last few years researchers have been investigating the possibilities given by low-cost and low-power acoustic modems, in the attempt to provide a way to employ dense deployment of underwater nodes. Another major turn in long-range low-power communications is the introduction of Low-Power Wide-Area Networks (LPWAN), which can be regarded as one of the most crucial entries in Internet of Things (IoT) applications. With this dissertation, we propose a network infrastructure for the tracking and the study of water quality parameters, to understand the impact they have on biodiversity. Specifically, we envision a system where there are two types of sensor nodes; one underwater and another on the water surface, forwarding the data they aggregate to one or more gateways. The gateways are connected to the Internet so that the data can be saved in a database for further processing. Underwater nodes use a part of the surface nodes as relays basing on an acoustic communication protocol, while the remaining surface nodes generate sensor data themselves; LoRa (together with LoRaWAN) has been chosen as the core LPWAN, enabling the long-range communication between the surface nodes and the gateways. Finally, the gateways are connected to the Internet with LTE standard. Simulations have been run to estimate the traffic requirements of the network as well as the feasibility of the system and a functioning prototype of a surface node has been developed. We selected a section of the Venice lagoon as reference area where our network could eventually be put in place, thus the simulations have been set according to this scenario. Also, the low-cost prototype has been tested and proved its full operativity

EFFICIENT DYNAMIC ADDRESSING BASED ROUTING FOR UNDERWATER WIRELESS SENSOR NETWORKS

This thesis presents a study about the problem of data gathering in the inhospitable underwater environment. Besides long propagation delays and high error probability, continuous node movement also makes it difficult to manage the routing information during the process of data forwarding. In order to overcome the problem of large propagation delays and unreliable link quality, many algorithms have been proposed and some of them provide good solutions for these issues, yet continuous node movements still need attention. Considering the node mobility as a challenging task, a distributed routing scheme called Hop-by-Hop Dynamic Addressing Based (H2- DAB) routing protocol is proposed where every node in the network will be assigned a routable address quickly and efficiently without any explicit configuration or any dimensional location information. According to our best knowledge, H2-DAB is first addressing based routing approach for underwater wireless sensor networks (UWSNs) and not only has it helped to choose the routing path faster but also efficiently enables a recovery procedure in case of smooth forwarding failure. The proposed scheme provides an option where nodes is able to communicate without any centralized infrastructure, and a mechanism furthermore is available where nodes can come and leave the network without having any serious effect on the rest of the network. Moreover, another serious issue in UWSNs is that acoustic links are subject to high transmission power with high channel impairments that result in higher error rates and temporary path losses, which accordingly restrict the efficiency of these networks. The limited resources have made it difficult to design a protocol which is capable of maximizing the reliability of these networks. For this purpose, a Two-Hop Acknowledgement (2H-ACK) reliability model where two copies of the same data packet are maintained in the network without extra burden on the available resources is proposed. Simulation results show that H2-DAB can easily manage during the quick routing changes where node movements are very frequent yet it requires little or no overhead to efficiently complete its tasks

Medium Access Control in Distributed Networks with Large Propagation Delay

Most of the Earth is covered by water, so underwater acoustic networks (UWANs) are becoming increasingly popular in a variety of undersea applications. The needs to understand the underwater environment and exploit rich undersea resources have motivated a further development of UWANs. Underwater acoustic signals suffer from more difficult physical channel phenomena than terrestrial radio signals due to the harsh underwater environment, such as sound absorption, time-varying multipath spread, man-made and ambient noise, temperature and pressure dependent refraction, scattering and Doppler shift. Among all the challenges, the large ratio of propagation delay to packet duration (relative propagation delay (a)) is arguably the most difficult one to address in the Medium Access Control (MAC) layer. In this dissertation we focus on the examination and improvement of the MAC layer function in UWANs, based on a critical examination of existing techniques. Many MAC techniques have been proposed in recent years, however most of them assume the ratio of the propagation delay to the packet duration is negligibly small (a>1), these protocols perform poorly. This is because the large a leads to both a large negotiation delay in handshaking based protocols and the space-time uncertainty problem as the packets do not arrive at each node contemporarily. Some underwater-oriented protocols have been proposed which attempt to address these issues but the more successful rely on master nodes or a common understanding of geometry or time. We show by analysis and simulation that it is possible to eliminate collisions in ad-hoc networks with large relative propagation delay (a>>1) as well as improving the channel utilisation, without a common understanding of geometry or time. This technique is generally applicable, even for truly ad-hoc homogeneous peer-to-peer networks with no reliance on master nodes or other heterogeneous features. The mechanism is based on future scheduling with the inclusion of overhearing of RTS messages and allowing third-party objections to proposed transmissions. This MAC mechanism is immediately applicable in underwater acoustic networks (UWANs), and may find other uses, such as in space or very high rate terrestrial wireless networks. In summary, the key contributions of this study are: investigating the causes of the poor performance of existing MAC protocols in ad-hoc UWANs with large relative propagation delay, fully detailing the problem in order to propose analytic solutions, demonstrating how the MAC layer of an ad-hoc UWAN can eliminate packet collisions as well as improve channel utilisation without time synchronization or a network’s self-configuring phase to gain knowledge of the geometry, and verifying the utility of the proposals via both theoretical analysis and simulations