Abstract
Ocean bottom sensor nodes can be used for oceanographicdata collection, pollution monitoring, offshore explorationand tactical surveillance applications. Moreover, Unmannedor Autonomous Underwater Vehicles (UUVs, AUVs),equipped with sensors, will find application in exploration ofnatural undersea resources and gathering of scientific data incollaborative monitoring missions. Underwater acoustic networkingis the enabling technology for these applications. UnderwaterNetworks consist of a variable number of sensors and vehiclesthat are deployed to perform collaborative monitoring tasks overa given area.In this paper, several fundamental key aspects of underwateracoustic communications are investigated. Different architecturesfor two-dimensional and three-dimensional underwater sensornetworks are discussed, and the underwater channel is characterized.The main challenges for the development of efficientnetworking solutions posed by the underwater environment aredetailed at all layers of the protocol stack. Furthermore, openresearch issues are discussed and possible solution approachesare outlined.
I. INTRODUCTION
Ocean bottom sensor nodes are deemed to enable applicationsfor oceanographic data collection, pollution monitoring,offshore exploration and tactical surveillance applications.Multiple Unmanned or Autonomous Underwater Vehicles(UUVs, AUVs), equipped with underwater sensors, will alsofind application in exploration of natural undersea resourcesand gathering of scientific data in collaborative monitoringmissions. To make these applications viable, there is a needto enable underwater communications among underwater devices.Underwater sensor nodes and vehicles must possess selfconfigurationcapabilities, i.e., they must be able to coordinatetheir operation by exchanging configuration, location andmovement information, and to relay monitored data to anonshore station.Wireless Underwater Acoustic Networking is the enablingtechnology for these applications. UnderWater Acoustic SensorNetworks (UW-ASN) consist of a variable number ofsensors and vehicles that are deployed to perform collaborativemonitoring tasks over a given area. To achieve this objective,sensors and vehicles self-organize in an autonomous networkwhich can adapt to the characteristics of the ocean environment.The above described features enable a broad range ofapplications for underwater acoustic sensor networks:² Ocean Sampling Networks. Networks of sensors andAUVs, such as the Odyssey-class AUVs, can performsynoptic, cooperative adaptive sampling of the 3D coastalocean environment.² Pollution Monitoring and other environmental monitoring(chemical, biological, etc.).² Distributed Tactical Surveillance. AUVs and fixed underwatersensors can collaboratively monitor areas forsurveillance, reconnaissance, targeting and intrusion detectionsystems.Acoustic communications are the typical physical layertechnology in underwater networks. In fact, radio wavespropagate at long distances through conductive sea water onlyat extra low frequencies (30 ¡ 300 Hz), which require largeantennae and high transmission power. Optical waves do notsuffer from such high attenuation but are affected by scattering.Thus, links in underwater networks are based on acousticwireless communications [1].The traditional approach for ocean-bottom or ocean columnmonitoring is to deploy underwater sensors that record dataduring the monitoring mission, and then recover the instruments[2]. This approach has the following disadvantages:² Real time monitoring is not possible. This is criticalespecially in surveillance or in environmental monitoringapplications such as seismic monitoring. The recordeddata cannot be accessed until the instruments are recovered,which may happen several months after thebeginning of the monitoring mission.² No interaction is possible between onshore control systemsand the monitoring instruments. This impedes anyadaptive tuning of the instruments, nor is it possible toreconfigure the system after particular events occur.² If failures or misconfigurations occur, it may not bepossible to detect them before the instruments are recovered.This can easily lead to the complete failure ofa monitoring mission.² The amount of data that can be recorded during themonitoring mission by every sensor is limited by thecapacity of the onboard storage devices (memories, harddisks, etc).Therefore, there is a need to deploy underwater networksthat will enable real time monitoring of selected ocean areas,remote configuration and interaction with onshore humanoperators. This can be obtained by connecting underwaterinstruments by means of wireless links based on acousticcommunication.Many researchers are currently engaged in developing networkingsolutions for terrestrial wireless ad hoc and sensornetworks. Although there exist many recently developed networkprotocols for wireless sensor networks, the unique characteristicsof the underwater acoustic communication channel,such as limited bandwidth capacity and variable delays, requirefor very efficient and reliable new data communication protocols.The main differences between terrestrial and underwatersensor networks can be itemized as follows:² Cost. Underwater sensors are more expensive devicesthan terrestrial sensors.² Deployment. The deployment is deemed to be moresparse in underwater networks.² Spatial Correlation. While the readings from terrestrialsensors are often correlated, this is more unlikely tohappen in underwater networks due to the higher distanceamong sensors.² Power. Higher power is needed in underwater communicationsdue to higher distances and to more complexsignal processing at the receivers.Major challenges in the design of Underwater Acoustic Networksare:² Battery power is limited and usually batteries can not berecharged, also because solar energy cannot be exploited;² The available bandwidth is severely limited [3];² Channel characteristics, including long and variable propagationdelays, multi-path and fading problems;² High bit error rates;² Underwater sensors are prone to failures because offouling, corrosion, etc.In this survey, we discuss several fundamental key aspectsof underwater acoustic communications. We discuss the communicationarchitecture of underwater sensor networks as wellas the factors that influence underwater network design. Theultimate objective of this paper is to encourage research effortsto lay down fundamental basis for the development of newadvanced communication techniques for efficient underwatercommunication and networking for enhanced ocean monitoringand exploration applications.The remainder of this paper is organized as follows. InSection II, we introduce the communication architecture ofunderwater acoustic networks. In Section III, we investigatethe underwater acoustic communication channel and summarizethe associated physical layer challenges for underwaternetworking. In Section IV we discuss the challenges associatedto the design of a new protocol stack for underwater communications,while in Section V we draw the main conclusions.
II. UNDERWATER ACOUSTIC SENSOR NETWORKS(UW-ASN) COMMUNICATION ARCHITECTURE
In this section, we describe the communication architectureof Underwater acoustic sensor networks. The reference architecturesdescribed in this section are used as a basis for discussionof the challenges associated with underwater acousticFig. 1. Architecture for 2D Underwater Sensor Networks.sensor networks. The underwater sensor network topology isan open research issue in itself that needs further analyticaland simulative investigation from the research community.In the remainder of this section, we discuss the followingarchitectures:² Static two-dimensional UW-ASNs for ocean bottommonitoring. These are constituted by sensor nodes thatare anchored to the bottom of the ocean. Typical applicationsmay be environmental monitoring, or monitoringof underwater plates in tectonics [4].² Static three-dimensional UW-ASNs for ocean columnmonitoring. These include networks of sensors whosedepth can be controlled by means of techniques discussedin Section II-B, and may be used for surveillance applicationsor monitoring of ocean phenomena (ocean biogeo-chemical processes, water streams, pollution, etc).


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