Major Challenges of Underwater Acoustic Sensor Networks (UASNs)
Author: Gimer Cervera
A Underwater Acoustic Sensor Network (UASN) consist of devices or vehicles enabled with acoustic communication capabilities that are deployed underwater at different depths to perform collaborative monitoring tasks . Moreover, Autonomous Underwater Vehicles (AUVs), equipped with sensors, will enable the exploration of natural undersea resources and gathering of scientific data in collaborative monitoring missions. UASNs are envisioned to enable applications for oceanographic data collection, pollution monitoring, offshore exploration, disaster prevention, assisted navigation and tactical surveillance applications . Thus, underwater nodes should gather and send information to a sink that is also equipped with a radio to communicate with other network components located on the surface.
“UASNs consist of devices enabled with acoustic communication capabilities deployed underwater to perform collaborative monitoring tasks”
Research in UASNs focuses on both physical and link layers [3, 4], whereas research on the network layer is still in an early stage. The design of an efficient routing mechanism should consider the limitations of the medium. The underwater acoustic channel is characterized by a high bit error rate, low data rate and large propagation delay. Underwater routing protocols must be energy-aware since the deployment and maintenance of underwater devices are particularly difficult. Moreover, UASNs are formed by nodes in constant motion that leads to continuous changes in the network topology.
Major Challenges of UASN
Electromagnetic and optical waves do not work well underwater due to the nature of the medium, especially in the case of seawater. Acoustic waves are used for underwater communication due to the relatively low attenuation (i.e., signal reduction) of sound in water, mainly in thermally stable, deep water settings. In shallow water, acoustic waves are severely affected by temperature, site specific noise and multipath propagation due to reflection and refraction.
“UASNs are envisioned to enable applications for oceanographic data collection, pollution monitoring, offshore exploration, disaster prevention, assisted navigation and tactical surveillance applications.”
The speed of sound in water varies according to the depth and is affected by temperature, salinity and pressure. The speed of acoustic waves is about 1500 m/s  close to the ocean surface, which is more than four times faster the speed of sound in air, but five orders of magnitude smaller than the speed of light . Compared with electromagnetic and optical waves in terrestrial networks, the speed of acoustic waves is significantly lower. As a consequence, underwater channel communication is also affected by a severe Doppler effect.
Routing in UASNs
Several routing protocols in a UASN are based on a greedy hop-by-hop method for packet delivery . Unlike the end-to-end routing, greedy hop-by-hop routing approaches select as next hop those one-hop neighbors that have positive progress toward the sink. Nevertheless, greedy routing protocols do not guarantee to find a path toward the sink, e.g., data packets reach a node with no positive progress . This problem is known as communication void. Routing protocols for a UASN are classified as location-based or location-free.
Location-based approaches assume that nodes know both their geographical position and the sink position. However, finding the location information of nodes is a main challenge due to the inapplicability of GPS under the water. Location-free approaches are classified in pressure-based or beacon-based categories. In pressure-based routing protocols, the depth information (i.e., pressure) is used to identify the positive forwarding area. Beacon-based approaches implement beacon messages with information to reach the sink, e.g., distance in hops. Generally, greedy routing protocols in UASNs do not consider the quality of the links.
There exist many major challenges in the design of a UASN. We should consider that the available bandwidth is extremely limited, the underwater channel is highly impaired, especially due to multi-path and fading; consider that underwater propagation delay is five orders of magnitude higher than in radio frequency terrestrial channels, and extremely variable, high bit error rates and temporary losses of connectivity can be experienced; battery power is limited and usually batteries cannot be recharged, also because solar energy cannot be exploited. Finally, there is also important to take into account that underwater sensors are prone to failures because of fouling and corrosion.
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