Abstract
Underwater networking technologies have brought us unforeseen ways to explore the unexplored aquatic environment and this way provided us with a large number of different kinds of applications for environmental, scientific, commercial, and military purposes. Although precise and continuous aquatic environment monitoring capability is highly important for various underwater applications, due to the unique characteristics of underwater networks such as low communication bandwidth, high error rate, node mobility, large propagation delay, and harsh underwater environmental conditions, existing solutions cannot be applied directly to underwater networks. Therefore, new solutions considering the unique features of underwater environment are highly demanded. In this chapter, the authors mainly focus on the use of wireless micro-electromechanical systems for underwater networks and present its advantages. In addition, the authors investigate the challenges and open research issues of wireless MEMS to provide an insight into future research opportunities.
TopIntroduction
Thanks to Underwater Networks (UNs), the desire and significant interest in monitoring aquatic environments are satisfied. UNs are used for various applications requiring precise and continuous data transfers including oceanographic data collection, pollution monitoring, scientific exploration, commercial exploitation, and coastline protection (Akyildiz, Pompili, & Melodia, 2005; Erol-Kantarci, Mouftah, & Oktug, 2010; Erol-Kantarci, Mouftah, & Oktug, 2011; Ren & Cheng, 2010; Han, Zhang, Shu, & Rodrigues, 2015). Although UNs and ground communication networks have common properties, they have two distinct differences. First and foremost, radio communications do not work well under the water and hence they are replaced by acoustic communications (Heidemann, Stojanovic, & Zorzi, 2011;
Akkaya & Newell, 2009). Second, although most ground nodes are static, underwater nodes are mostly mobile since they may move with water currents, waves, and other underwater activities (Heidemann, Stojanovic, & Zorzi, 2011).
Micro-Electromechanical Systems (MEMS) is a rapidly growing, innovative technology which combines micro computers with various tiny mechanical devices such as sensors, actuators, valves, gears, mirrors, and other parts in semiconductor chips. Basically, MEMS devices contain micro-circuitry on tiny silicon chips into which mechanical devices have been manufactured. MEMS is a foundational technology at the present and will continue to be so into the next decade; this technology has already successfully replaced many other devices in several applications since MEMS devices are built in large quantities at low cost and thereby are cost-effective for many uses.
Recently, the use of wireless MEMS in UNs has emerged as a powerful, reliable and low cost technique for many UN applications addressing monitoring, measurement, control, and surveillance needs (Yu, Ou, Zhang, Zhang, & Li, 2009; Chaimanonart & Young, 2006; Sarisaray-Boluk, Gungor, Baydere, & Harmanci, 2011). Due to the significant differences and unique features of UNs, this has received unprecedented interest, although the idea of applying wireless MEMS into monitoring and/or control applications is not new.
MEMS are miniature devices, which integrate processors, sensors, and actuators and their functional subsystems could be electronic, mechanical, optical, fluidic, or thermal. MEMS technology has numerous attributes, which can mitigate challenges to meet future military and non-military application requirements. For instance, MEMS allows miniaturization, reduced cost of fabrication through the use of microelectronics processing technologies, real-time control, and control of macro physical processes through micro action. Specifically, MEMS can enable enhanced functionality for the following applications:
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Inter-linked communication channels (radio frequency (RF) or optical);
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Optical devices and systems, and displays;
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Micro-satellite and unmanned surveillance systems;
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Multi-sensing capabilities; distributed, agent-based, or sensor array systems;
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Inertial Navigation Systems (INSs);
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Autonomous, unmanned ground sensors, detection and treatment systems
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Intelligent/unmanned operations;
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Power generators and management;
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Integrated fluidic systems including fluid sensing, control & transport;
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Nuclear and bio/chemical sensing;
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Micro thrusters;
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Fuze/safety & arming;
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Health monitoring and utilization monitoring systems;
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Logistic tagging systems;
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Self assembly/healing and reusable modules/subsystems.
Key Terms in this Chapter
Autonomous Underwater Vehicle: An Autonomous Underwater Vehicle is a programmable, robotic vehicle that, depending on its design, can drift, drive, or glide through the ocean without real-time control by a human operator.
Fuel Cell: A fuel cell is a device which generates electricity by a chemical reaction. Every fuel cell has two electrodes, the anode and cathode. Electricity is produced by the reactions taking place at the electrodes.
Underwater Network: A network which consists of a number of underwater nodes with onboard modems and a limited number of surface stations and/or autonomous underwater vehicles, and enables the accurate and energy efficient monitoring of aquatic environment.
Single-Chip RF Circuit: By the single-chip RF circuit approach, MEMS devices can be directly integrated on the RF chip itself, and this way several discrete components such as RF switches, varactor diodes, high Q-factor resonators and filters can be replaced.
Acoustic Modem: An acoustic modem offers the possibility of wireless communication under water. However, underwater acoustic communication is relatively slow compared to radio communication. This has to do mainly with the speed of sound in water which is roughly 1500 meters per second. The result is a relatively low baud rate.
MEMS above IC: MEMS above IC technology has developed in order to allow the elaboration of post-processed micro-machined passive components on top of other circuits. It presents many advantages, as it uses conventional equipments of microelectronics and is in adequation with high frequency applications.
Sensor Array: A sensor array is a group of sensors, usually deployed in a certain geometry pattern. Using a sensor array adds new dimensions to the observation, helping to estimate more parameters and improve the estimation performance.