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3 - PHY and MAC layer optimization for energy-harvesting wireless networks
- from Part I - Communication architectures and models for green radio networks
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- By Neelesh B. Mehta, Indian Institute of Science, India, Chandra R. Murthy, Indian Institute of Science, India
- Edited by Ekram Hossain, University of Manitoba, Canada, Vijay K. Bhargava, University of British Columbia, Vancouver, Gerhard P. Fettweis, Technische Universität, Dresden
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- Book:
- Green Radio Communication Networks
- Published online:
- 05 August 2012
- Print publication:
- 05 July 2012, pp 53-77
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Summary
Introduction
In typical wireless communication systems, nodes are deployed with pre-charged batteries. The energy stored in the battery is used by the node for communication, sensing, and signal processing tasks. However, when the battery drains out, the node “dies” and is no longer available in the network. When a sufficient number of nodes die, the network itself becomes dysfunctional. Therefore, periodic maintenance and battery replacement are necessary to ensure that the network continues to operate. Such maintenance is often operationally challenging or even impossible in several network deployments. The alternative option of running power cables to the nodes is also often infeasible.
An emerging green alternative that circumvents this problem is the use of energyharvesting (EH) functionality in the nodes [1]–[7]. An EH node harvests energy from the environment using solar, thermoelectric effects, vibration, and other phenomena. Unlike a conventional node, an EH node that drains out its battery can harvest energy later and, thus, again become “alive.” Energy harvesting eliminates the need for periodic battery replacements and significantly decreases the network maintenance overhead. This also makes it an attractive alternative to wireless networks that are powered by external power cables. Consequently, EH networks are finding applications in monitoring systems in aerospace, automobile, and civil applications, environmental/habitat monitoring, intrusion detection, inventory management, etc.
Unlike a conventional battery-operated node, a potentially infinite amount of energy is available to an EH node, albeit over an infinite duration of time. Hence, the focus of the physical layer and multiple access (MAC) layer protocols shifts to judiciously utilizing the harvested energy and ensuring that the energy is available when required – to the extent possible. Reducing the energy consumption and improving the spectral efficiency now become secondary goals of the design of these protocols. This motivates a redesign of the physical and multiple access layers of the network. For example, increasing the transmit energy improves the reliability of transmissions by a node as it counters noise. However, it also drains the node’s battery faster and lowers the odds that the node can transmit later. In a multi-node network, an additional multiple access problem that arises is the determination of which node(s) should transmit and when to transmit.
11 - Cooperative communications for reliability
- from Part II - Selected topics for improved reliability
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- By Andreas F. Molisch, University of Southern California, California, USA, Stark C. Draper, University of Wisconsin-Madison, Wisconsin, USA, Neelesh B. Mehta, Indian Institute of Science (IISc), Bangalore, India
- Edited by Ismail Guvenc, Sinan Gezici, Bilkent University, Ankara, Zafer Sahinoglu, Ulas C. Kozat
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- Book:
- Reliable Communications for Short-Range Wireless Systems
- Published online:
- 01 June 2011
- Print publication:
- 24 March 2011, pp 293-325
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Summary
Chapter 11 describes how teams of wireless nodes can work together to improve the reliability of signaling. Due to the inherent uncertain, time-varying, and shared nature of the wireless environment (reflected in shadowing, small-scale fading, and interference), it is difficult to achieve extremely high reliability over a single wireless link even when advanced signal-processing techniques such as diversity and multiuser detection are employed. However, since wireless transmissions are inherently broadcast – overheard by all nodes within range – a natural approach to reliability is to develop cooperative techniques. Cooperative techniques exploit in parallel many helper nodes, called relays, to increase the diversity of the available wireless links. These techniques can yield large improvements in reliability and throughput as well as large decreases in energy consumption.
The chapter starts with an overview of various cooperative communications methods that can be employed depending on the level of channel state information (CSI) and device synchronization. The chapter then considers two techniques in more detail: relaying using virtual beamforming and rateless codes. In both cases, we start out with an analysis of a “fundamental building block” that consists of one source, a number of parallel relays, and one destination. In the virtual beamforming technique, the relays rebroadcast the source signal that they have decoded. Relays adjust their transmission amplitudes and phases to ensure that their transmissions interfere constructively, maximizing the destination's signal-to-noise ratio (SNR). In the rateless coding approach, the relays individually decode the source message.
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