Opportunistic Carrier Sensing for Energy-Efficient Information Retrieval in Sensor Networks
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Opportunistic Carrier Sensing for Energy-Efficient Information Retrieval in Sensor Networks Qing Zhao Department of Electrical and Computer Engineering, University of California, Davis, CA 95616, USA Email: [email protected]
Lang Tong School of Electrical and Computer Engineering, Cornell University, Ithaca, NY 14853, USA Email: [email protected] Received 26 January 2005 We consider distributed information retrieval for sensor networks with cluster heads or mobile access points. The performance metric used in the design is energy efficiency defined as the ratio of the average number of bits reliably retrieved by the access point to the total amount of energy consumed. A distributed opportunistic transmission protocol is proposed using a combination of carrier sensing and backoff strategy that incorporates channel state information (CSI) of individual sensors. By selecting a set of sensors with the best channel states to transmit, the proposed protocol achieves the upper bound on energy efficiency when the signal propagation delay is negligible. For networks with substantial propagation delays, a backoff function optimized for energy efficiency is proposed. The design of this backoff function utilizes properties of extreme statistics and is shown to have mild performance loss in practical scenarios. We also demonstrate that opportunistic strategies that use CSI may not be optimal when channel acquisition at individual sensors consumes substantial energy. We show further that there is an optimal sensor density for which the opportunistic information retrieval is the most energy efficient. This observation leads to the design of the optimal sensor duty cycle. Keywords and phrases: sensor networks, distributed information retrieval, opportunistic transmission, energy efficiency.
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INTRODUCTION
A key component in the design of sensor networks is the process by which information is retrieved from sensors. In an ad hoc sensor network with cluster heads/gateway nodes, sensors send their packets to their cluster heads using a certain transmission protocol [1, 2, 3]. For sensor networks with mobile access [4, 5], data are collected directly by the mobile access points (see Figure 1). In both cases, a population of sensors (those in the same coverage area of an access point) must share a common wireless channel. Thus, an information retrieval protocol that determines which sensors should transmit and the rates of transmissions needs to be designed for efficient channel utilization. Distributed information retrieval allows each sensor, by itself, to determine whether it should transmit and the rate of transmission. One such example is ALOHA in which each sensor flips a coin (possibly biased by its channel state) to This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
determine whether it should transmit [6, 7]. Another example is a fixed TDMA schedule by which each senso
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