Cross-Layer Framework for Power Conservation in Wireless Ad Hoc Networks: A Case Study

Introduction

A critical design issue for future wireless ad hoc networks is to develop suitable communication architectures, protocols, and services that efficiently reduce power consumption of wireless devices and thus increasing their operational lifetime in the network. In wireless ad hoc networks, power conservation can be achieved in two different ways: power control algorithms and power management algorithms. Power management in ad hoc networks means that a station can decide when to switch its radio transceiver off to save energy. However, turning its transceiver off means that other stations cannot send their packets to the station, if any. Therefore, it should be done with caution to avoid possible delays. The power management issue is challenging especially in a multihop ad hoc network because hosts may sometimes lose synchronization. On the other hand, power control refers to the technique of tuning hosts transmission powers to the proper range to conserve energy. In addition, power control can also reduce interference and improve spatial reuse of wireless channels. The scope of the chapter is on power control algorithms in wireless ad hoc networks.

A wireless node in an ad hoc network has limited battery power; therefore, it is important to reduce its energy consumption. A wireless node has four modes: transmit, receive, idle, and sleep. It consumes most power in the transmit mode. In the idle mode, it needs to sense the medium and consumes a little less than in the receive mode. The sleep mode consumes very little and can be ignored compared to the other modes (Nilson, 2001). To save energy, one direction is to force a node to enter the sleep mode when it is not necessary to be awake (Jamieson, Balakrishnan, & Morris, 2001; Jung & Vaidya, 2002). The other direction is to apply a Transmission Power Control (TPC) protocol (Gomez, Campbell, Naghshineh, & Bisdikian, 2001; Agarwal, Krishnamurthy, Katz, & Dao, 2001; Pursley, Russell, & Wysocarski, 2000; Jung & Vaidya, 2002). This chapter is focused on the second direction. TPC looks for the minimum required transmit power between the transmitter and the receiver.

TPC is a simple and fundamental technique which is used for communications and impacts the operational lifetime of devices in different ways. Reducing the transmission power to significantly improve operational life time for some devices may not be significant if the transmission power accounts only for a small percentage of the overall power consumed, (e.g., a Wireless Local Area Network [WLAN] radio attached to a notebook computer). In contrast, for small computing devices with built-in or attached radios (e.g., sensors) reducing the transmission power may significantly extend the operational lifetime of a device, thus, improving the overall user experience.

The power control problem in wireless ad hoc networks is that of choosing the transmit power for each packet in a distributed fashion at each node (Kawadia & Kumar, 2005). Selecting power level fundamentally affects the network at various layers as follows: