Neighbor Discovery in Wireless Sensor Networks

Abstract

Wireless sensor networks (WSNs) are widely used in many applications such as air pollution monitoring, natural disaster prevention, health care monitoring, etc. The sensor nodes are deployed in the monitored area and they form a wireless network through communication with nearby sensors. In this chapter, we introduce the fundamental process in constructing a wireless sensor network, which is called neighbor discovery, where the sensors can find nearby neighboring sensors when their distance is within a threshold. There are two main reasons for studying the neighbor discovery problem. First of all, the deployed sensors as a configuration may vary dynamically according to different reasons. For example, new sensors may be added and old ones removed; and some types of sensors have the ability to move around inside the area. Therefore, a sensor may need to find nearby neighboring sensors when they move into its communication range or some new sensors are deployed in the vicinity. The second reason is the limited power supply. In most situations, sensors are powered by battery and their energy is very limited. For example, suppose a sensor is powered by a 1200 mAh battery, and the processor consumes 2 mA under full power and the radio consumes 20 mA when it is turned on. If the sensor keeps the radio on and computing never stops, the lifetime of the sensor is a little more than two days. Therefore, sensors need some special method to save energy in order to extend their lifetime. A trivial way is to add sleeping mode, where a sensor keeps silent in a sleep state for most of the time, wakes up for work for only a small fraction of the time. Practically, in sleep mode, the processor’s power consumption drops to $2\,\upmu$A and the radio power drops to $1\,\upmu$A, and thus lifetime can be extended significantly. If the sensor wakes up for only $1\%$ of the time and sleeps for $99\%$ of the time, the estimated lifetime would be half a year. If the wake-up portion is $0.1\%$ of the time, the lifetime can be over five years. Therefore, we assume sensors are silent for most of the time, and they wake up mainly for data collection and communication. The neighbor discovery problem is to design the schedules of the sensors’ sleep mode and wake mode, such that two nearby sensors can be in wake mode at the same time to find each other, which is a kind of rendezvous problem. In this chapter, we first propose a motivational example in Sect. 19.1, and formulate the problem in Sect. 19.2. We introduce the trivial brute force algorithm for the neighbor discovery problem in Sect. 19.3 and another two algorithms: relaxed difference set based algorithm and co-prime algorithm in Sects. 19.4 and 19.5 respectively. Finally, we summarize the chapter in Sect. 19.6.