Range-Based Density Control for Wireless Sensor Networks Yang-Min Cheng Li-Hsing Yen Dept. Computer Science & Information Engineering Chung Hua University Hsinchu, Taiwan 300, R.O.C. {cs88625, lhyen}@chu.edu.tw Abstract Density control in a wireless sensor network refers to the process of deciding which node is eligible to sleep (enter power-saving mode) after random deployment to conserve energy while retaining network coverage. Most existing ap- proaches toward this problem require sensor’s location in- formation, which may be impractical considering costly lo- cating overheads. This paper proposes a new density con- trol protocol that needs sensor-to-sensor distance but no location information. It attempts to approach an optimal sensor selection pattern that demands the least number of working (awake) sensors. Simulation results indicate that the proposed protocol is comparable to its location-based counterpart in terms of coverage quality and the reduction of working sensors. 1. Introduction Rapid progress in wireless communications and micro- sensing MEMS technology has enabled the deployment of wireless sensor networks. A wireless sensor network con- sists of a large number of sensor nodes deployed in a region of interest. Each sensor node is capable of collecting, stor- ing, and processing environmental information, and com- municating with other sensors. The positions of sensor nodes need not be engineered or predetermined [1] for the reason of the enormous number of sensors involved [3] or the need to deploy sensors in in- accessible terrains [1]. Due to technical limitations, each sensor node can detect only events that occur within some range from it. A piece of area in the deployment region is said to be covered if every point in this area is within the sensory range of some sensor. The area that are collectively covered by the set of all sensors is referred to as network coverage. As sensor nodes are usually powered by batteries, power- conserving techniques are essential to prolong their opera- tion lifetimes. In this paper, we are considering powering off redundant sensors temporarily after random deployment to conserve energy while retaining network coverage. Den- sity control is a process deciding which node is eligible to sleep (entering power-saving mode), while node scheduling arranges the sleep time. Existing approaches toward density control are mostly location-based [8, 7, 6, 12, 4, 9], meaning that these ap- proaches require location information of sensors. Location- based density control algorithms can ensure 100% network coverage. However, the requirement of location informa- tion may not be practical if energy-hungry GPS (Global Po- sitioning System) device is assumed for this purpose. There are other approaches that control density based on the count of working neighbors [10], the current node density [6], or the network coverage expected [11]. These approaches de- mand no locating devices and are thus more suitable for small-size sensors. However, it is intrinsic that 100% net- work coverage cannot be guaranteed. This paper proposes a new density control protocol that needs no location information. It attempts to approach an optimal sensor selection pattern that demands the least number of working (awake) sensors. Our approach needs sensor-to-sensor distance information, which can be ac- quired by some range measurement technique. We con- ducted extended simulations for performance comparisons among our protocol and other counterparts. The results in- dicate that our protocol performs nearly well as a location- based scheme can do in terms of coverage quality and the reduction of working sensors. The rest of this paper is organized as follows. The next section reviews existing density control protocols and Sec- tion 3 details our work. Experimental results are presented in Section 4. The last section concludes this paper. 2. Related Work and Motivation PEAS [10] is a node density control protocol that de- mands no location information. In PEAS, all nodes are