Adaptive Contention Window-based Cluster Head Election Mechanisms for Wireless Sensor Networks Li-Chun Wang * , Chung-Wei Wang * , and Chuan-Ming Liu + , * National Chiao Tung University, Taiwan + National Taipei University of Technology (NTUT), Taiwan Email : lichun@cc.nctu.edu.tw Abstract — The clustering architecture is essential in achieving the goal of energy efficiency for a wireless sen- sor network. In general, a clustering algorithm consists of the cluster head election and the cluster member as- signment mechanism. This paper proposes an adaptive contention window (ACW)-based cluster head election mechanism. Unlike other legacy cluster head election mechanisms such as LEACH (Low Energy Adaptive Clustering Hierarchy) protocol, the proposed ACW algorithm can achieve four major goals in cluster head election for wireless sensor networks: 1) high successful probability of cluster head election, 2) appropriate number of cluster heads, 3) uniform distribution of cluster heads, and 4) equal times to be a cluster head for each sensor, simultaneously. I. Introduction To design a cluster-based wireless sensor network (WSNs), a basic problem is how to distributively organize a larger number of sensor nodes into different clusters. In WSNs, in order to achieve the objective of energy- efficiency, the cross-layer design is necessary to achieve energy saving in each sensor node [1]. In general, a cluster formation algorithm consists of the cluster head election and the member assignment mechanism. In this work, we focus on the cluster head election problem in WSNs. The major goals of a cluster head election are four folds. We define the lifetime of a sensor network to be the time elapsed between the start of the system and the death of the first node (FND). First, the successful probability of head election must be as high as possible in order to save energy. Second, the number of elected cluster heads should be appropriate to enhance the network reliability. Third, the distribution of heads should be uniform. Fourth, each sensor node should becomes a cluster head with the same times in order to even the energy consumption. When the energy consumption is evened among all sensor nodes, no sensor node consumes more energy than other ones. Therefore, the lifetime can be extended. In this paper, we propose an adaptive contention win- dow (ACW)-based cluster head election mechanism to guarantee theses four concerns simultaneously. The legacy cluster head election mechanisms such as LEACH (Low The work was supported jointly by the National Science Council and the MOE program for promoting university excellence under the contracts EX-91-E-FA06-4-4, 93-2219-E009-012, and 93-2213-E009- 097. Energy Adaptive Clustering Hierarchy) protocol [2], only focuses on the forth concern, i.e., the equal times to be a cluster head for each sensor. We compare three different schemes based on the ACW-based head election algorithm: the short-term fairness, the medium-term and long-term fairness schemes. We simulate the upper bound and the lower bound of the lifetimes in the proposed ACW algorithm. From our results, we find that the short-term fairness scheme of ACW algorithm performs better than the medium-term and long-term fairness schemes of ACW algorithm in terms of network lifetime. The rest of this paper is organized as follows. In Section II, we analyzes the performance of head election in LEACH protocol. Section III shows our ACW-based cluster head election mechanism. Section IV analyzes ACW’s designing principle and shows some numerical results. Finally, we give our concluding remarks in Section V. II. Motivation and Cluster Head Election Criteria In this section, we discuss the design criteria for the cluster head election. For comparison, we analyze the performance of the head election algorithm in LEACH protocol. It is well know that the LEACH protocol can only guarantee the equal times to be heads for each sensor node. However, LEACH protocol cannot simultaneously guarantee the other concerns during the process of head election. A. Background on the Head Election Mechanism in LEACH Protocol In the LEACH protocol, each sensor node become the cluster head according to the probability related to the accumulative times of not being head before this round. The i th sensor in r th round be head with probability: T i (r)=  P 1-P ∗(r mod 1 P ) , C i (r)=1 0 , C i (r)=0 (1) where P is the desired percentage of cluster heads among all sensor nodes in the entire network, r ∈ [0, ∞] is the current round if the holding energy of each sensor node is infinite, and C i (r) is the indicator function determining whether the i th sensor node had been head in recent (r modulo 1/P ) rounds (i.e., C i (r)=0 if i th sensor had been 1819 0-7803-9152-7/05/$20.00 © 2005 IEEE