Abstract- This paper presents an energy efficient selection of cooperative nodes with respect to their geographical location and the number of nodes participating in cooperative communications in wireless sensor networks. The cooperative communication in wireless sensor networks (WSN) gives us leverage to get the inherent advantages of its random node’s locations and the direction of the data flow. Depending on the channel conditions and the transmission distances, the number of cooperative nodes is selected, that participate in an energy efficient transmission/reception. Simulation results show that increasing the cooperative receive diversity, decreases the energy consumption per bit in cooperative communications. It has also been shown that the network backbone capacity can be increased by controlled displacement of antennas at base station at the expense of energy per bit. Index Terms- Cooperative communication, sensor network, energy efficiency, capacity, receive diversity. I. INTRODUCTION ireless sensor network consists of hundred or thousand of small, inexpensive wireless nodes responsible for monitoring a physical activity and reporting to the base station (BS), where end user can access the reported data. Cooperative communication has gain an obdurate place in wireless networks. It has been shown [1] that multiple-input multiple- output (MIMO) systems require less transmission energy than the single-input single-output systems. It’s still intricate to build multiple antennas on low-cost, small-sized sensor nodes; therefore MIMO techniques cannot be applied directly on wireless sensor networks. However it is possible to implement MIMO techniques in WSN without physically having multiple antennas at the sensor nodes via cooperative communication techniques [2][3], such distributed MIMO techniques can offer considerable energy saving even after allowing some extra circuit power, communication and training overheads. The use of cooperative communications in wireless sensor networks allows energy savings through spatial diversity gains [2]. Traditional MIMO utilizes fixed antenna arrays in transmitter/receiver. These arrays are of definite geometric shapes, e.g., one dimensional array, circular array, rectangular array etc, but in cooperative MIMO communications there is no pre-defined array of antennas, the cooperative nodes cooperate with each other at run time to send/receive data. Most of the recent research work in wireless sensor networks, modeled the wireless channel with rich scattered or Rayleigh fading channel model [2,3,4], which is suitable model for wireless communications in urban areas where dense and large man made buildings act as rich scatterers. Sensor networks are usually deployed in the areas far away from human population, e.g., in plain desert areas for surveillance, on volcano hills for early alerts and near sea-shore for storm alerts etc. In this paper, we have divided the WSN into two functional parts, i) whole sensor network except the backbone link is modeled with Ricean fading channel and ii) the backbone link (the link between base station and the cooperative nodes near the base station) is modeled by pure deterministic channel. Remainder of this paper is organized as follows. System model is presented in section II. Section III exploits the inherent advantages of wireless sensor network. Performance analysis in terms of energy efficiency and capacity of sensor network is presented in section IV. We show simulation results in Section V, and conclude in Section VI. II. SYSTEM MODEL The system model shown in figure.1, it consists of a cluster based sensor network. Sensor nodes are grouped for cooperative communications. The selection of group nodes is described in next section. In typical wireless sensor network scenarios as narrated above, large number of nodes are randomly deployed in an unattended area. Due to the presence of scatterers as well as line-of-sight component in those areas, Ricean fading channel model has been used to modeled the WSN, Ricean fading channel has both LOS and NLOS components [1] ( ) ( ) /1 1/ 1 H= H H w K K K + + + (1) Optimal Number of Energy Efficient Cooperative Nodes Selection in Wireless Sensor Networks- In Ricean Fading Environment Irfan Ahmed, Mugen Peng, Prof. Wenbo Wang Wireless Signal Processing and Network Lab, Beijing University of Posts and Telecommunications irfanahmed44@gmail.com, pmg@bupt.edu.cn, wbwang@bupt.edu.cn W 1-4244-1312-5/07/$25.00 © 2007 IEEE 2384