## 1 Introduction  
The self-organizing nature of micro sensors in Wireless Sensor Networks (WSNs) and their ability to operate without support of predetermined infrastructure makes them effective for data gathering in a variety of areas, even in harsh environments. Energy consumption however remains one of the main design challenges in WSNs, due to the limited energy resource that is supplied by the batteries in the sensor node. It is usually unfeasible to recharge or replace the batteries once the sensor nodes have been deployed due to inaccessible terrains and enormous deployment scale [1]. A thoughtful design of WSNs is required to provide significant benefit to network lifetime by being energy efficient. Research has shown that the sensor node utility that drains the most energy is the radio module during communication mode [2]. The causes of energy waste in the radio module of the sensor node have been identified mainly as idle listening, collisions, overhead and overhearing.  
MAC protocols are employed to arbitrate access to the shared medium to avoid different causes of energy waste, and at the same time to efficiently share the medium resources among multiple sensor nodes [3]. Energy efficient MAC protocols control the duty cycle of the sensor nodes, based on the availability of traffic, minimizing idle listening leading to reduced energy waste [1,4]. MAC protocols use efficient schedulers to adapt to different traffic patterns of the network. Most schedulers are based on the sensor node traffic without considering the energy remaining in the nodes. The use of the remaining energy of sensor nodes in determining sensor nodes' schedule is important in improving the network energy performance. The large scale deployment of sensor nodes also contributes to the high communication packets overhead in WSNs, as all nodes report their sensed data to the base station, which results in a lot of energy waste. To alleviate this problem, clustering has been widely adopted in the design of WSNs [5]. Clustering reduces the number of transmissions to the base station as the CH is responsible for the communication of each cluster. Clustering is known for its scalability as it provides load balancing and efficient resource utilization by grouping the nodes within a geographical neighborhood into a cluster.  
WSNs utilize the wireless channel for their communication, and therefore are prone to fading due to the unreliable nature of the wireless channel. This leads to packet errors, which requires packet retransmission, resulting in energy waste. Currently researchers employ cooperative diversity to mitigate the effects of fading in WSNs, by using relay nodes to cooperate in the communication [6]. The enhanced reliability through the use of cooperating terminals reduces the need for retransmission which effectively leads to energy conservation. Cooperative diversity utilizes relay selection to choose the best relay nodes based on the channel quality of the relay link, and seldom based on the combination of distance and residual energy of the relay nodes which is also a vital instrument for the efficiency of WSNs.  
The aim of this work is to design an energy efficient MAC protocol for cluster-based wireless sensor networks, based on distance, residual energy, and channel quality, to improve the energy and delay performance of WSNs. The remainder of this paper is organized as follows: related work is discussed in Section 2, followed by the system model and detailed description of proposed MAC protocol in Section 3. Section 4 presents description of the analytical channel and energy model for proposed protocol. The simulation based performance and analytical performance evaluation is discussed in Section 5, and lastly the conclusions are presented in Section 6.