# An Energy Efficient MAC Protocol for Wireless Passive Sensor Networks
## 1 Introduction  
Wireless Sensor Networks (WSNs) are a kind of multi-hop ad-hoc network and are composed of large amounts of tiny and low-cost sensor nodes. Nodes have the capability to communicate and compute. They connect with each other wirelessly [1]. In WSNs, sensor nodes cooperate with each other under a certain topology to accomplish the goals of monitoring environments, gathering data, processing data, and communicating [2]. WSNs are classified into two types according to two different kinds of sensor nodes: active wireless sensor networks and passive wireless sensor networks [3]. At present, active wireless sensor networks called classic WSNs in this paper are widely used. Active wireless sensor networks are composed of active sensor nodes that are powered by batteries, so their energy is limited [4]. Meanwhile, Wireless Passive Sensor Networks (WPSNs) are composed of passive sensor nodes. Passive sensor nodes are powered via an external Radio Frequency (RF) source that provides infinite power [5]. In a word, the fundamental difference between classic WSNs and WPSNs is the way of power supply [6]. In order to ensure nodes to maintain work as much as possible when the RF source can not provide enough energy, the super capacitor has been applied to the design of passive sensor nodes [7], but it is not sure that nodes can acheive communication. In WSNs, Medium Access Control (MAC) protocols determine the usage of the wireless channel, which will ensure a reasonable allocation of cmunication resources for a large number of wireless nodes and avoid the collision of nodes while they transmitting data at the same time [8]. At present, nodes mainly consume the energy for hardware platforms. MAC protocols directly control the activities of the radio frequency transceiver and determine when to send out data or receive data [9]. The sameMAC protocol can be used in both classic WSNs and WPSNs. However, the same protocol has different effects in two networks. For example, the S-MAC protocol used in classic WSNs saves more energy than in WPSNs. WPSNs only focus on saving energy. Therefore, MAC protocols that cannot save much energy are not fit for WPSNs. Although the energy of WPSNs is infinite, the infinite energy is relative to infinite time. The energy per unit time is limited. For example, WPSNs can only provide 10 mA current, but 100 mA current is needed to achieve communication, so it is necessary to minimize energy to maintain daily work. MAC protocols for WPSNs only need to consider saving energy and do not need to consider delay, throughput, or other parameters. This is the difference in the design of MAC protocols for WPSNs.  
The four main sources of energy consumption in the MAC layer are idle listening, data collision, overhearing, and control overhead. Idle listening and data collision are the main factors leading to excessive energy consumption [10]. This paper mainly discusses data collision. One of the strategies to avoid data collision is to design effective backoff algorithms. Based on dynamically changing the contention window of communication, some classic backoff algorithms were previously proposed, but they mainly focused on throughput.  
This paper presented an energy efficient MAC protocol called the Improved Sensor MAC (IS-MAC) protocol based on dynamically changing the contention window. It compares the IS-MAC protocol, the Sensor MAC (S-MAC) protocol, and the IEEE 802.11 protocol using the NS2 simulator from two aspects of energy consumption and throughput. The remainder of this paper presents related work in Section 2, a brief description of the IS-MAC protocol in Section 3, the analysis of the simulation in Section 4, and concluding remarks in Section 5.