# A Power-Optimized Cooperative MAC Protocol for Lifetime Extension in Wireless Sensor Networks
## 1 Introduction  
Wireless sensor networks (WSNs) are widely used in many applications, such as environmental monitoring, traffic control, product quality monitoring, mineral survey and disaster relief [1]. One of its typical features is that each node can continuously work for a few months or up to a few years and its battery cannot be replaced after the exhaustion of battery energy. Therefore, how to prolong network lifetime in the case of constrained power and limited energy is one of key issues in WSNs. Most energy consumption of nodes in WSNs is in the process of communication, i.e., mainly in the aspects of multiple access for nodes to access and use channel, and their corresponding packet transmissions [2]. Therefore, the design of energy-efficient medium access control (MAC) protocols has become a research hotspot in WSNs [3; 4; 5]. To reduce the energy consumption and improve energy efficiency on multiple access, most MAC protocols use duty cycle mechanism [3]. Duty cycle can be adjusted to adapt to variable traffic load. In the variable load adaptive MAC (VLA-MAC) protocol [4], each node can estimate its traffic load through previous packet inter-arrival time and adaptively adjust its duty cycle. At high traffic load, each sender can transmit multiple data packets to its recipient without sleeping after the successfulexchange of request-to-send (RTS) and clear-to-send (CTS) packets. Other nodes go to sleep period after sensing the control packets or data packets. This mechanism reduces delay and unnecessary overhead on overhearing and reservation. At low traffic load, the sender can reserve channel in sync phase, and after successful reservation other unrelated nodes can go to sleep period early, which can adapt to dynamic traffic load and improve energy efficiency. To reduce energy consumption on packet transmissions, in the rate adaptive MAC (RA-MAC) protocol [5], each node estimates the channel gain and successful transmission probability of data packets by overhearing other nodes' packet transmissions, and sends data packets using the data rate with the minimum energy consumption, thus improving network energy efficiency. Meanwhile, in order to adapt to channel state and improve energy efficiency, the protocol reduces the data rate after a data packet transmission failure and increases the data rate after successful data packet transmission for a certain times. To achieve good energy efficiency and obtain low delivery latency for both normal and emergency packets, a TDMA-based quasi-sleep-preempt-supported (QS-PS) MAC protocol is proposed [6]. In the protocol, a tree structure is built for slot allocation in the starting phase. All nodes transmit their packets in the allocated slots, while entering the Q-Sleep mode in the other slots, which can reduce the clock rate of circuit for preserving some detecting ability. The node with emergency packets can broadcast a special designed m-sequence-based awakening message to wake up the whole network and preempts the right to use the current slot to transmit the packets without interference from other nodes by using a slot preemption strategy. This process continues until the emergency packets reach the root, thus fulfilling the fast delivery of the emergency packets. In recent years, both multi-input multi-output (MIMO) and cooperative communication techniques can efficiently improve energy efficiency at a certain throughput and bit error rate (BER), by taking advantage of the broadcasting nature of wireless communication and independent fading characteristic of wireless channel to obtain spatial diversity. The MIMO technique employs multiple transmitting and receiving antennas to achieve spatial diversity and thus improves the system capacity and reduces energy consumption. However, in WSNs, due to the limitation of size and cost, it is difficult to install multiple antennas on each node. Therefore, it cannot be widely used in WSNs.