# A Power-Optimized Cooperative MAC Protocol for Lifetime Extension in Wireless Sensor Networks
## 1 Introduction
In this case, the cooperative communication technique can transform the direct transmission link to the cooperative transmission link with better channel quality by exploiting cooperative nodes to help the sender forward data packets to its recipient, which constructs a virtual MIMO system and reduces energy consumption of data packet transmissions. Moreover, it can optimize local nodes' transmission power and energy consumption as a whole and reasonably allocate transmission energy consumption among the sender and its cooperative node(s), thereby balancing local nodes' energy consumption. Therefore, the cooperative communication technique can be used in WSNs to improve energy efficiency and extend network lifetime. Early research work on cooperative communication techniques primarily focuses on the physical (PHY) layer [7; 8]. In order to improve system capacity, decrease BER or reduce energy consumption, cooperative nodes can relay the data packet from the sender to its recipient through amplify-and-forward or decode-and-forward schemes [7], or collaborate with the sender to transmit the data packets to the recipient with the help of distributed space-time coding (DSTC) scheme [9]. Recently, more and more research work has applied cooperative communication techniques to high layers, such as MAC layer [10; 11; 12; 13] and network layer [1; 13; 14], thereby transforming the diversity gain obtained by the PHY layer to the higher layers. The MAC layer is close to PHY layer and its related protocol is adopted to coordinate multiple nodes sharing the channel, exchanging information and realizing cooperative transmissions, and thus avoiding packet collisions, improving throughput and energy efficiency and reducing delay [15]. Therefore, the design of cooperative MAC protocols has significant influence on the effect of transforming diversity gain to the higher layers. Cooperative MAC protocols for wireless local area networks and mobile ad hoc networks are well studied. However, only a few cooperative MAC protocols are designed for WSNs to improve energy efficiency and extend network lifetime. Motivated by this, we proposed a power-optimized cooperative MAC (PO-CMAC) protocol for WSNs in this paper. The main work includes two aspects. First, to prolong the lifetime of the WSNs with limited energy, we propose the lifetime-extended transmission power optimization (LETPO) algorithm based on the greedy algorithm and linear programming algorithm. This algorithm balances the energy consumption of the local network by maximizing the minimum residual energy of all the nodes participating in each data packet transmission. Second, based on the algorithm, we propose the PO-CMAC protocol resolving the problem that coordinates multiple nodes to access and use the wireless channel. With the help of a proposed cooperative node selection mechanism, the protocol can choose the nodes with the best channel quality to take part in the cooperative transmissions, thereby reducing the energy consumption of data packet transmissions and improving the nodes' energy efficiency. The main contributions of this paper are summarized as follows:  
* We consider both energy efficiency (i.e., the average consumed energy during each data packet transmission and the average number of data packets transmitted by each node) and energy utilization (i.e., the ratio of consumed network energy during network lifetime to the total network energy) to extend network lifetime and transmit more data packets in our proposed PO-CMAC protocol. With the help of the proposed LETPO algorithm, the protocol can balance the energy consumption of different nodes and improve the network energy utilization. Additionally, by using the proposed cooperative node selection mechanism, the protocol can select the cooperative nodes with the highest channel gain to participate in each cooperative transmission to reduce the average energy consumption of data packet transmissions and improve the energy efficiency of every node.