# A Power-Optimized Cooperative MAC Protocol for Lifetime Extension in Wireless Sensor Networks
## 2 Related Work
In early cooperative MAC protocols, such as the CoopMAC protocol [16] and the relay-enabled distributed coordination function (rDCF) protocol [26], the sender is responsible for the selection of the cooperative nodes based on a best cooperative node table (i.e., CoopTable) maintained by overhearing the packet transmissions of its neighbor nodes. Before the beginning of data packet transmissions, the sender looks up the CoopTable and applies the preselected best cooperative node to the cooperative transmission. In the recipient-specified cooperative MAC protocols, the recipient is responsible for selecting the best cooperative node for cooperative transmissions, like the on-demand cooperation with a selection of relays initiated by the destination equipment (OXIDE) protocol [27]. In this protocol, each recipient maintains a best cooperative node table based on its overhearing of packet transmissions. If the direct transmission failed, the recipient selects the best cooperative node from the table and informs it to retransmit the data packet. The drawback of using the best cooperative node table is that constructing the table consumes the extra computing and storage resources, and the information of the best cooperative nodes is possibly out of date. In recent cooperative MAC protocols, such as 2 rcMAC protocol [17], CoopMACA protocol [18], the rapid cooperation-differentiated MAC (RCD-MAC) protocol [28], the priority-differentiated cooperative MAC protocol with contention resolution (CRP-CMAC) [29] and the cooperative MAC protocol with rapid relay selection (RRS-CMAC) [30], each potential cooperative node contends to participate in the cooperative transmission based on the instantaneous channel information. The better the channel quality is, the more possible it becomes the final cooperative node. This method is easy to operate and the information of the best cooperative node is not out-of-date. To avoid the packet collisions in the process of cooperative node contention and improve the MAC performance of cooperative transmissions, collision resolution schemes [28; 29; 30] are used in this method. From the above, we can see that most current cooperative MAC protocols mainly focus on improving the throughput and transmission reliability other than improving the energy efficiency. Considering that saving energy is the main challenge in WSNs, the cooperative MAC protocols for the purpose of improving the energy efficiency and extending the network lifetime are more and more proposed. The objective of energy-efficient cooperative MAC protocols is twofold: optimize energy consumption for each data packet transmission and optimize the entire network lifetime. The first takes advantage of diversity gain generated by node cooperation to reduce the energy consumption for each data packet transmission by choosing the best cooperative node and thus improves the energy efficiency of entire network at a certain throughput and packet error rate (PER) [31; 32; 33; 34]. Opportunistic energy-efficient (OEE) relay selection cooperative communication scheme [31] defines the energy efficiency of data packet transmissions as the ratio of the channel capacity to the transmission power, and selects the nodes with the highest energy efficiency for cooperative transmissions to achieve the objective of improving the energy efficiency. [PERSON] et al. [32] select two groups of neighboring nodes near the sender and the recipient, respectively, to construct a virtual MIMO system. In this process, there may be several different groups of nodes satisfying the PER requirement. The protocol can select the two cooperative groups with the least energy consumption for cooperative transmissions to improve energy efficiency. The link-utility-based cooperative MAC (LC-MAC) protocol [33] considers both throughput and energy efficiency. It calculates the weighted sum of the achieved throughput and needed energy by cooperating with a certain cooperative node and defines it as the link-utility of this cooperative link.