# A Power-Optimized Cooperative MAC Protocol for Lifetime Extension in Wireless Sensor Networks
## 2 Related Work  
Cooperative MAC protocols tackle two key problems based on MAC protocols, i.e., when to cooperate and how to select the cooperative node(s). To resolve the first problem, it is critical to assess whether the node cooperation could improve the network performance and which kind of network performance is improved. The existing research work mainly focuses on two aspects of network performance, i.e., the network throughput and the transmission reliability [15]. According to different opportunities of selecting cooperation, cooperative MAC protocols can be divided into two categories. The first category decides whether node cooperation is needed during reservation phase of MACprotocols according to whether the node cooperation can obtain cooperative gain. The second category adopts cooperative retransmission after the failure of direct transmission. In the former cooperative MAC protocols, the sender or the cooperative nodes need to determine whether the cooperative transmission is adopted before the transmission of data packet. If at least one cooperative node can improve throughput or decrease the transmission delay, the cooperative transmission is initiated. Otherwise, the data packet is directly transmitted from the sender to the recipient. Typical protocols include the cooperative MAC (CoopMAC) protocol [16] and the two-relay-based cooperative MAC (2 rcMAC) protocol [17]. Furthermore, a packet piggyback mechanism is used in the cooperative MAC-aggregation (CoopMACA) protocol [18] and the active relay-based cooperative MAC (AR-CMAC) protocol [15] to allow the cooperative node to send its own data packet without reservation, thus avoiding reservation collision and further improving throughput. Based on the traditional cooperative node selection criterion (i.e., throughput improvement), the optimal relay selection MAC (ORS-MAC) protocol [19] also takes into account the transmission error probability, thus guaranteeing the reliability while improving the throughput. The latter cooperative MAC protocols adopt the concept of cooperative automatic repeat request (ARQ). If the direct transmission failed and there existed cooperative nodes to successfully receive the data packets, the cooperative transmission is employed to improve successful transmission probability. Typical protocols are the cooperative diversity MAC (CD-MAC) protocol [20], the differentiated cooperative MAC (DC-MAC) protocol [21], the cooperative MAC protocols used in time division multiple access (TDMA) systems [22; 23], the persistent relay carrier sensingmultiple access (PRCSMA) protocol [24] and the network coding-based cooperative ARQ MAC (NCCARQ-MAC) protocol [25]. In CD-MAC protocol, each sender selects its cooperative node by overhearing packet transmissions of its neighbors with respect to link quality, and retransmits the data packet using DSTC with the cooperative node if the direct transmission fails. In the cooperative TDMA protocols [22; 23], the idle cooperative node of the sender retransmits the failed data packets in its timeslot, which guarantees the transmission reliability and avoids the throughput reduction. The PRCSMA and NCCARQ-MAC protocols modify the basic rules of the IEEE 802.11 MAC protocol to execute a distributed cooperative ARQ scheme in wireless networks in order to enhance their performance [24; 25]. Network coding techniques are employed to benefit the energy efficiency of the NCCARQ-MAC protocol, as the number of the delivered bits increases by combining multiple data packets [25]. According to the method to select cooperative nodes, cooperative MAC protocols can be divided into three categories: the sender-specified method, the recipient-specified method, and the cooperative node contention method.