# A Power-Optimized Cooperative MAC Protocol for Lifetime Extension in Wireless Sensor Networks
## 2 Related Work
Each candidate calculates the optimal transmission power and data rate for participating in cooperation to make its link-utility the largest. The node with the largest link-utility among all candidates will access channel firstly and take part in cooperation after contention, which results in the optimum throughput and energy efficiency of the entire network. The authors in [34] adopt the model of cooperative retransmission and random network coding. The sender repeatedly transmits its data packet until its recipient or one of its cooperative nodes successfully receives it. If the recipient receives the data packet firstly, the data packet transmission ends. Otherwise, the cooperative node repeatedly transmits the data packet until the recipient successfully receives it. The protocol can obtain the average energy consumption based on the successful transmission probability of data packets among the sender, recipient and cooperative node, and minimize it by optimizing the transmission power of the sender and cooperative node. Thus, it can achieve the objective of energy efficiency improvement. However, in this type of cooperative MAC protocols, residual energy information (REI) of nodes is not considered and part nodes in the network drain out their energy more quickly than the others because of more frequent cooperative transmissions, thus decreasing the network lifetime. The second category of cooperative MAC protocols takes advantage of cooperation among nodes to balance local network energy consumption, avoids some nodes consuming their energy too quickly and makes full use of each node's energy in the whole network, thereby improving network energy utilization and extending network lifetime [35; 36; 37]. Given that the nodes with higher transmission rate consume more energy than those with lower transmission rate due to more frequent cooperative transmissions, the lifetime extended cooperative MAC (LECMAC) protocol [35] reduces the opportunity of a node to participate in cooperation transmissions when its residual energy is low by increasing its cost function to extend the network lifetime. However, this protocol balances the local network energy consumption only when the residual energy is very little. Therefore, its effect to extend the network lifetime is limited. The authors in [36] employ the greedy algorithm to optimize transmission power of the sender and a single cooperative node in order to balance the energy consumption of the local network, improve energy utilization and extend network lifetime. However, this protocol only considers the residual energy of nodes and does not consider the average energy consumption on each data packet transmission. Therefore, it cannot guarantee that the entire network can transmit more data packets in the presence of the same initial energy. That is to say, the protocol does not consider the energy efficiency of data packet transmissions. The delay- and energy-aware cooperative MAC (DEC-MAC) protocol [37] does not adopt transmission power optimization and attempts to balance the energy consumption of the sensor nodes by taking into account a node's residual energy and packet delivery delay as part of the relay selection metric, thus increasing the network lifetime. Its relay selection algorithm exploits the process of elimination and the complementary cumulative distribution function for determining the most optimal relay within the shortest time period. Both the wireless sensor network cooperative MAC (WcoopMAC) protocol [38] and the total power control MAC (TPC-MAC) protocol [39] consider both the energy efficiency of each node and the energy utilization of the entire network. In these distributed cooperative MAC protocols, the transmission powers are optimally allocated to source and relay nodes with the objective of minimizing the total transmission power under the average BER constraint.