# A Power-Optimized Cooperative MAC Protocol for Lifetime Extension in Wireless Sensor Networks
## 5 Power-Optimized Cooperative MAC Protocol
### Data Packet Transmission Phase  
In the data packet transmission phase, the data packets are transmitted from the sender to its recipient through two-hop manner, i.e., from the sender to the cooperative nodes and then from the cooperative nodes to the recipient. As shown in Figure 6, at the beginning of the data packet transmission phase, the sender broadcasts its data packet to its cooperative nodes and recipient using the optimal transmission power \(P_{\text{S}}\) calculated during the cooperative node selection phase. Since the optimal transmission power \(P_{\text{S}}\) is lower than \(P_{\text{D}}\), which is a threshold of correctly decoding in direct transmission, the recipient cannot decode the data packet except receiving and saving it. The cooperative nodes forward the data packet to the recipient using the optimal transmission powers declared in the OPD packet. Then the recipient decodes the data packet based on the data packet sent from the sender and those forwarded from the cooperative nodes by using MRC technique. If the recipient successfully decodes the data packet, it will reply with an acknowledgment (ACK) packet to the sender to confirm its successful data packet reception.  
If the recipient cannot decode the data packet successfully, the data packet needs to be retransmitted. Because the recipient has saved the data packets from the sender and the cooperative nodes, it only needs an extra data packet transmission in a certain power in order to correctly decode the packet without wasting too much energy. In the cooperative node selection phase, the cooperative nodes transmit their HTS packets to the sender in an order of their cooperative ability to participate in the cooperative transmission. Therefore, we should choose the cooperative node with the highest cooperative gain to retransmit the data packet.  
As shown in Figure 7, if a data packet is not correctly decoded, a negative ACK (NACK) packet is transmitted by the recipient. On receiving this packet, all nodes that participate in the current data packet transmission record that this is the first NACK packet transmitted by the recipient. Then, the first cooperative node to transmit its HTS packet during the cooperative node selection phase retransmits the data packet. Furthermore, the recipient decodes the data packet combined with the previously-received data packets by MRC technique. If succeeded, the recipient replies with an ACK packet. Otherwise, as shown in Figure 8, the recipient continues to transmit a NACK packet, and the second cooperative node to transmit its HTS packet during the cooperative node selection phase retransmits the data packet. This retransmission process repeats until the data packet  
Figure 6: Data packet transmission phase.  
is successfully decoded. If all selected cooperative nodes have retransmitted the data packet and the recipient still fails to decode the packet, the current data packet transmission fails and the sender will directly transmit the data packet to its recipient using the transmission power \(P_{\mathrm{D}}\).