# A Power-Optimized Cooperative MAC Protocol for Lifetime Extension in Wireless Sensor Networks
## 3 Network Model  
In WSNs, all nodes share a wireless channel to exchange control packets and data packets. Each node can estimate the channel gain between itself and other nodes based on the signal-to-noise ratio (SNR) of received control packets. For simplicity, we assume that the communication channels between any two nodes are bidirectionally symmetrical, i.e., they have the same channel gain, and the channel is invariant during the transmission session of each data packet. The recipient can decode data packets sent from the sender and its cooperative nodes by maximum ratio combining (MRC) technique.  
Each node in WSNs has a half-duplex transceiver with the maximum transmission power \(P_{\text{max}}\). Its transmission power can be continuously adjusted within the range of \(P_{\text{max}}\) according to the actual requirement. All nodes have the same initial energy, and their batteries cannot be charged if exhausted. If the residual battery energy of a node is insufficient to send the current packet, we consider that its battery energy is exhausted. For simplicity, this paper only considers the energy efficiency of WSNs in the transmission of information, and ignores the energy consumed in sensing and data processing.  
Figure 1 shows the cooperative transmission model adopted in this paper. During the current data packet transmission, there are several one-hop neighbor nodes (i.e., R\({}_{1}\), R\({}_{2}\) and R\({}_{3}\)) of both the sender S and its recipient D, and the current energy of these nodes are \(E_{\text{S}}\), \(E_{\text{D}}\), \(E_{\text{R}_{1}}\), \(E_{\text{R}_{2}}\) and \(E_{\text{R}_{3}}\), respectively. In traditional direct transmission model, the sender S transmits its data packets to its recipient D directly. The corresponding channel gain is \(h_{\text{SD}}\), and can be obtained by the recipient from receiving cooperative RTS (CRTS) packet and specified in its cooperative CTS (CCTS) packet for the sender and cooperative nodes. In the cooperative transmission model, the sender S transmits its data packets with a lower transmission power than that in the direct transmission model, and the recipient cannot decode the data packets but receive and save the data packets. Some common neighbor nodes of S and D (such as R\({}_{1}\), R\({}_{2}\) and R\({}_{3}\) in Figure 1) can decode the data packets successfully, and forward them to the recipient. The corresponding channel gains are \(h_{\text{SR}_{i}}\) and \(h_{\text{R}_{\text{D}}}\) (\(i=1,2,3\)), respectively. Channel gain \(h_{\text{SR}}\) can be obtained by the cooperative node from receiving CRTS packet and specified in its help-to-send (HTS) packet for the sender and recipient. Channel gain \(h_{\text{RD}}\) is assumed to be equal to channel gain \(h_{\text{DR}}\), which can be obtained by the cooperative node from receiving CCTS packet. For asymmetric wireless channels, the sender and cooperative node can obtain channel gain \(h_{\text{RD}}\) by this way: the recipient obtained channel gain \(h_{\text{RD}}\) from recently received control packets or data packets from the cooperative node and specifies this value in its CCTS packet. The recipient can use the MRC technique to decode the data packets from the sender and these cooperative nodes. This two-hop transmission method is the cooperative transmission model of multiple nodes used in this paper.  
In order to guarantee the recipient successfully decoding the data packets, improve the node energy efficiency and extend the network lifetime, the key issues in this cooperative transmission model include how much transmission power the sender should use to transmit data packets, which common neighbor nodes should take part in the cooperative transmission, and how much transmission power each cooperative node should use to forward the data packets. To resolve these issues, we propose the LETPO algorithm and PO-CMAC protocol.  
Figure 1: Cooperative transmission model.