# A Power-Optimized Cooperative MAC Protocol for Lifetime Extension in Wireless Sensor Networks
## 6 Simulation Results
### Network Performance at Uniform Traffic Load  
Figures 9-11 show the performance of the DT, EE-CR and PO-CMAC protocols at uniform traffic load, respectively. Herein, the number of nodes \(N\) in the network changes from 50 to 150. As shown in Figures 9 and 10, the PO-CMAC protocol outperforms the DT and EE-CR protocols in both network lifetime and the number of data packets transmitted by each node. Furthermore, in the case of uniform traffic load, the PO-CMAC protocol with \(M=1\) has the longest network lifetime and the most data packets transmitted by each node. When \(N=150\) and \(M=1\), the network lifetime and the number of data packets transmitted by each node of the PO-CMAC protocol are 56.17% and 39.24% higher than those of the EE-CR protocol, respectively. The advantage of the PO-CMAC protocol mainly originates from the following two aspects. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Parameter** & **Value** & **Parameter** & **Value** \\ \hline \(B\) & 10 kHz & \(P_{\text{max}}\) & 50 mW \\ \(N_{0}\) & \(-\)80 dBm & \(T_{\text{W}}\) & 100 \(\upmu\)s \\ \(T_{\text{R}}\) & 50 \(\upmu\)s & \(L_{\text{MAC}}\) & 272 bits \\ \(L_{\text{PHY}}\) & 192 bits & \(L_{\text{CRTS}}/L_{\text{NRTS}}\) & 160 bits \\ \(L_{\text{CCTS}}/L_{\text{HITS}}/L_{\text{ACK}}/L_{\text{NACK}}\) & 112 bits & \(L_{\text{OPD}}\) & 160 bits \\ \(L_{\text{DATA}}\) & 1000 bits & SIFS & 10 \(\upmu\)s \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Parameter settings. Figure 9: Network lifetime with varying \(N\) at uniform traffic load. 1. The cooperative transmission obtains spatial diversity gain by considering transmission link quality and transforming the direct transmission link from the sender to its recipient to the two-hop cooperative transmission links, i.e., from the sender to the cooperative nodes and from the cooperative nodes to the recipient. If the direct transmission link between the sender and its recipient is in deep fading, in traditional direct transmission, the sender can only increase the transmission power to guarantee the transmission reliability. However, in the cooperative transmission, considering the independent fading channel, the sender can select the cooperative nodes with the best cooperative transmission link to cooperate. Therefore, it can obtain diversity gain and guarantee the transmission reliability with low transmission power. As shown in Equations (22) and (23), in the cooperative node selection phase of the PO-CMAC protocol, the cooperative nodes with higher channel gain is chosen with higher priority, which ensures that the cooperative transmission fully explores the spatial diversity gain obtained by node cooperation and consumes less energy than the direct transmission. 2. Meanwhile, the PO-CMAC protocol considers the impact of the residual energy balance of the local network on the network lifetime. By balancing the energy consumption of nodes in local network, it avoids decreasing network lifetime due to the early energy exhaustion of some nodes. Figure 11: Network energy utilization with varying \(N\) at uniform traffic load. Figure 10: Number of data packets transmitted by each node with varying \(N\) at uniform traffic load. Usually when the cooperative transmission achieves spatial diversity, it also leads to extra energy consumption of some nodes due to their cooperative transmissions for other nodes, which aggravates the non-uniform energy consumption of the nodes in local network. However, in the PO-CMAC protocol, the LETPO algorithm makes the nodes with little residual energy consume energy as less as possible in the current data packet transmission by maximizing the minimum residual energy of each associated node, thus avoiding part of nodes consuming their energy too soon and balancing the energy consumption of the local network. Therefore, the PO-CMAC protocol outperforms the EE-CR protocol, which also adopts cooperative transmission manner. The results in Figure 11 also validate the above conclusions.