# A Power-Optimized Cooperative MAC Protocol for Lifetime Extension in Wireless Sensor Networks
## 6 Simulation Results
### Network Performance at Nonuniform Traffic Load  
Figures 12-14 show the network lifetime, the number of data packets transmitted by each node and network energy utilization of the DT, EE-CR and PO-CMAC protocols at non-uniform traffic load, respectively. Herein, the packet arrival rates of half nodes are 1.5 packets/s and those of the other half are 0.5 packets/s. From the figures, at non-uniform traffic load, the PO-CMAC protocol still outperforms the DT and EE-CR protocols. When \(N=150\) and \(M=1\), the network lifetime and the number of packets transmitted by each node of the PO-CMAC protocol are 41.11% and 25.40% higher than those of the EE-CR protocol, respectively. Furthermore, when \(N=150\) and \(M=2\), they are 88.53% and 59.98% higher than those of the EE-CR protocol, respectively. This can be explained as follows. The non-uniform traffic load intensifies the energy consumption difference of different nodes and results in that the nodes with higher packet arrival rate exhausted their energy too early due to more data packet transmissions, thus reducing the network energy utilization and decreasing the network lifetime. However, the PO-CMAC protocol not only improves node energy efficiency through diversity gain obtained by node cooperation but also balances the local network energy consumption through the LETPO algorithm. Any nodes with low packet arrival rate can use more energy to cooperate the nodes with high packet arrival rate or other nodes. Therefore, the PO-CMAC protocol can reduce the difference of energy consumption caused by different packet arrival rates, make full use of the network energy and extend the network lifetime. Figure 14: Network energy utilization with varying \(N\) at non-uniform traffic load. Figure 12: Network lifetime with varying \(N\) at nonuniform traffic load. Figure 13: Number of data packets transmitted by each node with varying \(N\) at non-uniform traffic load. In addition, there are still some differences between the performance at uniform network traffic load and that at non-uniform network traffic load. Compared to the results at uniform network traffic load in Figures 9-11, the DT and EE-CR protocols have less performance at non-uniform traffic load. This is because different packet arrival rates of different nodes lead to the difference in energy consumption, and some nodes will exhaust their energy too fast due to too many data packet transmissions. The performance of the PO-CMAC protocol with \(M=1\) at non-uniform traffic load is also lower than that at uniform traffic load. According to Equations (22) and (23), when \(M=1\), the PO-CMAC protocol chooses only one cooperative node with the best link quality in cooperative node selection phase, and then the sender and the cooperative node transmit data packets in the optimal transmission powers calculated by the LETPO algorithm. However, at non-uniform traffic load, the residual energy of nodes differs greatly from each other. The selected cooperative node with the highest channel gain in cooperative node selection phase probably has consumed too much energy due to too many data packet transmissions. In this case, if the node still takes part in the cooperative transmission, it will speed up its energy consumption and will not balance the energy consumption of the local network, thus reducing the network lifetime. As seen from Figure 14, the energy utilization of the PO-CMAC protocol with \(M=1\) drops to between 0.6 and 0.7. As shown in Figures 12 and 13, the PO-CMAC protocol with \(M=1\), 2 and 3 outperforms other protocols because the performance advantage of the PO-CMAC protocol is derived not only from the diversity gain obtained by cooperative transmissions, but also from the effect of the LETPO algorithm on balancing the local network energy consumption. The LETPO algorithm balances the energy consumption of different senders and cooperative nodes in the process of each data packet transmission, which finally balances the energy consumption of the whole network.