# A Power-Optimized Cooperative MAC Protocol for Lifetime Extension in Wireless Sensor Networks
## 6 Simulation Results
### Throughput  
From the above simulation results, the PO-CMAC protocol has a significant improvement on energy performance. However, in order to coordinate the contention and access of cooperative nodes, it introduces extra overhead, which unavoidably increases the transmission delay and reduces the network throughput. To resolve this decrease in throughput, for a high throughput requirement of the network, we can improve the data transmission rate between the sender and the recipient in the PO-CMAC protocol (i.e., improve the bandwidth utilization in the same bandwidth). The increase of the data transmission rate will increase energy consumption of each data packet transmission, thus leading to the reduction in network lifetime. However, compared to the DT protocol with a low data transmission rate, the PO-CMAC protocol still extends the network lifetime efficiently in this case. To illustrate this case, we give the results of the DT protocol with \(R=2\) bit/s/Hz, and the PO-CMAC protocol with \(R=2\) and 4 bit/s/Hz, respectively.  
Figure 20 shows the network throughput with the variation of offered traffic load. When \(R=2\) bit/s/Hz, the throughput of the PO-CMAC protocol is 0.6 and lower than that of the DT protocol (i.e., 0.68) due to extra control packet transmissions. However, when \(R=4\) bit/s/Hz, its throughput is 1.25, which is distinctly higher than that of the DT protocol.  
Figure 21 shows the corresponding energy efficiency performance. Because the network lifetime will contain a certain idle time when the network is unsaturated, we use the average number of data packets transmitted by each node to reflect the energy efficiency performance and network lifetime of each protocol. From the figure, when \(R=2\) bit/s/Hz, the PO-CMAC protocol has much higher average number of data packets transmitted by each node than the DT protocol. When \(R=4\) bit/s/Hz, the throughput of the PO-CMAC protocol increases, but its average energy consumption of each data packet transmission also increases, which leads to a decrease of the average number of data  
Figure 19: Throughput vs. \(M\).  
packets transmitted by each node. However, in this case, it is still higher than that of the DT protocol with \(R=2\) bit/s/Hz.  
Figures 20 and 21 also illustrate two conclusions. Firstly, when the data transmission rates between the sender and its recipient are the same, the throughput of the PO-CMAC protocol slightly decreases, but its network lifetime is significantly extended. Secondly, for a high requirement in terms of throughput, we can improve the equivalent data transmission rate between the sender and its recipient (i.e., improve the transmission powers of the sender and its cooperative nodes to achieve the expected bandwidth utilization). In this case, although the network lifetime of the PO-CMAC protocol decreases, it is still higher than that of the DT protocol, and its throughput is also much higher than that of the DT protocol. Namely, besides the better performance on network lifetime, the PO-CMAC protocol proposed in this paper can also balance the performance between the throughput and energy efficiency to achieve a significant overall advantage compared to the DT protocol.