# An Energy-Efficient and Robust Multipath Routing Protocol for Cognitive Radio Ad Hoc Networks
## 5 Performance Evaluation
### Simulation Results and Discussion
#### 5.3.2 Varying the Number of SUs  
We vary the number of SUs from 10 to 45 with a step size of 5 in a fixed network size and study its impact on the average network throughput, PDR, average energy consumption per bit, and average end-to-delay. The number of active sessions in each case is fixed at 10, making SUs mobile with speed randomly chosen between 0 and 5 m/s.  
As shown in Figure 10, it is observed that the average throughput of both protocols follows the same trend with increasing number of SUs. The throughput increases quite noticeably in the first half, resulting in the almost steady state in the second half. As the number of SUs increases, the throughput of ERMR is increasingly better than that of L2 ER. That is, at low SU density, the proposed ERMR outperforms the existing L2 ER protocol by approximately 15% and, at high SU density, the gap becomes approximately 30%. This allows us to infer that ERMR has better adaptability and robustness to the increase in the node density of SUs. This is mainly because the proposed ERMR with multiple paths effectively negates the impact of possible route breakages in mobile networks, and the number of channel switching events is significantly reduced in the multichannel environment.  
Figure 10: Network throughput vs. the number of secondary users (SUs).  
Figure 9: Average end-to-end delay vs. the number of sessions.  
Figure 11 depicts PDR as a function of SU density. With increasing number of SUs, the trend of PDR is very similar to that of throughput shown in Figure 10. That is, the plots gradually increase in the beginning, are seemingly steady in the middle, and slightly drop in the end. As in the case of throughput, the gap in the two PDR lines is low at low SU density and tends to widen with increased SU density because of better resilience to mobility effects. The lower the route failure, the greater the number of packets reaching the destination. In addition, the longer spectral duration also contributes to improving the successful packet transmissions because it provides healthy routes for a longer period.  
Figure 12 shows the energy consumption per bit with the variation in the number of SUs. It shows a gradual decline in the energy consumption per bit for both ERMR and L2 ER. This is because, although the average energy consumed per node is increased, the number of successfully delivered data packets is increased even further. That is, the increasing rate of data delivery is much better than that of energy consumption. Clearly, the proposed ERMR protocol proves its advantage in saving much energy owing to its reduced route discovery processes, robustness, and reduced channel switching activities.  
It is shown in Figure 13 that, with increasing number of SUs, the average end-to-end delay suffers a rapid increase in both protocols. Thanks to the better adaptability to the mobile network scenarios, however, the proposed ERMR has lower latency, fewer link failures, lower delays involved in the data transmission process, and less switching delay in the entire routing process.  
Figure 11: Packet delivery ratio vs. the number of SUs.  
Figure 12: Energy consumption per bit vs. the number of SUs.