# Energy-Efficient Device Discovery Mechanism for Device-to-Device Communications in 5G Networks +
## 5 Evaluation Results
While in the direct D2D discovery scheme, the collision probability is irrelevant to \(r\). However, when \(r\) increases, more target UEs are located within the discovery range and thus the discovery success rate grows with \(r\) in the direct D2D discovery scheme. Notably, in Figure 8 the discovery success rate cannot be greater than 25% because only those target UEs located within the probing range of source UEs can be discovered and the ratio of such target UEs is bounded by \(d^{2}/R^{2}\) = 0.25. Using the analytic result in Equation (8), the discovery success rate significantly grows with the ratio \(d^{2}/R^{2}\), as shown in Figure 9, where all parameters are similar to Figure 8 except \(R\). That is, the proposed discovery mechanism is more suitable to be implemented in the network scenarios with smaller cells such as 5G networks. Figure 7: Discovery delay of different schemes. Based on the results of Figures 6-8, the following conclusions are made. First, the proposed two-phase hybrid D2D discovery mechanism significantly improves the performance of the direct D2D discovery scheme, no matter in the energy consumption, discovery delay or discovery success rate. Second, the binary exponential backoff algorithm without frozen backoff time achieves better efficiency of resolving collisions and shorter discovery delay than the inverse proportional transmission probability algorithm does. Therefore, the proposed two-phase hybrid D2D discovery mechanism is an outstanding design. In the second example, the impact of retry limit \(R_{T}\) on the performance of D2D discovery mechanisms is studied. Figures 10-12 depict the analytic results of the energy consumption, discovery delay, and discovery success rate of the proposed D2D discovery mechanism under various retry limits \(R_{T}\)'s. In Figure 10, the average number of transmitted beacons of a UE increases almost linearly with the retry limit \(R_{T}\). While the discovery delay in Figure 11 is almost an exponential function of \(R_{T}\). Furthermore, there exists a tradeoff between the discovery success rate and discovery delay (or energy consumption) according to Figures 10-12. However, using the analytic results, one can find a proper retry limit \(R_{T}\) to maximize the discovery success rate under the constraint of energy consumption or discovery delay. For example, in Figure 12 the improvement in the discovery success rate  
Figure 8: Discovery success rates of different schemes. Figure 9: Impact of the ratio \(d^{2}/R^{2}\) on discovery success rate (\(d=100\) m). becomes insignificant when \(R_{T}\) is over 5. Hence, it is suggested that \(R_{T}\) is set no more than 5 in order to preserve a better discovery delay and energy consumption performance. In the third example, the impact of radio interference from Wi-Fi access points (APs) and their connected devices on the performance of the proposed D2D discovery mechanism is studied. There are six Wi-Fi APs uniformly distributed over each cell. Each channel (among channels 1, 6, and 11) is used by two APs. There are six fully loaded UEs connected to each Wi-Fi AP all the time. Other parameters not mentioned here are the same as those in the first example except \(R_{T}=3\). The performance of the proposed D2D discovery mechanism with Wi-Fi AP interference is compared to that without Wi-Fi AP interference. Figures 13-15 show the energy consumption, discovery delay, and discovery success rate of the proposed D2D discovery mechanism under the scenario with or without Wi-Fi AP interference. In Figures 13-15, the performance of the proposed D2D discovery mechanism under the scenario with Wi-Fi AP interference slightly degrades, compared with that without Wi-Fi AP interference. This is because more contentions and collisions may occur in the scenario with Wi-Fi AP interference, thus leading to performance degradation. However, since the binary exponential backoff algorithm used in the proposed scheme can effectively resolve collisions, the performance degradation due to radio interference is alleviated.