# An Experimental Study on D2D Route Selection Mechanism in 5G Scenarios
## 6 Results and Discussion
### Packet Delivery Ratio Comparison between D2D and MC  
Figure 8 compares the packet delivery ratio of a D2D route (i.e., route 1 (\(fc_{s}\)-\(fc_{1}\)-\(fc_{4}\)-\(fc_{d}\)) with 450 s out of 600 s (or 75%) of PUs' OFF time) and the backbone route (i.e., \(fc_{s}\)-\(mc\)-\(fc_{d}\)) in order to understand the intrinsic differences between these two types of routes and their effects to packet delivery ratio. In this scenario, there is a single D2D route with three hops and its packet delivery is affected by PUs' activities. The backbone route has two hops and packet delivery is not affected by PUs' activities. In the D2D route, each intermediate node, uses a separate RP3 processing unit. Each intermediate node, which is placed apart from other intermediate nodes, has different transmission range and uses different transmission frequencies within the VHF bands and has different transmission ranges. For instance, in route 1, the first intermediate node \(fc_{1}\) transmits in 860 MHz and has a proximity of 5 meters to the source node \(fc_{s}\) and to another intermediate node \(fc_{4}\), respectively. Comparing the two routes, the packet delivery ratio is 76.6% in the D2D route and 89.8% in the backbone route. Hence, the packet loss rate is 23.4% in the D2D route and 10.2% in the backbone route. The difference is caused by three main reasons: (a) the D2D route has a higher number of hops (i.e., three hops) compared to the backbone route, which can increase packet loss (b) intermediate nodes (i.e., RP3) in the D2D routes have a lower processing capability compared to MC BS (i.e., PC) in the backbone route causing a higher heterogeneity; and (c) the D2D route uses cognitive channels with a certain amount of white spaces, while the backbone route uses cellular channels without the presence of PUs' activities, which can increase end-to-end delay and packet loss. It is noteworthy that control messages are transmitted via the wireless medium in D2D routes, while they are transmitted via cables in the backbone route. Therefore, there is less interference from the operating environment to control message exchange in the backbone route. However, due to the small packet size of a control message, this effect is negligible. Figure 7: Packet delivery ratio of the backbone and D2D routes. This investigation presents the cumulative end-to-end delay of both D2D and backbone routes. The cumulative end-to-end delay increases as packets are delivered from a source node to a destination node with increasing number of hops. There are two main investigations, and their results are presented in Figure 9. First, the cumulative end-to-end delay over each of the three available D2D routes in the absence of PUs' activities. Route 1 has an end-to-end delay of 0.1063 s, route 2 has 0.1176 s, and route 3 has 0.1298 s. Secondly, the cumulative end-to-end delay over each of the three available D2D routes in the presence of PUs' activities. Each D2D route has different levels of PUs' activities (see Section 6.1). Route 1 has an end-to-end delay of 0.1501 s, route 2 has 0.1653 s, and route 3 has 0.1792 s. Meanwhile, the backbone route does not have PUs' activities in all cases, and it has an end-to-end delay of 0.079 s. The backbone route has the least cumulative end-to-end delay due to its lower number of hops and the higher processing capability of the MC BS. Comparing the scenarios with and without PUs' activities is higher due to the random reappearance of PUs' activities and route switches. Table 4 presents a summary comparison of the measurements made in both D2D and backbone routes with and without PUs' activities.