## 5 Performance Evaluation
### Tuning of the Multihop P2 MP Settings
#### 5.1.1 Varying Application-Level Settings
Of course, larger Trickle intervals result in larger delays, since UEs wait longer before relaying a message. The value of \(K\) has a negligible impact on the latency of the broadcasting. If our primary objective is to provide fast diffusion of a message, we should then select a short Trickle interval and a small value of the threshold \(K\). For the above reasons, we now set the Trickle parameters to \(K=3\) and \(I=10\) ms, and compare the performance of the with and without Trickle broadcasts. Figure 13, left, shows that adding Trickle increases the mean delay: in fact, Trickle adds delay before a relaying is attempted, and prevents some UEs from relaying the message at all. These delays add up at each hop. However, Trickle allows significant resource savings, quantifiable in about half the RBs. Figure 11: Average number of application-level transmissions with different settings of the Trickle algorithm. Figure 12: 95 th percentile of application-level delay with different settings of the Trickle algorithm. We then evaluate the performance of MDB against unicast eNB relaying. As far as the latter is concerned, we assume that the source UE sends the message to the eNB, which relays it within its cell using unicast DL transmissions, and sends it to its neighboring eNB using the X2 interface. We envisage two options to geofence the broadcast: one (which is called eNB relaying) assumes that the eNBs relay the message to all UEs in their cell, hence the target area consists of a number of cells. The other solution (called enhanced eNB relaying) consists in selecting the subset of UEs to target in each cell based on their geographic position. In both cases, the eNBs must be endowed with an application layer that sends and receives messages. With eNB relaying, the eNB application in the originating cell must read the message payload, and--specifically--the GPS coordinates of the source UE and the target radius, to understand which other neighbors to contact, if any. The eNB applications in the other cells will just receive the message and request the unicast forwarding. With the enhanced eNB relaying, instead, all eNB applications must also read the GPS coordinates and radius, find out which UEs are within the target area, and request the unicast relaying. This entails knowing the position of the UEs in the cell. As already stated, this can be achieved leveraging MEC solutions, but it can be expected to have a cost, in terms of added communication latency and overhead. Figure 14 reports the 95 th percentile of the delay and the allocated RBs in the DL subframe. As expected, the broadcast is completed sooner using unicast relaying, since UEs can be reached in two radio hops plus, possibly, a (fast) X2 traversal. However, the cost in terms of allocated resources is non-negligible: a broadcasting occupies 230 RBs in the DL subframe, whereas MDB does not require the eNB to use the DL spectrum at all. Note that, in a 10 MHz deployment, using 50 RBs per TTI, unicast relaying would stall the DL for 4-5 consecutive TTIs, which is unadvisable. Figure 14: 95 th percentile of delay (**left**) and average allocated RBs in downlink (DL) (**right**). Figure 13: Average delay (**left**) and average allocated RBs (**right**), w and w/o Trickle.