# Modeling Network-Controlled Device-to-Device Communications in SimuLTE
## 6 Performance Evaluation
### Evaluation of D2D-Enabled Simulation Scenarios  
Adding D2D communications to SimuLTE allows one to analyze how different parameters and algorithmic choices affect the performance at different layers of the protocol stack. We discuss three exemplary scenarios: In Scenario 1, we show that selecting different values of the interference radius affects the allocation of resources and the resulting SINR when frequency reuse among D2D transmissions is enforced. In Scenario 2, we highlight the interplay between mode switching of D2D communications and upper protocol layers, i.e., transport and application layers. More in detail, we observe that different mode-switching policies within the LTE protocol stack imply different performance of applications running on top of UDP and TCP protocols. In Scenario 3, we show that a careful selection of the transmission horizon for P2 MP D2D communications leads to significant speedup of the execution time, without affecting the accuracy of the results. **Scenario 1--Selection of the Interference Radius**  
We now evaluate the effects of using a Conflict Graph (CG) approach to make decisions about frequency reuse in D2D resource allocation. Figure 19 shows the simulation scenario, where we observe the SINR perceived by the receiver of one D2D connection, located 50 m from its transmitter. The observed D2D connection (the black UEs in the figure) is surrounded by two tiers of interferers, deployed at 100 and 200 m, respectively, from the receiving UE. For ease of reading, only the transmitting endpoints of the interfering connections are displayed in the figure. Each transmitter sends a 100-byte packet every 20 ms. We vary the number of interferers from 8 to 32. The main simulation parameters are reported in Table 4, column _Scenario 1_. To highlight the effects of the CG,  
Figure 18: Execution time for an increasing system packet rate, obtained with a fixed number of 30 UEs. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Base Value** & \multicolumn{2}{c}{431.45825} \\
**95\% Conf. Int.** & \multicolumn{2}{c}{\(\pm\)3.41} \\ \hline  & **Relative** & **Absolute** \\ \hline \#UE-pairs & 97.98\% & 255.18 \\
**System packet rate** & 2.01\% & 36.58 \\
**\#UE-pairs x System packet rate** & 0.005\% & \(-\)1.77 \\
**Unexplained** & 0.001\% & - \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Factorial analysis for execution time. we disable channel fading, so that the received power depends only on the distance to the transmitter. We vary the interference radius \(r_{i}\) used to build the CG, setting it at 50, 150 and 250 m: with \(r_{i}=50\) m, the CG is empty and the tagged D2D connection can share resources with all the other transmitters; with \(r_{i}=150\) m, the first tier of interferers is marked as conflicting; and, with \(r_{i}=250\) m, both tiers are marked as conflicting, hence the tagged D2D connection transmits in mutually exclusive RBs. Figure 20 shows that setting \(r_{i}\) to 150 m allows a relatively large number of simultaneous transmissions with only a limited decrease in the SINR. Figure 20 also confirms that, although the CG allows multiple transmitter whose distance is larger than \(r_{i}\) to share the same RBs, the cumulative interference is correctly accounted for in the simulations. D2D-capable UEs. The initial distance between the two UEs is 20 m, and they start their communications on the SL, denoted by the dashed arrow. During the simulation, they move in opposite directions until they reach a distance of 200 m, hence their SL CQI decreases. After about 3 s (marked by a vertical dashed line in the figures), the SL CQI drops below the UL one, which prompts the eNB to order a MS of the connection to the infrastructure path. We compare two MS policies on the UEs, which we call Instantaneous MS and Deferred MS: in the former, the content of the transmitter's RLC buffer is discarded, and new IP packets are sent along the new path.