# Modeling Network-Controlled Device-to-Device Communications in SimuLTE
## 7 Conclusions and Future Work  
In this paper, we have presented what--to the best of our knowledge--is the only model to date of network-controlled D2D communications in LTE-A networks. Our D2D model extends SimuLTE, a simulation library for LTE-A based on OMNeT++, compatible with the INET library. We have discussed the fundamental modeling choices underlying SimuLTE architecture, namely the split between logical and physical transmission, which allows us to include arbitrary interference models, without the burden of simulating broadcast air transmissions, as well as the modular framework for resource scheduling at the eNB. These modeling choices make it easy to incorporate new functionalities,  
Figure 30: Simulation time. Figure 28: Reception probability in the multihop case, \(h=500\) m.  
Figure 29: Reception probability in the multihop case, \(h=750\) m.  
namely D2D communications, into SimuLTE. We have shown that protocol layers for D2D-enabled UEs can be obtained via module inheritance. D2D-enabled resource allocation requires instead generalizing the one for standard cellular communication to allow for frequency reuse, which entails ownership of the same RBs by more than one transmitting UE at a time on the UL spectrum. We have described the data structures that we employ at the eNB to constrain frequency reuse, which are designed to minimize the complexity of checking for conflicts. We have shown how to limit the computational burden of broadcast P2 MP transmissions, by using multicast groups and transmission horizons to limit the number of logical transmissions, while still computing the interference correctly. Our profiling shows that D2D communications are not significantly costlier in terms of simulation time. On the one hand, single-hop transmission costs less than relaying at the eNB. On the other hand, computation and storage of SL CQIs (which are additional to UL CQIs) do add some overhead. Factorial analysis shows that the simulation duration is mainly determined by the number of UEs, whereas the overall traffic volume in the cell provides a negligible contribution, also in a D2D scenario. We have shown that limiting the transmission horizon for P2 MP transmissions may warrant considerable reductions in simulation time, and that a reasonable trade-off between accuracy and simulation time can be reached. Moreover, we have shown the effects of different mode switching policies on the higher layers of the communication stack, and demonstrated that, even though our support for reuse-aware scheduling of D2D connections employs a binary interference model, interference is always correctly accounted for. The ultimate purpose of releasing an open-source simulation library that models network-controlled D2D communications is to foster further research. More specifically, applications using D2D communications can now be prototyped and tested in a simulative environment. Their performance can be evaluated from both the user perspective--i.e., what performance the latter can expect from the underlying LTE-A network--and the network operator's--i.e., how many resources they consume, and how to engineer the network to support them. Burgeoning fields for applications exploiting D2D are Vehicular Networks and Internet of Things. Moreover, our work allows competing schemes (e.g., for resource allocation) to be evaluated fairly and comprehensively, factoring in the (often counterintuitive) combination of effects of all the protocols in the LTE stack. For instance, it is actually the interplay of ciphering and numbering (at the PDCP) and buffering (at the RLC) that makes mode-switching a risky operation, where data loss may happen. Therefore, when designing mode-selection algorithms, the benefits of frequent mode selection should be assessed together with the potential switching-induced losses. Our simulation library is--to the best of our knowledge--the only existing tool that allows all of the above. Our future work on this topic will involve developing a model for network-unassisted D2D communications.