## 6 Conclusions  
In this paper we have presented a solution for message broadcasting in LTE-A, using multihop P2 MP D2D transmissions. This solution relies on application-level intelligence on the UEs, and leverages standard D2D resource allocation schemes in the network. It allows UEs to specify the target area, without being constrained by cell boundaries. We performed simulations in both a static and a vehicular environment, in a multi-cell network. Our results show that this type of broadcast is fast, taking 80-120 ms to cover a 1000 m target radius at the 95 th percentile. Moreover, it is highly reliable, meaning that the percentage of UEs actually reached by the message is close to 100%. Last, but not least, it is cheap in terms of resource consumption. In fact, it does not occupy the DL frame (the SL being carved out of the UL spectrum); this means that no service disruption occurs in the DL, and that no additional power is consumed by the eNB to support this service. Moreover, the amount of UL resources consumed is quite limited, thanks to the possibility of frequency reuse and proximate transmissions with higher CQIs. The RBs consumed for a broadcast, on average, are less than one per UE, which makes MDB quite economical, and minimally disruptive of other services that an LTE network would need to carry on simultaneously. Further research on this topic, ongoing at the time of writing, includes at least two directions: first, investigating a deeper involvement of the infrastructure, in particular of the eNBs, in the broadcast relaying process. If the eNB is aware that a multihop transmission on the SL is being required by the sending applications, then it may allocate grants proactively to speed up the process. The second direction is to leverage network intelligence and network-wide information to characterize the target area. With reference to the vehicular case, the alert application could be made more efficient by being more selective as to which destination UEs it targets. For instance, if the message notifies that the originating vehicle is suddenly slamming on the brakes, the alert should reach the vehicles that are following it, and not those preceding it or across a block. This highlights the problem of building context-aware broadcast domains. On one hand, defining a more detailed area may occupy more space in the application-level message, which is something to consider carefully if the above-mentioned benefits are to be retained. On the other, we argue that a context-aware definition of the target area may not be defined by the vehicles themselves, since they may lack the knowledge of the surrounding environment and position of neighboring vehicles. Acquiring this knowledge using distributed means (i.e., inter vehicle communications) may not be viable either, because it would take a non-negligible message exchange and time, whereas broadcasting alerts should be accomplished in real-time to meet strict deadline requirements. The emerging MEC paradigm can play an important role in this respect. With MEC, vehicles could acquire the information about the intended geographical reach of one message by querying the corresponding service running at an application server located at the edge of the mobile network. The latter can leverage the location services provided by the network operator to define which vehicles should be receiving the message, and define a target area on behalf of the originator. Low latency would be guaranteed by the proximity of the MEC server to the vehicles, and by single client-server interaction. The subject matter of this paper includes description of results of a joint research project carried out by Telecom Italia and the University of Pisa. Telecom Italia reserves all proprietary rights in any process, procedure, algorithm, article of manufacture, or other result of said project herein described.