# Cellular-V2X Communications for Platooning: Design and Evaluation
## 1 Introduction
Platooning is mentioned in the 3 GPP Release 15 [22] among the key safety use-cases to be targeted with future evolution of C-V2X. Platooning applications exhibit unique features in terms of: (i) _topology_, since platoon members are positioned in the same lane along a chain and keep a very small inter-vehicle distance; (ii) _CAM coverage range_, which is in the order of tens of meters, instead of hundreds of meters, as in classic safety applications, since communications typically occur among adjacent platoon vehicles; and (iii) _delivery requirements_ as the latency is expected to be much lower than 100 ms and the reliability greater than 90%. Such peculiarities make the analysis different from what already investigated in the literature for V2V communications and raise novel concerns which require to be addressed in a different manner. To the best of our knowledge, only very few works in the scientific literature have addressed the support of platooning applications through LTE/C-V2X and evaluated its performance quantitatively. In particular, in [23] a resource allocation approach is proposed that controls channel and power assignment to support V2V communications in underlaying cellular network in a scenario with a chain of platoons. The evolved multimedia broadcast multicast services (eMBMS) capability of LTE is used to disseminate the messages transmitted by the leading vehicle to all vehicles in the platoon. In [24], we designed a solution according to which pooled LTE resources are allocated to the platoon by the eNodeB, which exploits the short inter-vehicle distances and the low transmission power to enable reuse of LTE resources by those nodes for which the mutual interference is weak. In that work, early simulation results were provided to showcase the timeliness of message dissemination within the platoon under ideal settings, i.e., perfect frequency reuse, while neglecting the impact of signaling procedures for getting radio resources. In this work, we take a step forward in investigating the key design issues and choices related to the network-assisted mode specified by 3 GPP for C-V2X communications over the sidelink (i.e., the PC5 reference point) [18], and focus on the challenging case of platooning. No scheduling algorithm is currently mandated by 3 GPP for the eNodeB-assisted mode for C-V2X [21]. In such a context, this paper intends to provide the following main contributions:  
* Overcome the current lack of either a legacy or a dominant solution to manage platooning applications with LTE/C-V2X and analyse the viability and performance of the _dynamic scheduling_[25] for V2X sidelink platoon communications. We consider two possible ways of implementing dynamic scheduling; both modes are not currently specified by 3 GPP for C-V2X. With the first mode, called _sequential_, somehow reminiscent of TDMA solutions based on IEEE 802.11p--till now the only investigated access technology for platooning [6; 7; 8]--platoon vehicles ask the eNodeB for radio resources as soon as they receive a CAM from the preceding vehicle. The second mode, called _simultaneous_, allows all vehicles in the platoon to send their requests to the eNodeB soon after receiving the CAM from the leading vehicle, and supports _simultaneous CAM transmissions_ between couples of vehicles, sufficiently spaced apart within the platoon, by using the _same resource blocks_. * Derive theoretical bounds for the time required to disseminate CAM messages within a platoon, when varying the composition of the platoons, CAM size and network parameters, for the purpose of a preliminary comparison between the sequential and simultaneous transmission schemes. * Carry out extensive simulations using SimuLTE [26], a system-level simulator based on OMNeT++ [27] that accurately implements the whole LTE protocol stack, from the radio propagation dynamics and access technology to the application data pattern, which we extended so as to support the CAM exchange within a platoon and the proposed scheduling solutions.