# Cellular-V2X Communications for Platooning: Design and Evaluation
## 1 Introduction  
The recent advancements in the sensing, automation, computing, communication and networking technologies for vehicles hold the promise of improved road safety and traffic efficiency and reduced fuel consumption and emissions [1]. By exploiting sensing and communication capabilities, vehicles can cooperate and extend their context awareness beyond the visual field. Cooperative vehicles will share their driving intentions with other traffic participants, thus accurately predicting what other traffic participants are going to do, and optimizing their own decisions and maneuvers accordingly [2]. Platooning is among the key cooperative driving applications. In a platoon, autonomous/semi-autonomous vehicles organize themselves in order to travel along the same lane at the same speed in a train-like manner, i.e., each one keeping a small, constant distance to the vehicle ahead. Such a drivingpattern lays its foundation in adaptive cruise control (ACC) systems, which allow one to measure the distance from the preceding vehicles through automotive radars mounted on the front of the vehicle. However, ACC systems exhibit some limitations. The on-board sensors of an ACC system have a short sensing range (they can only see adjacent vehicles) and do not work reliably under adverse weather conditions [2; 3; 4]. Such limitations are overstepped by cooperative ACC (CACC) systems which leverage direct vehicle-to-vehicle (V2V) wireless communications. In CACC information about the maneuvering (e.g., position, speed, and acceleration, retrieved through sensor measurements) of one, i.e., the leading one, or more vehicles can be made available almost instantly to all other vehicles, in order to ensure quicker reaction to changes in traffic conditions, while avoiding platoon instability. The behavior and stability of the platoon is highly affected by the reliability and latency of messages exchanged among platoon vehicles. So far, the IEEE 802.11 standard [5] has been mainly considered as the communication technology for platooning. In order to support platooning requirements, many literature solutions have proposed modifications to the native contention-based distributed 802.11 access protocol, which is known to suffer from unbounded delay and low reliability at high network load. Time-division multiple access (TDMA) schemes on top of 802.11 have been mainly pursued so far [6; 7; 3; 8]. On the other hand, the Third Generation Partnership Project (3 GPP) evolved towards a network-assisted device-to-device (D2D) Long-Term Evolution (LTE) communication technology [9] in Release 12. D2D communications suit particularly well localized V2V communications, since--on one hand--they exhibit lower latency than classic infrastructure-based communications, which instead traverse both an uplink and a downlink leg, and--on the other--they are still scheduled by the infrastructure, which implies that the benefits of interference management and collision avoidance are still present. Plenty of literature solutions have investigated the feasibility of D2D for vehicular communications [10; 11; 12; 13; 14; 15], identifying the potentials as well as the limitations of the 3 GPP-specified D2D technology to support the low latency requirements of vehicular applications in a high-speed context. Recent efforts by industries and academia coalesced into the development of the Cellular Vehicle-to-Everything (C-V2X) technology [16], lately specifically conceived by 3 GPP in Release 14 [17; 18] to optimize D2D communications among vehicles under high mobility conditions. Early studies have been published mainly focusing on assessing the performance of C-V2X in supporting the delivery of cooperative awareness messages (CAMs) [19] in the one-hop neighborhood of the sender nodes [20; 21] for cooperative road safety purposes. The challenges of the latter type of vehicular application are less strict than those raised by platooning, although it also relies on localized V2V communications and CAMs exchange.