# Cellular-V2X Communications for Platooning: Design and Evaluation
## 3 Cellular V2X: A Primer
#### V2V Communications: From LTE D2D to C-V2X  
In an LTE network, UEs are under control of one or more eNodeBs, which are responsible for allocating time-frequency resources to UEs. Scheduling of transmissions is performed in a centralized manner at the eNodeB's Medium Access Control (MAC) layer. At every Transmission Time Interval (TTI) of 1 ms, the eNodeB allocates Resource Blocks (RBs) to backlogged UEs according to its own scheduling policy. The number of RBs allocated to one transmission depends on the Modulation and Coding Scheme (MCS) chosen by the eNodeB for that transmission. To this aim, the eNodeB can make use of a Channel Quality Indicator (CQI) reported by the UE, which is a discretized measurement of the channel quality it perceives. In the downlink, the eNodeB performs the transmission to the intended receiving UE on the allocated RBs, whereas in the uplink the UE transmits to the eNodeB as soon as it has received a transmission grant from the eNodeB including the information about the MCS and the RBs it can use. The conventional LTE architecture requires that every communication goes through the cellular infrastructure (via the LTE-Uu interface), hence it does not natively support direct V2V communications (crucial for cooperative driving services); end-to-end latency could increase due to message passing through infrastructure nodes before being redistributed back to destination vehicles [1]. This issue can only be circumvented by resorting to device-to-device (D2D) communications [30]. Since LTE Release 12, the 3 GPP has provided specifications to allow direct message delivery between terminals in proximity through D2D communications (also known as Proximity Services, ProSe) [9]. ProSe are tailored to public-safety applications and have been optimized for robust operation and long lifetime of battery-power devices, with a focus on broadcast/multicast communications in low-density scenarios. Starting from Release 14, 3 GPP focused on specifically supporting V2V communications. They exhibit peculiar data traffic patterns and challenging requirements in terms of high reliability and low latency to be met under high-speed and high-density conditions. Similarly to ProSe, V2X communication utilizes single-carrier frequency division multiple access, while supporting 10 and 20 MHz-wide channels. Each channel is divided into 1 ms-long subframes, as the TTI. A group of RBs in the same subframe composes a _subchannel_ that can be used to transport both signaling and data transmissions. Two new high-level deployment configurations [17] have been designed for V2X (that extend Mode 1 and Mode 2 defined for ProSe):  
* _Mode 3_ (a.k.a. scheduled), according to which resource scheduling and interference management over the PC5 interface are assisted by eNodeBs via control signaling over the LTE-Uu interface. * _Mode 4_ (a.k.a. autonomous), according to which V2V resource scheduling and interference management over the PC5 interface are supported based on distributed algorithms implemented between the vehicles. There are no strict guidelines about the usage of the two modes. However, Mode 3 only applies to _in-coverage_ conditions, whereas Mode 4 could work both in _in-coverage_ and _out-of-coverage_ scenarios. Mode 3 guarantees collision avoidance, thanks to the centralized resource allocation, at the expenses of a larger signaling overhead. On the other hand, Mode 4 requires less signaling but cannot guarantee collision-free transmissions. Unlike with Mode 4, specifications do not detail a resource management algorithm for Mode 3, and each operator can implement its own solution [25]. A possible option is _dynamic scheduling_, according to which vehicles request subchannels to the eNodeB for each packet transmission. Such an approach incurs signaling overhead, and entails a latency due to the fact that vehicles must wait scheduling grants from the eNodeB before using subchannels. As an alternative, with _semi-persistent scheduling_ (SPS) the eNodeB can reserve subchannels for the periodic transmissions of a vehicle.