# Cellular-V2X Communications for Platooning: Design and Evaluation
## 4 Platooning in Cellular V2X
### Mapping to C-V2X  
The platoon CAM exchange described above is implemented on top of the C-V2X protocol stack. First, transmission of a CAM requires that the VUE has some RBs allocated. As mentioned in Section 3, resources can be allocated either dynamically or using SPS, with different performance. _Dynamic scheduling_ requires non-negligible signaling overhead for each CAM transmission. With dynamic scheduling, a VUE has to explicitly request a transmission grant to the eNodeB for each CAM, using the Random ACcess (RAC) mechanism. The delay between the RAC request by the transmitting VUE and the reception of the actual data transmission by the receiving VUE is unavoidable and lasts 15 ms, in the best case. With reference to Figure 3, the transmitting VUE must send a RAC request to the eNodeB, which in turn grants enough space for it to send its Buffer Status Report (BSR). The BSR reports the amount of bytes that the VUE needs to transmit and it allows the eNodeB to know how many RBs need to be granted to the VUE for data transmission. This handshake lasts \(T_{req}\) TTIs. Then, the eNodeB performs the scheduling operations, i.e., it allocates the required number of RBs in \(T_{sched}\) subsequent TTIs. \(T_{grant}\) is the time required by the eNodeB to issue the transmission grant to the VUE, which indicates which RBs to use for transmitting the CAM. Once the VUE has received the grant, it can transmit the data, which lasts \(T_{tx}\). While \(T_{grant}\) and \(T_{tx}\) can be considered constant, \(T_{req}\) and \(T_{sched}\) cannot. In fact, collisions during the RAC procedure may occur if different VUEs perform simultaneous RAC requests using the same resources. In this case, the eNodeB grants resources to one of them, whereas the other colliding VUEs, which do not receive a reply from the eNodeB within the expected time window, re-iterate the requests after a backoff. \(T_{sched}\), instead, depends on both the BSR and the MCS selected for the transmission, as well as the possible presence of other V2X transmissions. On the other hand, SPS drastically reduces the signaling overhead for RBs assignment, since VUEs can obtain periodic grants by the eNodeB, based on (periodic or on demand) collected information from the VUEs (about position, channel status, etc.). However, SPS is inflexible, since it is not able to accommodate rapidly the time-varying transmission needs of the VUEs, both from application and resource-allocation perspectives. In fact, the size of CAM and the periodicity of the transmissions can vary whenever the platoon status changes (e.g., when a vehicle joins/leaves the platoon; when changing speed/intra-platoon gap requires modification in the controller algorithm cycle). Likewise, interference or the channel conditions conditions may vary over time, hence the eNodeB should be able to timely change the MCS used by VUEs for transmission. Another potential advantage of using dynamic scheduling instead of SPS is that it allows the network to better exploit _spatial frequency reuse_. Frequency reuse schemes (like the one in [31]) allow two or more D2D (V2V in our study) transmissions to be scheduled on the same RBs, if their mutual interference is weak. This allows the eNodeB to save precious frequency resources that could be re-utilized for serving other V2X communications and reducing the delay before transmission. With dynamic scheduling, the eNodeB can better control the interference level by selecting, at every TTI, which VUEs can use the same resources, i.e., by avoiding to share RBs among highly-interfering VUEs. In this way, _inter-platoon frequency reuse_ can be exploited if two platoons are far from each other, and also frequency reuse can be enforced _among VUEs of the same platoon_ if their transmit power is sufficiently low and/or transmitting PMs are far enough. Intra-platoon frequency reuse is not applicable if PMs' transmissions are sequential, in which case they occur at different time instants. Note that frequency reuse could be enabled for SPS mode, too.