# Cellular-V2X Communications for Platooning: Design and Evaluation
## 4 Platooning in Cellular V2X
### Mapping to C-V2X
#### 4.3.1 PL-to-PMs Step  
This phase is realized through P2 MP V2V communications. This way, the PL only needs to send its CAM _once_ and all the interested receivers can decode it.  
The first problem is to determine the set of destinations, i.e., vehicles interested in receiving and decoding the message. In fact, PMs belonging to different platoons can be in the transmission range of the transmitting PL. It is not desirable that these PMs receive the CAM, since it poses additional workload on these devices and, in the worst case, might affect the controller algorithm (e.g., changing their speed to a wrong value). This problem can be addressed at the application layer. To this aim, a Platoon ID field is added to the CAM messages. Platoon ID is a unique string that is used to differentiate between different platoons. Similar to previous studies [32], we set Platoon ID to be vehicle ID of the PL. When a vehicle joins a platoon, besides other information, it acquires from  
Figure 3: Handshake for obtaining frequency resources.  
the PL the Platoon ID reserved for that platoon, in order to receive CAMs from the PL of that platoon. When a PL needs to send its CAM to all its PMs, it only specifies the Platoon ID in the transmitted CAM. All the PMs belonging to the platoon receive the CAM, whereas other vehicles discard it. A possible improvement could be reached by enlisting the aid of the network infrastructure: if a cell allocates a multicast MAC ID to each platoon traversing it, CAM messages of a platoon can be encapsulated into MAC-level transmissions having that MAC ID as a destination address. This way, other platoons would discard the CAM messages directly at the MAC layer, without having to go all the way up through the stack. Moreover, since cellular transmissions are cyphered, this solution could be used to enhance the security.  
Another issue is the reliable delivery of the PL's CAM to all the PMs of the platoon. In fact, it is desirable that all the PMs belonging to the platoon receive the CAM with at least a certain probability. In ideal conditions (i.e., with no interference), the reception range depends on the CAM size, the PL's transmit power and the selected MCS. Given these parameters, it is possible to find the maximum platoon size \(L\) that can be covered with a single transmission from the PL. If the reception range does not cover the entire platoon, one of the PM needs to relay the CAM to other PMs. This implies some mechanism that allows one PM to be elected as a relay (additional message exchange), or a dissemination mechanism like the one in [33]. Considering that the CAM size depends on the amount of information that needs to be sent and that transmit power is usually fixed by the standard or implementation constraints, the only way to adapt the reception range is to manipulate the MCS. The latter is not a trivial task, since there is a tradeoff between the reception range and radio resources consumption. Using lower MCSs allows the PL to reach farther PMs, but it comes at the cost of more RBs per transmission. In fact, we recall that the number of bits that can be sent in one RB decreases with the MCS. On the other hand, higher MCSs allow the eNodeB to reduce the number of RBs required for a transmission, but it will also decrease the reception range.  
Finally, P2 MP V2V transmissions are not acknowledged at the MAC level, i.e., there is no Hybrid Automatic Repeat Request (H-ARQ) to recover corrupted transmissions. This means that application-level retransmissions should be foreseen in order to improve the correct reception probability at the PM side. Clearly, adding retransmissions might increase the duration of the CAM cycle.