# Empirical Performance Models of MAC Protocols for Cooperative Platooning Applications
## 3 Motivation and Methodology for Empirical Models of MAC Protocol Performance
### The Three Tested MAC Protocols  
The first MAC protocol we consider in this study is **CSMA/CA** (the MAC of IEEE 802.11p), which uses carrier sensing and random backoff intervals to mitigate potential collisions. In this case, all vehicles transmit a periodic beacon with period \(T_{\text{round}}\) but without any control of their relative phases, i.e., all transmissions are independent periodic processes. Thus, it is possible to fall into pernicious scenarios (named critical intervals in [23]) in which several vehicles transmit with close phases (or, in other words, with high coherence and a small phase difference), creating frequent collisions until their clocks drift away. The second MAC protocol we consider is **PLEXE-slotted**[6], an overlay TDMA protocol for platoon communication that works directly on IEEE 802.11p and synchronizes the beacons of the vehicles in a platoon avoiding or further reducing potential collisions among them. The leader transmits its beacons with period \(T_{\text{round}}\). This period is then divided in \(N\) equal slots of width \(T_{\text{xwin}}=T_{\text{round}}/N\). The leader transmits at the beginning of the first slot and all follower vehicles synchronize upon the reception of the leader beacon and transmit their own beacons at the beginning of their respective slots. Consequently, all transmissions in the platoon will be triggered with \(T_{\text{xwin}}\) separation. The sequence of transmissions starts with the leader and continues with the first follower and then throughout the platoon until the last follower. We refer to this sequence of platoon beacon transmissions as downstream. Nevertheless, this protocol is agnostic to the traffic external to the platoon. Thus, it is still possible to fall in critical intervals in which different platoons, or independent vehicles, transmit with a phase similar to that of the vehicles in the platoon, again leading to frequent collisions and degrading the quality of the communications until the clocks drift away. In the presence of sufficient number of platoons (we will see that 16 is already enough), the effect of the intra-platoon synchronization that PLEXE-slotted does is very slim and this protocol ends up behaving very similarly to CSMA/CA on which it operates. The third MAC protocol we consider is another overlay protocol working over IEEE 802.11p, namely **RA-TDMAp** that we proposed in [7]. This is very similar to PLEXE-slotted in what concerns  
Figure 2: A platoon of four cars (red) and three external cars (blue), running separate applications, possibly another platoon. the transmissions inside each platoon but it includes a simple mechanism that enforces implicit global synchronization across all neighbouring platoons and independent vehicles. The fact that the mechanism is implicit makes it scalable to the capacity of the channel without needing explicit inter-platoon coordination. Such global synchronization in RA-TDMAp is implemented with an adaptive round duration mechanism capable of adjusting the relative phase of the platoon beacon transmissions to accommodate the transmissions of all nearby platoons and independent vehicles and thus reduce, or even eliminate in some cases, mutual collisions. In RA-TDMAp the beacon interval \(T_{\text{round}}\) is divided equally by the \(N\) vehicles currently engaged in the platoon, assigning a time slot to each node of duration \(T_{\text{xwin}}=T_{\text{round}}/N\). All followers receive the leader beacon at the beginning of every round and use it for synchronizing the transmission of their own beacon. The transmission order starts with the beacon of the platoon leader and continues with the farthest vehicle from the leader (last follower) and then throughout the platoon in ascending order until the first follower. We refer to this sequence of platoon communications as upstream (Figure 3). The distinctive feature of RA-TDMAp is the adaptive synchronization mechanism. This is based on measuring the delays suffered by the transmissions of the platoon beacons.