# Empirical Performance Models of MAC Protocols for Cooperative Platooning Applications
## 1 Introduction  
With the advance of autonomous driving and platooning technologies, it is expected that cooperative platooning will make for a considerable share of highway traffic, as it leads to considerable gains in safety and fuel efficiency. We can envision that, in the near future, a subset of highway lanes (or even entire highways) can be fully dedicated to platoons. Vehicular ad-hoc networks (VANETs) will play an important role in leveraging communications within the platoon and with other platooning and non-platooning traffic, enabling applications of safety (e.g., platoon member coordination, collision avoidance), intelligent transportation management (e.g., intersection management), and infotainment (e.g., video streaming) [1, 2]. However, the performance of these applications, particularly those safety-critical applications such as platooning, depends significantly on the quality of the communication channel. Two metricsassume special relevance, the rate of collisions, which imply message losses (the lower, the better), and the busy time ratio, which depends on the communication range and affects spatial reuse (the lower, the better, too). Application designers must rely on models of network performance to predict the reliability, volume and timeliness of data exchanges that can occur and define the adaptation the application should perform in response to the network service quality. Often such models will be defined as a function of contextual parameters, e.g., number of nodes in the system [3,4]. A key component in defining the quality-of-service (QoS) that the network can offer is the medium access control (MAC) protocol that is in place. A MAC protocol must observe a number of (often contradictory) requirements such as: promote efficient use of the medium, provide fair and balanced access to all nodes, adjust to varying densities of vehicles and support changes in VANET topology. The most established standard for vehicle-to-vehicle (V2V) communication, IEEE 802.11p/dedicated short range communication (DSRC) [5], uses the carrier sense multiple access with collision avoidance (CSMA/CA) mechanism for medium access which, due to the stochastic nature of the exponential backoff rule, can cause highly variable delays and high message losses in the presence of high network loads. For the particular scenario of platooning, it has been advocated (see Section 2) that the time-division multiple access (TDMA) protocols are a better fit to satisfy the referred design requirements since they coordinate intra-platoon communications, thus reducing contention at the MAC level. However, TDMA protocols still vary significantly in supporting the same design requirements regarding extra-platoon traffic. In this paper, we propose empirical models relating a contextual metric, namely the number of radio-frequency (RF) neighbours (defined further on), and relevant network performance metrics for MAC protocols of different classes: the native CSMA/CA MAC of IEEE 802.11p and two overlay TDMA protocols (PLEXE-slotted [6] and RA-TDMAp [7]) that can be readily implemented on IEEE 802.11p. The latter two protocols were designed for platoon-beaconing applications that explicitly synchronize beacons within each platoon. However, while PLEXE-slotted is agnostic to extra-platoon traffic, RA-TDMAp does an implicit synchronization among neighbouring platoons. We leverage simulations to produce observations of network metrics as we change selected parameters such as platoon size, number of occupied lanes and transmit power of the followers. The observations are fitted with an appropriate curve through regression: we show that an exponential rule can be observed for the rate of collisions and busy time ratio.