# Empirical Performance Models of MAC Protocols for Cooperative Platooning Applications
## 3 Motivation and Methodology for Empirical Models of MAC Protocol Performance
### Relevant Metrics and Parameters and Simulation Setup  
We now identify the metrics and parameters relevant to explore, in the process of building a more comprehensive set of empirical models relating selected network and topological metrics. We collect three metrics, classified into two categories: network topology and network performance. The number of RF neighbours is the only network topology metric and the one against which network performance metrics are characterized. The value is not under our direct control, thus being measured during the simulation. Our objective is to develop practical models that can be used by vehicles in real-world conditions:  
* **Number of RF neighbours**: The number of different sources of the beacons correctly received by a vehicle per second. The remaining metrics are related to network performance, and are useful to characterize the network service to the application. They are directly provided by the used simulation framework (Veins [25]):  
Figure 5: Network metrics vs. platoon size. * **Collisions rate**: The packets lost due to a collision between interfering transmissions per unit of time;
* **Busy time ratio**: The ratio of time that the physical layer at each node observes the channel busy. Both metrics (\(m_{ij}\) standing for either collisions or busy time ratio) are observed for each second \(j\) and by each vehicle \(i\). Then, they are averaged over the entire simulated time and among all vehicles according to the following equation, resulting in one value per simulation scenario. Note that _GlobalMetric_ stands for collisions rate or channel busy time ratio depending on the semantics of \(m_{ij}\). \[GlobalMetric=\frac{1}{\#_{\textit{cars}}}\sum_{i=1}^{\textit{\#cars}}\left( \frac{1}{\#_{\textit{secs}}}\sum_{j=1}^{\textit{\#ccs}}m_{ji}\right)\]  
We vary several scenario and communication parameters over a range of values, in order to simulate realistic scenario modifications that can lead to a change in the number of RF neighbours. * **Platoon Size:** this parameter change has several effects: (i) It increases the frequency of messages transmitted during a round (round duration \(T_{\text{round}}\) remains constant, while follower slots \(T_{\text{xwin}}\) decrease); (ii) it decreases the ratio of leaders to followers; and (iii) while the communication range is not filled with neighbours, it leads to an increase in the number of observable neighbours. * **Platoon Size Homogeneity:** The size homogeneity concerns whether platoons have the same size (**homogeneous platoon sizes**), or variations between them around a known average (**heterogeneous platoon sizes**). In the first case, all platoons have the same integer number of nodes \(N\). In the latter, we draw platoon sizes from a uniform distribution \(\mathcal{U}(a,b)\), in which the mid-point is \(N\); this strategy is expected to cause the average platoon size to be \(N\), allowing for a fair comparison with the homogeneous-size counterpart scenario. * **Number of Lanes with Traffic:** Increasing the number of lanes with traffic leads to an increase in the number of neighbours over a particular spatial layout. If a single lane is considered, increasing the platoon size may not lead to an increase in the number of neighbours (if neighbours exist up to the communication range); allowing traffic in adjacent lanes, however, immediately introduces a large number of neighbours to individual nodes. * **Transmit Power of Followers**: Changing the transmit power of followers causes the communication range of individual nodes to increase. Note that leaders transmit with high power, already reaching all nodes in the simulated area. The observations reported in the following section were obtained from simulations in the discrete event simulator OMNeT++, paired with the road traffic simulator SUMO, the vehicular IEEE 802.11p simulation framework Veins [25] and the Veins extension for platooning, PLEXE [24]. As in [19], we have simulated a stretch of a 10 km of a highway with five lanes where (only) 16 platoons travel.