# Coordination of Congestion and Awareness Control in Vehicular Networks
## 3 Methodology for Cross-Layer Coordination
The figure represents two-dimensional CSs (\(C=2\)) where the communication parameters to be configured are the packet transmission frequency and transmission power. Additional parameters could be considered in multi-dimensional configuration sets. As it can be observed, the CS of the congestion control protocol limits the maximum packet transmission frequency and transmission power that can be used. On the other hand, the CS of the awareness control protocol limits the minimum values that can be used. The ICS defines all possible packet transmission frequencies and transmission power levels that the vehicle could use to satisfy the requirements of both the congestion and awareness control protocols. For each vehicle, the COMPASS methodology operates as follows:  
1. Each protocol operates independently, calculates its own CS, and reports it to COMPASS. 2. COMPASS calculates the Intersection CS based on the received CSs for each protocol. 3. COMPASS configures the communication parameters to be compliant with the identified ICS. Different criteria could then be applied to select the communication parameters within the identified ICS. COMPASS dynamically adapts its operation to the conditions of the protocols it coordinates. If the CS for any of these protocols changes (e.g., because the channel load changes or the application requirements are modified), COMPASS calculates the new ICS and adapts the communication parameters based on the new ICS. The proposed COMPASS methodology constitutes a generic framework, and is independent of the protocols to be coordinated, their specific implementations, the method used to compute the CSs, the considered communication parameters, and the configuration of the communication parameters following the identified ICS. The operation of COMPASS is here illustrated considering the coordination of A-DCC and MINT. COMPASS is independent of the mechanism used to configure the communication parameters. However, to evaluate its performance, it is necessary to specify how the communication parameters are configured once the ICS has been identified. Different options are possible, but in this study, the parameters are initially configured to the minimum values pertaining to the ICS (minimum \(P_{t}\) and  
Figure 4: COMPASS configuration sets when coordinating congestion and awareness control protocols. \(T_{f}\)). The parameters are then dynamically adapted following an AIMD process (Additive Increase and Multiplicative Decrease). All vehicles piggyback the difference between their current \(P_{t}\) and \(T_{f}\) and the minimum \(P_{t}\) and \(T_{f}\) of their ICS, as well as the minimum values received from all neighboring nodes. This process modifies the DCC_NET piggybacking scheme since instead of exchanging channel load information, vehicles exchange information about the configuration of their communication parameters with respect to the minimum required. The information exchanged is used in the AIMD process to dynamically modify the configuration of the communication parameters within the ICS. \(P_{t}\) and \(T_{f}\) are increased if their increment is compliant with the computed ICS, or decreased otherwise. We consider additive factors of 0.25 dB and 0.05 Hz for increments, and a multiplicative factor of 0.025 for decrements. Additive factors are multiplied by 4 for vehicles for which the difference between \(P_{t}\) and \(T_{f}\) and the minimum \(P_{t}\) and \(T_{f}\) of the ICS is lower than the average values of vehicles within 2 hops. Similarly, the multiplicative factor is doubled for vehicles for which the difference between \(P_{t}\) and \(T_{f}\) and the minimum \(P_{t}\) and \(T_{f}\) of the ICS is higher than the average values of vehicles within 2 hops. The following algorithms formally describe using pseudo-code how COMPASS derives the ICS for a given vehicle (Algorithm 1) and updates its communication parameters (Algorithm 2).