RRI Adaptive: A Standards Compliant Approach for Equitable and Stable Congestion Control in C-V2X Networks

A key topic of interest in all vehicular networking technologies is their ability to deal with dense radio environments as widespread deployment becomes a reality. This requires congestion control mechanisms to maintain adequate communication performance. The current Cellular Vehicle-to-Everything (C-V2X) and its successor New Radio V2X (NR-V2X) standardised congestion control approach is table-based, utilising packet dropping. As shown in past works by the authors of this paper, these approaches exhibit congestion instability and require extensive configuration. To overcome this, algorithmic approaches were standardised, namely ETSI's DCC Adaptive. Although this can be effective for wireless vehicular communications, it cannot be applied directly to NR-V2X/C-V2X due to incompatibility with the underlying radio scheduling approach, Sensing-Based Semi-Persistent Scheduling (SB-SPS). In previous work, the authors of this paper investigated in detail why this is the case and proposed an algorithmic approach that was compatible with the SB-SPS scheduler, namely RRI_Adaptive. This paper provides an in-depth evaluation of RRI_Adaptive. Importantly, its efficacy is evaluated not simply from the perspective of maintaining a desired channel load, but also from the perspective of maintaining effective application quality of service. This paper also describes the first study of fairness and stability in the context of C-V2X/NR-V2X congestion control. These are of increased importance, given dynamic channel conditions in vehicular scenarios, and reduced awareness due to degraded quality of service or vehicles starved of radio resources, may increase the likelihood of collisions.