A Selective Re-Evaluation Mechanism for 5G NR V2X Mode 2 Communications

The Internet of Vehicles (IoV) vision requires the pervasive capacity of connected and automated vehicles to communicate with their driving environment. To this aim, direct or sidelink Vehicle-to-Everything (V2X) communications should complement network-based communications with reliable and low latency local connectivity. 5G NR V2X has been designed to support advanced V2X services for connected and automated driving. These services can generate aperiodic traffic of variable size that can cause packet collisions when vehicles autonomously select their radio resources under 5G NR V2X mode 2. To prevent such collisions, the standard introduces a re-evaluation mechanism that vehicles use to detect and avoid packet collisions prior to their transmission on selected resources. This study first shows that re-evaluation is not fully effective in avoiding packet collisions because many of the detected collisions ultimately do not happen under the presence of aperiodic traffic of variable size, and selecting new resources increases the probability of packet collisions. To address these challenges, we propose a selective re-evaluation mechanism that changes resources only when the vehicle is certain that a collision detected with re-evaluation will occur, which is the case when a re-evaluation detection is triggered by a reservation for a retransmission of a TB. This study shows that the proposed selective re-evaluation mechanism improves the reliability and latency of 5G NR V2X mode 2 communications.