Enhancing the Reliability of NR-V2X Sidelink Broadcast Through Relaying and Dynamic Retransmission Control

NR-V2X sidelink broadcast is an important communication mode for delivering safety-related information to surrounding vehicles with low latency. However, in non-line-of-sight (NLOS) environments such as intersections, direct links are blocked, and the blind retransmission mechanism defined for NR-V2X provides limited reliability improvement. This paper extends our previously reported relay-assisted retransmission scheme, RRT + ACK, which combines relay selection using link-quality indicators aggregated per direction (AvgLQIs/Dir) with ACK-assisted transfer of retransmission responsibility to the selected relay. The extension is a dynamic retransmission control (RT-Ctrl) method based on the Channel Busy Ratio (CBR), intended to limit retransmission-induced congestion. Simulations using Scenargie in NLOS intersection environments, under the assumption of ideal error-free ACK delivery, show that the basic RRT + ACK scheme improves the evaluation packet delivery ratio within 200 m (EPDR200m) by approximately 8.6% compared with conventional blind retransmission. Under high vehicle density, the RT-Ctrl method with the best parameter setting improves EPDR200m from 0.783 to 0.867 and reduces the average CBR from 0.594 to 0.329 compared with RRT + ACK using a fixed retransmission count.