Opportunistic Retransmissions: A Novel Distributed Adaptive Algorithm for NR C-V2X Mode 2

In this article, we present a novel distributed blind retransmission algorithm for fifth generation (5G) new radio (NR) cellular vehicle-to-anything (C-V2X) in out-of-coverage (mode 2) scenarios. Our contribution enhances the standard (3GPP) specified sensing-based semi-persistent scheduling (SPS) resource re-allocation by opportunistically reusing blind retransmissions to improve average per-user throughput for mode 2 transmissions. We initially developed a detailed cross-layer model for (per user) 5G NR mode 2 throughput, supported by simulations using the reputable open-source ns-3 network simulator. This model and simulation analysis provided essential insights into the underlying causative trends. It led to two novel distributed adaptive algorithms for resource selection: an initial standards-compliant dynamic retransmission (D-Re) and a final standards-noncompliant opportunistic retransmission (O-Re) algorithm. We show that both D-Re and O-Re render average per user equipment (UE) throughput robust even in the absence of direct knowledge of 5G network parameters, while O-Re significantly improves the averaged per-UE throughput at high UE densities.