Toward 6G Sidelink Reliability: MAC PRR Modeling for NR Mode 2 SPS and ns-3 Validation

Abstract

5G New Radio (NR) Sidelink (SL) Mode 2 has enabled decentralized, infrastructure-less direct communications which is evolving to serve reliability-critical services in 6G SL. Particularly, the channel access in NR SL Mode 2 relies on the Sensing-based Semi-Persistent Scheduling (SPS) whose key features significantly influence the packet reception ratio (PRR). While SPS has been widely studied, existing analytical models typically abstract or omit several NR-specific SPS features that are standardized in the 3rd Generation Partnership Project (3GPP), limiting their ability to explain how SPS parameters shape MAC collision dynamics and PRR. This paper develops an analytical MAC-layer PRR model for broadcast NR SL mode 2 by explicitly modeling SPS-driven MAC collision events. The model captures (i) Collisions caused by simultaneous resource reselection and (ii) Persistent collisions induced by resource keeping across resource reservation intervals (RRIs). Based on the event-level characterization, we derive closed-form expressions for the steady-state MAC collision probability and PRR. We further extend the analysis to incorporate under-explored SPS features, including the duplicate transmissions per RRI and the minimum resource-availability requirement for reselection, and quantify their impact on PRR in under-saturated regimes. The analytical results are validated using ns-3 simulations based on the 5G-LENA framework, showing close agreement under under-saturation and revealing deviations as the system approaches saturation. The proposed model provides mechanistic insight and design guidance of tuning the SPS parameters to improve 6G SL reliability.