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.

翻译：5G新空口（NR）侧链路（SL）模式2已实现去中心化、无基础设施的直接通信，并正演进以服务于6G侧链路中可靠性要求严苛的业务。特别地，NR侧链路模式2的信道接入依赖于基于感知的半持续调度（SPS），其关键特性显著影响数据包接收率（PRR）。尽管SPS已被广泛研究，现有分析模型通常抽象化或忽略了第三代合作伙伴计划（3GPP）标准化的若干NR特有SPS特性，这限制了它们解释SPS参数如何塑造MAC层碰撞动态和PRR的能力。本文通过显式建模SPS驱动的MAC碰撞事件，为广播式NR侧链路模式2开发了一个分析性的MAC层PRR模型。该模型捕捉了（i）由同时资源重选引起的碰撞，以及（ii）由跨资源预留间隔（RRI）的资源保持所引发的持续碰撞。基于事件层面的刻画，我们推导了稳态MAC碰撞概率和PRR的闭式表达式。我们进一步扩展了分析，以纳入尚未被充分探索的SPS特性，包括每个RRI内的重复传输以及重选所需的最小资源可用性要求，并量化了它们在欠饱和状态下对PRR的影响。分析结果使用基于5G-LENA框架的ns-3仿真进行了验证，在欠饱和条件下显示出高度一致性，并揭示了系统接近饱和时的偏差。所提出的模型为调整SPS参数以提高6G侧链路可靠性提供了机理性的见解和设计指导。