NR Sidelink Mode 2 in Unlicensed Bands: Throughput Model and Validation

Sidelink is a 3GPP-standardized technology that enables direct UE-to-UE communication. Initially designed for licensed 4G spectrum (Release 15), it expanded to 5G NR (Release 16, 17). However, due to the recent availability of large swaths of unlicensed spectrum (notably 6 GHz in U.S.), 3GPP has initiated studies to support sidelink operation in such unlicensed bands (denoted as SL-U). SL-U specifications ride on adaptation of 5G NR operation from licensed to unlicensed bands (NR-U). This paper provides a first analytical framework to quantify SL-U network performance in saturation conditions, for comparative evaluation to NR-U. A Markov chain model for NR-U is first presented, as a basis for necessary (significant) modifications needed to capture SL-U-specific features - the representation of gap states in the SL-U model to account for the restriction that SL-U transmissions only occur at SL slot boundaries. The resulting analysis highlights the differences and consequent (negative) impact of current 3GPP-defined SL-U channel access rulemaking; the analytical estimates are validated by results from a custom simulation of channel access. The primary takeaway is that the SL-U throughput under current 3GPP access rules suffers significantly relative to NR-U, and thus needs amelioration for successful marketplace adoption in the future.