PROBE: Reactive MCS Adaptation from Summarized HARQ Feedback for NR Sidelink Groupcast

NR sidelink groupcast provides no group channel-quality report, yet one modulation and coding scheme (MCS) must serve every member. We propose PROBE, a reactive MCS adaptation scheme that operates directly on the summarized HARQ outcome. PROBE increases the MCS index by one after a group ACK and retreats by D after a group NACK. We show that, away from the index boundaries, this update has a zero-drift group-NACK probability q∗ = 1/(D + 1), which gives the retreat parameter a direct operating-point interpretation and provides a principled basis for selecting it from group-level feedback. We characterize BLER for MCS 0–28 with an NR SL-SCH coding chain and use the resulting curves throughout the evaluation. Across the evaluated group sizes, topologies, and transmission periods, PROBE remains within 1.26% of the per-condition best fixed retreat and improves goodput over the conventional 10%-target reference by 1.09–15.50 percentage points. PROBE also outperforms a feedback-matched discounted Thompson-sampling learner across the evaluated steady-state conditions. These results show that a constant-complexity controller driven only by summarized HARQ feedback can adapt the groupcast MCS without CSI or changes to the NR sidelink framework.