Delta MCS-based Enriched Hybrid ARQ Feedback Design for 6G Networks

Conventional hybrid automatic repeat request (HARQ) uses single-bit feedback to boost retransmission reliability; however, its limited granularity restricts advanced link adaptation (LA) for optimizing performance beyond reliability. This paper presents a novel LA approach for retransmissions in 6G networks, targeting spectral efficiency (SE) optimization through effective modulation and coding scheme selection within the HARQ framework. In a dense urban setting with urban macro cell deployment and file transfer protocol model 3 (FTP3) traffic, our optimal LA solution improves average downlink (DL) physical resource block utilization by 31.37 % over Chase combining (CC), a gain inherently linked to SE optimization. Following 3GPP release 18 study item technical report's recommendation to include LA information in HARQ feedback, we address feedback design limitations of the optimal solution with a quantized LA metric. This metric is incorporated into our novel enriched HARQ feedback (EHF) at the user equipment, enabling EHF to surpass single-bit conventional feedback that only conveys transport block decoding status. Evaluated under varying feedback bit counts and LA quantization levels, our design achieves significant gains-e.g., a 20.26 % increase over CC in average DL throughput per FTP3 packet with 4-bit feedback-offering flexibility in application-specific bit sizing.