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

Single-bit hybrid automatic repeat request (HARQ) feedback improves retransmission reliability but lacks the resolution necessary for advanced link adaptation (LA) to optimize performance. 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 load, our optimal LA solution shows an improvement of 31.37% over Chase combining (CC) in average downlink (DL) physical resource block utilization, which is inherently linked to SE. 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 integrates into our enriched HARQ feedback at the user equipment, going beyond the single-bit HARQ feedback offering limited decoding status about the transport block. Our design, evaluated under various feedback bit counts and LA quantization levels, achieves notable gains--for example, a 20.26% increase over CC in average DL throughput per FTP3 packet with 4-bit feedback--offering flexibility in application-specific bit sizing.