Nokia · 10.5.4.3

Nokia proposes studying HARQ enhancements that decouple UL and DL scheduling to reduce tight coordination requirements present in NR, specifically by removing the link between HARQ-ACK feedback transmission timing and DL assignment timing and decoupling DL scheduling information across cells or cell groups. They propose studying PDCCH detection feedback techniques, including multi-value NACK and explicit counts of detected PDCCHs or failed PDSCH decodings, to mitigate the severe impact of missed DL assignments on fully adaptive HARQ re-transmissions using reserved MCS or non-self-decodable RVs. They propose a HARQ-ACK payload range from 1 to 1536 bits as a working assumption, based on NR values for CBG sizes, TBs per PDSCH, and realistic assumptions on reported HARQ processes and carriers. They present an observation from system-level simulations that NACK-to-ACK errors at 0.1% probability only marginally degrade simulated application layer throughput except at the very low end of the distribution, supporting NR's design target. Nokia proposes studying finer granularity HARQ-ACK feedback and discussing DCI-based triggering of a faster ARQ procedure for UL to avoid RLC status report latency.

Nokia proposes that 6GR shall retain NR's asynchronous adaptive HARQ operation for UL-SCH and DL-SCH. They present a technical case against NR's tight coordination between UL and DL schedulers in HARQ-ACK reporting, observing that DAI dependencies in Type-2 HARQ-ACK codebooks and timing indication in DL assignments increase scheduling complexity, and propose studying mechanisms that decouple DL assignment from HARQ-ACK feedback triggering to enable parallel CA processing. They argue that a 6G flexible CA framework with decoupled UL and DL carriers requires HARQ-ACK reporting with decoupled scheduling, targeting a design that allows decoupling of DL scheduling information between cell groups. They also propose studying enhanced feedback techniques including multi-value NACK to differentiate PDSCH decoding failure from missed DL assignment, and indication of detected PDCCH count, to limit the impact of missed DL assignments on adaptive DL HARQ operation.