Feedback Enhancements for Semi-Persistent Downlink Transmissions in Ultra-Reliable Low-Latency Communication

Ultra-reliable low-latency communication (URLLC) is one of the new service categories specified by The 3rd Generation Partnership Project (3GPP) that will be used for the applications with the strict reliability and latency requirements. To satisfy these requirements in Release 16, URLLC downlink (DL) transmission can be scheduled in the semi-persistent scheduling (SPS) resources to reduce control overhead and a retransmission if necessary is triggered by hybrid automatic repeat request (HARQ) feedback transmitted in uplink (UL) resources. However, in time division duplex (TDD) mode, HARQ feedback is dropped when there are no available UL resources for physical uplink control channel (PUCCH) in the indicated slot/sub-slot because that slot/sub-slot contains DL symbols in semi-static TDD configuration or flexible symbols updated to DL symbols in dynamic TDD configuration. Dropping the feedback results in a degradation of DL SPS quality of service (QoS) and may not be acceptable for URLLC based services and applications. This paper provides a scheme to guarantee the transmission of HARQ feedback and potential data retransmission when there is UL-DL slot/sub-slot conflict at the indicated feedback resource. The scheme comprises a dynamic indication of feedback resource without using an associated downlink control information (DCI). An acknowledgement (ACK)-only feedback protocol is proposed to best suit the scenario in question. The combination of dynamic indication of feedback resource and ACK-only feedback structure guarantees higher reliability and brings flexibility to the transmission of HARQ feedback as confirmed by the simulations.