Outer Loop Link Adaptation Enhancements for Ultra Reliable Low Latency Communications in 5G

Abstract

The very low block error rate (BLER) targets required in ultra-reliable low-latency communications (URLLC) call for new channel state information (CSI) feedback enhancements for an accurate link adaptation (LA) in the radio interface. This paper describes and analyses two new feedback reporting types, in addition to the legacy positive and negative acknowledgements (ACK/NACK) feedback, based on a form of soft-ACK reports. The aim is to improve the outer loop link adaptation (OLLA) accuracy in 5G New Radio (5G NR) based URLLC wireless communications systems where the amount of NACK events will be negligible. These schemes are based on the physical downlink data channel (PDSCH) decoding performance. In particular, the methods are based on an indication of the decoding margin of a PDSCH transmission, for the so-called Scheme A, and on an indication of the estimated block error probability (BLEP) of a PDSCH transmission, for the so-called Scheme B. In addition, two interference measurement (IM) approaches are analysed and compared based on non zero power CSI reference signal (NZP CSI-RS) and CSI interference measurement (CSI-IM) resources. The results show that Scheme A does not converge to any predefined BLER target while Scheme B allows faster convergence times towards the predefined target compared to legacy OLLA scheme. These results indicate that a new feedback reporting approach based on the estimated BLEP is suitable to achieve the tight latency requirements in URLLC scenarios together with efficient radio link performance.