Multi-connectivity for Urllc and Coexistence with Embb in Time-varying and Fading Channels
Ultra-reliable and low latency communications (URLLC) is a novel use case of 5G. It has challenging requirements like such as low block error rates (BLERs) and stringent latency targets. Multi-connectivity, in which multiple base stations (BSs) transmit the same data to the user, is a key technique in 5G to meet the stringent reliability requirements. URLLC data is immediately transmitted by puncturing the ongoing enhanced mobile broadband (eMBB) transmission to satisfy the latency constraint. However, this results in an increase in the BLER of the eMBB users. We first propose a low complexity multi-connectivity MCS selection algorithm (MCMSA) to select the subset of co-operating BSs and the modulation and coding schemes (MCSs) they employ. The goal is to minimize the eMBB throughput loss while satisfying the URLLC constraints. We study two types of multi-connectivity: orthogonal transmission (OT) and joint transmission (JT). We derive tractable expressions to calculate the achievability, which is the probability that the URLLC reliability requirement can be satisfied by the multi-connectivity. We do so for flat fading and frequency-selective fading scenarios. Our results highlight the trade-offs between URLLC achievability, eMBB throughput loss, and channel state information (CSI) feedback overhead of OT and JT. We then consider time-varying channels, in which the CSI report from the URLLC user to the BSs about the downlink channel gains becomes partially outdated by the time the BSs transmit data. We propose a novel stochastic BLER constraint for selecting MCSs at the time of transmission. We derive expressions for the conditional probability that the BLER of an MCS at the time of transmission is less than the target given the CSI fed back. These expressions enable the selection of MCS at the time transmission and meets the URLLC error target with high probability. Our results bring out the significant impact of feedback delays on reliability even at moderate Doppler spreads.