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Ultra-Reliable Low Latency Communications (URLLC) is a key service in fifth generation (5G) networks, that requires stringent Quality of Service (QoS) in terms of latency and reliability. As URLLC services may require specific numerology and/or specific channel access and re-transmission strategies, network slicing has been proposed as a solution for multiplexing them with other services such as enhanced Mobile Broadband (eMBB). Once the URLLC slice is configured and resources are dimensioned and allocated to it, URLLC performance targets should be attained thanks to the 5G New Radio (NR) low latency and high reliability features. However, in vehicular services such as safety message exchange, URLLC slice resource dimensioning cannot be static due to the varying number of vehicles in the cell. We show in this paper how the delay for slice reconfiguration alters the URLLC performance and propose a proactive resource reservation scheme that anticipates slice needs and allows ensuring URLLC targets. In order to reduce the impact of this proactive reservation on eMBB performance, we make use of vehicle trajectory prediction and show that limiting anticipated reservation to fewer cells allows reaching the target URLLC QoS with a limited degradation of the network capacity.

This work proposes a new joint link adaptation and HARQ (Hybrid Automatic Repeat Request) scheme for URLLC (Ultra-Reliable Low-Latency Communication) services. We consider the case where the transmitter knows only the average SNR (Signal-to-Noise Ratio) and not the instantaneous one. In the proposed scheme, the optimal maximum number of allowable HARQ transmissions and the optimal MCS (Modulation and Coding Scheme) level are determined for each packet to maximize the spectral efficiency. Our adopted approach exploits the channel diversity and increases the flexibility of the scheduling mechanism. Simulation results show that the proposed retransmission policy and link adaptation scheme increases the system performance in terms of spectral efficiency, while satisfying the latency and reliability constraints.