Hybrid retransmission scheme for QoS-defined 5G ultra-reliable low-latency communications

One of the key challenges in next generation 5G networks is to deliver Ultra-Reliable Low-Latency Communications (URLLC). Recent advances in information theory about principles that govern short packet transmissions pointed out that, for the URLLC typical short packet dimension, achieving higher reliabilities comes at the price of a lower maximum achievable rate, thus reducing spectral efficiency. Hence, retransmissions are used in LTE and planned for 5G, in order to achieve reliability with a better resource consumption, at the price of increased packet latency. Keeping in mind the stringent requirements for URLLC, in this paper we analyze the tradeoffs and limitations of retransmission strategies considered in the literature, either too demanding in terms of wireless resources and aggressive URLLC performance or the contrary. Then we propose a novel scheme, whose purpose is to match the URLLC requirements, minimizing the resource consumption. We evaluate the schemes through simulations and highlight the advantages of the proposed scheme, providing also insights on the performance HARQ techniques in URLLC scenarios.