Machine-type wireless communications enablers for beyond 5G: Enabling URLLC via diversity under hard deadlines

URLLC is a key design aim in 5G and a mandatory prerequisite in the future uncountable number of industrial applications. In this regard, cooperative relaying and diversity sources in time and frequency domains are introduced as URLLC enablers to support higher reliability and lower latency. The objective of this work is to study two URLLC performance metrics namely, probability of time underflow where the aggregated transmission time is below the time threshold, and reliability which refers to successfully delivering the message within the time window. We examine the impact of cooperative relaying and exploiting time and frequency diversities on the aforementioned performance metrics. We provide the approximated upper bound of the probability of time underflow under time and frequency diversities. In addition, we indicate the maximum achievable reliability as a function of the time threshold for a given probability of time underflow. The performance advantage of cooperative diversity compared to the single transmission to meet URLLC requirements is also highlighted.