  Ultra-reliable and low-latency communications (URLLC) is expected to be
supported without compromising the resource usage efficiency. In this paper, we
study how to maximize energy efficiency (EE) for URLLC under the stringent
quality of service (QoS) requirement imposed on the end-to-end (E2E) delay and
overall packet loss, where the E2E delay includes queueing delay and
transmission delay, and the overall packet loss consists of queueing delay
violation, transmission error with finite blocklength channel codes, and
proactive packet dropping in deep fading. Transmit power, bandwidth and number
of active antennas are jointly optimized to maximize the system EE under the
QoS constraints. Since the achievable rate with finite blocklength channel
codes is not convex in radio resources, it is challenging to optimize resource
allocation. By analyzing the properties of the optimization problem, the global
optimal solution is obtained. Simulation and numerical results validate the
analysis and show that the proposed policy can improve EE significantly
compared with existing policy.
