Energy-efficient Training-assisted Transmission Strategies for Closed-loop MISO Systems

This paper studies energy-efficient transmission strategies for a closed-loop downlink multiple-input-single-output (MISO) system, where a communication period consists of three phases for uplink training, downlink data sending, and base station (BS) idling. For both delay-tolerant and delay-sensitive services, the durations of the three phases are optimized, aimed at maximizing the energy efficiency (EE) of the system. To this end, we derive the approximate average net spectrum efficiency (SE) and outage probability with imperfect uplink channel estimation, which are used to characterize the quality-of-service (QoS) requirements for the two kinds of services, respectively. The impact of QoS requirement, signal-to-noise ratio (SNR), and circuit power consumption on the optimal transmission durations is analyzed. For delay-tolerant services, analytical results show that the EE-oriented design leads to a longer training duration than the SE-oriented design in general. For delay-sensitive services, it is shown that introducing BS idling is crucial in improving the EE. The challenges and opportunities of applying the proposed transmission strategies in current and future cellular systems are discussed, and the transmission strategies are extended from single-user single-service to multiuser mixed-service scenarios. Simulation results demonstrate the significant EE gain of the EE-oriented design over the SE-oriented design in both single-user and multiuser scenarios.