Energy-Efficient Massive MIMO in Massive Industrial Internet of Things Networks

Massive multiple-input–multiple-output (MIMO) systems can support a large number of Industrial Internet of Things (IIoT) devices using many service antennas that are equipped at a base station (BS). Even though massive MIMO can increase both spectral efficiency (SE) and energy efficiency (EE), to support numerous IIoT devices, a drastic amount of downlink power consumption can be required. We investigate the performance of downlink signal transmission for massive IIoT networks using massive MIMO, and propose a downlink signal transmission scheme that can significantly reduce the transmission power consumption with little SE loss. We consider a system in which the number of IIoT devices is much larger than the number of service antennas, and in order to simultaneously support a large number of IIoT devices, every orthogonal uplink reference signal (RS) may be reused by a few IIoT devices. In this situation, we derive simple SE approximations, and based on the approximation, a target data rate is determined. The target data rate is used to determine a target signal-to-interference and noise ratio (SINR) and the corresponding signal power. In addition, the SE approximation can be used as an upper bound and the corresponding signal power can be determined by setting an adjustable parameter, which is multiplied to the upper bound. The maximum allowable uplink/downlink signal power is predefined to prevent unexpected high-power requirements. The simulation results are provided to show the effectiveness of the proposed schemes.