Spectral and energy efficiency of downlink MU-MIMO systems with MRT

This paper studies the achievable spectral efficiency (SE) of downlink multiuser multiple-input-multiple-output (MIMO) system, where the base station (BS) is deployed an arbitrary finite antenna number and communicates simultaneously with many users. We assume that the BS has accurate channel state information (CSI) and adopt maximum ratio transmission (MRT) precoding. An accurate analytical result for the achievable SE is obtained. Based on the analytical result on the achievable SE, we further study the achievable energy efficiency (EE) of multiuser MIMO system by considering an energy consumption model. Results indicate that the increasing number of BS antennas can boost the achievable SE of system, whilst the achievable SE tends to a saturated rate in the high signal-to-noise ratios (SNR) regime. Furthermore, an important conclusion is that the increasing number of users is beneficial for the achievable EE and there is an optimal antenna number to maximize the EE of system.