Energy efficiency of massive MU-MIMO with limited antennas in downlink cellular networks

In this paper, we analyze and optimize the energy efficiency (EE) of multi-user (MU) multiple-input and multiple-output (MIMO) in downlink multi-cell networks. We first analyze the spectral efficiency (SE) of MU-MIMO systems with a huge number of base station (BS) antennas by using maximal ratio transmission (MRT) and zero-forcing (ZF) precoders in downlink cellular networks. In the analysis, we assume that the number of the BS antennas is huge but limited, which is different from the conventional analysis. Then, we use the analytical SE result and the EE definition to obtain the system EE expression and further analyze the effect of the number of BS antennas and the number of served users on system EE. Since the system EE on the numbers of BS antennas and the number of served users is approximately concave, the corresponding iterative optimization algorithms are proposed to achieve the maximum system EE. The computer simulation results show that the analytical results are accurate and each served user needs about 2 active BS antennas to obtain the maximum system EE when the channel state information is available at the transmitter.