LAUSR

This letter investigates the area spectral efficiency (ASE) of a two-tier network with multi-antenna base stations (BSs). Based on a tractable approximation, some key properties of ASE are analytically revealed. We demonstrate that the additional deployment of small-cell BSs is efficient for improving ASE. For the purpose of ASE maximization, the optimal number of active users is derived for each tier, which has a linear relationship with the number of antennas. Furthermore, under the optimal condition, ASE will increase linearly with the number of antennas. Different from single-antenna networks, we observe that range expansion can have either a positive or a negative effect on ASE.