Antenna array configurations for massive MIMO outdoor base stations

Massive MIMO predicts unprecedented capacity and energy efficiency improvements in future networks, by the use of base stations with hundreds of antennas. However the crucial question of determining suitable, or even optimal, antenna hardware solutions for deploying such a large number of antennas in constrained physical spaces is still open. Here we analyse the potential benefits of using compact arrays, dual-polarized arrays, and 2D arrays in outdoor scenarios using a realistic 3D spatial channel model and accurate antenna electromagnetic simulations. In particular we show that it is detrimental to reduce the distance between elements below λ/3. Actually, better performances can be achieved by increasing the number of antennas using dual polarization instead (if random polarization of user antennas is assumed). We also show that good performance can be achieved by using 2D arrays, but at the expense of an increased number of total elements when compared to horizontal linear arrays.