On the selection of radiating elements for compact indoor massive-multiple input multiple output base stations

Massive-multiple input multiple output (MIMO) was recently proposed as a key solution for improving the performance of multi-user MIMO in future cellular generations. The main paradigm in this novel MIMO technique is to use a very large number of antenna elements at the base station (BS), which was shown to drastically increase capacity while saving transmission power. However, envisioning indoor BS with such number of antennas it is essential to assess the implementation of compact arrays based on different antenna candidates, which constitutes the main objective of this work. Antenna solutions ranging from miniaturised patches to triple-polarised radiators are proposed. Characteristics such as return loss and mutual couplings are obtained via full-wave electromagnetic simulations and as a function of elements density in a given limited BS real estate. The different antenna candidates are compared by computing the signal-to-interference plus noise ratio in both noise and interference-limited regimes and for different multi-user MIMO precoding strategies. This analysis allows reporting on optimal antenna densities above which performance degrades despite an increasing number of antennas, and identifying the main cause of this limitation for the different antenna types and scenarios. Finally, practical considerations such as antenna miniaturisation, bandwidth and cost are included in the discussion.