  Massive MIMO (MaMIMO) Channel State Information (CSI) based user positioning
systems using Convolutional Neural Networks (CNNs) show great potential,
reaching a very high accuracy without introducing any overhead in the MaMIMO
communication system. In this study, we show that both these systems can
position indoor users in both Line-of-Sight and in non-Line-of-Sight conditions
with an accuracy of around 20 mm. However, to further develop these positioning
systems, more insight in how the CNN infers the position is needed. The used
CNNs are a black box and we can only guess how they position the users.
Therefore, the second focus of this paper is on opening the black box using
several experiments. We explore the current limitations and promises using the
open dataset gathered on a real-life 64-antenna MaMIMO testbed. In this way,
extra insight in the system is gathered, guiding research on MaMIMO CSI-based
positioning systems using CNNs in the right direction.

Signal Processing