Spatial channel hardening is one of the key properties of a massive MIMO system. In this paper, we investigate the spatial channel hardening based on ray tracing channel simulations with different array antenna topologies deployed in different radio wave propagation conditions. Investigated are 9 different indoor deployment scenarios of massive MIMO. Considered are the empty room, the rich-furniture room and the room-to-room scenarios, where massive multi-antennas are deployed in the co-located, the split and the cell-free array topologies. The simulation results show that the cell-free topology results in the most-spatially-hardened channel in target area, while the co-located antennas the least-spatially-hardened. It is also found that the convergence of channel spatial variations is consistent with the number of paths estimated in multipath scenarios, i.e., the more multipath components are estimated in a propagation scenario, the smaller the number of antennas is required for the channel to reach the spatially hardened state. In our simulations, the rom-to-room scenario leads to the most-spatially-hardened channel, while for the empty room scenario it is the opposite.
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