Abstract: In this letter, a comparative analysis of the hardening effect for concentrated and distributed massive multiple-input-multiple-output channels (C-mMIMO and D-mMIMO, respectively) is presented. The analysis is carried out in two buildings with similar structural characteristics, considering two frequency bands of interest for 5G deployments, 3.5 and 26 GHz; taking into account both experimental data at 3.5 GHz and simulation results obtained in the 26 GHz band using a rigorous and well-tested ray tracing method. Both, measurements and simulations emulated C-mMIMO and D-mMIMO systems in an indoor cell in the framework of a time division duplex (TDD)-orthogonal frequency division multiplexing (TDD-OFDM) system. To quantify the level of hardening that the specific channels under analysis offer in the frequency domain, the standard deviation of the gain of the channels is used, as well as its evolution as the number of active antennas at the base station grows. The results obtained for both mMIMO systems show that a sufficiently high level of hardening occurs in indoor environments to contribute to the reliability of communication systems or sensor networks.

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