Abstract: This paper presents an experimental study devoted to the characterization and comparison of massive multiple-input multiple-output (mMIMO) channels in two sub bands of distinct frequency ranges: the upper frequency range 1 (FR1, 6.425-7.125 GHz) and in frequency range 2 (FR2, 25.7-26.3 GHz). A comprehensive measurement campaign was carried out in a research building at the University of Cantabria, emulating the uplink of a realistic indoor picocell scenario with the base station (BS) located in a meeting room and user terminals distributed across various indoor and outdoor positions. The measurements were performed in the frequency domain using a vector network analyzer, along with a planar electromechanical scanner used to emulate a mMIMO BS. The enhanced channel-sounding system integrates a radio-over-fiber link and combines at the BS a custom-designed 2 × 2 microstrip patch array with a four-port radio-frequency switch, thereby improving both realism and acquisition speed. The comparative analysis between the two frequency bands includes the evaluation of the overall and per-user spectral efficiency (SE), Jain's fairness index, root-mean-square delay spread, and coherence bandwidth. The obtained results provide valuable insights into the impact of the frequency range on achievable SE, channel dispersion, frequency selectivity, and spatial non-stationarity, highlighting practical implications for the design and optimization of mMIMO systems in future 5G and 6G networks.

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