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This paper presents an analysis of the massive multiple input and multiple output (MIMO) channel in an indoor picocell with a high number of active user terminals and a base station consisting of a virtual array with up to one hundred elements. The analysis is based on the results of a measurement campaign carried out in the 3.2 to 4 GHz band in a scenario of reduced size and with a symmetrical geometry, in which users are also placed in an orderly manner. The channel meets the condition of favorable propagation depending on several factors, one of the most important being the spatial distribution of users. Results concerning the inverse condition number as well as the channel sum capacity are included. Another factor that determines the performance of massive MIMO systems when operated in an orthogonal frequency division multiplexing (OFDM) framework is the frequency selectivity of the channel that limits the size of the coherence block (ChB). Focusing on the most significant results achieved, it can be concluded that the channel reaches a capacity of 89% with respect to an i.i.d. Rayleigh channel. Concerning the cumulative distribution function (CDF) of the sum capacity, it can also be observed that the tails are not very pronounced, which indicates that a homogeneous service can be given to all users. Regarding the number of samples that make up the ChB, although it is high in all cases (of the order of tens of thousands), it strongly depends on the degree of correlation used to calculate the coherence bandwidth.

In this paper, research results on the applicability of ray-tracing (RT) techniques to model massive MIMO (MaMi) channels are presented and discussed. The main goal is to show the possibilities that site-specific models based on rigorous RT techniques, along with measurement campaigns considered for verification or calibration purposes where appropriate, can contribute to the development and deployment of 5G systems and beyond using the MaMi technique. For this purpose, starting from the measurements and verification of the simulator in a symmetric, rectangular and accessible scenario used as the testbed, the analysis of a specific case involving channel characterisation in a large, difficult access and measurement scenario was carried out using the simulation tool. Both the measurement system and the simulations emulated the up-link in an indoor cell in the framework of a MaMi-TDD-OFDM system, considering that the base station was equipped with an array consisting of 10 × 10 antennas. The comparison of the simulations with the measurements in the testbed environment allowed us to affirm that the accuracy of the simulator was high, both for determining the parameters of temporal dispersion and frequency selectivity, and for assessing the expected capacity in a specific environment. The subsequent analysis of the target environment showed the high capacities that a MaMi system can achieve in indoor picocells with a relatively high number of simultaneously active users.