Ultra densification of the number of antennas combined with the use of large bandwidths in the millimeter wave (mmWave) spectrum is considered one of the main methodologies to achieve the quality of service requirements for future generations of wireless communications. Massive multipleinput multiple-output (mMIMO) cell-free (CF) systems have a large number of access points (APs) distributed in the coverage area, serving simultaneously a smaller number of users consuming the same timefrequency resources. In order to support realistic CF networks designs, this work proposes a performance analysis based on ray tracing simulations. The propagation modeling considers reflection, diffraction, diffuse scattering, atmospheric molecular absorption and foliage and rainfall losses. CF networks with APs equipped with multiple antennas operating in the 26 GHz, 38 GHz and 73 GHz bands are evaluated. From the simulation results, the communication channel is characterized and parameterized. Different performance analysis of CF networks are performed, based on downlink spectral efficiency. In addition, the performance of CF networks in rainy environments is evaluated, and it has been observed that this architecture promotes resistance to the effect of rain attenuation.INDEX TERMS cell-free, massive MIMO, millimeter wave, ray tracing, spectral efficiency.

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Resumo-O estudo de sistemas móveis sem células, em que não há divisão espacial e todas as unidades radiobase atendem a todos os usuários simultaneamente, se popularizou na última década. Neste trabalho, o desempenho do sistema sem células é comparado ao de sistemas baseados em células virtuais e em células pequenas. Tais comparações foram realizadas considerando diferentes modelos de perdas de percurso em larga escala, a fim de determinar sob quais condições de canal qual arquitetura apresenta melhor desempenho. Com base nos resultados de simulação, as células virtuais, em que os pontos de acesso escolhem quais usuários eles atenderão, atigem as taxas mais elevadas em todos os cenários, revelando que esta é a melhor arquitetura para os dois modelos de canal considerados.Palavras-Chave-Células pequenas, células virtuais, comunicações móveis, sistema sem células.Resumo-The study of cell-free mobile systems, in which there is no spatial division and all base units serve all users simultaneously, has become popular in the last decade. In this work, the performance of the cell-free system is compared to those based on virtual cells and on small cells. The comparison was performed considering different large-scale path loss models, in order to determine the channel conditions in which each architecture has better performance. Based on the results, the virtual cells, where the access points select which users they will serve, provide the highest rates in all scenarios, revealing that this is the best architecture for the two channel models considered.Palavras-Chave-Small cell, virtual cell, mobile communications, cell-free.

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