Evaluation of Cell-Free Millimeter-Wave Massive MIMO Systems Based on Site-Specific Ray Tracing Simulations

Abstract

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.

Energy and Spectral Efficiencies of Cell-Free Millimeter-Wave Massive MIMO Systems Under Rain Attenuation Based on Ray Tracing Simulations

“…The CF mMIMO system architecture, considered in this work, is illustrated in Fig. 1, in which M APs, each one equipped with an uniform linear array (ULA) of L antennas, are arranged in the sites serving K single-antenna user equipment (UE) consuming the same time-frequency resources [16], [32]. The channel gain vector between the mth AP and the k-th UE, denoted as g m,k , is given by [16], [33]…”

“…Despite being originally proposed to operate in the sub-6 GHz band [5], [11], several works presented in the literature analyze CF networks with carriers in the mmWave band [12]- [16]. The mmWave spectrum comprises the bands between 30 GHz and 300 GHz [17], [18].…”

“…Considering the importance of CF networks operating in the mmWave spectrum, the impact of rain attenuation on key performance metrics, such as spectral efficiency (SE) and energy efficiency (EE), needs to be investigated. To the best of authors knowledge, the effect of rain attenuation on CF networks has only been investigated in [16]. The mentioned work evaluates the influence of rainfall attenuation on the average SE in an environment with low urban density.…”

“…Therefore, this work presents a study of the impact of rain attenuation on the SE and EE of CF networks operating in the mmWave spectrum. A hybrid methodology has been applied, which combined the ray tracing deterministic method, used to characterize the channel large-scale parameters, with a random small-scale fading model [16], [27]. The site-specific three-dimensional ray tracing model is based on the shootingand-bouncing rays (SBR) method and consider the effects of reflection, diffraction, diffuse scattering, atmospheric molecular absorption, foliage attenuation and rain attenuation.…”

“…In contrast, this study evaluates CF networks by means of realdata ray tracing simulations applied to environments models comparable to typical urban settings. This methodology enables an accurate evaluation of the impact of different factors on the links [30], [31], which is crucial for designing local CF network projects [16], [27].…”