Stochastic Geometry-Based Analysis of Cell-Free Massive MIMO Systems With Aerial Users

Cell-free massive multiple-input-multiple-output (CF-mMIMO) systems are promising deployment paradigms for next-generation networks. They comprise a large number of base stations (BSs) and simultaneously serve all users over the same time and frequency resources. We analyze the performance of integrating aerial users, such as unmanned aerial vehicles (UAVs), into this novel system. Specifically, we consider a CF-mMIMO network containing both ground and aerial users and study the influence of system parameters on the signal-to-interference-plus-noise ratio (SINR) and rate coverage performance. Additionally, we use tools from stochastic geometry to capture the spatial randomness of users and BSs and compare the performance of the CF-mMIMO system to that of traditional small cell systems. Given the improvement of dedicated antennas, such as up-tilted/down-tilted antennas, on the performance of small cell systems, we include the up-tilted/down-tilted antenna model in this work and analyze the omnidirectional antenna model as a special case. We derive both the exact expressions and dominant-signal-based approximations for SINR and rate coverage. Our numerical results demonstrate that the CF-mMIMO system exhibits better performance at low values of SINR and rate thresholds and higher minimum achievable SINR compared to the small cell system. Furthermore, we observe that users benefit dramatically from increasing altitudes and establishing line-of-sight (LoS) channels with BSs.