Pilot Assignment for Cell-Free Systems Serving Ground Users and Unmanned Aerial Vehicles

This study evaluates the performance of cell-free massive multiple input multiple output systems serving UAVs subject to Rician fading channels. The studied cell-free systems can be implemented employing three different levels of cooperation between the access points, namely: level 3, which is a fully centralized approach; level 2, which is a partially centralized approach with large-scale fading decoding weights; and level 1, which is a fully decentralized approach resembling small-cell systems. We study the performance of five different pilot assignment methods, while considering a mix of served UAVs and ground user equipments. The results show how the presence of UAVs with good propagation conditions impacts the overall performance in terms of the achieved spectral efficiency. Specifically, our results show that one of the pilot assignment algorithms provides the best trade-off between the spectral efficiencies allocated to UAVs and ground users.