On the Uplink Distributed Detection in UAV-enabled Aerial Cell-Free mMIMO Systems

In this paper, we investigate the uplink signal detection in cell-free massive MIMO systems with unmanned aerial vehicles (UAVs) serving as aerial access points (APs). The ground users are equipped with multiple antennas and the ground-to-air propagation channels are subject to correlated Rician fading. To overcome huge signaling overhead in the fully-centralized detection in cell-free systems, we propose a two-layer distributed uplink detection scheme, where the uplink signals are first detected in AP-UAVs by using the minimum mean-squared error (MMSE) detector based on local channel state information (CSI), and then collected and weighted combined at the CPU-UAV to obtain the refined detection. By using the operator-valued free probability theory, the asymptotic expressions of the combining weights are obtained, which only depend on the statistical CSI and show excellent accuracy compared to the exact but intractable expressions. Based on the proposed scheme, we further investigate the impacts of different deployment scenarios on the spectral efficiency (SE). Numerical results show that in urban and dense urban environments, it is more beneficial to deploy more AP-UAVs to increase SE. Nonetheless, in suburban environment, an optimal combination of the number of AP-UAVs and the number of antennas per AP-UAV exists to maximize SE.