Low Altitude 3-D Coverage Performance Analysis of Cell-Free RAN for 6G Systems

In this article, we propose to utilize a more flexible and scalable cell-free radio access network (CF-RAN) for 6G systems to promote the coverage performance of the low altitude three-dimensional (3-D) scenarios, where unmanned aerial vehicles (UAVs) and ground users (GUEs) are coexisted. User-centric association strategy and scalable receiving schemes such as maximum ratio combing (MRC), partial zero-forcing (P-ZF) and partial minimum-mean-square error (P-MMSE) are adopted to reduce the interference between UAVs and GUEs and improve the scalability of signal processing. Closed-form expressions for uplink spectral efficiency (SE) are derived with considered receivers. Based on the derived closed-form expressions, we propose a power allocation strategy by solving a multi-objective optimization problem between maximizing the average SE of UAVs and GUEs simultaneously with intelligent algorithm——Deep Q-Network (DQN). Numerical results verify the accuracy of the derived closed-form expressions and the effectiveness of the proposed power allocation schemes for low altitude 3-D coverage with coexisted UAVs and GUEs. The coverage performance analysis under various system parameters offers much of flexibilities for the optimization of the low altitude CF-RAN systems.