A Cell-Free Scheme for UAV Base Stations with HAPS-Assisted Backhauling in Terahertz Band

Abstract

In this paper, we propose a cell-free scheme for unmanned-aerial-vehicle (UAV) base-stations (BSs) to manage the severe intercell interference between aerial and terrestrial nodes. Since the cell-free scheme requires a huge bandwidth for backhauling, we propose to use the terahertz (THz) band for the wireless backhaul links between UAV-BSs and central-processing-unit (CPU). Also, because the THz band requires a reliable line-of-sight (LoS) link, instead of a terrestrial CPU, we propose to use a high-altitude-platform-station (HAPS) as a CPU. At the first time-slot of the proposed scheme, users send their messages to UAVs at the sub-6 GHz band. Then each UAV applies match-filtering to align the received signals from users, and performs power allocation for the aligned signal of each user. At the second time-slot, we allocate orthogonal resource-blocks (RBs) for each user at the THz band, and send signals towards HAPS. In HAPS, for aligning the received signals for each user from different UAVs, we perform analog beamforming. Finally, we demodulate and decode the message of each user at its unique RBs. We formulate an optimization problem that maximizes the minimum SINR of users, and find the optimum allocated powers for users in each UAV by the bisection method. Simulation results prove the superiority of the proposed scheme compared with aerial-cellular and terrestrial-cell-free baseline schemes. Simulation results also showed that utilizing HAPS as a CPU is useful when the huge path-loss between UAV-BSs and HAPS in the THz band is compensated by a high number of antennas at HAPS.

Topics & Concepts