Hierarchical IRS-Assisted User Association for Sum-Rate Maximization in Hybrid THz LEO-HAPS-Terrestrial Networks

In this paper, we propose a hierarchical intelligent reconfigurable surface (IRS)-assisted terahertz communication framework to ensure robust user equipment (UE) connectivity under severe line-of-sight (LoS) blockages. The system employs a three-tier IRS structure comprising terrestrial, aerial, and satellite-mounted IRSs. We derive a closed-form expression for power allocation and propose a binary path selection strategy to prioritize IRS layers. Furthermore, a stable matching-based algorithm optimizes UE-IRS associations using achievable data rates. Simulation results demonstrate that the proposed scheme achieves near-optimal sum rate performance, closely approaching the Exhaustive and Hungarian methods while outperforming the Nearest, Greedy, and Random baselines.