Cooperative MEC-Enabled HAPS and UAV-RIS Assisted Task Offloading in ITS Systems

Non-Terrestrial Networks (NTNs), comprising Unmanned Aerial Vehicles (UAVs) and High Altitude Platform Stations (HAPS) equipped with Mobile Edge Computing (MEC), offer promising solutions for network traffic and tasks offloading from ground users. To enhance the reliability and energy efficiency of such systems, Reconfigurable Intelligent Surfaces (RIS) can be deployed to control wireless signal propagation. In this context, we propose in this paper a novel MEC-enabled framework with HAPS and RIS-equipped UAVs (UAV-RISs) to optimize task offloading from ground users. Our objective is to minimize the tasks' average end-to-end (E2E) delay, under constraints of UAV and HAPS power capacity and E2E service delay threshold, through the optimization of task assignment and UAV-RIS phase-shift configuration. Given the NP-hardness of the problem, we decompose it into two subproblems. The first consists of optimizing the RIS phase shifts to minimize the RIS-assisted communication delay. The second tackles the task assignment problem using a Particle Swarm Optimization (PSO)-based approach, considering the RIS phase shifting solution previously developed. Through simulations, we validate the efficacy of our approach, which outperforms other benchmarks in terms of task average E2E delay and task offloading success rate.