Energy-Efficient Ultra-Reliable Low-Latency 6G Communication via Cooperative UAV-RIS-Assisted OTFS Networks and Hybrid PSO-DRL Optimisation

Ultra-reliable low-latency communication (URLLC) is a critical 6 G service, yet achieving <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$&lt;10 ~\text{ms}$</tex> latency, <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\geq 99.9 \%$</tex> reliability, and energy efficiency remains challenging in dense, dynamic environments. This work proposes a UAV-RIS cooperative architecture combining mmWave UAV relays and reconfigurable intelligent surfaces to meet URLLC demands. The joint optimization of UAV trajectory, RIS phase shifts, and resource allocation is formulated as a mixed-integer nonlinear program and solved using a hybrid particle swarm optimization-deep reinforcement learning (PSO-DRL) approach, achieving sub-0.5s decision updates. Hardware-in-the-loop urban testbed experiments and Monte Carlo simulations show up to 35 % energy savings, 42 % latency reduction, and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$99.9-99.95 \%$</tex> reliability at high user densities (<tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$1,000 \text{UE} / \text{km}^{2}$</tex>), outperforming ground-based baselines. These results validate the practicality and scalability of UAVRIS integration for real-time, energy-efficient 6G URLLC.