Beyond diagonal reconfigurable intelligent surfaces (BD-RIS) have emerged as a transformative technology for enhancing wireless communication by intelligently manipulating the propagation environment. This paper explores the potential of BD-RIS in improving cognitive radio enabled multilayer non-terrestrial networks (NTNs). It is assumed that a high-altitude platform station (HAPS) has set up the primary network, while an uncrewed aerial vehicle (UAV) establishes the secondary network in the HAPS footprint. We formulate a joint optimization problem to maximize the secrecy rate by optimizing BD-RIS phase shifts and the secondary transmitter power allocation while controlling the interference temperature from the secondary network to the primary network. To solve this problem efficiently, we decouple the original problem into two sub-problems, which are solved iteratively by relying on alternating optimization. Simulation results demonstrate the effectiveness of BD-RIS in cognitive radio-enabled multilayer NTNs to accommodate the secondary network while satisfying the constraints imposed from the primary network.

翻译：超越对角可重构智能表面（BD-RIS）作为一种变革性技术，通过智能调控传播环境来增强无线通信。本文探讨了BD-RIS在改善认知无线电多层非地面网络（NTNs）中的潜力。假设高空平台站（HAPS）建立了主网络，而无人驾驶飞行器（UAV）在HAPS覆盖范围内建立了次网络。我们构建了一个联合优化问题，通过优化BD-RIS相移和次网络发射机功率分配，同时控制次网络对主网络的干扰温度，以最大化保密速率。为有效求解此问题，我们将原问题解耦为两个子问题，并依靠交替优化方法进行迭代求解。仿真结果表明，BD-RIS在认知无线电多层NTNs中能有效容纳次网络，同时满足主网络施加的约束。