This paper investigates reconfigurable intelligent surface (RIS)-aided frequency division duplexing (FDD) communication systems. Since the downlink and uplink signals are simultaneously transmitted in FDD, the phase shifts at the RIS should be designed to support both transmissions. Considering a single-user multiple-input multiple-output system, we formulate a weighted sum-rate maximization problem to jointly maximize the downlink and uplink system performance. To tackle the non-convex optimization problem, we adopt an alternating optimization (AO) algorithm, in which two phase shift optimization techniques are developed to handle the unit-modulus constraints induced by the reflection coefficients at the RIS. The first technique exploits the manifold optimization-based algorithm, while the second uses a lower-complexity AO approach. Numerical results verify that the proposed techniques rapidly converge to local optima and significantly improve the overall system performance compared to existing benchmark schemes.

翻译：本文研究可重构智能表面（RIS）辅助的频分双工（FDD）通信系统。由于FDD系统中下行与上行信号同时传输，RIS处的相移需兼顾两种传输需求。针对单用户多输入多输出系统，我们构建了一个加权和速率最大化问题，旨在联合优化下行与上行系统的性能。为求解这一非凸优化问题，我们采用交替优化（AO）算法，并开发了两种相移优化技术以处理RIS反射系数带来的单位模约束。第一种技术基于流形优化算法，第二种则采用低复杂度的AO方法。数值结果验证了所提技术能够快速收敛至局部最优解，且相较于现有基准方案显著提升了整体系统性能。