Reconfigurable intelligent surface (RIS) has recently emerged as a promising technology for wireless communications, which intelligently controls the phase shift of each unit cell to form desired beams. Most prior works on RIS consider time-division duplexing (TDD) systems, in which the same phase shifts can be applied to both uplink and downlink due to the channel reciprocity. However, for frequency-division duplexing (FDD) systems, using the same phase shifts will result in beam misalignment, thereby leading to performance degradation. To address this issue, in this article, we study the practical RIS design and beamforming optimization for FDD systems. By representing the phase shifts of RIS with the equivalent circuit model which includes the resistance, inductances, and tunable capacitance, we propose a methodology to design the circuit parameters (i.e., inductances and capacitance) to meet the desired reflection requirements (i.e., phase tuning range, reflectivity, and zero phase slope) of both the uplink and downlink transmissions in FDD systems. Given the designed inductances, a practical binary RIS reflection model corresponding to two reflection states is then proposed. Furthermore, based on the proposed reflection model, a problem is formulated to jointly optimize the active and passive beamforming such that the minimum array response gain of the uplink and the downlink is maximized. An efficient iterative algorithm is proposed to obtain a suboptimal solution. Simulation results show that our proposed RIS design outperforms those benchmarks which design the circuits by only optimizing either uplink or downlink.

中文翻译：

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FDD 系统的可重构智能表面：设计与优化


可重构智能表面（RIS）最近成为一种有前景的无线通信技术，它可以智能地控制每个单元的相移以形成所需的波束。大多数 RIS 先前的工作都考虑时分双工 (TDD) 系统，其中由于信道互易性，相同的相移可以应用于上行链路和下行链路。然而，对于频分双工（FDD）系统，使用相同的相移将导致波束失准，从而导致性能下降。为了解决这个问题，在本文中，我们研究了 FDD 系统的实用 RIS 设计和波束成形优化。通过用包括电阻、电感和可调电容的等效电路模型表示 RIS 的相移，我们提出了一种设计电路参数（即电感和电容）以满足所需反射要求（即相位调谐）的方法。 FDD 系统中上行链路和下行链路传输的范围、反射率和零相位斜率。给定设计的电感，然后提出对应于两种反射状态的实用二元RIS反射模型。此外，基于所提出的反射模型，提出了联合优化有源和无源波束形成的问题，使得上行链路和下行链路的最小阵列响应增益最大化。提出了一种有效的迭代算法来获得次优解。仿真结果表明，我们提出的 RIS 设计优于仅通过优化上行链路或下行链路来设计电路的基准。