The conventional reconfigurable intelligent surface (RIS) assisted far-field communication systems can only implement angle beamforming, which actually limits the capability for reconfiguring the wireless propagation environment. To overcome this limitation, this paper proposes a newly designed frequency diverse RIS (FD-RIS), which can achieve joint distance-angle beamforming with the assistance of the time modulation technology. The signal processing model for FD-RIS-aided wireless communications is first derived. Then, an optimization problem aimed at maximizing the achievable rate is formulated where the frequency-time modulations are jointly optimized to achieve distance-angle beamforming. Furthermore, a novel iterative algorithm based on the cross-entropy optimization (CEO) framework is proposed to effectively handle the non-convex optimization problem. The numerical results validate that the proposed FD-RIS assisted communication scheme can achieve a notable performance improvement compared with the baseline scheme utilizing traditional RIS. In addition, the effectiveness of the proposed CEO algorithm is further verified by comparing with the benchmark using the genetic algorithm (GA).

翻译：传统的可重构智能反射面辅助远场通信系统仅能实现角度波束成形，这实际上限制了其重构无线传播环境的能力。为克服这一局限，本文提出一种新设计的频率分集智能反射面，该方案借助时间调制技术可实现联合距离-角度波束成形。本文首先推导了FD-RIS辅助无线通信的信号处理模型，随后构建了以最大化可达速率为目标的优化问题，其中通过联合优化频率-时间调制来实现距离-角度波束成形。进一步地，提出了一种基于交叉熵优化框架的新型迭代算法，以有效处理该非凸优化问题。数值结果表明，与采用传统RIS的基准方案相比，所提出的FD-RIS辅助通信方案能实现显著的性能提升。此外，通过与采用遗传算法的基准方案进行对比，进一步验证了所提CEO算法的有效性。