Introducing reconfigurable intelligent surfaces (RISs) greatly improves the wireless propagation environment. Limited to feedback overheads and computing power, the low-complexity precoding and the cost-effective beamforming implementation are urgently demanded in RIS-empowered millimeter-wave cloud radio access networks (mmWave C-RANs). Herein, an optical true time delay pool-based hybrid beam-forming (OTTDP-based HBF) scheme, enabling centralized analog beamforming control, is proposed for RIS-empowered mmWave C-RANs. By using codebook quantization techniques, we develop a non-iterative limited feedback (NI-LF) precoding algorithm for the RIS-empowered mmWave C-RAN using the proposed OTTDP-based HBF. In the NI-LF precoding algorithm, the complex joint optimization problem of hybrid precoding and phase-shifting of RIS is equivalently formulated as a maximum ratio transmission (MRT) problem and a sparse reconstruction problem. For an active antenna unit (AAU) equipped with eight antennas, a designed example of the proposed OTTDP is presented. Furthermore, for a RIS-empowered mmWave C-RAN at 28 GHz, the received signal-to-noise ratio (SNR) curves obtained via different precoding schemes are compared and discussed. Compared with the precoding without RIS, the proposed NI-LF precoding by using different codebooks to quantize the phase-shifting of the RIS respectively achieves 20.56, 19.55, and 18.93 dB performance improvement for the user located near the 49-element RIS. Moreover, performance degradations due to limited feedback are analyzed. The computational complexity and overhead of the proposed NI-LF precoding algorithm are also given and discussed.

可重构智能表面 (RIS) 的引入极大地改善了无线传播环境。受限于反馈开销和计算能力，RIS 支持的毫米波云无线接入网络 (mmWave C-RAN) 迫切需要低复杂度的预编码和经济高效的波束成形实现。在此，针对 RIS 授权的毫米波 C-RAN 提出了一种基于光学真实时间延迟池的混合波束成形（基于 OTTDP 的 HBF）方案，可实现集中模拟波束成形控制。通过使用码本量化技术，我们使用提议的基于 OTTDP 的 HBF，为 RIS 授权的毫米波 C-RAN 开发了一种非迭代有限反馈 (NI-LF) 预编码算法。在 NI-LF 预编码算法中，RIS的混合预编码和相移的复杂联合优化问题等价地表述为最大比传输（MRT）问题和稀疏重构问题。对于配备八个天线的有源天线单元 (AAU)，提出了所提出的 OTTDP 的设计示例。此外，对于 RIS 支持的 28 GHz 毫米波 C-RAN，比较和讨论了通过不同预编码方案获得的接收信噪比 (SNR) 曲线。与没有 RIS 的预编码相比，所提出的 NI-LF 预编码通过使用不同的码本对 RIS 的相移进行量化，分别为位于 49 元素 RIS 附近的用户实现了 20.56、19.55 和 18.93 dB 的性能提升。此外，还分析了由于有限反馈导致的性能下降。