Effectively supporting millimeter-wave (mmWave) beamforming is still a major challenge in 5G cloud radio access network (5G C-RAN) systems with evolved common public radio interface-based (eCPRI-based) fronthaul. Herein, an optical true time delay pool based hybrid beamforming (OTTDP-HBF) scheme, enabling centralized beamforming control, is proposed for mmWave 5G C-RAN systems. The weight control of the OTTDP-HBF is physically implemented by a pre-designed optical wavelength matrix which is mapped from a series of optical carriers. After introducing optical true time delay, this optical wavelength matrix then maps to the defined OTTDP. In this scheme, all physical implementation and the computational processing of analog beamforming can be centrally deployed into a centralized unit or distributed unit (CU/DU). Each active antenna unit (AAU) therefore becomes very simple. For single-user and multi-user scenarios, the OTTDP-based hybrid precoders are formulated respectively. In the developed OTTDP-based multi-user hybrid precoder, the spectral efficiency is improved by making use of all RF chains. For a 9-element uniform planar array deployed at an AAU, a designed example of the OTTDP-HBF is presented, where spectral efficiency curves obtained via different precoding schemes for single-user and multi-user scenarios are compared and discussed respectively.

有效支持毫米波 (mmWave) 波束赋形仍然是 5G 云无线接入网络 (5G C-RAN) 系统的主要挑战，该系统具有基于公共无线电接口 (eCPRI-based) 演进的前传。在此，针对毫米波 5G C-RAN 系统提出了一种基于光学真实时延池的混合波束成形 (OTTDP-HBF) 方案，可实现集中式波束成形控制。OTTDP-HBF 的权重控制物理上是通过预先设计好的光波长矩阵来实现的，该矩阵从一系列光载波映射而来。在引入光真实时间延迟后，该光波长矩阵然后映射到定义的 OTTDP。在该方案中，模拟波束成形的所有物理实现和计算处理都可以集中部署到集中单元或分布式单元（CU/DU）中。因此，每个有源天线单元 (AAU) 变得非常简单。对于单用户和多用户场景，分别制定了基于 OTTDP 的混合预编码器。在开发的基于 OTTDP 的多用户混合预编码器中，通过利用所有射频链提高了频谱效率。对于部署在 AAU 的 9 单元均匀平面阵列，提出了 OTTDP-HBF 的设计示例，其中分别比较和讨论了通过不同预编码方案获得的单用户和多用户场景的频谱效率曲线。