Codebooks have been indispensable for wireless communication standard since the first release of the Long-Term Evolution in 2009. They offer an efficient way to acquire the channel state information (CSI) for multiple antenna systems. Nowadays, a codebook is not limited to a set of pre-defined precoders, it refers to a CSI feedback framework, which is more and more sophisticated. In this paper, we review the codebooks in 5G New Radio (NR) standards. The codebook timeline and the evolution trend are shown. Each codebook is elaborated with its motivation, the corresponding feedback mechanism, and the format of the precoding matrix indicator. Some insights are given to help grasp the underlying reasons and intuitions of these codebooks. Finally, we point out some unresolved challenges of the codebooks for future evolution of the standards. In general, this paper provides a comprehensive review of the codebooks in 5G NR and aims to help researchers understand the CSI feedback schemes from a standard and industrial perspective.

翻译：自2009年长期演进技术首次发布以来，码本一直是无线通信标准不可或缺的组成部分。它们为多天线系统获取信道状态信息提供了一种高效手段。如今，码本不再局限于一组预定义的预编码器，而是指代日益复杂的信道状态信息反馈框架。本文系统综述了5G新无线电标准中的码本设计。展示了码本的发展时间线与演进趋势，详细阐述了每个码本的动机、相应的反馈机制以及预编码矩阵指示符的格式。通过深入剖析这些码本的底层设计逻辑与直觉理念，本文给出了启发性见解。最后，指出了码本在未来标准演进中面临的若干未解决挑战。总体而言，本文对5G新无线电中的码本进行了全面综述，旨在帮助研究人员从标准与产业视角理解信道状态信息反馈方案。