ciated with a UE, each link corresponds to a beam. The second goal is addressed by designing a modular and scalable CSI framework. High-resolution spatial channel information is provided via two-stage high-resolution precoding. The first stage involves the choice of basis subset, and the second stage comprises a set of coefficients for approximating a channel eigenvector with a linear combination of the basis subset. Note that while beam management and CSI acquisition can be operated independently, they can be used together for mobile UEs. In NR, the DM-RS and the TRS are used to estimate the timing offset and frequency errors. To guarantee that the timing-offset and frequency-error of the antenna panels can be esti- mated independently, the DM-RS and TRS are grouped, and the grouping information is signaled to the UE. The UE distinguishes the time-frequency resource of reference signals 588 Chapter 4 allocated for different data streams. In this way, timing-offset and frequency-errors can be independently estimated for different MIMO layers in non-coherent MIMO transmission to improve the performance. For downlink transmission with multiple antenna panels, the accuracy of CSI acquisition is critical for the system performance. To characterize the channel directional information, the codebook and the related feedback mechanism are typically designed for CSI acquisition, especially for FDD systems. For users in the cell edge of TDD systems, reciprocal channel estimation based on uplink reference signals is also inaccurate, and codebook-based CSI feedback can help improve the performance. Codebook design for massive MIMO should be flexible for different antenna array structures and applicable to various numbers of anten- nas. For a uniform panel array, the codebook is designed similar to the one used for FD- MIMO. While a single DFT vector can only characterize one spatial channel path, an advanced codebook taking the combination of two or more DFT vectors as the precoder can capture the characteristics of multipat