CSI Type-II Codebook of Codebooks

Large MIMO arrays can improve network performance when configured with accurate channel state information. 5G supports two formats, or types, for feeding back channel state information to the transmitter based on codebooks. Type-I is a direct codebook format containing a small set of predefined codewords, while type-II feedback is a construction-based feedback format where the channel state information is represented as a sum of multipath components with complex gains. Type-II feedback involves more overhead than type-I feedback, but it is more computationally expensive and neither format is sufficiently rich enough for low- and mid-band deployments. Type-II feedback can be extended to support richer feedback, but the computational complexity grows exponentially. In this paper, we propose two new methods for calculating type-II precoding matrix indicator (PMI) feedback using the inherent structure of oversampled DFT codebooks. The first method is based on iteratively decomposing the PMI, while the second method uses a codebook-of-codebooks to integrate the correlation of oversampled DFT codebooks into the beam selection process. We show in simulation that the proposed codebook-of-codebooks method can increase the effective spectral efficiency by up to 25% in multi-user settings with the same computational cost.