A Codebook-Based Concept for Hybrid CSI Feedback in FDD Massive MIMO Systems

In this paper we discuss the pros and cons of non-precoded and beamformed Channel State Information (CSI) downlink (DL) reference signal schemes for Frequency Division Duplex (FDD) massive MIMO systems. We propose a hybrid mode of operation between the two, factorizing the antenna ports into a set of slow and fast ports with different update rates using a codebook with factorized precoders designed for a dual-polarized Uniform Planar Array (UPA). We show with simulation results that the proposed hybrid CSI feedback scheme can increase both average and 5th percentile spectral efficiency compared to the non-precoded CSI reference signal scheme. Compared to the beamformed CSI reference signal scheme, it can significantly increase the 5th percentile spectral efficiency with only a small loss in average spectral efficiency.