Grassmannian Product Codebooks for Limited Feedback Massive MIMO With Two-Tier Precoding

In frequency-division duplex full-dimension massive MIMO systems, one of the main challenges is to obtain sufficiently accurate channel state information (CSI) at the transmitter to enable efficient multi-user MIMO transmission. In this paper, we propose a novel dual stage Grassmannian product quantization approach that is suitable for high-dimensional CSI quantization and feedback. We apply the proposed method for outer-tier CSI feedback in two-tier precoding architectures, which employ channel-subspace based outer-tier precoding strategies, such as maximum eigenmode transmission. The proposed method is especially effective if the channel can be decomposed in the angular domain, such that DFT codebooks enable an efficient CSI compression. Our dual stage product codebook quantization approach mitigates the well-known inefficiency of oversampled DFT codebooks for growing codebook sizes, by varying the intermediate quantization dimension of the proposed quantizer.