Two-stage precoder for massive MIMO systems with limited feedback

Abstract

A two-stage precoder is able to solve the feedback overhead problem in frequency division duplex (FDD) massive MIMO systems since an outer precoder reduces the dimension of the effective channel which is fed back to the base station for inner precoder design. We show that optimization of an outer-precoder dimension is consequential for two-stage precoder design with limited feedback. A smaller dimension of the outer precoder is beneficial to both inter-group interference (IGI) cancelation and the amount of feedback, but restricts same-group interference (SGI) cancelation. Based on a lower-bound of the expected signal to interference plus noise power ratio (SINR), we optimize the dimension of an outer precoder to maximize the average sum-rate. Since the derived lower-bound depends only on the channel covariance matrix and the dimension of the outer precoder, the dimension of an outer precoder can be pre-determined at the BS without knowing the instantaneous dimension-reduced effective channel via feedback. Our simulations results show that our proposed scheme achieves near-optimal average sum-rate.