Dimension-optimized Two-Stage Precoder Design for Massive Mimo Systems with Limited Feedback

Abstract

A two-stage precoder is a widely considered strategy that solves the feedback overhead problem in frequency division duplex (FDD) massive MIMO systems because an outer precoder reduces the dimension of the effective channel, which is fed back to the base station (BS) for inner precoder design. In this paper, we proposed the dimension-optimized outer precoder design considering a quantization for the cases when the two-stage precoder design is adopted with limited feedback. A dimension of the outer precoder is important for both of inter-group interference (IGI) and same group interference (SGI) cancellations, and for the amount of feedback overhead. Moreover, a quantization error which restricts an average sum rate performance, depends on the channel dimension. Based on a lower bound of the expected signal-to-leakage plus noise power ratio (SLNR), where the effects of the quantization errors are reflected, we derive the dimension-optimized outer precoder to maximize average SLNR. Since the derived lower bound depends only on channel covariance matrices, the outer precoder and feedback bit allocation, the dimension-optimized outer precoder can be pre-determined at the BS without knowing the instantaneous dimension-reduced effective channel via feedback. Our simulation results show that our proposed design well increases the average sum rate compared to existing schemes.