Improve Downlink Rates of FDD Massive MIMO Systems by Exploiting CSI Feedback Waiting Phase

In this paper, we consider a massive multiple-input-multiple-output (MIMO) system, where the base station (BS) is equipped with a large number of antennas while serving a much smaller number of users simultaneously. Though massive MIMO systems can provide significant spectral and energy efficiency via simple signal processing, the required channel state information (CSI) overhead is still a huge challenge, especially for the FDD mode. The basic frame structure of the FDD massive MIMO does not fully exploit the CSI feedback waiting phase since the BS needs to wait some time for the CSI feedback sent by users and then transmit data in downlink with the estimated CSI. The proportion of the CSI feedback waiting phase in the downlink transmission would be high as the MIMO scaling up, which reduces downlink rates for the FDD massive MIMO systems to some extent. In this paper, we propose two novel downlink precoding and transmission (DPT) schemes for FDD systems by exploiting the CSI feedback waiting phase. The corresponding performance comparisons between our proposed DPT methods, the contemporary DPT scheme and the ideal DPT one are also provided based on the COST 2100 outdoor channel model. Numerical results show that one of our proposed DPT schemes can achieve higher downlink rates than the contemporary scheme in relative low-mobility scenarios. The other proposed DPT scheme performs much better and is robust to user mobility.