Time-Series-Aware User Scheduling for Multi-user Massive MIMO Null-Space Expansion

This paper proposes a user scheduling scheme for multi-user MIMO (MU-MIMO), particularly effective in time-varying channel environments. This scheme utilizes not only the current channel state information (CSI) but also the past ones for scheduling. MU-MIMO enables to enhance spectral efficiency by multiplexing a number of user terminals (UTs) in spatial domain. Due to the finite number of base station (BS) antennas, the simultaneous transmission capacity is constrained in MU-MIMO. Therefore, BS is required to divide UTs into distinct subsets and transmit signals for each respective subset. Various user scheduling techniques have been proposed to optimize these subsets for overall maximum throughput. When UTs move at high speed, it causes inter-user interference (IUI) and subsequently reduces the spatial multiplexing capability. Null-space expansion (NSE) fully exploits the spatial degree of freedoms of BS and enhances IUI suppression capability by steering several nulls to interfering UTs. Effective multi-user scheduling suitable for NSE has not been established so far. Our proposed approach leverages past CSI of UTs and strategically selects UTs to obtain their maximum individual gains.