Two-Stage Interference Management for Multicell Multiantenna Cellular Downlink Systems

The sum degrees of freedom (DoFs) of L-cell multiantenna cellular downlink systems are studied. Each base station (BS) equipped with M-t transmit antennas sends independent messages to its K serving users equipped with M-r reconfigurable antennas, each of which can choose its receiving mode from N preset modes at each time. It is assumed that each BS and user only knows channel state information (CSI) of its own cell. A novel two-stage interference management scheme is proposed in which intra-cell interference is first nulled out by zero-forcing outer beamforming based on intra-cell CSI and inter-cell interference is then aligned by inner beamforming in a blind manner without inter-cell CSI. It is proved that, for M-t > KMr and N > 1, the proposed scheme achieves an improved sum DoF compared with the conventional time division multiple access among cells. We further demonstrate that such DoF improvement yields an improved sum rate at the finite signal-to-noise ratio regime.