Transmission mode switching for two-user downlink systems

In this paper, we study adaptive transmission mode switching between statistical and instantaneous channel state information (CSI) aided single-user (SU) and multiuser (MU) precoding for a two-user downlink system, where two transmit antennas are equipped at the base station and each mobile user has one receive antenna. In the case where only statistical CSI (SCSI) is available at the transmitter, a statistical-eigenmode space-division multiple-access (SE-SDMA) scheme is proposed by maximizing a lower bound of the ergodic signal-to-leakage-and-noise ratio. An exact analytical expression of the ergodic achievable rate is derived for the proposed SE-SDMA and compared with SU schemes such as SE transmission (SET) and instantaneous CSI (ICSI)-aided beamforming (BF), as well as the MU schemes such as ICSI-aided zero-forcing BF (ZFBF). Assuming the ICSI obtained at the transmitter is imperfect, the operating regions of these schemes are determined for different signal-to-noise ratio regions, channel correlation levels and ICSI inaccuracy levels.