  Multiple transmitting antennas can considerably increase the downlink
spectral efficiency by beamforming to multiple users at the same time. However,
multiuser beamforming requires channel state information (CSI) at the
transmitter, which leads to training overhead and reduces overall achievable
spectral efficiency. In this paper, we propose and analyze a sequential
beamforming strategy that utilizes full-duplex base station to implement
downlink data transmission concurrently with CSI acquisition via in-band closed
or open loop training. Our results demonstrate that full-duplex capability can
improve the spectral efficiency of uni-directional traffic, by leveraging it to
reduce the control overhead of CSI estimation. In moderate SNR regimes, we
analytically derive tight approximations for the optimal training duration and
characterize the associated respective spectral efficiency. We further
characterize the enhanced multiplexing gain performance in the high SNR regime.
In both regimes, the performance of the proposed full-duplex strategy is
compared to the half-duplex counterpart to quantify spectral efficiency
improvement. With experimental data [1] and 3D channel model [2] from 3GPP, in
a 1.4 MHz 8X8 system LTE system with the block length of 500 symbols, the
proposed strategy attains a spectral efficiency improvement of 130% and 8% with
closed and open loop training, respectively.
