Comparison of multiuser diversity using STBC and transmit beamforming with outdated feedback

For multiuser diversity systems employing adaptive modulation, we compare the spectral efficiency for space-time block coding (STBC) and transmit beamforming (BF) with outdated channel feedback. Taking into account the channel feedback delay, we derive the optimal thresholds to maximize spectral efficiency subject to an average BER constraint, and illustrate the impact of feedback delay on the achievable multiuser diversity gain with either STBC or BF. We observe that more transmit antennas bring higher spectral efficiency for BF But this becomes inverted using STBC, due to the effect of channel-hardening. With a small feedback delay, the BF scheme outperforms the STBC scheme. However, when feedback delay becomes large enough, the STBC scheme achieves higher spectral efficiency due to the reduced diversity order of BF.