Adaptive vs. Diversity Transmission for Multiuser MISO Systems with Imperfect CSIT

Adaptive transmission techniques including transmit beamforming, preceding, and opportunistic scheduling offer high spectral efficiency when channel state information at the transmitter (CSIT) is accurate, but suffer performance loss when the CSIT quality is poor. Diversity transmission techniques such as space-time coding and frequency interleaving, in contrast, are capable of capturing spatial and spectral diversity without CSIT, hence can provide good performance when CSIT degradation is severe. Between a pair of adaptive and diversity techniques, there exists a switching point in terms of the CSIT-quality; a CSIT- quality higher than this point favors the adaptive technique, but a lower CSIT-quality prefers the diversity one. This paper analyzes several adaptive and diversity schemes for a multiuser multiple-input single-output (MISO) system. Comparative performance in terms of the outage capacity is studied and the CSIT-quality switching points between adaptive and diversity schemes are analyzed. Results are supportive of adaptive transmissions for mobile applications, such as in the downlink of an orthogonal frequency division multiple access (OFDMA) system.