Extension of space-time coding to beamforming WCDMA base stations

Space-time coding methods are being extensively investigated since they appear to provide levels of performance appropriate for next and future generations of wireless communication systems. Coding is typically carried out in the spatial domain (across the antenna) as well as in the temporal domain. In this paper we extend the space time transmit diversity (STTD) approach to the case of an antenna array, where angular (beam) domain is exploited. The relatively poor downlink performance (compared with uplink) is significantly improved by applying this technique to a code division multiple access (CDMA) base station. Both downlink and uplink simulations were carried out for flat-fading channel models with narrow, moderate and wide angular spreads. Co-channel interference was assumed as spatially and temporally white. It was found that, when transmitting with two space-time coded beams, the downlink performance improves by 1-5 dB (at 10^-3 BER) compared to the references cases of a conventional single-beam and two-antenna transmitting scheme. Performance figures are strongly dependent on the angular spread of the radio channel. The additional array gain resulting from the beamforming approach reduces the transmitted power and hence increases the spectral efficiency (capacity) of the base station. Also, the directional downlink transmission produces less spatial cochannel interference