Comparison of multiple antenna techniques for UMTS downlink capacity enhancement

Multiple transmit antennas can be used to enhance downlink capacity of wideband CDMA, either by improving the signal quality when employing space-time block coding or by reducing interference when employing beamforming. Although the antenna separation and the spatial distribution of scatterers have a significant impact on the antenna correlation, the diversity schemes based on space-time block coding rely on the assumption of sufficiently uncorrelated signals transmitted from different antennas. On the other hand, the interference cancellation efficiency of downlink beamforming degrades for large angular spreads and spatially inseparable users. Based on spatio-temporal channel modeling, a comparison is performed for various array configurations, scatterers distributions and multipath scenarios, in order to address the question of under which circumstances each of the two techniques would provide the most capacity enhancement