Comparing adaptive beamforming in micro- and macro-cells

This paper shows how wideband and directional channel physical constraints fundamentally condition adaptive beamforming, comparing between micro- and macro-cells. We define the angular and temporal densities, and multiuser multipath frequency parameters. Their meaning roots from the way that temporal and angular dispersions affect the original correlation properties of signals transmitted among links, reflecting channel richness in the multiuser sense. Their trends are consistent with beamforming performance divergences between those cell environments, helping to establish the main physical constraints. Among key micro- and macro-cell multiuser propagation scenarios, multiuser multipath frequency values are near one order of magnitude larger in micro-cells, compared to macro-cells. This is in agreement with the better beamforming obtained for the micro-cells channel. In the case of terminal grouping, where multiuser channels contribute with the same angles- and times-of-arrival, beamforming gains are limited to being close to 0 dB in macro-cells. In similar conditions, micro-cells´ results reach up to 11.6 dB for 4 users, or 9.6 dB for 16, and also render larger beamforming robustness. The physical-level reasoning behind this work is flexible to be applied to most wideband directional propagation models. The presented parameters are general and useful in evaluating spatial filtering techniques, together with those models.