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M. Steinbauer, A. F. Molisch, E. Bonek:
"The Double-Directional Radio Channel";
IEEE Antennas and Propagation Magazine, vol. 43 (2001), no. 4; 51 - 63.

We introduce the concept of the double-directional mobile radio channel. It is so called because it includes angular information at both link ends, e.g. the base station and the mobile station. We show that this angular information can be obtained with synchronized antenna arrays at both link ends. In wideband high-resolution measurements, we used a switched linear array at the receiver and a virtual cross array at the transmitter. We evaluated the raw measurement data with a technique that uses estimation and beamforming alternatingly, and that relies on ESPRIT (Estimation of Signal Parameters via Rotational Invariance Techniques) to obtain superresolution in both angular domains and the delay domain. In sample microcellular scenarios (open and closed courtyard, Line-of-Sight and obstructed Line-of-Sight), up to 50 individual propagation paths were determined. The major multipath components could be matched precisely to the physical environment by geometrical considerations. Up to three reflection/scattering points per propagation path were identified and localized, lending insight to the multipath spreading properties in a microcell. The extracted multipath parameters allow unambiguous scatterer identification and channel characterization independent of a specific antenna, its configuration (single/array) and its pattern. The measurement results demonstrate a considerable amount of power being carried via multiply reflected components thus suggesting to revisit the popular single-bounce propagation models. It turned out that the wideband double-directional evaluation is a most complete method to separate multipath components.
Due to its excellent spatial resolution, the double-directional concept provides accurate es timates of the channel's multipath-richness, which is the important parameter for capacity of multiple-input multiple-output (MIMO) channels.