The double-directional radio channel

We introduce the concept of the double-directional mobile radio channel. It is called this because it includes angular information at both link ends, e.g., at the base station and at the mobile station. We show that this angu lar information can be obtained with synchronized antenna arrays at both li nk ends. In wideband high-resolution measurements, we used a switched linea r array at the receiver and a virtual-cross array at the transmitter. We ev aluated the raw measurement data with a technique that alternately used est imation and beamforming, and that relied on ESPRIT (Estimation of Signal Pa rameters via Rotational Invariance Techniques) to obtain superresolution in both angular domains and in the delay domain. In sample microcellular scen arios (open and closed courtyard, line-of-sight and obstructed line-of-sigh t), up to 50 individual propagation paths were determined. The major multip ath components could be matched precisely to the physical environment by ge ometrical considerations. Up to three reflection/scattering points per prop agation path were identified and localized, lending insight into the multip ath spreading properties in a microcell. The extracted multipath parameters allow unambiguous scatterer identification and channel characterization, i ndependently of a specific antenna, its configuration (single/array), and i ts pattern. The measurement results demonstrated a considerable amount of p ower being carried via multiply reflected components, thus suggesting revis iting the popular single-bounce propagation models. It turned out that the wideband double-directional evaluation is a most complete method for separa ting multipath components. Due to its excellent spatial resolution, the dou ble-directional concept provides accurate estimates of the channel's multip ath-richness, which is the important parameter for the capacity of multiple -input multiple-output (MIMO) channels.