I. Introduction
In THE last years, there has been a renewed interest for modeling the spatial wireless channel in the framework of multiple-input multiple-output (MIMO) broad-band wireless communication systems. MIMO systems combine the use of antenna arrays at both the base station (BS) and the subscriber end with space–time signal-processing techniques in order to improve the link quality or increase the system capacity. A common assumption in the study of such systems is to consider the fading channels between pairs of transmit and receive antennas as independent identically Rayleigh distributed (Rayleigh i.i.d.). In practice, fading channels might not be Rayleigh distributed due to the existence of a dominant component. Also, channels as seen by different antennas are often correlated, depending on the antenna spacings and polarizations, the possible existence of mutual coupling, the richness of scattering, etc. Fading correlation is shown in [1] and [2] to have a significant impact on the capacity and performance of MIMO systems. A number of recent papers have presented measurement results regarding directional and MIMO channels [3]– [6], each focusing on different aspects. In [3], empirical results characterizing the joint statistical properties of azimuth spread at the BS, delay spread, and shadow fading are analyzed in various environments, using the space-alternating generalized expectation (SAGE) maximization algorithm. In [4], measurements of the directional wide-band channel in downtown Helsinki are reported. Multipaths at the BS are resolved in both space and angle by means of the Unitary ESPRIT algorithm. In [5], azimuth-delay profiles at the BS have been recorded in Japan, by means of a wide-band sounder, together with a rotating receive antenna. In [6], empirical results for fixed macrocellular wireless local area networks (WLANs) are described, with a highlight on the range-dependence behavior of the channel parameters. Regarding the microcellular channel, usual measurements do not simultaneously highlight range-dependent behaviors and link the channel correlation to propagation parameters such as the Ricean K-factor, azimuth, and delay spreads. Finally, most are narrow-band measurements.