7 MHz. The complex correlation coefficient ρc , power correlation coefficient ρp and envelope correlation coefficient ρe are defined with respect ˜ f |2 and |H ˜ f | respectively for a fixed frequency but at different positions. Discone ˜ f , |H to H antennas with efficient radiation characteristics over the UWB band are used. 1 shows the measured results. 4) is observed when the antenna distance is larger than ±5 cm. 1 also shows that the square-law relation [152] ρp ≈ ρe ≈ |ρc |2 does not hold for the UWB channels, which is valid for Rayleigh fading channels.

In [159], a fixed region of scattering environments for UWB-MIMO systems is considered. Thus, the number of spatial degrees of freedom of the scattered field, denoted η, is limited. It is shown in [159] that the system capacity is fundamentally limited by the three numbers NT , NR and η. This is not strange, since η will place a limit on the rank of the UWBMIMO channel matrix; hence, it will affect the number of the independently separate channels. In [197], it is shown that if several different antennas (a loop antenna and two orthogonal bow-tie antennas there) are placed in the same place instead of separately in different places with sufficient distance as in the traditional spatial antenna array, the spectral efficiency of such a system approaches that of the traditional array system.

It is particularly interesting that, as shown in [65, 114, 152, 155, 156], the antenna angular orientation and the signal polarization can be used to decrease the correlation of the spatial channels or to improve the system performance. Another approach describing the correlation property is from the deterministic viewpoint [247]. It is defined as the average value of all the crosscorrelation functions of different spatial channels normalized by the autocorrelation functions of corresponding spatial channels.