e other hand, the angle of spread refers to the spread in the angle of departure (AoD) of the multipath components that leave the transmit 296 Chapter 3 antennas. If the angle spectrum function O(A) denotes the average power as function of AoA, then the RMS angle spread can be estimated as follows: The angle spread causes space-selective fading, which manifests itself as variation of signal amplitude according to the location of antennas. The space-selective fading is characterized by the coherence distance Dc which is the spatial separation for which the autocorrelation coefficient of the spatial fading reduces to 0.7. The coherence distance is inversely propor- tional to the angle spread Dc 1/ORMS. In Fig. 3.3 a duality between multipath-intensity function A(T) and Doppler power spectrum A(v) is shown, which means that the two func- tions exhibit similar behavior across time domain and frequency domain. As the A(T) func- tion identifies expected power of the received signal as a function of delay, A(v) identifies expected power of the received signal as a function of frequency. Similarly, spaced- frequency correlation function (f) and spaced-time correlation function (t) are dual func- tions. It implies that as (f) represents channel correlation in frequency, (t) corresponds to channel correlation function in time in a similar manner [33]. The AS in radians can be expressed based on the circular standard deviation in directional statistics using the following expression [24,25]: where Pnm denotes the power of the mth subpath of the nth path and 4nm is the subpaths angle (either AoA, AoD, elevation angle of arrival [EoA], elevation angle of departure [EoD]). In order to model large signal bandwidths and large antenna arrays, the channel models have been specified with sufficiently high resolution in the delay and angular domains. There are two important aspects related to large antenna arrays. One is the very large size of the antenna array and the other is the large number of antenna elements. These features re