Introduction 15 .The path resolv ewmk5eolionchbe am aths with different vs is confirmed by the obse othat the he interarrival times of the paths wer e modeled by the w the number of paths the modified Beta ne Rayleign,Riclan and that th distri umbe:Th e parameters o aths fo the aand th ew findings,for example when the dynamic range is not selected,the path-gain c nts follow the lognorma dB,the Rayleigh distribution provides a better fit.Thus,the estimated pdf for gain coefficients dependson the level of lued int of channel has been investigated in 1970s since polarization diversity was used for combating with the multipath fading propertyo hann Employing orthogonally polarize chann er the eame mic o0971 1,Andrews et a (poned ourht chaelwhny correlarion can immersely mpov trans capacity or w tion syste nng-nic d air of ve n8popeg6 different polarizations. horizontally polarized channels are more correlated than the vertically polarized channels;the correlation of the co arized hannels inc esas the Ri K factor increases;channel by the environ To be added Spatial channel models Estimating the directions or bearings of incoming signals has been a research topic for years.The original obiective for detection an ion.The methods signal finding,omnidirectional antenna with vertical Introduction 15 versus delay and the distribution of path strength versus delay have been studied for outdoor channels in urban environments Cox (1973). The small-scale characteristics of the channel, i.e. the channel property at specific delays, became an important subject for modeling. Some important observations were obtained through extensive measurements. For outdoor urban environments, the excess delay of a channel in 900 Mhz can be up to 9 to 10 µs Cox (1973), delay spread, defined as the square root of the second central moment of the power delay profile, is 2 to 2.5 µs. The path resolved with 0.1 µs resolution exhibits Rayleigh distribution, inferring that the fading coefficients for the first arrival path can be modeled as Gaussian random process. The uncorrelated scattering among paths with different delays is confirmed by the observation that the Doppler frequency power spectra are quite different for the paths with different delays. The conclusion that the paths with different delays are uncorrelated seems more appropriate for the urban clutter environments. Some literature proposed to use correlated paths to construct the discrete models, which is contradictory with the observations in Cox (1973). For indoor manufacturing environments, Yegani and McGillem (1991) provided with the statistics of the channels in different sites in a factory under four scenarios among the settings of LoS/OBS, light/heavy clutter. It was found that the interarrival times of the paths were modeled by the Weibull distribution, the number of paths the modified Beta distribution, path gain coefficients the Rayleigh, Rician and Lognormal distributions. The values of the parameters of these distributions are reported. It is interesting to observe that the average number of paths for different sites at a fixed threshold of signal strength is about the same, an indication that the statistics of the number of paths arriving at the receiver is not very sensitive to the topography of the factory site. Furthermore, the geometry of the factory and the layout of the working area have a strong influence on the distribution of the path gain coefficients. There are also some new findings, for example, when the dynamic range is not selected, the path-gain coefficients follow the lognormal distribution regardless of the LoS, OBS, light- and heavy-clutter scenarios. When the threshold is considered, when the threshold is greater than −10 dB, the path gain distribution follows the Rician pdf and for threshold lower than −10 dB, the Rayleigh distribution provides a better fit. Thus, the estimated pdf for gain coefficients depends on the level of the dynamic range set at the receiver. The research of channels for SISO has been evolved into multiple areas. For example, the polarization characteristics of channel has been investigated in 1970’s since polarization diversity was used for combating with the multipath fading property of channel. Employing orthogonally polarized channels over the same microwave link for satellite communications can obtain two times system capacity as much as single polarized antennas Lee and Yeh (1972). In 2001, Andrews et al. (2001) pointed out that six channels without any correlation can immensely improve the transmission rate and system capacity of wireless communication system by polarization in scattering-rich environment. Channel models have been proposed that can be used to generate the channel responses with arbitrary pair of vertical and horizontal polarizations in both transmitter and receiver sites Spatial channel model for Multiple Input Multiple Output (MIMO) simulations (Release 7) (2007) Jeon et al. (2012). Besides the cross-polarization discriminations (XPDs) of individual propagation paths, these models also involve the responses of antennas in different polarizations. In ?, the correlation coefficients for both co-polarized and cross-polarized channels are studied. Some phenomena have been reveals, i.e. polarization decorrelation outperforms the spatial decorrelation in strong LOS scenario; the horizontally polarized channels are more correlated than the vertically polarized channels; the correlation of the co￾polarized channels increases as the Ricean K factor increases; channels have very higher correlation in the elevation domain. A strong conclusions was made that is, the cross-correlation of the cross-polarized channels are not affected by the environment, while the performance of the co-polarized channels is scenario dependent. To be added Spatial channel models Estimating the directions or bearings of incoming signals has been a research topic for years. The original objective for the study is for signal detection and estimation, including radar target tracking, component separation. The methods used for estimating directions of arrival are similar with the time-series spectral analysis methods. The former is applied specifically with the samples obtained from spatially distributed array of sensors, including antennas for receiving electromagnetic waves, and microphones for acoustic signals. The study of the arrival angles of the signals for design of the communication systems can be traced back to 1970’s. For example, in Lee and Brandt (1973) it was found from field measurements of mobile radio signals that signal arrival is concentrated in elevation angles lower than 16◦ . Based on this finding, omnidirectional antenna with vertical directivity is usually selected to increase average received signal strength