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Abstract-In this paper, we investigate the polarization characteristics of the multiple-input multiple-output (MIMO) channels. The cross polarization discrimination (XPD) is found to change over time, while its variation in frequency domain is relatively small. The probability density function (PDF) of the XPD follows a Gaussian distribution. Analysis and measurement results show that the polarization preserves better in the line of sight (LOS) scenario. In the case of non line of sight (NLOS) scenario, the polarization of the signals is destroyed due to multiple reflections, diffractions or scattering. The difference between the XPDs of the vertically polarized and horizontally transmission highly depends on the specific propagation environment between the transmitter and the receiver. Comparison of the capacity in different scenarios further shows that the polarization diversity gain is less in NLOS scenarios compared with that in LOS scenarios.

In future wireless networks, cooperation between the base stations may reduce the effects of intercell interference and enhance the capacity per cell. In this paper, the potential of such cooperation is evaluated using measurements of the informationtheoretic channel capacity at 5.2 GHz in 4 overlapping urban macrocells with 5 users. In each link, a 10-transmit 16-receive antenna configuration is used. Results for individual links between terminals and base stations, valid for 60% of the scenarios, indicate that the direct signal has a significant impact on the channel structure. The direct signal contributes two spatial degrees of freedom due to the polarization. Rich scattering is observed when the direct signal is blocked. Results for the multiuser multicell scenario demonstrate for the first time experimentally that the intercell interference leads to an enhanced rank of the joint multiuser multicell channel matrix. Compared to a conventional cellular system, where a fixed assignment of different radio resources is used in adjacent cells in order to avoid the interference, it is better to exploit the interference by cooperation. This enhances the spectral efficiency by a factor of five in our measurement scenario.

In this paper, we study the spatial structure of multiple-input multiple-output (MIMO) links in small urban macro-cells at 2.6 GHz. We resolve the double-directional structure of the radio channel. Also we compute common metrics used to characterize MIMO links, i.e. the structure of singular values and the resulting capacity. In a line-of-sight (LOS) link, we find that local scattering is not enough to create full-rank MIMO channels. Even behind a sector, we observe a low-rank channel. In a non-line-of-sight (NLOS) scenario, although we resolve several multi-paths with individual delay, direction of departure (DoD) and direction of arrival (DoA), the impact of local scattering is limited. Singular values indicate few more degrees of freedom for NLOS channels, but less than for a random matrix. We can model the spatial degrees of freedom in small cells better by assuming a random but small number of nearly specular paths feeding some local scattering widely spread in azimuth and elevation. Based on our results, we discuss modeling aspects and the value of large antenna arrays in mobile networks.