Shadow fading (slow fading) effects play a central role in mobile communication system design and analysis. Experimental evidence indicates that shadow fading exhibits log-normal power distribution almost universally, and yet it is still not well understood what causes this. In this paper, we propose a versatile sum-product signal model as a physical basis for shadow fading. Simulation results imply that the proposed model results in log-normally distributed local mean power regardless of the distributions of the interactions in the radio channel, and hence it is capable of explaining the log-normality in a wide variety of propagation scenarios. The sum-product model also includes as its special cases the conventional product model as well as the recently proposed sum model, and improves upon these by: a) being applicable in both global and local distance scales; b) being more plausible from physical point of view; c) providing better goodness-of-fit to log-normal distribution than either of these models.

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This paper presents the results achieved with a dualpolarized multiple-input multiple-output (MIMO) measurement system in the 2 GHz range. Results from continuous measurement routes were used in evaluating and comparing different MIMO antenna configurations. Different pattern and polarization diversity possibilities were studied using two methods: elements were selected from the antenna arrays used in measurements, and as another option, in the mobile station the incident waves were estimated and used in different dipole antenna arrays. The capacity limit seems to be higher in an indoor picocell than in an outdoor microcell environment. At the mobile station, directive elements result in 35% higher average capacities than those of the omnidirectional elements; however, the capacity of the directive elements also depends on the azimuth direction of arrival of the incident field. Dual-polarized antenna configurations have approximately 14% higher capacities than copolarized configurations. Increasing the number of mobile antenna elements increases the capacity in those environments where the angular spread of the incident field is large. Increasing the distance between elements at the fixed station increases the capacity-especially in microcells where signals arrive from specific directions.

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