  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.
