Chapter 5 treats the fundamentals of small-scale fading and the propagation mechanisms that cause multipath, doppler spread, time dispersion, and distortions to transmitted signals in the radio propagation channel. Detailed theoretical derivations and explanations for the small-scale channel impairments are presented with numerous examples. Flat and frequency selective fading, as well as fast and slow fading, are defined and analyzed. Key distributions found in the real world, such as Raleigh fading, Rician fading, and the classical Clarke and Gans model for multipath, are presented. Shape factor theory shows how the classical small-scale fading results may be replicated with excellent accuracy using the first thee Fourier coefficients of the spatial distribution of energy arriving at an antenna.

When designing a wireless communication system, it is essential to have a channel model that can quickly and accurately generate channel impulse response needed for system simulations. Deterministic models such as ray-tracing offer an accurate model of the propagation environment, but their high computational complexity prohibits the intensive link or system-level simulations required during system design. Hence, models with lower computational complexity that could emulate a large class of radio-propagation environments are preferred. These requirements have led to stochastic channel models, which are often classified into geometry-based stochastic models (GSCMs) and nongeometrical stochastic models. In this chapter we focus on the GSCM models.