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Small‐scale fading refers to the random fluctuations of amplitude and phase of the received signal over a short distance or a short period of time due to constructive or destructive interference of the MPC. For different propagation environments and wireless systems, different distribution models are suggested to analyze the random variations in the received signal envelop. The Rician and Rayleigh distributions are widely used models in the literature of wireless communications, where both are based on the central limit theorem. The Rician distribution provides better fit for the AA and AG channels, where the impact of LoS propagation is stronger. On the other hand, when the MPC impinges at the receiver with random amplitude and phase, the small‐scale fading effect can be captured by the Rayleigh distribution [6].

Geometrical analysis, numerical simulations, and empirical data are used to obtain the stochastic fading models [38–40]. Geometry‐based stochastic channel model (GBSCM) is the most popular type of small‐scale fading model. GBSCM is subdivided into regular‐shaped geometry‐based stochastic channel model (RS‐GBSCM) and irregular‐shaped geometry‐based stochastic channel model (IS‐GBSCM). Time‐variant IS‐GBSCM was presented in [41] and RS‐GBSCM was presented in [42] and [43].These works illustrated Rician distribution for small‐scale fading. In [44], non‐geometric stochastic channel model (NGSCM) was provided, where small‐scale effects of AG propagation were modeled by using Rician and Loo models. Table 2.3 provides the measured characteristics of small‐scale fading of AG propagation in different environments.

Table 2.3 Measured small‐scale fading of AG propagation in different environments.

References	Scenario	Frequency band	Fading distribution
Khawaja et al. [11]	Suburban/Open field	Ultra‐wideband	Nakagami
Newhall et al. [12]	Urban/Suburban	Wideband	Rayleigh, Rician
Tu and Shimamoto [13]	Urban/Suburban	Wideband	Rician
Matolak and Sun [14]	Urban/Suburban	Wideband	Rician
Simunek et al. [45]	Urban/Suburban	Narrowband	Rician
Cid et al. [46]	Forest/Foliage	Ultra‐wideband	Rician, Nakagami
Matolak and Sun [47]	Sea/Fresh water	Wideband	Rician