Fading

Fading arises primarily from multipath propagation, where a transmitted signal reaches the receiver via multiple reflected, diffracted, or scattered paths. These paths combine constructively or destructively, causing rapid fluctuations in received amplitude and phase. The two main categories are large-scale fading (shadowing) and small-scale fading (multipath). Large-scale fading is caused by obstacles like buildings and hills, leading to slow variations in mean signal strength. Small-scale fading occurs over distances comparable to the wavelength, producing rapid fluctuations. It is further classified as flat or frequency-selective, and as fast or slow, depending on the relationship between signal bandwidth, symbol duration, and channel coherence parameters.

Engineers model fading using probability distributions to predict performance. The Rayleigh distribution describes fading in non-line-of-sight environments with no dominant path, while the Rician distribution applies when a strong line-of-sight component exists, characterized by the K-factor. The Nakagami-m distribution offers a flexible model that can represent a range of fading severities. These models are essential for simulating and analyzing wireless systems, enabling the calculation of outage probability and bit error rates. The Clarke–Gans model and Jakes' model are widely used to generate time-correlated fading waveforms for testing.

To combat fading, systems employ diversity techniques that exploit independent fading paths. Space diversity uses multiple antennas, polarization diversity uses different polarizations, and frequency diversity transmits over multiple carriers. Time diversity interleaves data across time slots. Modern systems also use error correction coding, adaptive equalization, and orthogonal frequency-division multiplexing (OFDM) to mitigate frequency-selective fading. In cellular networks, power control and handoff algorithms help maintain link quality. These techniques are fundamental to the design of robust wireless communications.

Fading also affects underwater acoustic communications, where sound waves experience multipath due to reflections from the surface and bottom, leading to severe inter-symbol interference. In optical fiber communications, modal dispersion can cause a form of fading in multimode fibers, though it is less common. The phenomenon of fading was first systematically studied in the early 20th century, with early radio engineers observing signal fluctuations on shortwave links. The term "fading" also appears in the context of scintillation in satellite communications, caused by ionospheric irregularities. Additionally, the Jakes' model, developed in the 1970s, remains a standard simulation tool despite its simplifying assumptions.

Multipath propagation

The phenomenon where a transmitted signal reaches the receiver via multiple paths, causing interference.

Rayleigh fading

A statistical model for fading where the received signal envelope follows a Rayleigh distribution, typical in non-line-of-sight conditions.

Rician fading

A fading model that includes a dominant line-of-sight component, with the envelope following a Rician distribution.

Diversity

Techniques that combine multiple independently fading signal paths to improve reliability.