edVirtus Reference

What Is Fading?

Why Do Radio Signals Fade?

Fading is the variation in the strength and quality of a received communication signal caused by changes in the propagation environment. Unlike attenuation, which produces a gradual reduction in signal power with distance, fading causes the received signal to fluctuate, sometimes rapidly and sometimes over much longer periods. Fading is one of the principal impairments affecting wireless communication systems and plays a major role in determining communication reliability.

Radio waves rarely travel to the receiver by a single path. Instead, they are reflected from buildings, the ground, vehicles, and other objects, producing multiple signal paths that arrive with different delays and phases. As these signals combine at the receiver, they may reinforce one another through constructive interference or cancel one another through destructive interference. Even a small movement of the transmitter, receiver, or surrounding objects can change these phase relationships, producing significant variations in received signal strength.

A useful analogy is listening to music from several loudspeakers positioned around a room. At some locations the sound is louder because the sound waves reinforce one another, while at other locations they partially cancel, making the music noticeably quieter. Radio waves behave in much the same way when multiple propagation paths combine at the receiving antenna.

Fading is generally classified as either large-scale fading or small-scale fading. Large-scale fading results from gradual changes in average signal strength caused by distance, terrain, or large obstacles such as hills and buildings. Small-scale fading results from multipath propagation and produces rapid fluctuations over distances of only a fraction of a wavelength or over very short periods of time.

Several statistical models are used to describe fading. Rayleigh fading occurs when there is no dominant line-of-sight path and the received signal consists entirely of multiple reflected components. Rician fading occurs when a strong direct signal exists in addition to the scattered signals. Other models, such as Nakagami fading, are used to represent a wider range of propagation conditions.

Fading can significantly reduce communication performance by lowering the received signal-to-noise ratio (SNR) or signal-to-interference-plus-noise ratio (SINR), increasing the bit error rate (BER), and occasionally causing complete signal loss. Modern communication systems employ numerous techniques to combat fading, including diversity, Multiple-Input Multiple-Output (MIMO), beamforming, adaptive modulation, forward error correction (FEC), and interleaving. These techniques enable reliable communication even when the received signal fluctuates considerably.

It is important to distinguish fading from attenuation. Attenuation is the predictable reduction in signal strength caused by propagation losses, while fading is the fluctuation of signal strength caused by changing propagation conditions. A signal may experience both attenuation and fading simultaneously, but they arise from different physical mechanisms.

Today, fading is a fundamental consideration in the design of wireless communication systems. Mobile telephone networks, satellite links, Wi-Fi systems, microwave radio, and radar all employ propagation models that account for fading when determining coverage, link margins, and system reliability. Understanding fading enables engineers to design communication systems that continue to operate reliably despite the constantly changing nature of the radio propagation environment.