What are multipath and fading?
Multipath propagation occurs when a signal reaches the receiver by several paths via reflection, diffraction and scattering. The paths add with different phases, causing fading: rapid fluctuations over distances of about half a wavelength. Without a dominant path the envelope follows a Rayleigh distribution; with a line-of-sight component, a Rician distribution. Antenna countermeasures include diversity, MIMO and directive antennas.[1][2]
What parameters describe a multipath channel?
- Delay spread: the spread of arrival times; its inverse is roughly the coherence bandwidth.
- Doppler spread: due to motion, fD = v/λ maximum; its inverse is coherence time.
- Angular spread: the spread of arrival directions, which determines needed antenna spacing for diversity.[1][3]
How deep are fades?
In Rayleigh fading the received power is below 1% of its mean (−20 dB) about 1% of the time, and below 10% (−10 dB) about 10% of the time; hence fade margins of tens of dB are needed without diversity.[1]
How do antennas mitigate fading?
Diversity combining of decorrelated antennas, MIMO spatial multiplexing that exploits multipath, directive antennas that reject off-axis paths in fixed links, and circular polarization that rejects odd-bounce reflections in GNSS.[4][5][6]
Frequently asked questions
Is multipath always bad?
No; MIMO relies on rich multipath to support multiple streams.
What is small-scale fading?
Fluctuation over distances of about a wavelength due to multipath interference.
What is shadowing?
Slower, larger-scale variation from obstructions, often log-normal.
See also
References
Online sources accessed October 21, 2026; printed works cited by edition. See all sources.
- “Wireless Communications, 2nd ed.”. A. F. Molisch, Wiley. 2011.
- “Antennas and Propagation for Wireless Communication Systems, 2nd ed.”. S. R. Saunders and A. Aragón-Zavala, Wiley. 2007.
- “3GPP TR 38.901: Study on channel model for frequencies from 0.5 to 100 GHz”. 3GPP. current release.
- “Antenna Diversity in Mobile Communications”. R. G. Vaughan and J. B. Andersen, IEEE Transactions on Vehicular Technology, vol. 36, no. 4, pp. 149-172. November 1987.
- “On Limits of Wireless Communications in a Fading Environment when Using Multiple Antennas”. G. J. Foschini and M. J. Gans, Wireless Personal Communications, vol. 6, no. 3, pp. 311-335. March 1998.
- “Understanding GPS/GNSS: Principles and Applications, 3rd ed.”. E. D. Kaplan and C. J. Hegarty (eds.), Artech House. 2017.