What causes multipath fading?
Multipath occurs when a signal reaches the receiver by several paths with different delays and phases. Their sum fluctuates rapidly with position and frequency (small-scale fading). With no dominant path, the envelope follows a Rayleigh distribution; with a strong line-of-sight component it is Rician with K-factor equal to the ratio of direct to scattered power.[1]
What are delay spread and coherence bandwidth?
RMS delay spread στ describes the spread of path delays. Coherence bandwidth, over which the channel is roughly flat, is approximately Bc ≈ 1/(50στ) for 0.9 correlation or 1/(5στ) for 0.5 correlation. Signals wider than Bc see frequency-selective fading; OFDM handles this with many narrow subcarriers.[1]
What is Doppler spread?
Motion at velocity v produces maximum Doppler shift fd = v/λ; coherence time is roughly Tc ≈ 0.423/fd. At 2 GHz and 100 km/h, fd ≈ 185 Hz.[1]
How is fading mitigated?
- Diversity: space (multiple antennas), polarization, frequency, time.
- Coding and interleaving.
- Equalization or OFDM with cyclic prefix.
- MIMO: exploits multipath for spatial multiplexing.
- Fade margin in the link budget.
Frequently asked questions
How deep are Rayleigh fades?
Deep fades of 20–30 dB occur a small percentage of the time.
What is a two-ray model?
A direct path plus ground reflection; beyond a breakpoint distance, power falls as d⁻⁴.
Does multipath help MIMO?
Yes; rich scattering enables multiple independent spatial streams.
See also
References
Online sources accessed October 21, 2026; printed works cited by edition. See all sources.
- “Wireless Communications: Principles and Practice, 2nd ed.”. T. S. Rappaport, Prentice Hall. 2002.