What causes multipath fading?

When a transmitted signal reaches the receiver via multiple paths (direct, reflected off buildings, diffracted around corners, scattered from vehicles), the signals arrive with different delays, amplitudes, and phases. If two paths differ by exactly half a wavelength (lambda/2), they cancel completely (destructive interference), creating a deep fade. At 900 MHz (lambda = 33 cm), moving just 8 cm can change the signal by 40 dB. This rapid variation is called small-scale fading or Rayleigh fading (when there is no dominant line-of-sight path). The Rayleigh distribution describes the probability of fade depth: P(signal less than X dB below median) = 1 minus exp(minus 10 to the (X/10)). The probability of a 20 dB fade is 1 percent; a 30 dB fade is 0.1 percent. For vehicle speeds of 60 km/h at 2 GHz, the fade rate is approximately 111 fades per second (at the minus 10 dB level), with an average fade duration of 0.9 milliseconds. This is why modern systems use diversity, equalization, and coding to combat fading.

What path loss models are used?

Free-space path loss (Friis): PL = 32.44 + 20 log(d_km) + 20 log(f_MHz). Only valid for unobstructed line-of-sight with no reflections. Okumura-Hata model: empirical model for cellular coverage in urban, suburban, and rural environments from 150 to 1500 MHz. PL = 69.55 + 26.16 log(f) minus 13.82 log(h_b) minus a(h_m) + (44.9 minus 6.55 log(h_b)) log(d). Includes correction factors for mobile antenna height and environment type. COST-231 extends Hata to 2 GHz. 3GPP TR 38.901: the standard model for 5G NR coverage planning. Covers 0.5 to 100 GHz with separate models for urban macro (UMa), urban micro (UMi), rural macro (RMa), and indoor hotspot (InH). Includes LOS and NLOS probabilities, O2 and H2O absorption, and spatial consistency for MIMO channel modeling. ITU-R P.530: for point-to-point microwave links, including rain, gas, and multipath fade statistics.

How does frequency affect propagation?

Higher frequencies experience more free-space path loss (6 dB per octave), more atmospheric absorption, more rain attenuation, and less diffraction (less bending around obstacles). At 60 GHz, oxygen absorption adds 15 dB/km, limiting range to about 1 km for practical links. At 22 GHz, water vapor absorption peaks at approximately 0.2 dB/km. Rain attenuation becomes significant above 10 GHz: at 28 GHz in heavy rain (50 mm/hr), attenuation is approximately 10 dB/km. At 77 GHz: approximately 30 dB/km. Building penetration loss increases with frequency: 10 dB at 900 MHz, 20 dB at 3.5 GHz, 30 to 40 dB at 28 GHz for modern low-E glass. However, higher frequencies have shorter wavelengths, which means: antennas can have higher gain for the same physical size (more antenna elements per area), resolution improves for radar and imaging, and more bandwidth is available. The shift to mmWave in 5G accepts the propagation challenges in exchange for the massive bandwidth (hundreds of MHz to GHz).

Radio Physics

Propagation

/prop-uh-gay-shun/

FSPL = 32.44+20log(d_km)+20log(f_MHz). +6 dB/octave freq, +6 dB/doubling distance. Multipath: Rayleigh fading 20-40 dB. Models: Friis, Hata, COST-231, 3GPP 38.901. Atm: O₂ @60GHz (15 dB/km), H₂O @22GHz. Rain @28GHz: 10 dB/km. Building: 10 dB @900M, 30-40 @28G. Higher freq = more loss + more BW + more gain/area.

FSPL: +6 dB/oct

Rayleigh: 20-40 dB

O₂: 60 GHz

Understanding Propagation

Propagation is the central challenge of wireless communication. The signal undergoes free-space spreading, reflection, diffraction, scattering, and absorption on its way from transmitter to receiver. Understanding and modeling these effects is essential for designing systems that work reliably in the real world.

The RF engineer cannot control propagation; they can only account for it in the link budget. The path loss model predicts the median signal level, and the fade margin accounts for the statistical variations. Getting both right is the difference between a system that works on paper and one that works in the field.

Propagation Equations

Free-space path loss:
FSPL = 32.44 + 20log(d_km) + 20log(f_MHz)

Hata (urban, 150-1500 MHz):
PL = 69.55 + 26.16log(f) − 13.82log(h_b)
− a(h_m) + (44.9−6.55log(h_b))log(d)

Rayleigh fade probability:
P(fade > X dB) = 10^−X/10
20 dB: 1%, 30 dB: 0.1%, 40 dB: 0.01%

Rain attenuation:
A = k×R^α dB/km
28 GHz, 50mm/hr: ~10 dB/km

Propagation Effects by Frequency

Frequency	FSPL @1km	Rain (50mm/hr)	Building	Application
900 MHz	91.5 dB	Negligible	10 dB	IoT, cellular
2.4 GHz	100 dB	Negligible	12 dB	Wi-Fi, BT
5.8 GHz	108 dB	0.1 dB/km	18 dB	Wi-Fi 6E
28 GHz	121 dB	10 dB/km	30-40 dB	5G FR2
77 GHz	130 dB	30 dB/km	N/A	Automotive radar

Common Questions

Frequently Asked Questions

Multipath?

Multiple paths: direct + reflected + diffracted. Phase differences cause constructive/destructive interference. Rayleigh: no LOS, 20-40 dB fades. 8cm movement @900MHz = 40 dB change. Fade rate @60km/h @2GHz: 111/sec. Duration: 0.9ms. Combat: diversity, equalization, OFDM, coding.

Models?

Friis: free-space only. Hata: empirical, urban/suburban/rural, 150-1500MHz. COST-231: extends to 2GHz. 3GPP 38.901: 5G standard, 0.5-100GHz, UMa/UMi/RMa/InH, LOS/NLOS probability, spatial consistency. ITU-R P.530: point-to-point microwave with rain/gas/fade stats.

Frequency effect?

Higher freq: +6 dB/octave FSPL, more atm absorption (O2 @60G, H2O @22G), more rain (28G: 10 dB/km), less diffraction, more building loss (30-40 dB @28G). But: more BW, higher antenna gain/area, better resolution. 5G mmWave: accepts propagation cost for massive bandwidth.

Related Terms

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