Propagation Loss
Understanding Propagation Loss
Propagation loss determines whether a wireless link can close, meaning whether the received signal is strong enough for reliable communication. Understanding and accurately predicting propagation loss is essential for any wireless system design.
Propagation Loss Components
- FSPL: 20log(4*pi*d/lambda). The fundamental spreading loss. Increases 6 dB per doubling of distance or frequency.
- Atmospheric absorption: Oxygen (60 GHz peak), water vapor (22 GHz, 183 GHz). 0.01-15 dB/km depending on frequency.
- Rain attenuation: Significant above 10 GHz. 1-20 dB/km in heavy rain at Ku/Ka-band.
- Terrain/building: Wall losses (5-15 dB per wall), floor losses (15-20 dB per floor), foliage (0.2-1 dB/m at microwave).
Frequently Asked Questions
What is propagation loss?
Propagation loss is the total signal attenuation from transmitter to receiver, including FSPL, atmospheric absorption, rain, terrain, and multipath. It is the dominant factor in link budget calculations that determine whether a wireless link can close.
What is FSPL?
Free Space Path Loss is the fundamental spreading loss: FSPL = 20log(4*pi*d/lambda) dB. It increases 6 dB per doubling of distance and 6 dB per doubling of frequency. At 28 GHz over 100 m: FSPL = 101 dB.
How does rain affect propagation?
Rain attenuation is negligible below 5 GHz. At 12 GHz (Ku-band): 1-5 dB/km in heavy rain. At 28 GHz (5G mmWave): 5-15 dB/km. At 60 GHz: even higher. Rain fade is the primary availability concern for satellite and mmWave links.