What is the effect of rain attenuation on a millimeter wave link at 60 GHz versus 28 GHz?
Rain Attenuation
Rain attenuation follows the ITU-R P.838 model: specific attenuation γR = k × R^α (dB/km), where R is the rain rate in mm/hr, and k and α are frequency-dependent coefficients. At 28 GHz: k ≈ 0.124, α ≈ 1.06. At 60 GHz: k ≈ 0.49, α ≈ 0.87. The attenuation is roughly proportional to the rain rate raised to a power slightly less than 1.
| Parameter | Free Space | Urban | Indoor |
|---|---|---|---|
| Path Loss Model | Friis (1/r²) | Okumura-Hata | IEEE 802.11 |
| Fading Margin | 0 dB | 10-30 dB | 5-15 dB |
| Multipath | None | Severe | Moderate-severe |
| Typical Range | Line of sight | 1-30 km | 10-100 m |
| Shadow Fading (σ) | 0 dB | 6-12 dB | 3-8 dB |
Frequently Asked Questions
How does this differ from Ka-band satellite?
Ka-band satellite links traverse the full rain column (rain heights of 2-5 km depending on latitude). The total rain attenuation on a slant path can be 10-30 dB in heavy rain at Ka-band. This is why Ka-band satellite systems use adaptive coding and modulation (ACM) to trade data rate for reliability during rain events.
Does snow cause attenuation?
Dry snow causes minimal attenuation at mmWave (< 1 dB/km). Wet snow and sleet cause moderate attenuation (5-10 dB/km at 60 GHz in heavy conditions). Hail causes attenuation comparable to rain of the same equivalent liquid water content.
How do I design for rain?
Include a rain fade margin in the link budget: calculate the specific attenuation at the rain rate exceeded for the desired availability percentage, multiply by the path length (with a path reduction factor for links > 1 km), and add this to the clear-sky path loss. The fade margin determines the excess EIRP or receiver sensitivity needed.