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.
The rain rate is a statistical quantity: different rain rates are exceeded for different percentages of time. ITU-R P.837 provides rain rate statistics for regions worldwide. For link design: the rain rate exceeded for a given percentage of time (e.g., 0.01% of the year ≈ 53 minutes) is used to calculate the rain attenuation that the link must tolerate to achieve the desired availability.
For mmWave terrestrial links: the short path lengths (100-500m for 5G cells) limit the total rain attenuation to manageable levels (1-5 dB even in heavy rain). For longer links (1-3 km backhaul): rain attenuation is the dominant impairment and determines the required fade margin and antenna gain.
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.