Ever wonder which antenna polarization is more susceptible to rain outage? Vertical? Horizontal? Which should you use for very long hops?
What would you do besides add extra fade margin to mitigate rain outage? Design a shorter path or use a lower frequency band?
Aviat Networks’ microwave radio guru, Principal Engineer Dick Laine, tackles these tricky questions and others in the latest episode of our Radio Head Technology Series of videos.
Dick also talks about rain outage—as calculated by ITU using a simple scientific calculator, or computer programs (Starlink) that use the Crane model. He goes through an ITU-R availability calculation in one example, noting specifically about rain attenuation calculation above and below 30 degrees latitude. Dick then proceeds into a deep dive on calculating outage when you know the fade margin, followed by a discussion on the Crane rain attenuation model.
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Rain fading (also referred to as rain attenuation) at the higher microwave frequencies (“millimeter wave” bands) has been under study for more than 60 years. Much is known about the qualitative aspects, but the problems faced by microwave transmission engineers—who must make quantitative estimates of the probability distribution of the rainfall attenuation for a given frequency band as a function of path length and geographic area—remains a most interesting challenge, albeit now greatly assisted by computer rain models.
A surprising piece of the puzzle is that the total annual rainfall in an area has almost no correlation to the rain attenuation for that area. A day with one inch of rainfall may have a path outage due to a short period of extremely high localized rain cell intensity, while another day of rain may experience little or no path attenuation because rain is spread over a long period of time, or the high intensity rain cell could miss the microwave hop completely.
Over the years, we have learned a lot about deploying millimeter wave microwave hops for our customers:
More information about assessing rain-induced attenuation is available in our white paper, Rain Fading in Microwave Networks.