09/10/2010
Are Microwave Communication Systems Affected by Rain?
RAIN, RAIN, GO AWAY or Are Microwave Radio Communication Systems affected by rainfall?
With the severe weather that we all occasionally experience (especially in New England), we in the microwave communication business take note.
Ever since the inception of microwave technology for point-to-point communication systems, the question of weather related attenuation is questioned. Especially in the 1980's, when there was a proliferation of 23Ghz microwave systems, the technology never seemed to be reliable. When it rained, the systems simply did not perform as advertised. Much of the bad press was justified in that systems were poorly engineered, and customers were misinformed.
Microwave communication systems are available in a range of frequencies, ranging from 1Ghz to 30 Ghz, with millimeter wave radios up to 80 Ghz and beyond. A basic principle of RF propagation is that weather related attenuation (rain fade) is related to frequency; usually the higher the frequency, the more rain attenuation is experienced (there are some exceptions to this principle in millimeter wave systems). The effect can be calculated based upon the statistical data (rainfall).
In engineering a system for optimal performance, microwave engineers take into account the projected amount of rainfall in any given location, the distance of the path, and the required reliability of the system. Based upon these factors, the system can be engineered to perform with a "5 nine's" reliability factor. What this means is that the user can expect to have (99.999%) availability of the system. This equates to about 315 seconds annually of unavailability (about 5 minutes). By the way, 99.999% is a conservative design goal, and many systems using IP protocols are very tolerant to less stringent criteria. Back in the day, when most systems were passing TDMA traffic (T1's), the reality was that microwave fading presented real problems for the protocol. Fortunately IP is much more forgiving, and the end user never experiences any problems. Be aware that this 99.999% is a number that refers to availability due to weather related phenomena, not equipment failure.
So, is microwave communication technology a reliable method for point-to-point systems? The answer is "yes", when systems are properly engineered. Reliability of (99.999%) can be achieved in all but the most severe rainfall areas, with performance being equivalent to or better than other options.
Jack Davis CEO
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