I am planning on deploying a test 802.11g point-to-point system to hopefully replace an old coax network. I am wondering what the impact can be from rain, fog and snow. Where I am located gets all three and in abundance and variety. A full deployment on site would consist eventually of multiple point-to-multipoint and a handful of point-to-point links with the distances ranging from a few hundred feet to almost a mile. The longest run would have to contend with other issues too such as NLOS due to a tree that we can't quite get high enough to shoot over.
I have read a few posts on the subject that suggest rain and snow don't have quite as much impact as fog but I am wondering if anyone can point me in the direction of quantified impact? I'm intending to make sure that the links have more than enough "power" but I was wondering how much would be an appropriate margin for the various types of weather events that can impact 2.4GHz signals. Also, what impact does light foliage cause? Can it be "blasted through" or is some other solution more appropriate?
You're always better off with wire, coax, or fiber. The problem is not rain fade. It's interference from other users on the 2.4GHz band. Interference is a problem you don't normally consider on a wired network, but which is of paramount importance on a wireless link. Excessive interference is also why many commercial users have switched from uncontrolled unlicensed operation, to licensed microwave links.
of good references and reading.
By the lack of specifics, I presume you only want additional reading material and not specific calculations. If you want help with calculations, numbers are usually helpful.
Once you get beyond a few hundred feet, you'll probably need to go with directional or sector antennas. This will make the link topology rather complex. You can probably survive with an omnidirectional antenna at the Point to Multipoint hubs if interference is minimal. The problem with omni antennas is that they pickup interference from all directions, while directional panels and sector antennas minimize the areas from which interference might arrive. If the "full" deployment involves a large number of outside radios, you are likely to create much of your own interference problem.
Trees are not fatal but difficult to deal with. A very rough guess is that one medium size tree that you can see through is good for about
0.5dB of attenuation per meter at 2.4GHz. It can be radically more or less depending on season and type of tree. The real problem with trees is that you can usually find a hole in the foliage to shoot through, but that the signal will not be stable because it infringes on the Fresnel Zone. At 2.4GHz, you need a much larger hole than what you can see through. For example, at 1 mile, at midpoint, you need about a 19ft radius zone that's clear. |
In my limited experience, rain, fog, and such are minor problems at
2.4GHz as compared to interference, lack of fade margin (SOM), and sloppy installations. For example, deploying a wide are network in an area infested with a municipal wireless network. The fade margin problem is that many links are deployed with limited calculations. I try to aim for an minimum of 20dB fade margin. This is rather difficult to obtain with low power hardware. Try your longest link on this procedure from the FAQ: |
see how close you get. For the time being, ignore fog, trees, and such. When you arrive at a first approximation for fade margin, just subtract out your worst case estimates for these in dB. If you end up with a lower number than 20dB, you may have problems.
Note that you can relate fade margin to link reliability: Reliability Fade Margin 90% 8 dB 99% 18 dB 99.9% 28 dB 99.99% 38 dB 99.999% 48 dB
Sloppy installation is the most common problem. Water in the coax connectors and cable can really ruin a wireless link. I have reinstalled a few wireless links for waterproofing (and alignment errors) that suggest that the problem is all too common.
Possibly wrong. Increasing transmit power at one end of a link creates an "alligator" which is an animal with a big mouth and small ears. It transmits much farther than it can hear unless you also increase the power at the other end of the link. If this Point to Multipoint system is properly designed, it will have equal xmit power at all points of the system. That's not always possible or economical. Anything less creates a wide area jammer. As I previously noted, you may be your own primary source of self-interference.
use 0.1dB/mile for fog. No big deal.
results for L band should be sufficiently close. Otherwise, use the hints for frequency scaling.
No. Or rather, yes it will work for a while, but it will not be stable unless there is sufficient fade margin.
Cables, wires, or fiber. Licensed microwave. I think FSO (free space optics) are out due to fog.