Wireless link frequency choice (900Mhz vs 5.8Ghz) for 2-3km distance

Question

I have recently been contracted by a client of mine to facilitate the wireless communication of his "home" offices and a secondary site.

The primary site is the top two floors of a 5-story office building (15m height more or less) an the secondary is one of two open "lots" (which one is TBD by management). The ground distance from the secondary sites is a little more than 2km for the one closer and around 2.9km for the one furthest.

The link will be used to transmit the video feed of 1 (or even possibly two) IP cameras and some kind of Ethernet-enabled environmental or weather sensor. I have checked the necessary b/w for the cameras and both 900Mhz and 5.8Ghz are more than adequate for even 4 of them, much more for 2. I have also verified that there is clear line-of-sight to both possible installation points and that the 60% Fresnel Zone clearance is more than covered. Bear in mind that this is my first long distance link (long with or without quotes) and I hate to admit that wireless physics is far from my strong suit.

The ultimate point of my question is that although I have read a lot about frequency choice the last few days, I continue to find some ambiguity (I know it is just me that finds it ambiguous). Most sources, like this one, agree that although the lower frequencies have less losses over a given distance (free-space-loss I learned it is called) they need larger antennae for the same "strength" of trasmission (is "gain" really the same as "strength"?).

So, for the given distance of 2-3km and given also that all typical requirements are met, which is preferable (or do I dare say "better") frequency? Should I choose 900Mhz with a relatively "small" antenna on the basis that 3km is not really "long distance" and that it will provide a link with less attenuation ergo less retransmits ergo higher overall speed? Or should I choose the 5.8Ghz option for the superior b/w (I am still not very sure about this, please correct me if wrong) on the basis that at this distance there is no real difference so why not take the "better" one?

On a side note, should I stay to the beaten path of true WiFi or should I consider proprietary bridging solutions like the ones from Ubiquiti? I have a lot of experience with their Access Points and am really satisfied, so I would not mind integrating one more of their products in my client. In any case, I am looking for an optimal solution, choice of vendor is of very little concern at this point.

Forgive my ignorance and the possible mistaken use of language.

UPDATE: I arranged to have a spectrum analyzer on loan for a couple of days. I will make sure that the 900Mhz band is reasonably clear and proceed down that way.

UPDATE 2: I had the aforementioned equipment available to play with for a day and a half. The conclusive finding is that the 9Mhz band is almost "empty" in the area, as one suggested here, so that takes care for the frequency choice issue.

Concerning the equipment now, I am going with Ubiquiti AirMax Yagi antennae and matching RM900 2x2 radios. Preliminary testing on my part and from the client's employees shows that performance exceeds expectations.

On a side note, the chosen "lot" is the one that is 3km away.

Answer 1

3km is approaching the limit of 5.8GHz equipment with reasonable size antennas and typical WiFi radios. With some of the best equipment you can only get ten times that distance with some caveats.

900MHz can easily go 3km, and much much farther. 900MHz is used for a wide variety of devices for this reason, so there's a good level of background noise. This might be a problem, especially if your neighbor has a 900MHz phone or similar (not a popular frequency for phones these days, but people hold on to technology for a long time too). I would avoid this frequency unless you're in the middle of nowhere (which you aren't). The 2.4GHz spectrum has worse problems with this, our microwave at work blasts 2.4GHz (I'm sure it violates FCC something or other, but nobody really cares as we have 5GHz WiFi).

Answer 2

Chris S's answer is incorrect in several areas.

3km is not approaching the limit of 5.8GHz. Similarly, that's not as big of a distance as for 2.4GHz wifi either. Chris says 10X, I say 100X the distance is possible. The longest range 5.8GHz wifi that I've heard of is 304KM with a 1.2M hand made antenna (See: Long Range Wifi). I believe it went over water so there were not any thing in the way of the signal. It used Ubiquiti radios. I don't know if it was reliable, but to get a connection and send data over that distance is nothing short of amazing.

I used to work for a wisp carrier and had wifi radios about twice the size of your hand easily going 10KM with good line of site. I personally had wifi on my house going 8KM without any issues. Amazingly, they were only 500mW.

We were using 5GHz radios and a mixture of Ubiquiti and Microtik hardware. They work almost flawlessly.

While it is true that with lower frequencies you need bigger antennas. You'll notice that your home wifi doesn't have large antennas. Neither do 5GHz ones.

In reality, 2.4GHz should perform better, but 5GHz in my experience works just as well. In theory, 5GHz could be affected by rain fade, but I didn't find that even over the 8KM link.

As for 2.4GHz and other devices like microwaves and cordless phones inside the office, it doesn't even factor in. The reason it doesn't factor in is that you'll be using directional antennas. Better still, if the antenna is on an iron roof you will be shielded from all that noise coming from inside the office/house.

Having said all of that, I will say that it's not perfect for every situation. You need:

1. Line of site between the two buildings. Factoring in the freznel zone (good open clearance)
2. A suitable place to mount the antenna to achieve point 1.
3. Hopefully not much wifi traffic facing your directional antennas (no gaurantees there, but it should be workable) even in a reasonably crowded wifi space.

Freznel zone looks like this:

By the way, I've had trees in the way of the freznel zone and in some cases right in the middle and it still worked, although it does affect the performance a lot. Install your antennas as high as practical/possible.

You can still get interference issues. Usually because of other devices transmitting in your direction at the same frequency. So it's not perfect. Directional antennas help a lot with that.

I can recommend the 5.8GHz radios simply because I know they work well. And yes, provide good bandwidth! From memory I used a Ubiquity Bullet M5 with a patch antenna (another brand). The other end was a less directional antenna feeding multiple clients but used a similar Microtik radio.

I don't know of anyone who uses 900MHz. But I do see you can buy them at around 3X the cost than 2.4 or 5.8. Stick away from it, it seems uncommon. In several countries the 900MHz spectrum is quite crowded anyway.

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Update: You've been looking at the airmax antennas and the 900MHz radios. While I think performance should be great. I think you may be spending much more than you need to... perhaps a little overkill!? 3KM is not a long distance. I'd try some of these if you can.

http://www.gowifi.co.nz/antennas/5-ghz/directional/5.8-ghz-27-dbi-cast-reflector-grid-antenna.html combined with a Bullet M5.

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Update2: You can also mitigate interference issues using polarized antennas. Here's a good write up that explains the concept. From memory, we used horizontal polarization. As I recall sometimes if I set the client radio to vertical polarization I could see maybe 12 AP's. But when setting it to horizontal I only saw 3 or 4 and half of them were ours.