Unlicensed frequencies mean that interference is a way of life in most major cities. The question is what to do when all the frequencies you plan on using or even radios that are in operation start having errors. I just came from an installation like that. In addition, then I was asked to design an expansion to the system. One of the clues was that interference is a problem is when things work for a couple weeks and packet errors in the log jump from nothing to tens of thousands.
By default, most of us set up our networks with 20MHz wide channels. That’s the default for 2.4GHz WiFi and usually the default for most 5.8GHz deployments. However, what happens when we do a site survey and 500 APs show up on the list? If the design is already deployed, your options are limited. You basically have three:
- Find the least interfered channel. I usually don’t hold much hope out for that.
- Increase power at the possible expense of reduced modulation rates – which should also be titled “How to make new friends”
- Reduce channel width down to 10 or even 5 MHz – not an option with WiFi hotspots
- Play FCC Russian Roulette and and use channels you aren’t supposed to be on with way more power than is legally allowed in those bands anyway. It seems that for those people violating FCC rules that if you are going to break one rule, might as well break two. I have even seen systems set up by companies for police departments on these frequencies. If the FCC ever drove through towns with sniffers they way Google did with cameras, the fines alone would solve the National Debt.
If you control both sides of the radio equipment equation, a combination of these together might squeeze out better performance and minimize the interference with a little patience. Sometimes you just have to take the best option out of bad choices. For example, is it better to have a 20MHz wide channel with a few errors or a 10MHz wide channel with no errors? That same argument applies with a 20MHz wide channel with more power which might reduce modulation versus a 10MHz wide channel with lower power and higher modulation. Keep in mind that you are now running your design on the edge so that any new interference will probably degrade your system even further. On the other hand, you might be disrupting someone else enough that they move to another band.
Designing the system from the beginning provides more options than having to fix an existing system. Let’s get something out right now. If tomorrow you decided to deploy a PTMP system with wide angle sector antennas in an unlicensed frequency band in a major city, you are not going to be the first one in those bands. This means expect interference and plan for it. Use the first three ideas and let someone else who should be working at McDonalds instead of consulting in the wireless industry, use option 4. If you get bored, site survey the band and give the FCC a little heads up on what you found in the 5.0-5.6GHz bands.
The strategy you develop from here is going to depend on the type of application. If the application is WISP services where you are trying to cover a general area, good luck. I’m not saying it can’t be done, it’s just going to be about as easy as washing a cat in the tub. Throwing up 90 degree or more sector antennas or even worse, omni-directional antennas is simply inviting problems. Not only are you going to tick off everyone around you, they are going to return the favor out of pure self-preservation.
Let’s start with the idea that deploying anything with 802.11a/b/g today is also simply a waste of bandwidth. Go right for 802.11n and try to go for 2×2 MIMO. However, keep in mind that as hard as the standards body tried, 802.11n simply doesn’t work well with legacy devices. It’s not that they won’t function, it’s just the legacy devices slow down 802.11n radios and simultaneously 802.11n will interfere with 802.11a/b/g radios. Throw in some manufacturer proprietary settings and things get even more interesting. Other than the basic ideas above, is there a way to work in a high-interference environment?
Start by thinking of your English 101 class in college. Charlemagne said, “let my armies be the rocks and the trees and the birds in the sky”. Let’s change that to “let my shields be the trees and the buildings”. Even though RF engineers in 2.4GHz frequencies and above look at vegetation and buildings as the enemy, they can also be allies in a design. Instead of fighting RF interference in the open air 10 floors or more above the city, take the battle to the ground where the buildings and trees block interference. Taking an RF survey 300’ in the air is a whole lot different than taking one standing on a corner block surrounded by trees and buildings.
So now you must be thinking that we are back to metropolitan WiFi. Actually no. WiFi assumes 2.4GHz and even at ground level is probably pretty congested. However, there is an FCC rule called the 3-1 rule pushed through by Vivato with the FCC. We pretty much know the current omnidirectional rule for WiFi, 30dBm radios with 6dBi omni-antennas. However, the 3-1 rule means that in a PTP link we can increase the antenna gain by 3dB if we reduce the power output by 1dB. If we are going PTP at ground level in 2.4GHz where we know interference is going to be an issue, some really directional equipment might work pretty well. It shouldn’t be too hard to get a 52dBm signal to go 2 blocks with directional antennas 15’ off the ground. This can be achieved with a 30dBm antenna and a power output of 22dBm. Obviously I’m joking but the point I’m trying to make is that highly directional antennas at ground level in any band will be very effective, even in high-noise environments. Drop the channel width down to 5MHz with a 2×2 MIMO 802.11N radio and your interference issues will probably become a memory. Some poor schmuck in the middle of this link might get hammered however, especially if he is using a legacy device. Let your conscience be your guide here.
5.8GHz works even better in this environment if the trees aren’t in the way. Testing has shown that 2.4GHz 2×2 MIMO dual-polarity will punch through trees but 5.8GHz works about as well as a political candidate does once he gets elected. It might work or it might not. Either way it’s not reliable. The buildings will also kill 5.8GHz signals from above, leaving the ground wide open. There aren’t a lot of indoor APs using 5GHz bands yet but with up to 53dBm of signal output, interference will be a memory. In reality, you really don’t need or actually want 50+dBm of signal to go 2 blocks unless you just want to wipe out baby monitors and cordless phones that are in your path. However, the fundamental concept of using highly directional antennas at ground levels might solve some problems.
Taking that further, if you are using a proprietary polling scheme like Motorola or Ubiquiti or frequency hopping, the noise those APs will generate will be massive. Using this technique in a financially feasible manner, assuming you don’t have the government’s open pocketbook backing you, requires either a mesh type radio with directional antennas or a generic radio that can be either an AP or a CPE device, depending on settings. There are also many variations of this that can work quite well also.
I am finding in almost every environment that I’m going into is challenging me to come up with new variations of designs to be successful either technically or financially. The old hub and spoke model is much more difficult to deploy in a city today when there are thousands of radios already deployed with many of them not following the FCC rules in either frequency or power output. Rising or high internet costs coupled with lower equipment costs are now opening up new competitive markets for wireless.
This design is specifically oriented towards a fixed location model in a high interference environment with talk buildings. Dynamic location designs have to be done differently because you don’t have fixed CPE locations. We covered this in some of the previous articles. Omnidirectional antenna use in a municipal environment just doesn’t fly any more. In lower population density environments, things are different. In the middle of a major city however, use the environment to your advantage.
On a side note, it’s been pointed out to me that I should probably proofread my posts a little further. I notice a few “minor” typos that I miss in my haste to get these done. Unfortunately this isn’t my day job and my wife, who clearly proofread the first few, just doesn’t have time to keep me from looking like I never took a technical writing class before. She also hates the first person style in a technical document and gave me the Rollie Eye every time she took out a “you” and “we”. So, I’m down to proofing myself and I’m hoping that content is more important than grammar. Proofing these takes twice as long as writing them. So, if you the reader are willing to let the grammatical errors slide by, I’ll try and get them out a little quicker. And if you use any of these techniques or have some unique ideas I haven’t covered, drop me a note and let me know.