Interference problem

[wirelessboy]

I am using around 8 different frequencies, for a ISP location. all are cisco aironet RF Modems, i am getting heavy nearend interference, how do i solve this.
different PN codes should work in theory, but practically?
i want your inputs to solve this nightmare for any wireless field engineer
at a time only 5 out of 8 links are working fine

[oshea85]

Hmm....I think you've answered your own question.

If you're using 8 different frequencies, then you have 8 APs set up, each on it's own antenna and frequency, correct? If you are serving a 360 degree swath around your site, then you should be using antennae with a -3dB 45 degree beamwidth, correct?

Even if you do this, your design is fatally flawed.

You only have 3 (a recent paper I read argued that you might be able to get 4, but not in a WISP-type deployment) non-interfering channels that you can co-locate. If you can successfully use cross-polarization rejection by rotating half of your antennae 90 degrees and utilizing adequate spatial separation, you still only could get possibly 6 separate "channels", but I doubt this is easily achieved. Otherwise, as soon as you use a fourth AP, you're interfering with yourself. By using 8 channels, you are guaranteeing that your base station is interfering with itself.

Set 3 APs to channels 1, 6, and 11. Shut down the other 5 APs, and see if you're problem goes away. I bet it does.

You have to re-use the 3 non-interfering/non-overlapping channels in such a way that you isolate identical or overlapping channels from each other, either by staggering/separating them, or perhaps you could isolate them by using some type of shielding coupled with alternating antenna polarizations.

Please don't take this the wrong way, but you need to understand the basics of how 802.11b (and RF) works before you start deploying revenue-generating services using it. Study up.

[wirelessboy]

thanks a lot oshea, i will definetely study and will get back .
thanks a lot, boy RF is tough!

[oshea85]

Tell me some more about how you have this set up:
-what types of antennae are you using?
-where/how are the antennae mounted/pointing
-how many client devices or locations are you trying to serve?
-how far from your base stations are your clients?
-how are you connecting your APs (I'm assuming your using APs) to the internet?
-what model Cisco devices are you using?

[bj_orn]

oshea

is there any good site where i can read how it works and stuff ??

[wirelessboy]

dear oshea85

i am sorry i was not specific about the site, these are 8 different point-to-point links, to various customers(radius of 10 Kms), from an ISP.
we are using 802.11 modems(cisco). aironet 350 series.
The point is how come those old RF modems with V.35 output would work very well even with overlapping channels, altough different PN codes.
There lies the problem, these 802.11 AP's are good for WLAN's , but i beleive if we know how to deploy these modems, they are pretty good for campus and even metro LAN's.
surely those companies would not offer 15-20 Dbm power output without any research on this right.
Apart from 1,6,11 any other additional channel would result in high latency.
The spread for these modems is 22 Mhz i understand, but even with 40 Mhz channel bandwidth i deployed 4 CYLINK Modems succesfully at one site, we have 22 numbers of 64-128 kbps cylink modems at one site, but these 802.11B are a mystery
Antenna and installation is not a problem: 18 Dbi with 7.5 degree 3D beamwidth, polarisation have been taken care off.
As a last resort we can convert half links to a higher band, but i really want to solve this problem in S band itself oterwise these higher frequencies are like antibiotics the more you invent the more stronger these viruses get!
thanks
wirelessboy

[oshea85]

Aha! You've replaced an existing Cylink Airlink system with Cisco Aironet? I think that's your problem.

The Cylink 'modems' are using frequency hopping (haven't looked up the documentation, so I'm not sure). You can co-locate around 15 of these links before they start interfering with each other, but they typically each have low bandwidth (historically around 2 Mbps, but now FH can do 10 Mbps, I think).

Direct sequence only allows 3 non-overlapping channels, but you can get 11 Mbps out of each one.

There are two methods to achieving spread spectrum transmission, FH and DS. You need to research the difference between frequency hopping and direct sequence, too long to go into in a post.

You can probably find one of Jim Geier's tutorials on this website.

[wirelessboy]

The cylinks use DSSS, very much the same as these APs, only difference is APs use 802.11B, supposed to be used only indoors, i wonder why?
does anyone know what standard do those RF modems, cylink,alcatel,skyplex and a whole lot of those isaraeli modems use?
you are right oshea, FHSS cause less interference but only 1 Mbps BW or a max of 2 rarely
These are new installations, i am planning a few things today, i will keep u posted.
i am getting a few attenuators for some links which have a fade margin of > 35 Db!!!
regards

[wirelessboy]

i shifted three links to a new location, the other end of the terrace floor infact.
using a few attenuators for a couple of links, and by different polarisation, and channels, we got 2 Mbps for all links.
still working on it.
thanks
wirelessboy

[Steve Chen]

Wirelessboy,

Did overcome headache?

How about thinking directional antenna, since an OMNI antenna radiates with any direction, so its very noisy in 8 different frequencies site. The noise includes interference, wave reflection, multipath, co-channel(?), if you can relocate the devices, and use directional antenna for the same area, you will improve the situation. I'm an antenna expertise, feel free to contact me on this issue please.

Steve Chen