Frequency Reuse settings for Omni towers

Hello. For any large tower sites, we use 4 sectors and for back-to-back sectors, we do select a front sector and a back sector on opposite AP's as described in the ABAB reuse guides. For smaller sites that only serve 10-15 people max, we only deploy an omni antenna. These sites are usually 3-6 miles from a sector site. My question is on these omni sites, should we have the Frequency Reuse setting on "Off, Front, or Back sector compared to adjacent sites.

I have not yet found a clear answer to this and we're just starting to Elevate our network so looking to see what the recommended scenario is.

Thanks

I'd leave it 'OFF', as you're not using back to back... that being said, for all the other relavent GPS settings, if you match them up with the adjacent sector, you might see some benefits in being able to use a smaller guard band between adjacent channels between the omni and the sector.

It depnds on if you think the SMs off that Omni will see another AP on the same channel.  For example, if you have an East/West sector set to front/back respectivly and then put an omni a few miles to the west of that sector tower on the same channel, you'd want to set the omni to "front" if you think there's a chance the SMs off that Omni will see the west sector.

1 Like

That is exacly our scenario that you described and we have been using "off".  We have a couple omni sites that can not see eachother due to geography but some of the SMs can see both omni's.  Do we set both omni's to "front" or just one of them?

And on a bigger question, what specifically does making that "front" or "back" setting do in regards to sync?

I would like to understand something more about that.
We’re starting to having multiple SMs in some zones that can hear 3-4 or more AP.
Even if these AP have one Front and one Rear, same max range, same frequency, the SMs aren’t able to operate correctly in these cases. Is that a normal behavior? How can sync help when you have multiple APs in your radio range?

The Front and Back settings prevent two APs re-using frequency from interfering with each other. This question has been asked several times but It is a proprietary implementation that allows sync on ePMP and we are not able to go into the details on a public forum. Chad, visit us and I promise you a long technical discussion on this :)

guiseppe4, are your SMs hearing 3-4 APs using the same frequency? If so, you should rethink the frequency plan. It is not recommended that two APs facing each other use the same frequency, specifically when the SMs on one AP can hear the other AP. The Front or Back setting will not help in this situation. 

Thanks,

Sriram

There are some places where you have only some free channels and you can’t really plan a frequency plan… sometimes you can hear a nearby AP even if you’re pointing another direction, maybe because it’s close to the SM, and you can’t do so much to change the frequency :frowning:

Hello giuseppe4,

In such cases, you may want to consider increasing your antenna downtilt or reducing AP Tx power, to reduce the range of your sector and how much of its signal interferes with neighboring sectors operating in the same frequency.


@Au Wireless wrote:

That is exacly our scenario that you described and we have been using "off".  We have a couple omni sites that can not see eachother due to geography but some of the SMs can see both omni's.  Do we set both omni's to "front" or just one of them?

And on a bigger question, what specifically does making that "front" or "back" setting do in regards to sync?


Set one omni to front and the other to back.  When a SM can see a second AP on the same channel you need to make sure:

  • It sees it's AP 30db stronger than the other AP (If you want MCS15)
  • The other AP isn't set to the same F/B setting

If an SM sees another AP on the same channel and it has the same F/B setting, or is set to off, that SM will sporadically disconnect from the AP.  Basically, it's getting confused by the 2nd AP.

Ok someone correct me if I'm wrong here. GPS sync is strictly for preventing Access Points from interfering with other Access Points, it does nothing to prevent subscriber radios from interfering with Access Points or stopping Access Points from interfering with subscriber radios.


The Front/Back setting has nothing to do with preventing subscriber radios from be being interfered with by the sector facing away from them (or facing any other direction) as that is the job of front/back ratio of the sector and down tilt (which becomes up tilt on the back)


If you have a subscriber radio that can see two access points and those access points are on the same frequency then the 2nd AP will interfere with the subscriber module and the subscriber module will interfere with the AP. There is nothing GPS sync can do about this. The solution is don't install subscriber radios where they can see two APs running the same frequency.


@brubble1 wrote:

Ok someone correct me if I'm wrong here. GPS sync is strictly for preventing Access Points from interfering with other Access Points, it does nothing to prevent subscriber radios from interfering with Access Points or stopping Access Points from interfering with subscriber radios.

Correct.


@brubble1 wrote:
The Front/Back setting has nothing to do with preventing subscriber radios from be being interfered with by the sector facing away from them (or facing any other direction) as that is the job of front/back ratio of the sector and down tilt (which becomes up tilt on the back)

Yes and no.  You use the antenna f/b ratio and downtilt to get as much SNR as possible over the other AP.  30db will get you MCS15, which is why it works with Cambium's sector antenna.  But even with a 30db f/b ratio your SMs can still sometimes see (i.e. demodulate management frames) the AP facing the other direction.  The front/back setting is needed any time a SM can demodulate the other AP's management frames.


@brubble1 wrote:
If you have a subscriber radio that can see two access points and those access points are on the same frequency then the 2nd AP will interfere with the subscriber module and the subscriber module will interfere with the AP. There is nothing GPS sync can do about this. The solution is don't install subscriber radios where they can see two APs running the same frequency.


1 Like

>Yes and no.  You use the antenna f/b ratio and downtilt to get as much SNR as possible over the other AP.  30db will get you MCS15, which is why it works with Cambium's sector antenna.  But even with a 30db f/b ratio your SMs can still sometimes see (i.e. demodulate management frames) the AP facing the other direction.  The front/back setting is needed any time a SM can demodulate the other AP's management frames.

I'm pretty sure the front/back setting has nothing at all to do with SM's seeing the sector facing away from them (or facing any other direction). As a matter of fact in this very thread Cambium_Sri  posted "The Front and Back settings prevent two APs re-using frequency from interfering with each other." I've never seen a post by cambium claiming that the front/back setting has anything at all to do with SMs.

 I stand corrected.

My understanding is that if you have two APs that can see eachother on different towers, you should be able to run both APs on the same channel (assuming all the sync settings match) with out any issues as long as none of the SMs can hear more than one of the APs (and the APs can't hear any of the SMs on the other AP)... is that correct?

Mathew, that's correct.

brubble1, I'd explain it in more detail, but per Sri's comment, this is getting into their "secret sauce".  I'll leave it to Cambium to divulge or not divulge more info on the F/B setting.  Perhaps I've already said too much.

brubble1,

Mathew, dkeltgen and Luis are correct and they are essentially suggesting two specific things for your deployment:

  1. When an SMs sees another AP operating on the same frequency as its own AP, then these two APs have to have the "Frequency Reuse" setting set to the opposite of each other, i.e if one is set to Front, the other needs to be set to Back or vice versa. This is irrespective of which direction the other AP is facing, away or towards the SM. 
  2. The signal level from the other AP must be at least 30 dB weaker than the signal level from its own AP, if you want to achieve MCS15 on the SM. For this scenario, the Front or Back setting is irrelevant. This is why Luis suggested additional downtilt or lowering the Tx power. I know of customers who have used the terrain to their advantage to isolate two sectors from each other. 

Both of the above is desriable to have a clean sync'd frequency reuse deployment.

If you have two AP on the same tower facing 180 degrees from each other reusing frequency, the antennas should have good F/B ratio (> 30 dB) so the two APs do not interfere with each other. You will want these APs to also use opposite Frequency Reuse settings of Front and Back. 

There is a section in the ePMP User Guide that explains a few of these scenarios and what settings to use (just search for "Frequency Reuse"). 

Thanks,

Sriram

1 Like

Sriram, thanks for the input. This is a very helpful topic and I think some further discussion or diagrams by Cambium - like those you developed for tower co-location scenario - would be very helpful. As bandwidth needs are increasing, I think more of us are deploying these "mini-pop" antenna sites closer to the SMs. There are certainly cases where APs will see each other and SMs might see multiple APs.

Clearly, using different frequencies is the optimal solution, but as 5GHz gets more crowded, this won't be possible and knowing how the different settings on the APs (and SMs) can help us in a non-tower deployment is really helpful.

At the same time, I'd love to see some additional RF shielding developed into the Force 180 and 200 replacements. Being mostly plastic, especially on the back, I am guessing there is little, if any, shielding built in?  This is going to become more and more important as we (and competitors) deploy more 5GHz in different directions in suburban areas. No longer are all the APs in one direction. We try to use the house to shield us from other sites but that is not always possible.

Would love to have some RF Armour options for the Force 180 and 200.

2 Likes

I had a Force180 that got dropped off a roof, so I got a good look at the internals... you'd be surprised at the amount of metal inside those, they actually appear to be shielded pretty well (it's very different from the guts of other little plastic radios I've torn apart). Unfortunately, I forgot to take a picture before I glued it back together.

Well, drop it off rhe roof again - and take a picture for us this time.

Well, I'd like to, but it's cold outside...

I have an ePMP 2000 that got dropped from about 300', if that helps.