ePMP + Horn Antennas ---- Freq Reuse with GPS Sync

Hi Guys,

Just wondering for those of you that have the ePMP with GPS Sync attached to an RFelements antenna. Is it possible to re-use freqs when ap's are about 120 degrees apart instead of 180 degrees using 60 degree symmetrical sectors? Since its states it doesn't have side lobes or what not. Just trying to figure out if this is doable.



Hi.  Well, the Cambium GPS Syncing is remarkable, but to answer your specific question, I'll try to explain a little about how it does it's ''magic''.

So - all radios have some sort of 'frame', where they transmit for a while and then they receive for a while. With all other non-Cambium type of WiFi(ish) devices, such as UBNT or MikroTik or whatever - this ''frame'' is internally generated, and every AP will have it's own frame start and stop time. This means that while one of your AP's is in receive mode and trying to listen to some client 5 miles away - at that exact same instant, some other AP is in transmit mode, and if it is a nearby AP (for example, one of your other AP's on your tower) the first AP has no chance to hear it's client 5 miles away, since it's incoming signal might be a -69, but your other AP's which are broadcasting at the same instant might be a -36

SO, with Cambium's GPS sync, there are two main benefits.

1) All your AP's will TX at the same instant, and they will all RX at the same instant.  This essentially eliminates self interference between the AP's.  You no longer have the situation where one AP is trying to listen to some distant quiet client, right while another AP is yelling in it's ear.  GPS Syncing alone virtually eliminates self AP to AP interference.

2) You can then ALSO do frequency reuse - BUT this requires understanding and planning - and to answer your question about 120 degree frequency reuse...  the answer is no, not really.  The reason is - for frequency reuse to be effective, every SM needs to really only hear it's own AP, and every AP really needs to hear only it's own SM's.  So - if a SM is right on the boundary between two 120 degree sectors, and if both of those sectors are using the same frequency... then that SM will hear both AP's at the same time, and also when that SM talks to it's AP - both AP's will hear it about equaly well.  So - if you try to reuse frequencies with 120 antennas, you'll essentially defeat everything that Cambium built in.

So - GPS Sync alone, without any real planning or understanting, is a really really neat thing.  However, in order for you to really do frequency reuse, you do need to both have a working GSP Synced setup, but also to understand and design things correctly.

I can say you CAN do frequency reuse if you're absolutely sure you won't connect any client that can hear both APs.

30°-40° horns in particular are very directive so if you are sure about that, and you're sure the horns won't hear both clients, you can do frequency reuse.

Hi. YES - I missed the fact that you're wanting to use horns that are a LOT less than 120 degree beams. :) So, everything I said still applies, and still for the same reasons.

But, like Giuseppe says, if the clients only hear their own AP and if the APs only hear their own clients... then this likely would work fine. The trick is having a lot of issolation between the antennas and ensuring that the AP's and client's are all issolcated to their own Sector.

So, if you only have these 3 horns sharing the same channel, and if you connect a.client 120 degrees off center.. the same problem will exist... it'll still hear both APs about the same, and two APs will hear it about the same - and it'll be a mess. BUT, if you only connection clients inside the beam of the horn and if it doesn't hear the other sectors, and if the other APs only hear their own cleints.. . then I don't know why that couldn't work.

GPS sync is part of the frequency reuse magic, but SNR and isolation are the key to frequency reuse.

If your client hears another AP at 30+ db lower signal you will still have an issue if both APs are set to the same Front/Back setting. You may be able to get around this by shifting one of the APs by 5mhz.

Thanks for the input guys. I won't do freq re-use on these deployments then. Instead I'll use different channels separated by at least 10 mhz from the edge of the "skirts".