Hi Fezdmn, Well, much of the 'magic' in the ePMP is ways to mitigate self interference - which can substantially improve performance. The ability for the AP's to sync via GPS is pretty much key to the way the ePMP works, and syncing is the 'magic' of reducing self-interference.
So - if you understand the way that is achieved, in the simplest form, all four of your AP's will transmit at the same time, and then they will all listen to their clients at the same time. That helps dramatically with self interference, since AP-North isn't trying to listed to some distant client at the same instanat that AP-West is transmitting - which would overwhelm any hope of AP-North hearing it's clients. That is the self-interference nightmare that most systems have, but with GPS Syncing (and by having the same download/upload ratios, the same frame siz, the same distance setting, etc) then all the AP's can be exactly synced for when they TX and when they RX.
Now - from the client side of things, it's also important to mitigate self-interference as well. That is automatically greatly helped because of the syncing - so that no client is trying to transmit to it's AP at the same time as any other AP is transmitting and deafening the target AP. But, it's also important to issolate each AP as much as possible from them being heard by the clients of the other AP's. You can do that by channel separation, and you can also do that by good front-back ratios, and by physical separation and shielding and so on.
So - if for example, you have a client conneceted to 'north' and if he's about 45 degrees off the beam, he'll hear 'north' and 'west' about the same - and it's important to have some channel separation so that 'north' and 'west' don't screw each other up. Likewise, if you have a client right in the middle of the beam 'north' and another right in the middle of the beam on 'south' and if you're doing frequency resuse, then it's important to have 'north' and 'south' issolated from each other as much as posisble. If you don't, then the client on 'south' that hears 'south' as a -60 and if it also hears 'north' as a -70 - then you've defeated much of Cambium's 'magic'. I think Cambium recommends >30 dBm of issolation - so that's Front-Back ratios and shielding and physical separation and so on. That way, the client on 'south' hear 'south' at a -60, and it'll only hear 'north' at a -90 and all is well and right and just in the world.
In my humble opinion - FRONT-BACK IS THE MOST IMPORTANT THING - either a good ratio in the antenna, or via shielding as well. I'm not sure if the RF Elements sectors will have enough F/B to be the best choice or not.
So - if you can only do 1' of separation and there's no possibiity to do more, or to add shielding - then it is what it is. :) It may still work OK, but you're defeating some of the magic that Cambium's engineers put in there. But, if you can get them to 30 dB of front/back issolation, I'd think you should be good. :)