I'm in the middle of updating an extremely outdated FSK infrastructure and decided to invest in the ePMP line. We recently deployed a four-sector GPS-synced ePMP 1000 AP cluster on a local water tower.
Our bandwidth needs warrant the use of nothing more than a 20MHz channel width. I have selected manual frequency assignment and have picked out the following frequencies for each sector:
The primary N/E/S/W frequency carriers are each 20MHz apart (N5250/E5270/S5290/W5310). Upon the need for channel shift, the secondary frequency carriers start back over 20MHz away from the last assignment (N5510/E5530/S5550/W5570) and so on for the third frequency carrier (N5590/5610/5630/5650).
However upon reading this post in the forum, I became confused. Rather than my planned 20MHz channel separation for adjacent sectors, do I need only 5MHz of real separation, resulting in a frequency plan akin to N5250/E5255/S5260/W5265? Or is this "guard band" in addition to the existing 20MHz frequency bandwidth, resulting in a frequency plan akin to N5250/E5275/S5300/W5325?
Yes, 5 MHz is all you need. Even more so, you don't need channel seperation betwen the back-to-back sectors, i.e E-W and N-S. So your channel plan could look like this, what we call an ABAB deployment:
All of the above is assuming that you are sync'ing all of them. And the sector antennas have > 30 dB Front to Back ratio which the Cambium sectors do. Although, in your case, the water tower should provide plently of isolation assuming these sectors are on the side of the water tower and not on top of it.
Unfortunately our deployment consists of the four sectors (Cambium branded 90/120 antennas) mounted on a single quad mount atop the water tower with ~24" back-to-back separation between the sectors. This is why I was hesitant to utilize the "frequency reuse" feature.
If you are using an antenna with good FB ratio, the quad mount should be fine. In the unlikely scenario where you see self interference, maybe tweak the downtilt a little bit to see if things get better. Most likely you wouldnt need to do that.