I have 2 PMP450i 900mhz sectors that face each other. They are about 3 miles apart, but they do not see each other due to the Jungle in between. Due to other factors I have to have them on the same frequency.
I have customers that are about 1.5 miles from each sector so they hear both a little bit. Obviuosly they hear one more then the other when their Yagis are facing that direction.
Would putting sync on both these towers help to mitigate interference that the customers in the middle get? I know its not the ideal situation, but it is what we are dealing with.
The point of GPS sync is that the timeslots when the APs are transmitting are simultaneous. (and the timeslots when SMs are transmitting)
Without sync, two APs that share a channel and see each other will interfere with each other, usually leading to intermittently horribly uplink MCS across the sectors as one AP stomps on the signal from the SMs trying to talk to the other AP, and waves of session drops if it's strong enough.
With sync, both APs will transmit at the same time (ergo neither is listening for SMs while the other is transmitting) and both will shut up and listen for SMs at the same time.
A client that is able to see both APs will have to have enough difference in signal strength to maintain a workable connection, but the two APs will no longer be interfering with each other with reliable sync.
In your case you say the APs cannot see each other, (I assume you mean spectrum, not simply eyeball LoS) in which case they're already not directly interfering with each other.
You'll also avoid SMs hearing each other with sync - so if two SMs aimed at opposite APs can see each other they'll not interfere with each other provided both sectors are on stable GPS sync. Occasionally geography leads you into this - e.g. we have two customers across the street from each other whose connections each go across the other house. Less problematic would be next-door-neighbors with one aiming front the other back - they're 90 degrees off-axis to each other, so as long as they're not physically near one another they should be OK. But with good sync it doesn't matter.
But sync will have essentially no effect if the sole source of interference is off the back end of the SM's Yagi - you're dependant on SNR at that point, ensuring there's enough difference in signal strength at a particular site to make one AP workable.
I'd still encourage you to do it, everything just ends up stabler and cleaner with solid sync across the network. It also lets you re-use channels on the same tower reliably - two APs 180 degrees apart can share the same channel happily, only clients far off-axis or very close to the tower will have a problem where they see both APs too strongly. (without sync the APs will hear each other, at least intermittently, and usually much stronger than any SMs since they're feet apart instead of miles apart, killing sessions)
It's a good idea to check for PMP100 and PMP450i 900 MHz APs from other companies, and make sure you're not interfering with one another.
I spent an evening putting together a spreadsheet that took the very different settings of the APs in one particular area, and told me if I was interfering with any of them. That allowed me to play with the numbers until I found something that worked with all of them. It made a huge difference in customer stability.
The math isn't complicated, both sides have to stop transmitting before the other side starts receiving. If you want to get a little fancier, the frame calculator will also tell you the time it takes the signal to travel to max range, so you can take into account the time it takes the signal to move from one tower to another. At 3 miles, it's negligible, but it can matter for longer distances.