Some additional questiosn:
If you use Synchronization in PMP 450 are you required to have any guard band between channels on Aps within the same tower?
If you use antennas less the 90 degrees, say 40 could you run frequency reuse without the antenna being fully front to back?
Do you need to configure specific parameters at all sites to match in order to have a truly synchronized network? If yes, what are those parameters?
Can you speak to the density per tower you’ve seen, or personally achieved with a synchronized network?
Will synchronization help you mitigate interference from an outside source such as another ISP?
I'll let cambium handle the guard band question.
In relation to the antenna pattern/frequency reuse question: It all comes down to the patterns for the antennas you choose. The engineering is actually rather simple. Take the antennas you are looking at. Look at the difference in signal gain between the two antennas based on the angle apart you are mounting them and where the SM's are going to be. For instance if you mount one antenna at a bearing of 0 degrees, and a second at 90, and a single SM at 0 degrees, you'll want to compare the signal gain of the first antenna at 0 degrees and the second at 90. If you had SM's out to say 15degrees, you'd want to compare the first antenna at 15 degrees and the second at 75 degrees.
Once you understand the difference in signal gain, you then have to determine whether that is sufficient or not. Of course you have to have at least as much difference as is needed to maintain the modulation you desire. Plus some for things like uplink signal strength differences (this is where power leveling helps a lot), and so on.
In an extreme case, you may be able to do several reuses of a given frequency with an extremely tight dish antenna - such as with the case with backhauls. Panels are much more difficult. But you could also mix and match - say a BH with an AP - assuming the BH is set to master - remember a slave is the same as a SM.
In relation to the "matching parameters' question: I'll let cambium be specific about what needs to be set to be truly synchronized, but yes, they need to be all set the same. Or more accuately, whatever parameters you set needs to result in a similar transmit/receive timing. There are frame calculator tools in the radios and white papers to help you get everything set correctly. Things like uplink/downlink ratios, control slots and max distance settings along with specific versions and radio types change the framing, and I've sort of lost track of all of the details since I haven't had to do the math for some time now.
In relation to density: Because at the WISP we generally are up against a mountain and are pointing all our AP's in the same direction, we don't get a per-tower reuse - but we do get to use all of the non-overlapping channels pointed in the same direction, and mounted very close together. We do get very good tower-to-tower reuse - never losing a channel to self interference (although we do have to pay attention to placement of AP's in relation to SM's when reusing frequencies - don't want two close AP's on the same channel pointing the same direction).
I also have several customers (of packetflux) who rely heavily on the frequency reuse inherent in the synchronization system. One customer in particular, who is in a fairly populated part of the US, regularly puts up a full ring of both 2.4 and 5.7 - that is, two radios on each non-overlapping frequency (12 radios on 5.7 and 6 on 2.4). He has now expanded into the rest of the 5.x band to get even more capacity.
In relation to outside ISP interference: The answer is 'depends'. If you both use the same gear and can coordinate, definitely. If not, then the synchronization doesn't help you reduce interference from them, but instead helps you use the remaining channels more efficiently.