Will gps sync not work with 120 degree sectors?
Hi. Well my answer would be: You 'can' turn GPS Syncing on with 120 degree sectors, so technically it 'does function', but what they mean is that you'll be defeating much of the benefit that GPS Sync is designed for.
You see, there are two sources of interference - competative interference and self interference. You can't control competative interference (although ePMP does an awesome job or workign in the face of competative interference) so what GPS Syncing is about is trying to control SELF interference, and trying to do Frequency Reuse, so that you can get the best use of the quietest channels. The single biggest source of interference is self interference, and you CAN control much of that with proper design. Using 3 sectors instead of 4 will defeat much of the design.
So, the idea with GPS is to Syncronize two Access Points which are pointed 180 degrees opposite ways from each other so that they both broadcast at the same instant, and so that they both listen at the same instant, and that they therefor can use the same channel. If two non-Syncronized AP's are on the same tower and if one is listening while another is transmitting (even if they are on different channels) the listening one can get deafened by the nearby AP and the broadcasting AP can stomp all over the clients which are trying to talk to the listening AP.
Also, when both AP's are in listening mode, the idea is that they need to be issolated as much as possible from the CPE's of any other AP. Again, if you have two Sectors which are pointed in exactly opposite directions, and if the F/B ratio is >30dB, then the idea is that clients on Sector A won't be very loud on Sector B (which is pointed 180 degrees away with only a 90 degree beamwidth and with a >30 dB F/B ratio). So, when both AP's are in their listening part of the frame, they (hopefully) should only hear their own CPEs very loudly.
Now, if you only put up three 120 degree sectors, and if you point them 120 degrees apart, and if their Front/Back ratios are ~20 dB instead of >30 dB - then a client on the edge of one sector may be hear by two sectors at about the same level. If you are trying to do frequency reuse on those, then that client will be talking and two of your Sectors will be hearing it at about the same level, and it'll be stomping on whomever is trying to talk to the other Sector at the same instant.
So - my answer to this question would be 'Yes and No'. Yes, you can still use GPS Syncing - meaning that you can stil turn it on, and it'll still Sync the Transmit and Receive frames of all the AP's in your network. There 'may' still be some benefit by having all your AP's transmit at the same instant, and all of them receive at the same instant. Logically, that would HAVE to be better than what every other manufacturer else does. HOWEVER, you won't really get frequency reuse - certainly not on any CPE's which are near the edge of your 120 degree beams. You see, when two AP's are talking, that CPE will hear both of them at the same time at about the same signal level, and when that CPE is talkign, both AP Sectors will hear it at the same time at about the same signal level, and that'll defeat much of the benefits of GPS Syncing & Frequency Reuse that you'll get with sectors which are pointed in opposite directions.