SNR isn’t good enough to hold 54Mb. It will just about hold 36Mb, but wanders around enough that 24 is more stable. We are getting a steady 3Mb/s or a little better which is fine for this customer and is considerably better than we have been able to achieve with Trango.
There are a number of tactics I want to try moving forward.
First, 5km is probably too far for 900MHz in a busy operating area. It isn’t possible to do serious NLOS links while retaining -60-ish RSSIs at that range. You need the -60 RSSI if you want an “interference-proof” link.
2km is probably a better target for a deep rural cell radius, maybe even 1km. The new price point for APs makes that both achievable and realistic with as low as 10-12 customers per AP.
Second, I think we are going to try a 10MHz channel instead to get the 6Mb/s we want for voice and video so we don’t need the more exotic modulations and can stay both at the SR9 power peak and achieve excellent receive sensitivity in the inevitable low SNR environment of 900MHz. The lower modulations are also likely to be better for punching through interference.
The big question is going to be whether we take a more severe hit from interfernce at 10MHz channel width, or if we would be better using a 5MHz channel and a higher modulation. Only time and testing will tell for sure.
Third, using WDS mesh would be an interesting alternative in our situation, probably using RSTP to let us do multiple bandwidth injections. We can feed the mesh with either 900, horizontal 2.4 or 5.x as appropriate. I’m fuzzy on the actual implementation at this stage, we’re just starting to try some different scenarios.
Generally speaking, it is hard to find enough clear spectrum to do a standard multi-channel, multi-polarity sectorized cell deployment in 900. Using WDS mesh and re-using the same 10MHz channel of horizontally polarized spectrum in every location may be a viable alternative.
We intend to creat microcells. 900MHz is so busy that the traditional “big-tower” macro-cell model doesn’t work very well in a lot of locations. To be really effective, getting down and dirty works better. Stay low to the ground, use trees and other terrain features to mask and attenuate other noise sources and take advantage of the low access point cost we can now achieve using MT and SR9s to get close to the customer with very high RSSIs that insulate your services from the inevitable noise problems experienced in 900MHz.
We now have the tools to deliver 6Mb/s or better low latency QoS’ed services to NLOS rural customers at a great price and with a very high level of reliability. This level of service will be highly competitive with cable and DSL in the near to medium term.
Its going to be a fascinating couple of years when Ubiquiti finally gets full production volumes available for the SR9s.
Vive la revolution!!
George