Recently a PMP450m 3GHz/CBRS AP started experiencing interference and despite quite a bit of troubleshooting, including swapping out the AP, I was unable to resolve it. As a last ditch effort, I decided to try Cambium’s LTE collocation tools.
Symptoms were:
- Poor uplink modulation to a high number of SM’s, especially during peak hours
- Downlink modulation was actually pretty good for most SM’s
Here’s what it looks like when a PMP450m 3GHz AP can experience from a nearby LTE eNb. AP-> Tools → Link Status page:
Here’s an aggregate speed test at non-peak hours:
Here’s an SA from the AP:
NOTE, we’re using a 40MHz carrier at the very bottom of the band with a center at 3570. The AP is seeing noise at about -60 in that part of the band.
What’s really strange is that all of the SM SA’s show very little noise, most are similar to this or just a tad bit noisier, but still very quiet by most measures:
We ran some scans in the area with a 3GHz LTE UE but it didn’t seem to find anything. Running a scan with an LTE UE can be useful because it will typically reveal what TDD config type and special subframe type the eNb is running. Here’s an example of what you can find:
NOTE! This is for a B41/2.5GHz eNb, but a 3GHz/CBRS eNb’s scan will look similar. The EARFCN can be inputted into a converter like this to give you the eNb’s center channel.
As a last ditch effort we decided to play around with Cambium’s LTE collocation tools. We reviewed the guide and downloaded the tool. In the collocation tool there are a number values to consider, and after playing around with all of them the most important setting appears to be the PMP450 “Downlink Data” value. The values we ended up choosing we believe are a good fit for our WISP and most WISP’s and give the greatest chance for timing success without knowing exactly what timing settings the LTE operator is using.
Most 4G LTE operators are going to use a frame configuration of 2 (aka TDD config 2), while some will choose a s frame configuration (aka SSF configuration) of either 5, 6, or 7… with 7 being the most likely. The good thing about the config we chose is that it will have proper timing with SSF’s of 5,6 and 7. Here is a shot of the settings we ended up using:
Here’s what that looks like on the AP → Configuration → Radio tab:
NOTE! you have to use 5ms frames!
After implementing these changes were were delighted to find that the uplink modulations for all SM’s had improved dramatically, along with some minor improvements for downlink modulations as well!
Aggregate speed tests are looking quite a bit better as well!
Here’s what latency/jitter and packet loss look before and after:
In closing, I wish I would have tried out Cambium’s LTE collocation tools sooner! For some reason, because we weren’t really seeing horrible noise and the LTE scans we ran didn’t come up with anything, we kind of ruled out these issues being caused by LTE interference. This should have been something I tried very early on 
This write up is dedicated to @LuciaCambium