Can signal be too good?

Recently I’ve run into a few customers with really good signal, SNR and modulation but constant loss of registration and drops in the SNR graph. I move them to an adjacent sector and their signal drops, but the problem goes away. The most recent case was a customer on a 450b high gain at 2 miles with a -54 signal and really bad dropouts. Between the dropouts though they were at 8X and really good SNR on both HPOL and VPOL.

I didn’t think the signal could be too strong until it was in the 30’s or 40’s. Could -54 be too hot?

What about the SM receive target, can there be a problem is the signal at the AP is above or right at the target? Usually I leave the default of -52, sometimes if I remember I change it to -62.

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YES!

Its called receiver over feed (some use the term saturation). It happens when the received signal is higher than what the rx amplifier was designed to handle and can not drop its output below the rx input gain threshold. This means that higher coding schemes get distorted as it is akin to having someone screaming in your ear while giving detailed complex instructions to do a dificult task.

To mitigate this, Cambium (and others) use an AP controlled SM-tx power control to reduce SMs that are received too strongly in comparison to other SMs. On the SM side, if you are receiving the AP too loudly, you can detune to SM a bit, usually downward is good, and this will still keep a reasonable SNR.

This is the part that is not taught enough to new techs and mostly forgotten by us older guys.

But could -54 be too hot? I assumed if the default SM receive target was -52, the overload point was quite a bit higher.

Also I thought the point of ATPC was to avoid unnecessarily interfering with another AP pointed in the opposite direction, which will only have so much antenna F/B ratio.

IMHO, -54dBm isn’t too hot a signal. I usually set my SM receive target to -60. Do you have this SM onboarded into cnMaestro so you can look at the SnR, modulation, etc. over time?

-54 is hot but not maxed, I had a link at -45 but it was not a good performing link until it was toned down a bit. -60 is usually the hottest you want with -65 being a great strength.

The ATPC is only partly involved for frequency reuse. Its main function is to equalize the AP received signal strength of all the SMs on a sector. It has a side effect of ensuring the SM of the back sector has a hard time connecting to the front sector, but that is not the main intent. Think of a sector like an auditorium, if you have someone upfront chatting away but an attendant in the back asks a question, the presenter will have a hard time hearing the question. Now the chatter up front keeps it quiet and to about the same level as the querant then there is no problem.
The way the manual teaches howbthis all works isnt wrong, its just not fully correct as it does not explain in any detail how these systems work together to mitigate interference and self-interference.
The fact is an SM will connect to an AP which has the better SNR if the signals strengths are reasonably close to each other. Thats why we have the preferred ap list.

Frequency reuse and self-interference are both a function of setting up the GPS timing system and making sure that all APs have the same setting other than what has to be kept different on each ap. This and the fact that your antennas should have a 30dB f/b ratio means that the two APs have 60dB of isolation but since they are transmitting at the same time this is moot and only the 30dB loss is considered for SM connections. If you had untimed APs then the 60dB isolation would limit your tx power so you wouldnt exceed the isolation threshold.

See if you can get a copy of the old Motorola MWFE and MWSE books, this has a very good explanation of RF mechanics short of an EE degree.

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Eric, this particular SM is not in cnMaestro, at this time we have only onboarded our CBRS radios. But I do have graphs via SNMP from the SM. It is harder to get AP side data.

The signal level is absolutely steady, so we’re not seeing multipath or trees or anything like that. I do see variations in the SNR, which led me to believe it was an interference problem, but I’m doubting that. And of course these radios don’t really measure SNR but rather BER from which they extrapolate SNR. I’m reluctant to say the SNR suddenly nosedived and caused the SM to deregister, and it’s difficult to tell if it’s an uplink or downlink problem causing loss of registration.

450b complicates things because of its higher xmt power, so the rcv signal at the AP is usually several dB higher than at the SM. But we have ATPC do deal with that. If the signal at the SM is too hot, there isn’t much we can do except use a smaller antenna, tilt the antenna up, or connect to a different AP on the tower.

Try lowering the target receive down to -60 or lower. Read on a forum post or the docs somewhere that greater than -60 can self interfere. Generally we set ours to around -63 with exceptions for APs with very long shots.

What version of software are you running on these? I’m seeing the same thing on 16.2.3 where 16.1.1.1 didn’t have a problem.

The problem I’m working on now, both AP and SM are still on 15.2.

I’ve also seen it on a CBRS sector and all our CBRS stuff is on 16.2.2. Actually we’re trying to take everything to 16.2.2 but haven’t gotten to all of them. With the situation during the pandemic, people get mad if we do firmware updates even at 3am.

But what I’m seeing is definitely not a 16.2.3 issue.

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I can report that changing the rcv target from -52 to -62 has resolved this problem.

Thanks for reporting back Ken. This information might help another community member with similar issues. I wouldn’t have thought that -52 is “too hot” either… and will ask engineering about this just as a follow up.