5GHz Wifi Spurious Emissions Between Access Points???

[Jovial_Jack]

Wireless newbie here…

I have a router (R1) with (4) vertical external antennas and an access point (AP2) with 4 vertical external antennas placed at ceiling height on the same wall to extend coverage to a couple of adjacent rooms. Literally all the antennas are lined up 10' apart at the same height on the wall such that it's

l l l l - - - - - - - - - - l l l l

When I purchased the router + AP combo the vendor swore up and down the appliances wouldn't interfere with one another so long as one was operated on channel 36 and the other channel 165.

When appliance (R1 or AP2) transmits traffic with a single client (C1 or C2) it does so at 55MB/s. Sadly when both (R1 or AP2) pass/transmit WLAN traffic for 2 clients at the same time the achieved bandwidth drops to 5-10MB/s. To be clear C1 is connected to R1 and C2 is connected to AP2. The clients are just doing an iperf3 file transfer among themselves.

As soon as I placed R1 on the floor and out of line of site of AP2 (some boxes of stuff attenuating) the achieved transfer is 55MB/s like one would hope.

I don't have any reason to believe the vendor had a batch of units with bad band-pass filters causing spurious emissions outside of the spectral mask. When I check the FCC docs for the appliances, the FCC shows pass for "frequency stability" and "spurious emissions" at a 3M or 9' test distance which is akin to my setup.

https://fccid.io/Q87-WRT1900AC/Test-Report/Test-Report-NII-pdf-2205639.pdf

Because of attenuation to necessary coverage areas I really do need to place these appliances on the same wall, near the ceiling no more than 10-14' apart.

Is there a wireless principle I'm unaware of that explains why R1 and AP2 are diminishing each other so greatly when transmitting at the same time even though the channels are "supposed" to be completely isolated???

Is placing a small strip of RF shielding fabric obstructing the line of site between the router and access point a feasible solution???

I think a strip of RF shielding fabric between them may look tacky but if that is the only solution I'll gladly do it. If that is a feasible solution does anyone here recommend a particular RF shielding fabric to buy???

 

[thiggins]

Why are you placing two APs so close together in the first place?

 

[TonyH]

Does your router has only 10 feet range on 5GHz? Tried to relocate router to improve coverage w/o AP?

 

[Jovial_Jack]

Because after much testing it was discerned the placement was optimal given coverage needs and the attenuation of the surrounding environment.

Folks on various forums are expressing surprise these are interfering with each other despite being so far apart on the spectrum. At first I was shocked to hear other folks talk about instances of stacking units atop each other so long as they avoided using adjacent channels...but then I had to give credence to what they're saying because millions of dual 5G radio routers are hitting the market and they all have (2) 5G radios broadcasting at the same time (on 36 and 149) and function well.

I am thinking 55MB/s is fantastic and so long as I sort out why 320mhz of spectrum differential is inadequate between these appliances, and if the only solution is RF shielding between them.

 

[thiggins]

Because after much testing it was discerned the placement was optimal given coverage needs and the attenuation of the surrounding environment.

Maybe you mean capacity requirements. Putting two APs essentially in the same spot makes little sense for coverage, i.e. range.

At any rate, tri-band routers with two 5 GHz radios have band filters on each radio to deal with potential interference issues, albeit as much shorter distances.

 

[thiggins]

Can you please provide more detail / diagram of your test setup?

Is it
C1 ***** R1=====AP2*****C2
**** is WiFi
==== is Ethernet

Do you get the same results uplink and downlink?

 

[Jovial_Jack]

Maybe you mean capacity requirements. Putting two APs essentially in the same spot makes little sense for coverage, i.e. range.
At any rate, tri-band routers with two 5 GHz radios have band filters on each radio to deal with potential interference issues, albeit as much shorter distances.

Well said. Yeah it's mostly about capacity even though in this particular building the secondary AP really does provide coverage to areas that would otherwise not be served at all.

I now realize what you shared about high and low filters in all the dual 5G makes perfect sense. I suspect the filters on these units being wide open from 36-161 is part of the challenge (yet by design otherwise they wouldn't be able to serve as full function routers)

I noticed the FCC docks reflect they test "frequency accuracy" and these models pass...yet the FCC doc I looked at didn't distinguish what is defined as passing for accuracy.

 

[Jovial_Jack]

Can you please provide more detail / diagram of your test setup?
Is it
C1 ***** R1=====AP2*****C2
**** is WiFi
==== is Ethernet
Do you get the same results uplink and downlink?

I wish I had been that clever to communicate the diagram that way. Absolutely...that's it.

Yes, when I run iperf3 normally or in --reverse the behavior is the same.

 

[thiggins]

Since the effect goes away with distance, I suspect RF interference is the problem

Even though you are setting one radio to Channel 36 and the other to Channel 165, since both use 80 MHz bandwidth, channels 36-48 and 165 - 153 are used. The center frequencies of Channel 165 (5.765 GHz) and Channel 48 (5.240 GHz) are only 500 MHz apart.

The 802.11ac spectral mask for 80 MHz bandwidth requires the signal be down only 40 dB 120 MHz either side of center channel. That doesn't mean the signal goes away after that or even gets lower.

The point I'm making is that there is still plenty of chance your AP and router are interfering with each raising the RF noise level enough to cause a high rate of retransmissions. This could be confirmed via wireless packet capture, which is probably more than you want to do.

Again, your setup is VERY unusual. When more coverage is required, APs are usually spaced significantly further apart.

If you MUST have two 5 GHz radios placed so closely together, I suggest you use two tri-band routers that have band filtering.

A few other things to try:
- Put both router and AP on the SAME channel. This would ensure they both see each other and properly coordinate airtime. I'd expect ~ 50% throughput reduction vs. the 500% you are currently experiencing.
- Try reducing transmit power on one or both AP/router

 

[Jovial_Jack]

"The 802.11ac spectral mask for 80 MHz bandwidth requires the signal be down only 40 dB 120 MHz either side of center channel. That doesn't mean the signal goes away after that or even gets lower."

Tim. I greatly appreciate your sharing this important info.

Each time I've spoken with Linksys reps I've been a little incredulous about their insistance these units transmit precisely enough so as to not bleed noise 320-500MHz up or down. When I glanced at the FCC docs I saw the model passes for "frequency stability" and "spurious emissions" yet didn't know how that's defined. Is that more or less the 40dBm requirement you've shared??

Correct me if I misunderstand this...so lets say AP1 is transmitting 5.240 GHz at 30dBm is supposed to lessen 40dBm (so 70dBm??) at the upper fringe 5.360 GHz and 70dBm of noise spills out on everything from 5.360 GHz and up

The same applies to AP2 transmitting 5.765 GHz at 30dBm. It's supposed to lessen 40dBm (so 70dBm??) at it's lower fringe 5.645 GHz and 70dBm of noise spills out on everything from 5.645 GHz and below.

So hypothetically each could be spilling 70dBm of noise over the other's channels if the spill doesn't drop more then 40dBm correct??

I don't have a spectrum analyzer on hand and respect if I should take the firmware GUI with a grain of salt. All I can say is I'd observed when the WRT WLANs are idle the GUI reflects the noise as between 85-95dBm. I was confused when both WLANs are transmitting the GUI doesn't reflect the noise increasing whatsoever.

You mention wireless packet capture. Yes I don't have any experience with wireshark yet and have only used Kali Linux once. Can anything be learned from the dropped packets of iperf3 UDP tests??

A Linksys engineer called me and seems pretty confident a pair of new WRT ACS units will fix the problem. They'll arrive by the end of the week. I readily accept they may not fix the problem given what you've shared.

The band filters on triband routers totally makes sense.

Overwhelmingly folks have recommended a pair of Ubiquity UAP-AC-PRO would not exhibit this behavior. However, I was thinking that since they do everything from CH 36-161 they cannot have high/low pass filters to prevent noise within that range...just anything outside of that range. Am I correct to think it possible the UAP-AC-PRO units could exhibit the same spill behavior if they too only have to adhere to the 40dBm drop as you've indicated??

As for transmit power...I'd tested the units at lock step transmit power of 0, 4, 7, 10, 13, 16, 19 dBm. Unfortunately the behavior remained unchanged. Only breaking the line of site between the units (metal baking sheet etc) achieves function of 55MB/s. As soon as I pull the metal baking sheet away restoring their line of site, transfers regress to 5-10MB/s.

If the Ubiquity units will also exhibit the same behavior, then no matter what I'll be breaking the line of site between WRTs or Ubiquitys by lining the walls of a floating shelf with RF absorbing fabric. This large one on Amazon http://a.co/1dSgsUY is a little taller and wider then the antennas themselves so I think it will work??

If the new WRTs and Ubiquity don't function differently, is there any RF absorbing fabric that you recommend I buy for lining the floating shelf??

 

[thiggins]

This is not a question of APs being in or out of spec. APs are just not meant to be spaced that closely together.

If you application demands it, then you'll have to play with shielding. I have no recommendation.

 

[Jovial_Jack]

New units Linksys shipped me behaved the same. I opted to try a pair of Ubiquity Unify UAP-AC-PRO instead.

At the same 10' distance the achieved throughput is only 20% diminished instead of 90% diminished with the Linksys units.

For those interested, I even placed the Unify units immediately beside each other and the achieved throughput was only 54% diminished.

My hat is off to Ubiquity!