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Is there any known electromagnetic / interference / [no idea] effect which can cause IEEE 802.11 (Wi-Fi) 2.4GHz devices located in very close proximity (one on top of the other) to severly interfere with each other, even though they are using completely non-overlapping channels of the available ISM 2.4GHz spectrum?

The general knowledge about 2.4GHz Wi-Fi suggests that networks operating on non-overlapping channels should not interfere: answer at Network Engineering explaining 2.4GHz Wi-Fi channels and their interference

2.4GHz Wi-Fi channel structure:

Having a network topology like this (dots mean wireless, dashes are ethernet wire):

laptop . . . router A ----- router B . . . router C ----- ISP's box (FTTH ONT)    
(client      (access        (client)       (access               
 to A)        point)          to C          point)     

Router A's network uses channel 6, router C's network was tested on both channels 1 and 13 (13 gives even more clearance than needed and is legal to use in my country).

If routers A and B are placed physically on each other, the maximum achievable throughput to/from the ISP's speedtest server is about 4Mbps, which would normally resemble an overcrowded channel or channel with many collisions. However, everyone seems to claim that networks in my setup should not interfere as they are assigned non-overlapping channels with enough clearance.

If I move router A a few meters away from B (not sure how much minimum distance would be enough though), the throughput rockets to about 20Mbps, which is around the expected maximum of both wireless networks and means they both perform close to their maximum capacity.

No other changes in either configuration or topology are made on any of the devices, the only change that affects the throughput is physically moving the router A away from B.

Tested multiple times over multiple days to rule out short-term conditions (neighbors), in two different rooms to rule out environment/furniture, no other wireless clients connected in either network during testing, no microwaves in the vicinity, no issues with bandwidth to/from ISP (100/100 Mbps fiber connection achieving full speed to the ISP's speedtest server when on ethernet wire).

Any ideas?

Other notes:

  • This happens on two different vendors' hardware which should fully conform to Wi-Fi specifications, so it should not be an issue with a single vendor or hardware piece or any non-standard 802.11 protocol modifications.
  • I'm not using the wider 802.11n 40MHz channels, this happens on pure 802.11g-only (22MHz) settings too.
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  • \$\begingroup\$ Perhaps it is just the shielding effect of having a lot of metal connected to ground (in the PCB of the other device) close by... \$\endgroup\$ – stefandz Aug 16 '15 at 9:25
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It depends on the internal architecture of the receivers in each box - these are designed to match a specification and down to a price.

A radio receiver has two functions - selectivity, and amplification - and a high performance receiver will achieve each function in several stages - alternating between selectivity and gain.

For example, a tuned filter, followed by an RF amplifier, followed by a tuned filter, followed by a mixer (which often has gain), followed by an IF filter, followed by an IF amplifier, and so on.

Note that the main selectivity (channel rejection) is provided by the IF filter, and the front end RF filters will pass the entire 2.4 GHz band, but protect the following amplifier from other strong signals.

A lower cost receiver may omit the first tuned filter ahead of the RF amplifier. It will actually be more sensitive because that filter had some attenuation. However, extremely strong signals - even outside the 2.4GHz band altogether - can overload the receiver, distorting the wanted signals and mixing in components of the unwanted signals.

That is essentially what is happening here : because the boxes are stacked on top of each other, one's transmission is seen by the next as an extremely strong interfering signal, (in the 2.4GHz band, though outside the desired channel) overloading the receiver.

You may find that Brand B's receiver is more tolerant to overload than Band A's, but really, without getting deeply into the receiver design there's not a lot you can do about in except prevent the overload, by physically separating the boxes.

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  • \$\begingroup\$ Thanks! I'm actually a network engineer, not an electrical one, but I took some EE subjects in the past, so I surely get the point. I also think you might be right about the receivers' tolerance since the case above (A and B) were two Mikrotiks with the same Atheros radio, but when I connected one of them as a wired slave to the C router (a much older D-Link) and created two wireless networks with independent laptops and data streams, the (IMHO better quality) Mikrotik had no problems while D-Link's wireless was unusable regardless of which one was the topology master connected to ISP's box. \$\endgroup\$ – user83355 Aug 16 '15 at 9:50
  • \$\begingroup\$ You could specify and design a receiver to overcome this problem - radio hams might, military radio and radar designers certainly would. The 1dB gain compression point of an amplifier or the 3rd order intercept point of a mixer would be two such specifications. But meeting such specs would be an unnecessary expense for 99% or more of their customers : separating units by a couple fo metres is usually cheaper! \$\endgroup\$ – Brian Drummond Aug 17 '15 at 10:12
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This is not unexpected.

While the intentional radiation of the WiFi device must fit within the 22 MHz channel, there will be unintentional radiation for many MHz beyond that. This radiation is subject to different limits, and must be much smaller than the intentional in-band radiation, but that doesn't mean it's insignificant.

From memory, the power in adjacent channels would be at least -50 dB of the power in the selected channel.

(the receiver in a Wifi device is not perfect either, and picks up some energy from neighbouring channels.)

The out-of-band radiation is not a problem if there are two APs of equal strength on different channels, some distance from the user. The user sees the wanted one, and the unintentional radiation in the wanted channel is very small, so doesn't affect the signal to noise ratio much.

The problem happens when the receiver, trying to detect a signal from a user at normal range, is very close to a transmitter on an adjacent channel. Now the margin between intentional and unintentional is used up by the unequal distances.

From the -50 dB figure above, if the interfering device is 100x closer than your laptop, then its signal in the wanted channel will be only 10 dB below the laptop signal in that channel, which will definitely cause interference.

So in summary, what you are seeing, while not ideal, is expected behaviour for wifi devices held very close together.

If you have access to channel 13 then you should use 1, 7, 13, that might help a bit. Move them apart. Keep the APs that are closest together, on the most separated channels.

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  • \$\begingroup\$ That would explain it. Yes, exactly - the interfering radios were like 3 centimiters from each other (Mikrotiks and their perfectly square stackable boxes...), while the laptop was a few meters away. I'm not going to use channel 7 though, since most neighbors' routers stick to the 1/6/11, so I would interfere with the 11s too. 13 instead of 11 should be fine though I guess. \$\endgroup\$ – user83355 Aug 16 '15 at 9:56

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