Channel interference scan and manual selection

[Jim Thompson]

There is no "1, 6, 11" anymore.  You're not listening.

Back in the day, the adjacent  (1 on 6, 6 on 1, 11 on 6, 6 on 11)
channel rejection spec for 802.11b was 35dBm at 11Mbps modulation
There never was an alternate (1 on 11, 11 on 1) channel rejection spec
in the days of 802.11b.

Most decent super-het designs back then used a SAW filter in the
middle of the downconversion chain, and got around 41dBm of adjacent
channel rejection.

But we all moved to direct-conversion receivers ... because: cost.  We
all loves us some cheap 802.11 gear.

The adjacent channel rejection spec for 802.11g (and 802.11a) is -1dBm
@ 54Mbps modulation.
The alternate channel rejection spec for 802.11g/a is 15dBm @ 54Mbps modulation.
Just FYI, minimum ACR for 802.11g/a at 6Mbps is 16dB (alternate is
16dB more, for 32dB)

If you're underwhelmed by the difference between 41dBm and -1dBm, then
I can't help you.

Free Space Path Loss = 20 log(4*p*r/λ) dB, where
r = distance between transmitter and receiver
λ = wavelength

Path loss in the first meter @ 2.4GHz is 41dB. At 10m it's 60dB.

Lets say you've got a garden-variety radio that puts up 32mW (15dBm)
of tx power, and ignore antenna gain for now (so 0 dBi antennas on
both radios).

Old 802.11b (super-het receivers) world:

15dBm - 60dB - 41dB = -86dBm This is the in-channel 'noise power' of
the adjacent channel radio.
Notice that it is at least 15dB above the thermal noise floor.
Translated: you've lowered your SINR.

New 802.11g/a (direct conversion receivers) world:
15dBm - 60dB - -1 dBm = -44dBm.   This is about 20dB higher than what
is necessary to recover a 54Mbps receiver, but remember, it's the
*noise power* of a radio operated on an adjacent channel.   (Most
802.11 OFDM receivers are EVM-limited at 48Mbps and higher, but I
digress.)

Note as well that we're 57dB above the thermal noise floor, at 10m (33')

Even if you back off to alternate channels (1 and 11), you're still at -60dBm.



On Tue, Dec 3, 2013 at 9:40 PM, Adrian Chadd <adrian at freebsd.org> wrote:
> On 3 December 2013 19:32, Tek Wiz <tx2023 at gmail.com> wrote:
>> Yes, the interference can be calculated by averaging the beacon RSSI (dB) of
>> the overlapping channels, e.g. for channel 6, the beacon RSSI of channels 4,
>> 5, 6, 7, 8 are averaged. Normally channels 1, 6, and 11 are chosen, although
>> other channels may have less interference, in order to be a 'good neighbor'.
>
> Sure. But we can also look at what kinds of frames we actually receive
> during a sample window.
>
> What I'd also like to do is finally add receive power histogram
> support. Ie, look at the RSSI of all the frames you receive, stick
> them in buckets, respond to the power histogram action request frames,
> use them for this.. that doesn't require any driver support. Just
> net80211 work in the RX path.
>
>
>
> -adrian