Discussion:
Is 5/10MHz wifi bandwidth legal in 2.4GHz (half/quarter-clocking)?
(too old to reply)
bkil
2018-10-08 20:18:39 UTC
Permalink
If this is not the right forum to discuss, could you please point me
in the right direction?

After all, channel spacing is indeed 5MHz here. Although using a new
raster instead of the 20MHz channel center frequencies would allow
full utilization of the band (16 or 8 channels respectively), using
the standard set of 11 (13) channels is better than nothing.

Is it a good idea to use HT instead of g for such links?

=
Some background and links for those who do not know this mode:

"the 2007 version of the IEEE 802.11 standard [1] specifies 5 and 10
MHz wide channels for use in the 4.9 GHz public safety bands"

Although according to my reading of section 17.1, it applies to the
17. Orthogonal frequency division multiplexing (OFDM) PHY specification
for the 5 GHz band
[...]
The OFDM system also provides a “half-clocked” operation using 10 MHz
channel spacings with data
communications capabilities of 3, 4.5, 6, 9, 12, 18, 24, and 27 Mb/s.
The support of transmitting and
receiving at data rates of 3, 6, and 12 Mb/s is mandatory when using
10 MHz channel spacing. The half-
clocked operation doubles symbol times and clear channel assessment
(CCA) times when using 10 MHz
channel spacing. The regulatory requirements and information regarding
use of this OFDM system in
4.9 GHz and 5 GHz bands is in Annex I and Annex J.<<

They probably did not highlight 2.4GHz usage because of mixed-mode
(non-OFDM) crowding, although nowadays we could actually move this
band to OFDM-only as well.

It is unfortunate that this allowance has disappeared in newer
versions of the standard. Was that intentional?

Reasons why downclocking is advantageous (up to +9dB link budget):

* longer GI = better protection against multipath fading;
* higher power density allowed (2x here) = better SNR;
* less chance for (adjacent-channel) interference;
* reduced TX & RX power consumption for idling and low load.

I know that 802.11ah/af are here, but there exist literally millions
of devices potentially supporting this old and trusty mode, software
permit.

Many Atheros chipsets support it, both old and new. OpenWrt has
debugfs patches applied to enable this, while Linux has some other
patches as well, although it is not user visible.

If this is a legal and preferred mode, it would be nice if we could
unify access.

https://openwrt.org/docs/guide-user/network/wifi/basic?s[]=chanbw
http://ccr.sigcomm.org/online/files/p135-chandra.pdf
https://kabru.eecs.umich.edu/papers/publications/2011/xyzhang_kgshin_mobicom11.pdf
https://www.etsi.org/deliver/etsi_en/300300_300399/300328/01.08.01_60/en_300328v010801p.pdf
https://www.cwnp.com/forums/posts?postNum=305220
https://forum.archive.openwrt.org/viewtopic.php?id=38590
https://forum.openwrt.org/t/5-mhz-bandwith-option/3615
Ryan Mounce
2018-10-09 03:52:56 UTC
Permalink
I'm not aware of anywhere this would be illegal. Worst case you will
need to reduce power by 3/6dB (10/5MHz) if there is a power spectral
density limit in a given jurisdiction and max EIRP @ 20MHz is already
at that limit.
Post by bkil
If this is not the right forum to discuss, could you please point me
in the right direction?
After all, channel spacing is indeed 5MHz here. Although using a new
raster instead of the 20MHz channel center frequencies would allow
full utilization of the band (16 or 8 channels respectively), using
the standard set of 11 (13) channels is better than nothing.
Is it a good idea to use HT instead of g for such links?
=
"the 2007 version of the IEEE 802.11 standard [1] specifies 5 and 10
MHz wide channels for use in the 4.9 GHz public safety bands"
Although according to my reading of section 17.1, it applies to the
17. Orthogonal frequency division multiplexing (OFDM) PHY specification
for the 5 GHz band
[...]
The OFDM system also provides a “half-clocked” operation using 10 MHz
channel spacings with data
communications capabilities of 3, 4.5, 6, 9, 12, 18, 24, and 27 Mb/s.
The support of transmitting and
receiving at data rates of 3, 6, and 12 Mb/s is mandatory when using
10 MHz channel spacing. The half-
clocked operation doubles symbol times and clear channel assessment
(CCA) times when using 10 MHz
channel spacing. The regulatory requirements and information regarding
use of this OFDM system in
4.9 GHz and 5 GHz bands is in Annex I and Annex J.<<
They probably did not highlight 2.4GHz usage because of mixed-mode
(non-OFDM) crowding, although nowadays we could actually move this
band to OFDM-only as well.
It is unfortunate that this allowance has disappeared in newer
versions of the standard. Was that intentional?
* longer GI = better protection against multipath fading;
* higher power density allowed (2x here) = better SNR;
* less chance for (adjacent-channel) interference;
* reduced TX & RX power consumption for idling and low load.
I know that 802.11ah/af are here, but there exist literally millions
of devices potentially supporting this old and trusty mode, software
permit.
Many Atheros chipsets support it, both old and new. OpenWrt has
debugfs patches applied to enable this, while Linux has some other
patches as well, although it is not user visible.
If this is a legal and preferred mode, it would be nice if we could
unify access.
https://openwrt.org/docs/guide-user/network/wifi/basic?s[]=chanbw
http://ccr.sigcomm.org/online/files/p135-chandra.pdf
https://kabru.eecs.umich.edu/papers/publications/2011/xyzhang_kgshin_mobicom11.pdf
https://www.etsi.org/deliver/etsi_en/300300_300399/300328/01.08.01_60/en_300328v010801p.pdf
https://www.cwnp.com/forums/posts?postNum=305220
https://forum.archive.openwrt.org/viewtopic.php?id=38590
https://forum.openwrt.org/t/5-mhz-bandwith-option/3615
_______________________________________________
Bloat mailing list
https://lists.bufferbloat.net/listinfo/bloat
bkil
2018-10-09 05:44:40 UTC
Permalink
Yes, that was my conclusion as well. There exist spectral masks of
maximal allowed side lobes, but if you are transmitting signals
narrower than that, the side lobes will be much below the limit.

Spectral density in Hungary and some other countries allows for
10mW/MHz, meaning twice the power density for narrow channels.

There can exist a possibility for starvation if two narrow channels
use the two sides of a wider channel in turn, similar to HT40, so a
neighborly mechanism would be nice.
Post by Ryan Mounce
I'm not aware of anywhere this would be illegal. Worst case you will
need to reduce power by 3/6dB (10/5MHz) if there is a power spectral
at that limit.
Post by bkil
If this is not the right forum to discuss, could you please point me
in the right direction?
After all, channel spacing is indeed 5MHz here. Although using a new
raster instead of the 20MHz channel center frequencies would allow
full utilization of the band (16 or 8 channels respectively), using
the standard set of 11 (13) channels is better than nothing.
Is it a good idea to use HT instead of g for such links?
=
"the 2007 version of the IEEE 802.11 standard [1] specifies 5 and 10
MHz wide channels for use in the 4.9 GHz public safety bands"
Although according to my reading of section 17.1, it applies to the
17. Orthogonal frequency division multiplexing (OFDM) PHY specification
for the 5 GHz band
[...]
The OFDM system also provides a “half-clocked” operation using 10 MHz
channel spacings with data
communications capabilities of 3, 4.5, 6, 9, 12, 18, 24, and 27 Mb/s.
The support of transmitting and
receiving at data rates of 3, 6, and 12 Mb/s is mandatory when using
10 MHz channel spacing. The half-
clocked operation doubles symbol times and clear channel assessment
(CCA) times when using 10 MHz
channel spacing. The regulatory requirements and information regarding
use of this OFDM system in
4.9 GHz and 5 GHz bands is in Annex I and Annex J.<<
They probably did not highlight 2.4GHz usage because of mixed-mode
(non-OFDM) crowding, although nowadays we could actually move this
band to OFDM-only as well.
It is unfortunate that this allowance has disappeared in newer
versions of the standard. Was that intentional?
* longer GI = better protection against multipath fading;
* higher power density allowed (2x here) = better SNR;
* less chance for (adjacent-channel) interference;
* reduced TX & RX power consumption for idling and low load.
I know that 802.11ah/af are here, but there exist literally millions
of devices potentially supporting this old and trusty mode, software
permit.
Many Atheros chipsets support it, both old and new. OpenWrt has
debugfs patches applied to enable this, while Linux has some other
patches as well, although it is not user visible.
If this is a legal and preferred mode, it would be nice if we could
unify access.
https://openwrt.org/docs/guide-user/network/wifi/basic?s[]=chanbw
http://ccr.sigcomm.org/online/files/p135-chandra.pdf
https://kabru.eecs.umich.edu/papers/publications/2011/xyzhang_kgshin_mobicom11.pdf
https://www.etsi.org/deliver/etsi_en/300300_300399/300328/01.08.01_60/en_300328v010801p.pdf
https://www.cwnp.com/forums/posts?postNum=305220
https://forum.archive.openwrt.org/viewtopic.php?id=38590
https://forum.openwrt.org/t/5-mhz-bandwith-option/3615
_______________________________________________
Bloat mailing list
https://lists.bufferbloat.net/listinfo/bloat
bkil
2018-10-09 05:52:28 UTC
Permalink
I wouldn't be surprised if we could patch this mode to another common
chipset other than Atheros.

I understand that this is not a standard mode in 2.4GHz if reading the
standard to the letter, but it is close enough. If it is legal to use
it, as lots of devices support it, it would still be a great choice
for certain point to (mult-)point links or mesh/IoT deployments and we
should "advertise" this capability better.

We could get 16 orthogonal channels instead of 4 (or 13 overlapping) -
so we do get more channels in the end.

Also we have lots of underutilized spectrum in 5GHz, we would need to
update regdb in most countries and handle additional channel numbers
to use this.
make-wifi-fast is better here.
anyway there was a long debate about making the public access channels
available to folk that needed it in the ath10k patchset, I think in
the end ben greer decided to leave it out lacking getting anyone at
the FCC to pay attention.
the second question, regarding 5Mhz channels in general - I had tried
that a lot (it has worked multiple times in ath9k's lifecycle) and I
*liked it*, but as it was non standard never got around to depending
on it existing on anything.
We definitely need more channels, not less
Post by bkil
If this is not the right forum to discuss, could you please point me
in the right direction?
After all, channel spacing is indeed 5MHz here. Although using a new
raster instead of the 20MHz channel center frequencies would allow
full utilization of the band (16 or 8 channels respectively), using
the standard set of 11 (13) channels is better than nothing.
Is it a good idea to use HT instead of g for such links?
=
"the 2007 version of the IEEE 802.11 standard [1] specifies 5 and 10
MHz wide channels for use in the 4.9 GHz public safety bands"
Although according to my reading of section 17.1, it applies to the
17. Orthogonal frequency division multiplexing (OFDM) PHY specification
for the 5 GHz band
[...]
The OFDM system also provides a “half-clocked” operation using 10 MHz
channel spacings with data
communications capabilities of 3, 4.5, 6, 9, 12, 18, 24, and 27 Mb/s.
The support of transmitting and
receiving at data rates of 3, 6, and 12 Mb/s is mandatory when using
10 MHz channel spacing. The half-
clocked operation doubles symbol times and clear channel assessment
(CCA) times when using 10 MHz
channel spacing. The regulatory requirements and information regarding
use of this OFDM system in
4.9 GHz and 5 GHz bands is in Annex I and Annex J.<<
They probably did not highlight 2.4GHz usage because of mixed-mode
(non-OFDM) crowding, although nowadays we could actually move this
band to OFDM-only as well.
It is unfortunate that this allowance has disappeared in newer
versions of the standard. Was that intentional?
* longer GI = better protection against multipath fading;
* higher power density allowed (2x here) = better SNR;
* less chance for (adjacent-channel) interference;
* reduced TX & RX power consumption for idling and low load.
I know that 802.11ah/af are here, but there exist literally millions
of devices potentially supporting this old and trusty mode, software
permit.
Many Atheros chipsets support it, both old and new. OpenWrt has
debugfs patches applied to enable this, while Linux has some other
patches as well, although it is not user visible.
If this is a legal and preferred mode, it would be nice if we could
unify access.
https://openwrt.org/docs/guide-user/network/wifi/basic?s[]=chanbw
http://ccr.sigcomm.org/online/files/p135-chandra.pdf
https://kabru.eecs.umich.edu/papers/publications/2011/xyzhang_kgshin_mobicom11.pdf
https://www.etsi.org/deliver/etsi_en/300300_300399/300328/01.08.01_60/en_300328v010801p.pdf
https://www.cwnp.com/forums/posts?postNum=305220
https://forum.archive.openwrt.org/viewtopic.php?id=38590
https://forum.openwrt.org/t/5-mhz-bandwith-option/3615
_______________________________________________
Bloat mailing list
https://lists.bufferbloat.net/listinfo/bloat
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CTO, TekLibre, LLC
http://www.teklibre.com
Tel: 1-831-205-9740
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