[Bloat] Is 5/10MHz wifi bandwidth legal in 2.4GHz (half/quarter-clocking)?

[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?

=
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
5GHz bands as well:

>> 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