Ath10k-ct fixed rate and bandwidth

Installing and Using OpenWrt
1

hardware: Qualcomm Atheros QCA9880
firmware: branch openwrt-19.07, getver r10928-95d5cbd
use case: AP <-> Client link, -56 dBm.
Im trying to set a fixed data rate of MCS9 by
iw dev wlan0 set bitrates vht-mcs-5 2:9 sgi-5
on both sides.
First question:
iwinfo wlan0 assoclist on both sides shows
RX: 360.0 MBit/s, VHT-MCS 8, 40MHz, VHT-NSS 2 12432359 Pkts.
TX: 360.0 MBit/s, VHT-MCS 8, 40MHz, VHT-NSS 2 91664178 Pkts.
Does the QCA9880 not do MCS9 (256-QAM 5/6)?

Instead I set MCS8 in the iw command.

At first, iwinfo wlan0 assoclist on both sides shows the same as above, but after i start some traffic using MikroTik bandwidth test, iwinfo wlan0 assoclist on the AP shows:
RX: 360.0 MBit/s, VHT-MCS 8, 40MHz, VHT-NSS 2 12782721 Pkts.
TX: 173.3 MBit/s, VHT-MCS 8, 20MHz, VHT-NSS 2 94093557 Pkts.
the bandwidth has dropped to 20MHz.
Second question:
Is there a way to set the bandwidth?
I tried Setting the bandwidth
echo 0x0000000800000002 > phy0/ath10k/ct_special
and bandwidth disable
echo 0x0000000E00000001 > phy0/ath10k/ct_special

but they cause iwinfo wlan0 assoclist to show
RX: 360.0 MBit/s, VHT-MCS 8, 40MHz, VHT-NSS 2 1106003 Pkts.
TX: 6.0 MBit/s 7084204 Pkts.

then
echo "wlan0 tpc=255 sgi=1 rix=8 nss=2 pream=3 retries=1 dynbw=0 bw=1 active=1" > phy0/ath10k/set_rate_override

but it causes the driver to crash on the AP

Thanks.

2

The radio automatically decides what to transmit, that would merely set the maximum. I would advise not attempting to change such settings.

Is there an issue that you're experiencing that's making you consider setting your rate and bandwidth to a fixed value?

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3

The concept of fixed data rates simply doesn't exist beyond 802.11g (and was already questionable there), even less with 802.11ac and MIMO. The rate control algorithm needs the ability to react dynamically to the environment, it changes quickly, even if location and distance don't change.

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