schon mal mit "iwconfig .... modu" probiert?
Hier der Auszug aus der Man-Page:
man iwconfig schrieb
modu[lation]
Force the card to use a specific set of modulations. Modern cards support various modulations, some which are
standard, such as 802.11b or 802.11g, and some proprietary. This command force the card to only use the spe‐
cific set of modulations listed on the command line. This can be used to fix interoperability issues.
The list of available modulations depend on the card/driver and can be displayed using iwlist modulation. Note
that some card/driver may not be able to select each modulation listed independantly, some may come as a group.
You may also set this parameter to auto let the card/driver do its best.
Examples :
iwconfig eth0 modu 11g
iwconfig eth0 modu CCK OFDMa
iwconfig eth0 modu auto