I'm trying to reduce the connection bit rate on my Atheros TP-Link TL-WN821N v3 usb wifi adapter due to frequent instability issues (reported connection speed goes down to 1Mb/s and I have to physically reconnect the adapter to regain a connection). I know this is a common problem with this device, and I have tried everything I can think of to fix it, including using drivers from linux-backports; compiling and installing a custom firmware (following instructions on https://wiki.debian.org/ath9k_htc#fw-free) and (as a last resort) using ndiswrapper.

When using ndiswrapper, the wifi adapter is stable and operates in g mode at 54Mb/s (whilst when using the default ath9k_htc module, the adapter connects in n mode and the bit rate fluctuates constantly). Unfortunately, with this setup I have to run my processor using only one core, since using SMP with ndiswrapper causes a kernel oops on my system.

So I want to lock my bit rate to 54Mb/s (or less, if need be) for connection stability, using the ath9k_htc module.

I've tried 'sudo iwconfig wlan0 rate 54M'; the command runs with no error but when I check the bit rate with 'sudo iwlist wlan0 bitrate' the command returns:

wlan0 unknown bit-rate information. 
          Current Bit Rate:78 Mb/s

Any ideas? Here's some info (hopefully relevant) on my setup:

Xubuntu (12.04.3) 64bit (kernel 3.2.0-55.85-generic) using Network Manager. My Router is from Virgin Media, the VMDG480.

lshw -C network :

  *-network               
       description: Wireless interface 
       physical id: 1 
       bus info: usb@1:4 
       logical name: wlan0 
       serial: 74:ea:3a:8f:16:b6 
       capabilities: ethernet physical wireless 
       configuration: broadcast=yes driver=ath9k_htc driverversion=3.2.0-55 firmware=1.3 ip=192.168.0.9 link=yes multicast=yes wireless=IEEE 802.11bgn

lsusb -v:

Bus 001 Device 003: ID 0cf3:7015 Atheros Communications, Inc. TP-Link TL-WN821N v3 802.11n [Atheros AR7010+AR9287] 
Device Descriptor: 
  bLength                18 
  bDescriptorType         1 
  bcdUSB               2.00 
  bDeviceClass          255 Vendor Specific Class 
  bDeviceSubClass       255 Vendor Specific Subclass 
  bDeviceProtocol       255 Vendor Specific Protocol 
  bMaxPacketSize0        64 
  idVendor           0x0cf3 Atheros Communications, Inc. 
  idProduct          0x7015 TP-Link TL-WN821N v3 802.11n [Atheros AR7010+AR9287] 
  bcdDevice            2.02 
  iManufacturer          16 ATHEROS 
  iProduct               32 UB95 
  iSerial                48 12345 
  bNumConfigurations      1 
  Configuration Descriptor: 
    bLength                 9 
    bDescriptorType         2 
    wTotalLength           60 
    bNumInterfaces          1 
    bConfigurationValue     1 
    iConfiguration          0 
    bmAttributes         0x80 
      (Bus Powered) 
    MaxPower              500mA 
    Interface Descriptor: 
      bLength                 9 
      bDescriptorType         4 
      bInterfaceNumber        0 
      bAlternateSetting       0 
      bNumEndpoints           6 
      bInterfaceClass       255 Vendor Specific Class 
      bInterfaceSubClass      0 
      bInterfaceProtocol      0 
      iInterface              0 
      Endpoint Descriptor: 
        bLength                 7 
        bDescriptorType         5 
        bEndpointAddress     0x01  EP 1 OUT 
        bmAttributes            2 
          Transfer Type            Bulk 
          Synch Type               None 
          Usage Type               Data 
        wMaxPacketSize     0x0200  1x 512 bytes 
        bInterval               0 
      Endpoint Descriptor: 
        bLength                 7 
        bDescriptorType         5 
        bEndpointAddress     0x82  EP 2 IN 
        bmAttributes            2 
          Transfer Type            Bulk 
          Synch Type               None 
          Usage Type               Data 
        wMaxPacketSize     0x0200  1x 512 bytes 
        bInterval               0 
      Endpoint Descriptor: 
        bLength                 7 
        bDescriptorType         5 
        bEndpointAddress     0x83  EP 3 IN 
        bmAttributes            3 
          Transfer Type            Interrupt 
          Synch Type               None 
          Usage Type               Data 
        wMaxPacketSize     0x0040  1x 64 bytes 
        bInterval               1 
      Endpoint Descriptor: 
        bLength                 7 
        bDescriptorType         5 
        bEndpointAddress     0x04  EP 4 OUT 
        bmAttributes            3 
          Transfer Type            Interrupt 
          Synch Type               None 
          Usage Type               Data 
        wMaxPacketSize     0x0040  1x 64 bytes 
        bInterval               1 
      Endpoint Descriptor: 
        bLength                 7 
        bDescriptorType         5 
        bEndpointAddress     0x05  EP 5 OUT 
        bmAttributes            2 
          Transfer Type            Bulk 
          Synch Type               None 
          Usage Type               Data 
        wMaxPacketSize     0x0200  1x 512 bytes 
        bInterval               0 
      Endpoint Descriptor: 
        bLength                 7 
        bDescriptorType         5 
        bEndpointAddress     0x06  EP 6 OUT 
        bmAttributes            2 
          Transfer Type            Bulk 
          Synch Type               None 
          Usage Type               Data 
        wMaxPacketSize     0x0200  1x 512 bytes 
        bInterval               0 
Device Qualifier (for other device speed): 
  bLength                10 
  bDescriptorType         6 
  bcdUSB               2.00 
  bDeviceClass          255 Vendor Specific Class 
  bDeviceSubClass       255 Vendor Specific Subclass 
  bDeviceProtocol       255 Vendor Specific Protocol 
  bMaxPacketSize0        64 
  bNumConfigurations      1 
Device Status:     0x0000 
  (Bus Powered)

iwlist wlan0 scanning:

wlan0     Scan completed : 
          Cell 01 - Address: C4:3D:C7:3A:1F:5D 
                    Channel:1 
                    Frequency:2.412 GHz (Channel 1) 
                    Quality=37/70  Signal level=-73 dBm  
                    Encryption key:on 
                    ESSID:"my essid" 
                    Bit Rates:1 Mb/s; 2 Mb/s; 5.5 Mb/s; 11 Mb/s; 18 Mb/s 
                              24 Mb/s; 36 Mb/s; 54 Mb/s 
                    Bit Rates:6 Mb/s; 9 Mb/s; 12 Mb/s; 48 Mb/s 
                    Mode:Master 
                    Extra:tsf=00000070cca77186 
                    Extra: Last beacon: 5588ms ago 
                    IE: Unknown: 0007756E69636F726E 
                    IE: Unknown: 010882848B962430486C 
                    IE: Unknown: 030101 
                    IE: Unknown: 2A0100 
                    IE: Unknown: 2F0100 
                    IE: IEEE 802.11i/WPA2 Version 1 
                        Group Cipher : TKIP 
                        Pairwise Ciphers (2) : CCMP TKIP 
                        Authentication Suites (1) : PSK 
                    IE: Unknown: 32040C121860 
                    IE: Unknown: 2D1AFC181BFFFF000000000000000000000000000000000000000000 
                    IE: Unknown: 3D1601080400000000000000000000000000000000000000 
                    IE: Unknown: DD7E0050F204104A0001101044000102103B00010310470010F99C335D7BAC57FB00137DFA79600220102100074E657467656172102300074E6574676561721024000631323334353610420007303030303030311054000800060050F20400011011000743473331303144100800022008103C0001011049000600372A000120 
                    IE: Unknown: DD090010180203F02C0000 
                    IE: WPA Version 1 
                        Group Cipher : TKIP 
                        Pairwise Ciphers (2) : CCMP TKIP 
                        Authentication Suites (1) : PSK 
                    IE: Unknown: DD180050F2020101800003A4000027A4000042435E0062322F00

iwconfig:

lo        no wireless extensions. 

wlan0     IEEE 802.11bgn  ESSID:"my essid"  
          Mode:Managed  Frequency:2.412 GHz  Access Point: C4:3D:C7:3A:1F:5D   
          Bit Rate=78 Mb/s   Tx-Power=20 dBm   
          Retry  long limit:7   RTS thr:off   Fragment thr:off 
          Power Management:off 
          Link Quality=36/70  Signal level=-74 dBm  
          Rx invalid nwid:0  Rx invalid crypt:0  Rx invalid frag:0 
          Tx excessive retries:0  Invalid misc:0   Missed beacon:0,