sudo apt-get install linux-headers-generic build-essential
   wget http://www.kernel.org/pub/linux/kernel/projects/backports/stable/v3.12.2/backports-3.12.2-1.tar.bz2
   tar xvf backports-3.12.2-1.tar.bz2
   cd backports-3.12.2-1
   make defconfig-ath9k
   make
   sudo make install

sudo rmmod ath9k
  sudo modprobe ath9k nohwcrypt=1