Polar codes for q-ary channels, q=2^r
  We study polarization for nonbinary channels with input alphabet of size q=2^r,r=2,3,... Using Arikan's polarizing kernel H_2, we prove that the virtual channels that arise in the process of polarization converge to q-ary channels with capacity 1,2,...,r bits, and that the total transmission rate approaches the symmetric capacity of the channel. This leads to an explicit transmission scheme for q-ary channels. The error probability of decoding using successive cancellation behaves as exp(-N^\alpha), where N is the code length and {\alpha} is any constant less than 0.5. 
Woomyoung Park and Alexander Barg
cs.IT math.IT