A Class of Transformations that Polarize Symmetric Binary-Input   Memoryless Channels
  A generalization of Ar\i kan's polar code construction using transformations of the form $G^{\otimes n}$ where $G$ is an $\ell \times \ell$ matrix is considered. Necessary and sufficient conditions are given for these transformations to ensure channel polarization. It is shown that a large class of such transformations polarize symmetric binary-input memoryless channels. 
Satish Babu Korada and Eren Sasoglu