Efficient Construction of Polar Codes
  The construction of polar codes for channels other than BECs requires sorting of all bit channels and then selecting the best $K$ of them for a block length $N=2^n$. In this paper, two types of partial orders (PO) of polar codes are incorporated in the construction process to decrease the required computations. Three sets, corresponding to the good bit channels ($\mathcal{I}$), the frozen bit channels ($\mathcal{F}$), and the undetermined bit channels ($\mathcal{U}$), are selected by applying PO relations. The POs are channel independent and are therefore universal for all binary-input discrete memoryless channels. For a given specific channel, a new process, called Dimension Reduction (DR), is proposed in this paper to further reduce the size of $\mathcal{U}$. Our studies show that for $N=10$ and the code rate $R=0.5$ (being the worst code rate), incorporating PO relations alone can determine 50% of the bit channels ($|\mathcal{I}| + |\mathcal{F}| \approx N/2$). With our proposed DR, this number of the determined bit channels goes up to 82%, which brings a significant reduction of computations in the construction of polar codes. 
Wei Wang and Liping Li
cs.IT math.IT