A novel partial order for the information sets of polar codes over B-DMCs
We study partial orders on the information sets of polar codes designed for binary discrete memoryless channels. We show that the polar transform defined by Arikan preserves 'symmetric convex/concave orders'. While for symmetric channels this ordering turns out to be equivalent to the stochastic degradation ordering already known to order the information sets of polar codes, we show that a strictly weaker partial order is obtained when at least one of the channels is asymmetric. We also discuss two tools which can be useful for verifying this ordering: a criterion known as the cut criterion and channel symmetrization.