On the scaling of polar codes: II. The behavior of un-polarized channels

We provide upper and lower bounds on the escape rate of the Bhattacharyya process corresponding to polar codes where transmission takes place over the the binary erasure channel. More precisely, we bound the exponent of the number of sub-channels whose Bhattacharyya constant falls in a fixed interval [a, b]. Mathematically this can be stated as bounding the limit lim_n→∞ 1/n ln P(Z_n ∈ [a, b]), where Z_n is the Bhattacharyya process. The quantity P(Z_n ∈ [a, b]) represents the fraction of sub-channels that are still un-polarized at time n.