  Polar coding was conceived originally as a technique for boosting the cutoff
rate of sequential decoding, along the lines of earlier schemes of Pinsker and
Massey. The key idea in boosting the cutoff rate is to take a vector channel
(either given or artificially built), split it into multiple correlated
subchannels, and employ a separate sequential decoder on each subchannel. Polar
coding was originally designed to be a low-complexity recursive channel
combining and splitting operation of this type, to be used as the inner code in
a concatenated scheme with outer convolutional coding and sequential decoding.
However, the polar inner code turned out to be so effective that no outer code
was actually needed to achieve the original aim of boosting the cutoff rate to
channel capacity. This paper explains the cutoff rate considerations that
motivated the development of polar coding.
