  A reduced complexity sequential decoding algorithm for polar (sub)codes is
described. The proposed approach relies on a decomposition of the polar
(sub)code being decoded into a number of outer codes, and on-demand
construction of codewords of these codes in the descending order of their
probability. Construction of such codewords is implemented by fast decoding
algorithms, which are available for many codes arising in the decomposition of
polar codes. Further complexity reduction is achieved by taking hard decisions
of the intermediate LLRs, and avoiding decoding of some outer codes.
  Data structures for sequential decoding of polar codes are described.
  The proposed algorithm can be also used for decoding of polar codes with CRC
and short extended BCH\ codes. It has lower average decoding complexity
compared with the existing decoding algorithms for the corresponding codes.
\end{abstract}
