  A framework is proposed that allows for a joint description and optimization
of both binary polar coding and the multilevel coding (MLC) approach for
$2^m$-ary digital pulse-amplitude modulation (PAM). The conceptual equivalence
of polar coding and multilevel coding is pointed out in detail. Based on a
novel characterization of the channel polarization phenomenon, rules for the
optimal choice of the bit labeling in this coded modulation scheme employing
polar codes are developed. Simulation results for the AWGN channel are
included.
