  Massive machine-type communications (mMTC) is one of the key application
scenarios for future 5G networks. Non-orthogonal multiple access (NOMA) is a
promising technique for the use in mMTC scenario. In this paper, we investigate
NOMA schemes based on polar codes. We compare two possible decoding techniques:
joint successive cancellation algorithm and joint iterative algorithm. In order
to optimize the codes (choose frozen bits) we propose a special and efficient
design algorithm. We investigate the performance of the resulting scheme in the
Gaussian multiple access channel (GMAC) by means of simulations. The scheme is
shown to outperform LDPC based solution by approximately 1 dB and to be close
to the achievability bound for GMAC.
