  In this paper, the potential benefits of applying non-orthogonal multiple
access (NOMA) technique in $K$-tier hybrid heterogeneous networks (HetNets) is
explored. A promising new transmission framework is proposed, in which NOMA is
adopted in small cells and massive multiple-input multiple-output (MIMO) is
employed in macro cells. For maximizing the biased average received power for
mobile users, a NOMA and massive MIMO based user association scheme is
developed. To evaluate the performance of the proposed framework, we first
derive the analytical expressions for the coverage probability of NOMA enhanced
small cells. We then examine the spectrum efficiency of the whole network, by
deriving exact analytical expressions for NOMA enhanced small cells and a
tractable lower bound for massive MIMO enabled macro cells. Lastly, we
investigate the energy efficiency of the hybrid HetNets. Our results
demonstrate that: 1) The coverage probability of NOMA enhanced small cells is
affected to a large extent by the targeted transmit rates and power sharing
coefficients of two NOMA users; 2) Massive MIMO enabled macro cells are capable
of significantly enhancing the spectrum efficiency by increasing the number of
antennas; 3) The energy efficiency of the whole network can be greatly improved
by densely deploying NOMA enhanced small cell base stations (BSs); and 4) The
proposed NOMA enhanced HetNets transmission scheme has superior performance
compared to the orthogonal multiple access~(OMA) based HetNets.
