  The performance of physical layer secrecy approach in non-orthogonal multiple
access (NOMA)-enabled heterogeneous networks (HetNets) is analyzed in this
paper. A $K$-tier multi-cell HetNet is considered, comprising NOMA adopted in
all tiers. The base stations, legitimate users (in a two-user NOMA setup), and
passive eavesdroppers, all with single-antenna, are randomly distributed.
Assuming independent Poisson point processes for node distribution, stochastic
geometry approaches are exploited to characterize the ergodic secrecy rate. A
lower bound on the ergodic secrecy rate along with closed-form expressions for
the lower bound on the ergodic rates of the legitimate users in a special case
are derived. Moreover, simpler expressions for the ergodic secrecy rate are
obtained in the interference-limited regime with vanishing noise variance. The
effect of multi-tier technology, NOMA, and physical layer secrecy are
investigated using numerical results. The results reveal that applying HetNet
to a secure multi-cell NOMA system improves the spectrum efficiency
performance.
