  In this paper, we investigate the physical layer security in downlink of
Power Domain Non Orthogonal Multiple Access based heterogeneous cellular
network in presence of multiple eavesdroppers. Our aim is to maximize the sum
secrecy rate of the network. To this end, we formulate joint subcarrier and
power allocation optimization problems to increase sum secrecy rate. Moreover,
we propose a novel scheme at which the eavesdroppers are prevented from doing
Successive Interference Cancellation, while legitimate users are able to do it.
In practical systems, availability of eavesdroppers Channel State Information
is impractical, hence we consider two scenarios: 1 Perfect CSI of the
eavesdroppers, 2 imperfect CSI of the eavesdroppers. Since the proposed
optimization problems are nonconvex, we adopt the well known iterative
algorithm called Alternative Search Method. In this algorithm, the optimization
problems are converted to two subproblems, power allocation and subcarrier
allocation. We solve the power allocation problem by the Successive Convex
Approximation approach and solve the subcarrier allocation subproblem, by
exploiting the Mesh Adaptive Direct Search algorithm. Moreover, in order to
study the optimality gap of the proposed solution method, we apply the
monotonic optimization method. Moreover, we evaluate the proposed scheme for
secure massive connectivity in 5G networks. Numerical results highlight that
the proposed scheme significantly improves the sum secrecy rate compared with
the conventional case at which the eavesdroppers are able to apply SIC.
