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In this paper, we investigate the physical layer security in downlink ofPower Domain Non Orthogonal Multiple Access based heterogeneous cellularnetwork in presence of multiple eavesdroppers. Our aim is to maximize the sumsecrecy rate of the network. To this end, we formulate joint subcarrier andpower allocation optimization problems to increase sum secrecy rate. Moreover,we propose a novel scheme at which the eavesdroppers are prevented from doingSuccessive Interference Cancellation, while legitimate users are able to do it.In practical systems, availability of eavesdroppers Channel State Informationis impractical, hence we consider two scenarios: 1 Perfect CSI of theeavesdroppers, 2 imperfect CSI of the eavesdroppers. Since the proposedoptimization problems are nonconvex, we adopt the well known iterativealgorithm called Alternative Search Method. In this algorithm, the optimizationproblems are converted to two subproblems, power allocation and subcarrierallocation. We solve the power allocation problem by the Successive ConvexApproximation approach and solve the subcarrier allocation subproblem, byexploiting the Mesh Adaptive Direct Search algorithm. Moreover, in order tostudy the optimality gap of the proposed solution method, we apply themonotonic optimization method. Moreover, we evaluate the proposed scheme forsecure massive connectivity in 5G networks. Numerical results highlight thatthe proposed scheme significantly improves the sum secrecy rate compared withthe conventional case at which the eavesdroppers are able to apply SIC.

Robust Physical Layer Security for Power Domain Non-orthogonal Multiple Access-Based HetNets and HUDNs, SIC Avoidance at Eavesdroppers