  A secure downlink transmission system which is exposed to multiple
eavesdroppers and is appropriate for Internet of Things (IoT) applications is
considered. A worst case scenario is assumed, in the sense that, in order to
enhance their interception ability all eavesdroppers are located close to each
other, near the controller and collude to form joint receive beamforming. For
such a system, a novel cooperative non-orthogonal multiple access (NOMA) secure
transmission scheme for which an IoT device with a stronger channel condition
acts as an energy harvesting relay in order to assist a second IoT device
operating under weaker channel conditions, is proposed and its performance is
analyzed and evaluated. A secrecy sum rate (SSR) maximization problem is
formulated and solved under three constraints: i) Transmit power; ii)
Successive interference cancellation; iii) Quality of Service. By considering
both passive and active eavesdroppers scenarios, two optimization schemes are
proposed to improve the overall system SSR. On the one hand, for the passive
eavesdropper scenario, an artificial noise-aided secure beamforming scheme is
proposed. Since this optimization problem is nonconvex, instead of using
traditional but highly complex, bruteforce two-dimensional search, it is
conveniently transformed into a convex one by using an epigraph reformulation.
On the other hand, for the active multi-antennas eavesdroppers' scenario, the
orthogonal-projection-based beamforming scheme is considered, and by employing
the successive convex approximation method, a suboptimal solution is proposed.
Furthermore, since for single antenna transmission the
orthogonal-projection-based scheme may not be applicable a simple power control
scheme is proposed.
