Optimization for Maximizing Sum Secrecy Rate in SWIPT-Enabled NOMA Systems

In this paper, we study secrecy simultaneous wireless information and power transfer (SWIPT) in downlink non-orthogonal multiple access (NOMA) systems comprising a base station (BS), multiple information receivers (IRs), and multiple energy receivers (ERs) that have potential to wiretap the IRs. The goal of this paper is to maximize the sum secrecy rate (SSR) of the system subject to the individual IR's minimum data rate requirement and the individual ER's minimum harvested energy requirement. The corresponding problem involves resource allocation via max-min function, which is non-convex and difficult to solve directly. In order to tackle this, we first transform the original non-convex problem to a sequence of convex sub-problems which can be solved simultaneously. Hence, a closed-form solution of the optimal power allocation policy is derived based on the Karush-Kuhn-Tucker conditions. Numerical results validate the theoretical findings and demonstrate that the significant performance gain over the orthogonal multiple access scheme in terms of SSR can be achieved by the proposed algorithm in a SWIPT-enabled NOMA system.