Power Allocation Optimization for Secure OFDM-NOMA Downlink Systems

Non-orthogonal multiple access (NOMA) is considered as an important enabling technique for the sixth generation wireless network. In this paper, we study a secure orthogonal frequency division multiplexing (OFDM)-NOMA downlink system, which combines OFDM modulation and NOMA technique. In the system, the central user (CU) is an entrusted user and the cell-edge user (EU) is a potential eavesdropper. Due to the use of OFDM modulation, the CU may have weak subcarriers and the EU may have strong subcarriers. Our objective is to maximize both the achievable secrecy rate of CU and achievable rate of EU through power allocation optimization. For the non-convex optimization problem, we propose a sequential parametric convex approximation (SPCA)-based algorithm, a Lagrangian dual transformation (LDT)-based algorithm, and a prime decomposition-based algorithm. Simulation results demonstrate that the proposed LDT-based and prime decomposition-based algorithms, while have much lower computation complexity than the SPCA-based algorithm, achieve the same performance as the latter. Furthermore, it is found that our proposed OFDM-NOMA system outperforms the conventional OFDM-orthogonal multiple access system.