Secure transmission OFDMA VLC in presence of external jammer: Perfect and imperfect CSI

Abstract

Secure transmission in visible light communication (VLC) is very important. Hence, in this paper, we investigate physical layer security (PLS) in downlink of orthogonal frequency-division multiple access (OFDMA), in the presence of multiple eavesdroppers (Eve), multi users, and an external jammer. It is necessary to mention that in the future networks the users need to service continually. Hence, the Time Division Multiple Access (TDMA) method is not compatible for this application, to tackle this issue, we use OFDMA technique. Moreover, due to its orthogonal subcarriers, OFDMA minimizes inter-user interference. This leads to a lower bit error rate compared to non-orthogonal multiple access in most scenarios. Specially, this metric is very vital in the medical application, hence, we investigate OFDMA in this work. Our intention is to improve sum secrecy rate (SSR); hence, we propose an optimization problem to assign the subcarrier and power to users, subject to power limitation and OFDMA requirements. Moreover, since Eve is passive, the Channel State Information (CSI) of Eves may be not available, hence, we investigate two scenarios, perfect and imperfect CSI. Sine the proposed optimization problem is non-convex and complicated, we utilize the well-known iterative algorithm called the Alternative Search Method (ASM). This algorithm breaks down the optimization problem into two subproblems: power allocation and subcarrier allocation. Since the power allocation optimization problem is non-convex, we employ difference of two concave functions (DC) approximation and epigraph method, to approximate it to convex one. Since subcarrier allocation optimization problem consists of binary variables, we employ the Mesh Adaptive Direct Search algorithm (MADS) to solve it. The simulation results show that in the proposed system model, it is not necessary to employ a photodiode with high physical area the reason is that secrecy rate is saturated in high value of physical area photodiode.