Outage Probability and Secrecy Capacity of a Non-orthogonal Multiple Access System

In this paper, we analyze the outage probability and secrecy capacity of a non-orthogonal multiple access (NOMA) system in the presence of an eavesdropper. In order to enhance spectral efficiency, a base station communicates with two users simultaneously in the same frequency band by superpimposing the transmit signals to the users in the power domain. Specifically, the user with the worse channel conditions is allocated higher power such that it is able to directly decode its signal from the received superimposed signal. At the user with the better channel conditions, the interference due to NOMA is processed by successive interference cancelation. Given these system settings and accounting for decoding thresholds, we analyze the outage probability of the NOMA system over Rayleigh fading channels. Furthermore, based on the locations of the users and eavesdropper, the secrecy capacity is analyzed to assess the level of security provided to the legitimate users in the presence of an eavesdropper. Here, the decoding thresholds of legitimate users and eavesdropper are also included in the analysis of the secrecy capacity. Through numerical results, the effects of network parameters on system performance are assessed as well as the the superiority of NOMA in terms of secrecy capacity over traditional orthogonal multiple access.