This paper investigates the physical layer security of non-orthogonalmultiple access (NOMA) in large-scale networks with invoking stochasticgeometry. Both single-antenna and multiple-antenna aided transmission scenariosare considered, where the base station (BS) communicates with randomlydistributed NOMA users. In the single-antenna scenario, we adopt a protectedzone around the BS to establish an eavesdropper-exclusion area with the aid ofcareful channel-ordering of the NOMA users. In the multiple-antenna scenario,artificial noise is generated at the BS for further improving the security of abeamforming-aided system. In order to characterize the secrecy performance, wederive new exact expressions of the security outage probability for bothsingle-antenna and multiple-antenna aided scenarios. To obtain furtherinsights, 1) for the single antenna scenario, we perform secrecy diversityorder analysis of the selected user pair. The analytical results deriveddemonstrate that the secrecy diversity order is determined by the specific userhaving the worse channel condition among the selected user pair; and 2) for themultiple-antenna scenario, we derive the asymptotic secrecy outage probability,when the number of transmit antennas tends to infinity. Monte Carlo simulationsare provided for verifying the analytical results derived and to show that:i)~The security performance of the NOMA networks can be improved by invokingthe protected zone and by generating artificial noise at the BS; and ii)~Theasymptotic secrecy outage probability is close to the exact secrecy outageprobability.
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