On Secrecy Performance of RIS-Assisted MISO Systems Over Rician Channels With Spatially Random Eavesdroppers

Reconfigurable intelligent surface (RIS) technology is emerging as a promising technique for performance enhancement for next-generation wireless networks. This paper investigates the physical layer security of an RIS-assistedmultiple-antenna communication system in the presence ofrandom spatially distributed eavesdroppers. The RIS-to-ground channels are assumed to experience Rician fading. Using stochas-tic geometry, exact distributions of the received signal-to-noise-ratios (SNRs) at the legitimate user and the eavesdropperslocated according to a Poisson point process (PPP) are derived,and closed-form expressions for the secrecy outage probability(SOP) and the ergodic secrecy capacity (ESC) are obtained to provide insightful guidelines for system design. First, the secrecy diversity order is obtained as 2/alpha 2, where alpha 2d enotes the pathloss exponent of the RIS-to-ground links. Then, it is revealedthat the secrecy performance is mainly affected by the numberof RIS reflecting elements,N, and the impact of the numberof transmit antennas and transmit power at the base stationis marginal. In addition, when the locations of the randomlylocated eavesdroppers are unknown, deploying the RIS closer to the legitimate user rather than to the base station is shown to be more efficient. Moreover, it is also found that the density of randomly located eavesdroppers,lambda e, has an additive effect on the asymptotic ESC performance given by log(2)(1/lambda e). Finally,numerical simulations are conducted to verify the accuracy of these theoretical observations