On the Effect of Phase Error on Physical Layer Security of RIS-Aided NOMA Network

Abstract

This paper presents a study on the effect of phase error on the secrecy performance of a reconfigurable intelligent surfaces (RIS)-aided non-orthogonal multiple access (NOMA) network. The network under consideration consists of a base-station (BS) that broadcast the information and a pair of NOMA nodes where one is at the cell-centre and the other at the cell-edge. There exists a passive eavesdropper with an intention to tap the information broadcast by the BS. Cell-centre user is served directly by the BS whereas, the cell-edge node is served through the RIS. All channels of different links in the network are modelled using independent but non-identically distributed Rayleigh/Nakagami-m fading random variables. The phase error introduced by the RIS due to non-ideal performance is modelled using von Mises distribution. Statistics for the composite channels are shown to follow Nakagami-m distribution whose parameters depend on the phase error concentration parameter. Closed form expression of the secrecy outage probability (SOP) is derived for all the users. To gain further insights, expression for the asymptotic SOP is also derived along with the secrecy diversity order. With the help of numerical results, the effect of phase error parameter on the secrecy performance is presented. The analytical results show that the secrecy diversity order of cell-centre user is independent of the number of RIS elements whereas for the far user it is proportional to the number of RIS elements. The effect of the power allocation factor on the secrecy performance is also analysed.