In this paper, an analytical framework for secure simultaneous transmitting
and reflecting reconfigurable intelligent surface (STAR-RIS) assisted
non-orthogonal multiple access (NOMA) transmissions in large-scale networks is
proposed, where users and eavesdroppers are randomly distributed. Both the
time-switching protocol (TS) and energy splitting (ES) protocol are considered
for the STAR-RIS. To characterize system performance, the channel statistics
are first provided, and the Gamma approximation is adopted for general cascaded
κ-μ fading. Afterward, the closed-form expressions for both the
secrecy outage probability and secrecy ergodic rate are derived. To obtain
further insights, the asymptotic performance for the secrecy diversity order
and the secrecy slope are deduced. The theoretical results show that 1) the
secrecy diversity orders of the strong user and the weak user depend on the
path loss exponent and the distribution of the received signal-to-noise ratio,
respectively; 2) the secrecy slope of the ES protocol achieves the value of
one, higher than the slope of the TS protocol which is the mode operation
parameter of TS. The numerical results demonstrate that: 1) there is an optimal
STAR-RIS mode operation parameter to maximize the system performance; 2) the
STAR-RIS-NOMA significantly outperforms the STAR-RIS-orthogonal multiple
access.Comment: 30 pages, 7 figure