  Non-orthogonal multiple access (NOMA) serves multiple users simultaneously
via the same resource block by exploiting superposition coding at the
transmitter and successive interference cancellation (SIC) at the receivers.
Under practical considerations, perfect SIC may not be achieved. Thus, residual
interference (RI) occurs inevitably due to imperfect SIC. In this work, we
first propose a novel model for characterizing RI to provide a more realistic
secrecy performance analysis of a downlink NOMA system under imperfect SIC at
receivers. In the presence of untrusted users, NOMA has an inherent security
flaw. Therefore, for this untrusted users' scenario, we derive new analytical
expressions of secrecy outage probability (SOP) for each user in a two-user
untrusted NOMA system by using the proposed RI model. To further shed light on
the obtained results and obtain a deeper understanding, a high signal-to-noise
ratio approximation of the SOPs is also obtained. Lastly, numerical
investigations are provided to validate the accuracy of the desired analytical
results and present valuable insights into the impact of various system
parameters on the secrecy rate performance of the secure NOMA communication
system.
