  By activating blocked users and altering successive interference cancellation
(SIC) sequences, reconfigurable intelligent surfaces (RISs) become promising
for enhancing non-orthogonal multiple access (NOMA) systems. The downlink
RIS-aided NOMA networks are investigated via stochastic geometry. We first
introduce the unique path loss model for RIS reflecting channels. Then, we
evaluate the angle distributions based on a Poisson cluster process (PCP)
model, which theoretically demonstrates that the angles of incidence and
reflection are uniformly distributed. Additionally, we derive closed-form
analytical and asymptotic expressions for coverage probabilities of the paired
NOMA users. Lastly, we derive the analytical expressions of the ergodic rate
for both of the paired NOMA users and calculate the asymptotic expressions for
the typical user. The analytical results indicate that 1) the achievable rates
reach an upper limit when the length of RIS increases; 2) exploiting RISs can
enhance the path loss intercept to improve the performance without influencing
the bandwidth. Our results show that 1) RIS-aided NOMA networks have superior
performance than the conventional NOMA networks, and 2) the SIC order in NOMA
systems can be altered since RISs are able to change the channel quality of
NOMA users.
