  Reconfigurable intelligent surfaces (RISs) become promising for enhancing
non-orthogonal multiple access (NOMA) systems, i.e., enhancing the channel
quality and altering the SIC orders. Invoked by stochastic geometry methods, we
investigate the downlink coverage performance of RIS-aided multi-cell NOMA
networks. We first derive the RIS-aided channel model, concluding the direct
and reflecting links. The analytical results demonstrate that the RIS-aided
channel model can be closely modeled as a Gamma distribution. Additionally,
interference from other cells is analyzed. Lastly, we derive closed-form
coverage probability expressions for the paired NOMA users. Numerical results
indicate that 1) although the interference from other cells is enhanced via the
RISs, the performance of the RIS-aided user still enhances since the channel
quality is strengthened more obviously; and 2) the SIC order can be altered by
employing the RISs since the RISs improve the channel quality of the aided
user.
