  A hardware model and a signal model are proposed for dual-sided
simultaneously transmitting and reflecting reconfigurable intelligent surfaces
(STAR-RISs), where the signal simultaneously incident on both sides of the
surface. Based on the proposed hardware model, signal models for dual-sided
STAR-RISs are developed. For elements with scalar surface impedance, it is
proved that their transmission and reflection coefficients on both sides are
identical. Based on the obtained symmetrical dual-sided STAR model, a
STAR-RIS-aided two-user uplink communication system is investigated for both
non-orthogonal multiple access (NOMA) and orthogonal multiple access (OMA)
schemes. Analytical results for the outage probabilities for users are derived
in the high transmit signal-to-noise ratio (SNR) regime. Numerical results
demonstrate the performance gain of NOMA over OMA and reveal that the outage
probability error floor can be lowered by adjusting the ratio between the
amplitudes of transmission and reflection signals.
