  This paper investigates the performance of vehicleto-vehicle (V2V)
communications assisted by a reconfigurable intelligent surface (RIS) and a
simultaneous transmitting and reflecting intelligent omni-surface (STAR-IOS)
under nonorthogonal multiple access (NOMA) and orthogonal multiple access (OMA)
schemes. In particular, we consider that the RIS is close to the transmitter
vehicle while the STAR-IOS is near the receiver vehicles. In addition, we
assume that the STAR-IOS exploits the energy-splitting (ES) protocol for
communication and the fading channels between the RIS and STAR-IOS follow
composite Fisher-Snedecor F distribution. Under such assumptions, we first use
the central limit theorem (CLT) to derive the PDF and the CDF of equivalent
channels at receiver vehicles, and then, we derive the closed-form expression
of outage probability (OP) under NOMA/OMA scenarios. Additionally, by
exploiting Jensen's inequality, we propose an upper bound of the ergodic
capacity (EC), and then, we derive an analytical expression of the energy
efficiency (EE) for both NOMA and OMA cases. Further, our analytical results,
which are double-checked with the Monte-Carlo simulation, reveal that applying
RIS/STAR-RIS in V2V communications can significantly improve the performance of
intelligent transportation systems (ITS). Besides, the results indicate that
considering the NOMA scheme provides better performance in terms of the OP, EC,
and EE as compared with the OMA case for the considered V2V communication.
