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Nonorthogonal multiple access (NOMA) and reconfigurable intelligent surface (RIS) methods are explored to enhance spectral and energy efficiency of a wireless network. In this article, we have proposed a downlink RIS-assisted index modulation NOMA for next-generation wireless systems, and it is referred to as R-INOMA. In the proposed R-INOMA system, a base station transmits information through RIS to three users, the first user is assigned in the spatial domain and the remaining two users are multiplexed using power domain NOMA. We provide an analytical analysis of bit error probability (BEP) for the proposed R-INOMA system. Impact of number of surfaces, transmit antennas, modulation order, diversity order, and imperfect channel estimation is analyzed for the R-INOMA. Simulation results show that the proposed R-INOMA achieved better BEP and sum-rate over existing NOMA techniques. In the R-INOMA, doubling of the reflecting surfaces provides a 4 dB gain for a fixed error probability, and the sum-rate improves as the number of transmitting antennas and reflecting surfaces increases. INTRODUCTIONReconfigurable intelligent surface (RIS) has emerged as a promising research topic for sixth-generation (6G) wireless communications. [1][2][3][4] RIS changes the behavior of the wireless propagation environment and helps to obtain high-quality signals at the receiver. An RIS is fabricated by meta-surfaces and it consists of a planar structure to have the ability to reflect with low-cost passive elements. 2,5,6 RIS smartly tunes the phase of the incident signals according to the channel environment, and the reflected signals can be constructively added to enhance signal-to-noise (SNR) toward receivers. 1,7 RIS creates a virtual line-of-sight (LOS) propagation path between transmitter and receivers. Therefore, RIS improves the network coverage, spectral, and energy efficiency. [8][9][10] On the other hand, index modulation (IM) and nonorthogonal multiple access (NOMA) are a promising technology for fifth-generation (5G) and beyond wireless networks to enhance spectrum efficiency and support massive connectivity. 11-16 IM-based spatial modulation (SM) transmits information using antenna indices along with modulation symbols of M-ary phase shift keying or M-ary quadrature amplitude modulation (QAM), therefore it increases the data transmission rate. Moreover, SM transmits information in a single activated antenna in multiple-inputs and multiple-output (MIMO) system by using a single radio frequency (RF) chain. 17,18 Further, a combination of IM and NOMA (IM-NOMA) can serve simultaneously more users at the same time/frequency without the effect of inter-user interference (IUI) [19][20][21] and it consists of superposition coding (SC) at the base station (BS) and successive interference cancelation (SIC) at the receivers. 22

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