A Comprehensive Survey on IRS-Assisted NOMA-Based 6G Wireless Network: Design Perspectives, Challenges and Future Directions

The propagation environment was uncontrollable in first-generation to fifth-generation (5G) wireless technologies. This behavior of the wireless propagation environment is one of the prime constraints in harnessing the performance of wireless networks. This problem can be addressed in sixth-generation (6G) wireless networks by deploying intelligent reflecting surfaces (IRSs). IRS’s amplitude and phase reflecting coefficient of reflecting units (RUs) can be adjusted via a programmable controller to meet the network requirements. On the other hand, in 5G and 6G wireless communication networks, non-orthogonal multiple access (NOMA) is a robust and well-admired multiple access scheme among the other multiple access counterparts in terms of spectrum efficiency and link capacity. NOMA allows many user equipment (UE) by utilizing non-orthogonal distribution of resources. Therefore, the combination of IRS and NOMA is one of the dominant technologies for 6G wireless networks. Based upon the importance of NOMA and IRS in the initial development of 6G wireless networks, this paper presents a comprehensive survey on IRS-assisted NOMA-based networks, considering their designs and challenges. In this work, the concept and structure of IRS-assisted NOMA have been explained with an in-depth analysis of the frameworks. It also includes some challenges of IRS-assisted NOMA in wireless communication networks. Further, applications and future research directions of IRS-assisted NOMA networks are discussed.

I. Introduction

The sixth-generation (6G)1 wireless networks must offer highly effective, robust, dependable, and adaptable network designs for future wireless applications. To meet these objectives, various antennas, sophisticated encoding and decoding algorithms at the communication link’s endpoints have been introduced [1], [2], [3] and extra network frameworks, such as relays are used for reliable communication [4]. However, these solutions enhance the network’s complexity, power consumption, and deployment cost. In recent years, intelligent reflecting surface (IRS) has gained much attraction since IRS offers low complexity, intelligence, and energy-efficient wireless network [5], [6], [7], [8]. IRS establishes virtual line-of-sight (LoS) communication links between the base station (BS) and the user equipments (UEs). IRS is an energy-efficient and low-cost solution compared to conventional relaying technologies as it does not consist of radio frequency lines or high-power spending elements. IRS can be smoothly incorporated into recent networks to achieve various goals, such as high energy efficiency and low inter-user interference [9]. For long-term evolution, fifth-generation (5G) and beyond fifth-generation (B5G) systems, non-orthogonal multiple access (NOMA) is a potential candidate among multiple access (MA) techniques. NOMA obtains greater spectral and energy efficiency and assists massive connectivity than OMA, as NOMA operates on a power level, many UEs access the resources in terms of time, frequency, and coding when co-scheduled. IRS provides a controllable path among users and enhances the efficiency of NOMA frameworks. IRS constructively or destructively adds the user multipath signals to suppress inter-user interference and improve the throughput of NOMA frameworks. IRS-assisted NOMA technology can improve spectral effectiveness, UE fairness, response time, and privacy requirements [10]. The rest of the related papers on the IRS and NOMA have been elaborated in the following subsection, representing a literature review of the IRS and NOMA.¹

The abbreviations used in this paper are collected in Table I.