Performance Analysis of HARQ-Enabled IRS-NOMA Downlink Systems

In this article, we explore the application of hybrid automatic repeat request (HARQ) within the intelligent reflection surface-assisted nonorthogonal multiple access (IRS-NOMA) system. We investigate the closed-form expressions for the outage probability of multiple users in the HARQ-assisted IRS-NOMA system, considering scenarios with perfect successive interference cancellation (pSIC) and imperfect successive interference cancellation (ipSIC), respectively. A definite integral approximation method for multidimensional functions based on Gauss-Chebyshev quadrature (GCQ) is proposed to conduct the performance analysis of the proposed HARQ-assisted IRS-NOMA system. Based on the asymptotic outage probability, the diversity order of multiple users for HARQ-assisted IRS-NOMA is obtained. Under the analytical results, the diversity order of the mth $(m\gt 1)$ user for the HARQ-assisted IRS-NOMA with ipSIC is zero, and that of the mth user for the HARQ-assisted IRS-NOMA with pSIC is in connection with the number of reflecting elements and the number of transmission rounds. The simulation results are presented to substantiate the accuracy of the analytical results. The results demonstrate that: 1) the HARQ-assisted IRS-NOMA systems can achieve a significant gain compared with the IRS-NOMA systems; 2) the HARQ-assisted IRS-NOMA outperforms the HARQ-assisted IRS-orthogonal multiple access in terms of outage probability and diversity order; and 3) the HARQ with incremental redundancy (HARQ-IR)-assisted IRS-NOMA system has a better performance than the HARQ with chase combining (HARQ-CC)-assisted IRS-NOMA system.

I. Introduction

With the explosive growth of wireless data traffic globally, the performance requirements of beyond fifth-generation and sixth-generation (6G) wireless systems have become more demanding. These requirements include ultrahigh data rates, extremely high reliability, and high spectrum utilization [2], [3]. Intelligent reflection surface (IRS) is a promising technology for 6G wireless communications, as it is an effective hardware technology that improves spectrum efficiency while consuming less power [4], [5]. The IRS planar array comprises numerous passive reflective elements that adjusts the phase shifts of electromagnetic waves through a programmable controller. This technology can increase the received signal energy and expand the coverage region. With the help of IRS, an additional reflected link is provided to transmit signals to the user from a different angle when the link between the base station (BS) and the user is blocked. The benefits of IRS led to research in different application scenarios, such as the physical layer secrecy [6] and indoor scenarios [7], and the simultaneous wireless information and power transfer (SWIPT) [8].