Outage Performance of HARQ-CC-Assisted IRS-NOMA Systems

This paper focuses on the development of intelligent reflection surface (IRS) assisted non-orthogonal multiple access (NOMA), where the hybrid automatic repeat request with chase combining (HARQ-CC) is exploited to improve the system's reliability. We first investigate the outage behavior of HARQ-CC-assisted IRS-NOMA, where the exact and asymptotic closed-from expressions of the outage probability of a pair of users are derived. Based on the asymptotic outage probability, the diversity order of a pair of users for HARQ-CC-assisted IRS-NOMA is obtained, and it is in connection with the number of reflection elements and the number of transmission rounds. The numerical results are shown to substantiate the accuracy of the derived analytical results and demonstrate that: 1) The HARQ-CC-assisted IRS-NOMA systems can achieve a significant performance gain compared with the IRS-NOMA systems; 2) The HARQ-CC-assisted IRS-NOMA system outperforms the HARQ-CC-assisted IRS-OMA system in terms of outage probability and diversity order.

I. Introduction

As an effective hardware technology to improve spectrum efficiency with lower power consumption, intelligent reflection surface (IRS) has recently been regarded as a prospective technology for 6G wireless communications [1], [2]. The IRS planar array consists of a large number of passive reflective elements, with the capability of adjusting phase shifts of electromagnetic waves through a programmable controller. It can increase the received signal energy and expand the coverage region. As one of the promising multiple access candidates, non-orthogonal multiple access (NOMA) allows multiple users to share the same resource block and can improve spectral efficiency and link density. The co-existence of IRS and NOMA offers great potential to achieve higher spectral efficiencies [3], [4]. In [5], the authors jointly optimized the beamforming vectors and phase shift matrix to reduce the transmitting power of IRS-NOMA systems. To further enhance the outage performance for IRS-NOMA networks, the impact of coherent phase shift and the random phase shift was analyzed in [6]. Combined with the 1-bit coding scheme, the performance of the IRS-assisted NOMA network was investigated with perfect successive interference cancellation (pSIC) and imperfect successive interference cancellation (ipSIC) [7].