This paper proposes an intelligent reflective surface (IRS) design scheme to improve the spectral efficiency (SE) for downlink multi-user (MU) multiple-input-single-output (MISO) system. IRS composed of low-cost reflecting elements is expected to be used in various scenarios in future wireless communication systems. It can be used even when the direct path is blocked by obstacles. Since the reflecting elements can only adjust the phase, the design of IRS matrix causes a non-convex problem. In this paper, the proposed schemes perform optimization by transforming a non-convex problem into a solvable convex function. Typically, base station-IRS (BS-IRS) channel can be assumed as the line-of-sight (LOS) channel environment. In this case, multi-user system suffers from performance degradation due to the LOS channel rank problem. To alleviate this problem, this paper considers deterministic scattering and sufficient spacing between the reflecting elements of the IRS. The simulation results show that the proposed schemes achieve better SE performance than the randomly generated IRS scheme. In addition, proposed minimum mean square error (MMSE)-based scheme can achieve high performance compared to other schemes even in a low-rank channel environment.INDEX TERMS Intelligent reflecting surface, multi-user MISO, phase shift, spectral efficiency.

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Hybrid beamforming (HB) is a solution for reducing radio frequency (RF) chains in millimeter-wave (mmWave) massive MIMO systems. By reducing RF chains, HB can achieve energy efficient transmission compared to fully digital beamforming of massive MIMO system. However, if the resolution of RF precoder is not quantized, the energy consumption of HB may become similar to fully digital beamforming. And, if the resolution of RF precoder is heavily quantized, the spectral efficiency of HB is decreased significantly. Non-orthogonal multiple access (NOMA) is one of the emerging technologies for the fifth generation (5G) or beyond fifth-generation (B5G) wireless communication to support high spectral efficiency. Especially, power domain NOMA (PD-NOMA) can share same resources to transmit signals and divide the signals in power domain by using successive interference cancellation (SIC). In this paper, the adoption of NOMA shows increase of spectral efficiency in a low-resolution RF precoder. To apply NOMA, two users with high channel correlation are selected according to the proposed criterion. Then, the RF precoder and digital precoder that are applied to both users at the same time are determined. Finally, intra-pair user fairness is achieved by power allocation. The ratio of channel power is compared for each user in each pair. Then, users with high channel power are assigned with low power. It is shown that the spectral efficiency of the proposed scheme is higher than the conventional scheme in heavily quantized RF precoder.INDEX TERMS Millimeter-wave, hybrid beamforming, NOMA, spectral efficiency, precoding.

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