Joint beamforming and phase shift optimization for EE and SE in multi-IRS beyond 5G/6G systems

The rapid evolution of mobile communication technologies has increased the demand for high-speed and energy-efficient wireless networks. Intelligent reflecting surfaces (IRSs) have emerged as a key enabler for beyond-5G (B5G) and 6G systems by enhancing coverage, spectral efficiency (SE), and energy efficiency (EE). However, the joint optimization of beamforming and IRS phase shifts under realistic channel conditions remains challenging, particularly in dynamic multi-user environments. This paper investigates multi-IRS-assisted downlink multiple-input single-output (MISO) systems with 1, 2, 4, and 6 IRS blocks, each with 40 reflecting elements. An alternating optimization (AO) algorithm is proposed to jointly optimize base station beamforming and IRS phase shifts under transmit power constraints, considering path loss, Rayleigh fading, and user mobility. Simulation results show that multi-IRS deployments significantly outperform single-IRS systems. SE increases from 11 bps/Hz (single-IRS) to 22, 45, and 66 bps/Hz for 2-, 4-, and 6-IRS configurations at short distances, and remains higher at d = 800 m with 3, 5, 10, and 16 bps/Hz, respectively. In terms of EE, the 6-IRS system achieves up to 5.1 bits/Joule compared to 0.9 bits/Joule for single-IRS, with non-IRS systems performing lower. Overall, multi-IRS-assisted systems with optimized beamforming provide a scalable and energy-efficient solution for future 6G networks, offering substantial improvements in both SE and EE over conventional architectures.