A Holistic Review of Holographic MIMO for 6G: Hardware Design and Implementation Perspectives

Holographic MIMO (HMIMO) antennas are emerging as a transformative technology for next-generation wireless networks, offering unprecedented control over EM wave propagation through continuous aperture modulation and intelligent surface integration. Based on the principles of EM holography, HMIMO systems enable dynamic beamforming, high spatial multiplexing, and real-time reconfiguration, surpassing the capabilities of conventional and massive MIMO systems. This review provides a comprehensive overview of HMIMO technologies, with an emphasis on their physical realizations, including metasurface-based designs, holographic leaky-wave antennas (HLWAs), dynamic metasurface antennas (DMAs), and liquid crystal-based reconfigurable architectures. The paper also explores advanced system-level integrations, such as hybrid beamforming, near-field (NF) propagation models, and photonic-assisted processing. By synthesizing recent advancements, we highlight the role of HMIMO in the realization of programmable, energy efficient, and scalable communication environments for 6G and beyond, including applications in smart cities, space-air-ground-sea (SAGS) networks, and secure defense systems. Finally, this review identifies critical challenges in hardware scalability, material limitations, environmental robustness, and system calibration, offering strategic insights and research directions toward the practical deployment of HMIMO-enabled intelligent wireless infrastructures.