Extremely Large Aperture Array (ELAA) Communications: Foundations, Research Advances and Challenges

Extremely large aperture array (ELAA) represents a paradigm shift in wireless antenna systems, poised to redefine the capabilities of future networks, particularly in the era of the sixth generation (6G) networks. The integration of multiple-input multiple-output (MIMO) technology with an unprecedented scale of antenna arrays enables transformative potential for enhancing spectral efficiency, increasing coverage, and enabling new applications. This paper provides a comprehensive overview of ELAA, delving into fundamental concepts, state-of-the-art technologies, and practical applications. We commence by presenting the fundamentals of ELAA technology, focusing on the various architectural categories and two fundamental properties of communication in the near-field region of ELAA, namely spherical wave propagation and channel spatial non-stationarity. We then illustrate the phenomenon of finite beam depth in the near field before presenting general distance boundaries based on various criteria and conducting a comprehensive performance analysis of this technology. Subsequently, in light of the distinctive electromagnetic characteristics of ELAA, we will examine the practical challenges that have emerged, including channel estimation, beamforming design, and the practical hardware issues that have arisen. Subsequently, we examine the diverse applications of ELAA across various domains, emphasizing its transformative potential in fields such as physical layer security, communication and sensing, and wireless power and information transfer. Finally, the paper concludes by outlining several promising avenues for future research and exploration within the realm of ELAA. These avenues are identified as areas ripe for investigation and innovation.