Toward Near-Field 3D Spot Beam Focusing: Possibilities, Challenges, and Use Cases

Spot beam focusing (SBF) is the process of focusing the signal power in a small spot-like region in the 3D space, which can be either hard-tuned (HT) using traditional tools like lenses and mirrors or electronically reconfigured (ER) using modern large-scale intelligent surface phased arrays. ER-SBF (simply called SBF) can be a key enabling technology for next-generation 6G wireless networks, offering benefits to many future wireless application areas, such as wireless communication and security, midrange high-power and safe wireless battery-charging systems, medical and health, physics, etc. Although near-field HT-SBF and ER-beam focusing have been studied in the literature and applied in the industry, there is no comprehensive study of different aspects of SBF and its future applications, especially for nonoptical [millimeter-wave (mmWave), sub-terahertz (sub-THz), and THz] electromagnetic (EM) waves in next-generation wireless technology, which is the aim of this article. The theoretical concepts behind SBF, different antenna technologies for implementing SBF, employing machine learning (ML)-based schemes as enablers of beam training for SBF, and different practical application areas that can benefit from SBF will be explored.