Near-Field Spot Beam Focusing for Multiuser Communication Using Holographic Antennas: Theory

This paper develops the theoretical foundations for near-field spot beam-focusing (SBF) in multi-user communication systems using holographic antennas. While holographic apertures have been extensively studied for far-field wave control, rigorous modeling of their near-field focusing behavior has remained limited. In this work, we derive a closed-form analytical formulation for the focal gap, providing a mathematically tractable framework for understanding focal placement in the radiative near-field region. The analysis establishes the physical principles that govern near-field holographic beam formation and offers insight into the fundamental constraints associated with surface-wave–based holographic architectures. The theoretical developments in this paper lay the groundwork for practical nearfield SBF implementation. Detailed characterization of beam-focusing properties—such as depth of focus, angular beamwidth, and multiuser interference behavior—along with design examples and experimental demonstrations are presented in the companion paper. © 1990-2011 IEEE.