Physical Layer Security With Near-Field Beamforming

Extremely large antenna arrays (ELAAs) are being proposed for sixth-generation (6G) systems as an evolution of massive multiple-input, multiple-output (mMIMO). Besides having a large number of antennas, ELAAs can be physically large, which makes communication in the radiative near-field very likely. This means the plane wave approximation does not hold and channel models that accurately capture the propagation effects in the near-field are required. Recent works have highlighted the beam-focusing effect available in the near-field region, where antenna arrays can generate beams not only limited in width but also limited in depth. This work leverages a precise channel model for ELAAs intending to explore the inherent physical layer security (PLS) features in the radiative near-field. For that purpose, it is presented a study on the power ratio between a legitimate and a malicious user, considering the effects of the field region, relative distance, and beamforming direction. By taking advantage of that analysis, it is shown that both the jamming rejection and the secrecy rate can be substantially improved when communicating in the near field, demonstrating that beam-focusing constitutes an interesting PLS technique for upcoming 6G communications.