Minimizing Vulnerable Region for Near-Field Covert Communication

The development of the extremely large-scale antenna array (ELAA) for the upcoming 6G technology indicates the significance of near-field communication. This work performs a near-field analysis to improve covertness when maximum ratio transmission (MRT) is employed with ELAA to send messages to the legitimate user. Specifically, we first derive the covertness constraint of the system by analyzing the beampattern. Based on this constraint, we introduce the concept of the vulnerable region, which is the region where covert communication is not achievable if a potential warden resides there. Furthermore, determining the vulnerable region involves deriving the range of distances by initially fixing the angle dimension, and then utilizing the covertness and the minimum effective throughput constraints to obtain the range of angle. The simulation results illustrate the efficacy of the determined vulnerable region in both distance and angle dimensions. As the azimuth angle or the distance between the legitimate user and the transmitter decreases, the area of the vulnerable region decreases. Additionally, increasing the number of warden's antennas or requiring a higher signal-to-noise ratio for legitimate user will expand the vulnerable region.