Secrecy Performance Analysis for Near-Field Communications

A closed-form expression for the secrecy capacity under near-field communications is derived and compared with its far-field counterpart. To gain further insights, the capacity scaling law is revealed by assuming an infinitely large transmit array and an infinitely high power. Both analytical and numerical results demonstrate that, different from the far-field scenario, i) nearfield communications expand the areas where secure transmission is feasible, specifically when the eavesdropper is located in the same direction as the intended receiver; ii) as the number of transmit antennas increases, the near-field secrecy capacity is capped at a finite value, adhering to the principle of energy conservation.