Robust Beamfocusing for FDA-Aided Near-Field Covert Communications with Uncertain Location

In this paper, we study the robust beamfocusing design in a frequency diverse array (FDA)-aided near-field communication system with uncertain user location. In order to maximize the covert rate, we formulated a non-convex problem by jointly optimizing the transmit beamfocusing vector and FDA-specific frequency increments. For the subproblem of transmit beamfocusing, by leveraging the method of convex hull, we manage to transform the original semi-infinite programming problem into a semi-definite one which is efficiently solved by the semi-definite relaxation method. Furthermore, with the aim of improving the covert rate by decorrelating the communications and detection channels, we resort to a multi-variable non-convex optimization framework, named block successive upper-bound minimization, to find the efficient frequency increments. Numerical results show that, compared to the phased-array counterpart, the proposed FDA-based beamfocusing scheme generates a more focused beampattern, which boosts the covert rate significantly in the FDA-aided near-field communication system.