On Beamforming for Transmitter Location Privacy in MIMO Systems

In this paper, we introduce a beamforming framework to ensure transmitter location privacy against sensing-capable MIMO receivers. We propose a novel privacy metric called the direction-of-arrival obfuscation power ratio (DoA-OPR) to design the transmit beamformer. The design approach reshapes the angular power distribution observed at the receiver so that a false direction appears dominant without nulling the line-of-sight (LoS) component. We derive closed-form bounds on the feasible range of DoA-OPR via generalized eigenvalue analysis and formulate an achievable rate-maximization problem under the DoA-OPR constraint. The resulting problem is non-convex but can be efficiently solved using semidefinite relaxation, eigenmode selection, and optimal power allocation. Numerical results demonstrate that the proposed DoA-OPR-based beamformer achieves a trade-off between location privacy and communication rate. The proposed design attains higher achievable rates than existing LoS-nulling approaches while maintaining comparable location privacy. A suboptimal design strategy is also proposed with reduced complexity. It achieves a near-optimal communication rate with a reduction of nearly 85%.