Physical-Layer Security in Mixed Near- and Far-Field Communication Systems

Extremely large-scale arrays (XL-arrays) have emerged as a promising technology to improve the spectrum efficiency and spatial resolution of future wireless systems. Different from existing works that mostly considered physical layer security (PLS) in either the far-field or near-field, we consider in this paper a new and challenging scenario, where legitimate users (Bobs) are located in the far-field while eavesdroppers (Eves) are located in the near-field for intercepting confidential information at short distance, referred to as the mixed near- and far-field PLS. Specifically, we formulate an optimization problem to maximize the sum-secrecy-rate of all Bobs by designing their power allocation. To shed useful insights, we first consider a one-Bob-one-Eve system and characterize the insecure-transmission region of the Bob. Interestingly, we show that the insecuretransmission region is significantly expanded as compared to that in conventional far-field PLS systems, due to the energy-spread effect in the mixed-field scenario. Then, we propose an efficient algorithm to obtain a high-quality solution to the non-convex optimization problem. Finally, numerical results demonstrate that our proposed algorithm achieves a higher sum-secrecyrate than the benchmark scheme where the power allocation is designed based on the (simplified) far-field channel model.