Near-Field Beam Focusing for Integrated Sensing and Communication Systems

This paper investigates near-field integrated sensing and communication systems, where both communication users and radar targets are located within the near-field region of a base station equipped with an extremely large-scale antenna array. The near-field operation introduces new challenges and opportunities for joint radar sensing and communication tasks, especially when the traditional assumption of far-field propagation no longer holds. In this work, we focus on the design of near-field beam-focusing, a technique that allows for more focused and efficient energy distribution in specific regions. Specifically, our goal is to maximize the minimum beam pattern gain for radar sensing, ensuring that the sensing capabilities meet the required performance while simultaneously satisfying the communication requirements of the users. The originally formulated problem is non-convex due to the complex coupling between the radar and communication tasks. Nevertheless, we solve the problem globally optimally by leveraging semi-definite relaxation. Subsequently, we propose a low-complexity suboptimal solution that reduces the computational burden. Numerical results demonstrate the potential of near-field beam-focusing, showing that it can successfully detect multiple radar targets located at identical angular directions.