Rotatable Antennas for Near-Field Integrated Sensing and Communication

In this paper, we propose leveraging rotatable antennas (RAs) to enhance near-field communication and sensing performance by exploiting a new spatial degree-of-freedom (DoF) offered by array rotation. Specifically, we investigate an RA-aided near-field integrated sensing and communication (ISAC) system, where the transmit beamformers and the array rotation angle at the base station (BS) are jointly optimized to minimize the Cramér-Rao bounds (CRBs) for angle and range estimation, while ensuring a minimum signal-to-interference-plus-noise ratio (SINR) for communication users. To gain important insights into the impact of RAs on near-field ISAC, we analyze two special cases:communication-onlyandsensing-onlytransmission. For the communication-only case, we derive therotation-awarechannel path correlation using the Fresnel integrals and analytically demonstrate that RAs provide an additional rotation gain, thereby improving communication performance. For the sensing-only case, we derive closed-formrotation-awareCRBs for near-field angle and range estimation under bothisotropicanddirectionalbeamformers. It is theoretically unveiled that array rotation improves sensing performance by concurrently reducing both CRBs. Interestingly, the optimal rotation angles that minimize these CRBs are identical. Subsequently, to address the resultant non-convex optimization problem, we propose adouble-layeralgorithm to obtain a high-quality solution, where the inner layer optimizes the transmit beamformers using semidefinite relaxation (SDR), while the outer layer determines the array rotation through a one-dimensional exhaustive search. Finally, numerical results highlight the significant performance gains of the developed RA-aided near-field ISAC system over conventional fixed-antenna ISAC systems.