Near-Field Integrated Sensing and Communication Beamforming Considering Complexity

Integrated Sensing and Communication (ISAC) is a key technology for 6G and vehicle networking. With the continuous improvement of communication and sensing performance requirements, high frequency scenarios such as ultra large scale Multiple-Input Multiple-Output (MIMO), Extremely Large-scale Antenna Array (ELAA), and millimeter waves (mmWave) have become research hotspots in ISAC. In scenarios such as mmWave and terahertz (THz), with the increase of frequency and the application of ELAA, the electromagnetic environment shifts from far-field to near-field. Unlike the far-field, the near-field electromagnetic wavefront changes from a plane to a sphere, and the dimension of the antenna steering vector function changes from a single angle dimension to two dimensions: distance and angle. Therefore, suitable beamforming schemes need to be designed for near-field ISAC scenarios. Firstly, this paper models the near-field ISAC system. Then three ISAC beamforming schemes are proposed for near-field scenario: Conjugate Sensing Beamforming (CSBF), Null-space Sensing Beamforming (NSBF), and Joint Communication and Sensing Beamforming (JCSBF). The maximal SINR of NSBF is used as the SINR threshold for JCSBF. The complexities and the suitable application scenarios for the three proposed near-field ISAC beamforming schemes are analyzed also. The closed form solutions for two performance indicators which are communication rate (spectrum efficiency) and sensing rate are derived also. Finally, the performances of the three proposed beamforming schemes and the state-of-the-art near-field ISAC beamforming scheme are compared through simulation. The simulation results show that compared with the state-of-the-art scheme, the proposed three beamforming schemes all have lower complexity, and JCSBF also brings certain performance improvement with good robustness and single/multi-target detection ability.