Beamforming via Null Steering for ISAC Systems

Integrated sensing and communication (ISAC) is gaining attention as a core technology in the future sixthgeneration (6G) networks. As the same signal is used for both communication data transmission and sensing targets, the ISAC systems illuminate sensing targets with information-bearing signals, exposing the information to eavesdroppers. This necessitates joint waveforms and beamforming designs that support sensing accuracy while ensuring secure communication. This paper presents a closed-form null steering beamforming method that jointly supports communication, sensing and secrecy without iterative optimisation. The method steers energy towards both the sensing target and the communication user while placing a deep spatial null at the direction of an unintended receiver. To evaluate its performance, the method is compared with the zero-forcing (ZF) and genetic algorithm (GA) beamforming methods. Simulation results, including the resulting ISAC beam patterns, secrecy capacity, bit-error rate (BER), and probability of detection, are presented. The simulation results demonstrate that the proposed method achieves near-ZF performance and can achieve a trade-off between communication secrecy and sensing performance through a tunable trade-off parameter. It also achieves near-ideal null depths (> 70 dB), higher secrecy capacity, lower BER and improved target detection, compared to the GA benchmark. The proposed design provides a practical and low-complexity solution for real-time ISAC systems.