Constant Modulus Constrained Codebook Synthesis for Self-Interference Suppression in Integrated Sensing and Communication at mmWave

Self-interference (SI) is one of the most critical issues for an integrated sensing and communication (ISAC) device. This paper presents an efficient codebook synthesis method which suppresses the SI by using the analog phase shifters in a millimeter wave (mmWave) ISAC device. First, we formulate the problem as a signal-to-interference-and-noise ratio (SINR) maximization problem with constant modulus (CM) constraints. Then we propose a two-step method wherein the solution is initialized by a minimum variance distortionless response (MVDR) beamformer with imposed CM constraint, and then refined by a local search procedure using constant modulus gradient descent (CM-GD). Simulations have been conducted for realistic TX/RX antenna array placements for a cellular user equipment (UE). The results show that the proposed method achieves a much better performance than a sub-optimal MVDR beamformer which has imposed CM constraint. Meanwhile, the proposed method can almost approximate the performance of the optimal MVDR beamformer which does not have the CM constraint.