Transmit Beamforming for ISAC Based on Space–Time Coding in MU-MIMO Scenario

Integrated sensing and communication (ISAC) system utilizes multi-antenna arrays to transmit integrated signals, enabling multi-target sensing and simultaneous communication with multiple communication users (CUs). This requires the transmit signal to have sufficient degrees of freedom (DoF) for sensing and a high communication rate for effective communication. However, the DoF of the transmit signal is limited by the quantity of uncorrelated communication symbol streams (CSSs) in the ISAC system. To address this limitation, we employ space-time coding (STC) to extend CSSs, which not only provides additional DoF for sensing but also offers diversity gain for communication. Moreover, we optimize the transmit beampattern while ensuring that the signal-to-interference-plus-noise ratio (SINR) for each CU remains above a specified threshold. Consequently, we formulate an optimization problem with rank constraints and derive a suboptimal solution. The simulation results demonstrate that the proposed method achieves a better beampattern and a higher communication rate compared to previous work.