Low-Complexity CRB-Based Beamforming Design for Downlink Monostatic ISAC Systems

This paper addresses joint transmit beamforming for a monostatic integrated sensing and communication base station with separate transmit and receive arrays. The goal is to minimize the Cramér-Rao bound for target direction estimation while satisfying per-user downlink signal-to-interference-noise ratio (SINR) requirements and individual per-target radar SINR floors. We propose a Split-Adam algorithm with an augmented Lagrangian that decouples sensing and communication beam-formers, enforces downlink SINR constraints via closed-form iterative projection, and relies on analytic Wirtinger gradients of the Cramér-Rao bound that account for clutter coupling. Monte Carlo results confirm consistent gains over power-split bench-marks across a range of transmit powers and communication requirements.