Bi-Static ISAC Beamforming Design in Multi-User MIMO Systems

Integrated sensing and communications (ISAC) systems unify wireless sensing and communications functionalities, enabling reduced hardware costs and efficient resource utilization. However, mono-static ISAC systems, where transmit and receive antennas are co-located, suffer from self-interference, which degrades both sensing and communications performance. To address this limitation, bi-static ISAC architectures, where the communications transmitter and sensing receiver are decoupled, offer a promising alternative. In this paper, we propose a bi-static ISAC beamforming framework for multi-user multiple-input multiple-output (MU-MIMO) networks. We formulate the beamforming design as an optimization problem, addressing both single-target and multi-target scenarios. The objective is to minimize the Cramér-Rao lower bound (CRLB) for unbiased estimation of target parameters, subject to per-user signal-to-interference-plus-noise ratio (SINR) constraints and a transmit power budget. For the multi-target case, we employ epigraph reformulation and the Schur complement to ensure convexity. Additionally, we introduce a robust beamforming design that accounts for uncertainties in the communications channels. Simulation results demonstrate that our method achieves higher main-lobe gains and improved accuracy in target parameter estimation compared to existing benchmarks, while the proposed robust design consistently ensures superior communications performance across all users, even under channel variations.