This work initiates the study of a beyond-diagonal reconfigurable intelligent surface (BD-RIS)-aided transmitter architecture for integrated sensing and communication (ISAC) in the millimeter-wave (mmWave) frequency band. Deploying BD-RIS at the transmitter side not only alleviates the need for extensive fully digital radio frequency (RF) chains but also enhances both communication and sensing performance. These benefits are facilitated by the additional design flexibility introduced by the fully-connected scattering matrix of BD-RIS. To achieve the aforementioned benefits, in this work, we propose an efficient two-stage algorithm to design the digital beamforming of the transmitter and the scattering matrix of the BD-RIS with the aim of jointly maximizing the sum rate for multiple communication users and minimizing the largest eigenvalue of the Cramer-Rao bound (CRB) matrix for multiple sensing targets. Numerical results show that the transmitter-side BD-RIS-aided mmWave ISAC outperforms the conventional diagonal-RIS-aided ones in both communication and sensing performance.

翻译：本研究首次探讨了超对角可重构智能表面（BD-RIS）辅助的发射端架构在毫米波频段集成感知与通信（ISAC）中的应用。在发射端部署BD-RIS不仅减少了全数字射频链的依赖，同时提升了通信与感知性能。这一优势得益于BD-RIS全连接散射矩阵带来的额外设计自由度。为实现上述性能提升，本文提出了一种高效的两阶段算法，用于联合设计发射端的数字波束成形与BD-RIS散射矩阵，旨在同时最大化多通信用户的总速率并最小化多感知目标的克拉美-罗界（CRB）矩阵最大特征值。数值结果表明，与传统对角RIS辅助方案相比，发射端BD-RIS辅助的毫米波ISAC在通信与感知性能上均表现出更优效果。