  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.
