Transmitter Side Beyond-Diagonal RIS for mmWave Integrated Sensing and Communications

TL;DR

This paper tackles mmWave ISAC with a transmitter-side beyond-diagonal RIS (BD-RIS) to reduce RF-chain requirements while boosting both communication and sensing. It proposes a two-stage approach: Stage 1 designs the BD-RIS scattering matrix $\bm{\Psi}$ via a relaxed unitary solution followed by a symmetric unitary projection, and Stage 2 optimizes the transmit beamformers $\mathbf{W}$ with SCA to maximize the sum-rate $\sum_k R_k$ while minimizing the largest eigenvalue of the CRB $\lambda_{\max}(\mathbf{F}^{-1})$ under convexified constraints. The method yields a sequence of convex subproblems (P3) solved by CVX, delivering improved trade-offs over diagonal RIS, with performance scaling with BD-RIS size and sensor count. The results demonstrate the practical potential of transmitter-side BD-RIS to enhance ISAC performance in mmWave systems while reducing hardware complexity.

Abstract

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.

Figures (4)

• Figure 1: The system model of BD-RIS-aided transmitter architecture for ISAC.
• Figure 2: The trade-offs under different numbers of BD-RIS elements. $K=4$, $Q=1$, $N_{T}=6$, $N_{S}=8$, $M=128$, $P=20\ \mathrm{dBm}$.
• Figure 3: The trade-offs under different numbers of sensor elements. $K=4$, $Q=1$, $N_{I}=32$, $N_{T}=4$, $M=64$, $P=15\ \mathrm{dBm}$.
• Figure 4: The trade-offs under different numbers of targets. $K=2$, $N_{I}=16$, $N_{T}=4$, $N_{S}=4$, $M=64$, $P=20\ \mathrm{dBm}$.