Tri-Hybrid Holographic Beamforming for Integrated Sensing and Communication

TL;DR

The paper tackles the challenge of designing integrated sensing and communication (ISAC) systems that leverage low-cost large-scale arrays. It introduces an RHS-enabled tri-hybrid holographic ISAC framework that jointly optimizes digital, analog, and EM (RHS) beamformers to minimize sensing waveform error while guaranteeing user-rate constraints, using an adaptive penalty and iterative gradient-based methods. Key contributions include (i) a three-layer optimization scheme with SQP for the digital layer and adaptive gradient projection for the analog and RHS layers, (ii) theoretical insights showing that optimized RHS amplitudes cluster near 0/1 enabling 1-bit amplitude control, and (iii) analysis and measurements confirming that subarray-level phase control yields holographic beamforming gains and that performance improves with more RHS elements but saturates beyond a point. The results demonstrate controllable trade-offs between communication and sensing, show that tri-hybrid holographic ISAC can outperform conventional hybrid beamforming with far fewer phase shifters, and validate the approach with a prototype, underscoring its practical impact for scalable 6G ISAC deployments.

Abstract

Integrated sensing and communication (ISAC) can perform both communication and sensing tasks using the same frequency band and hardware, making it a key technology for 6G. As a low-cost implementation for large-scale antenna arrays, reconfigurable holographic surfaces (RHSs) can be integrated into ISAC systems to realize the holographic ISAC paradigm, where enlarged radiation apertures achieve significant beamforming gains. In this paper, we investigate the tri-hybrid holographic ISAC framework, where the beamformer comprises digital, analog, and RHS-based electromagnetic (EM) layers. The analog layer employs a small number of phase shifters (PSs) to provide subarray-level phase control for the amplitude-modulated RHSs. Tri-hybrid beamforming provides a pathway for low-cost large-scale holographic ISAC. However, compared to conventional ISAC systems, it is challenging to achieve joint subarray-level phase control via PSs and element-level radiation amplitude control via RHSs for holographic ISAC. To address this, we present a tri-hybrid holographic ISAC scheme that minimizes sensing waveform error while satisfying the minimum user rate requirement. A joint optimization approach for PS phases and RHS amplitude responses is designed to address inter-layer coupling and distinct feasible regions. Theoretical analyses reveal that the optimized amplitude responses cluster near boundary values, i.e., 1-bit amplitude control, to reduce hardware and algorithmic complexity. Simulation results show that the proposed scheme achieves a controllable performance trade-off between communication and sensing tasks. Measured RHS beam gain validates the enhancement of holographic beamforming through subarray-level phase shifting. Moreover, as the number of RHS elements increases, the proposed approach exceeds the performance of conventional hybrid beamforming while significantly reducing the number of PSs.

Figures (8)

• Figure 1: RHS-enabled holographic ISAC system.
• Figure 2: Tri-hybrid structural diagram of a single RF chain.
• Figure 3: Distribution of the optimized amplitude responses.
• Figure 4: ISAC performance vs. the number of iterations.
• Figure 5: White squares indicate user locations, red dots represent sensing targets, and green crosses denote interference suppression targets. As the threshold rate $R_{th}$ increases, the beam energy is progressively shifted from sensing targets to communication users, reflecting the inherent trade-off between communication and sensing performance.

Theorems & Definitions (3)

• Remark 1
• Remark 2
• Remark 3