Hybrid Beamforming Design for RSMA-enabled Near-Field Integrated Sensing and Communications

TL;DR

This work tackles NF-ISAC in extremely large antenna systems by proposing a RSMA-based scheme with hybrid analog-digital beamforming to jointly serve multiple users and sense multiple targets in the near-field. It establishes that dedicated sensing beams are unnecessary for NF multi-target detection via a rank-zero solution and develops a PDD-WMMSE-quadratic transform algorithm to optimize receive filters, beamformers, and rate allocation, with closed-form updates for key variables. Simulation results show near-field RSMA with hybrid beamforming achieves performance comparable to fully digital beamformers, while significantly reducing RF chains and outperforming conventional SDMA/NOMA and far-field ISAC schemes. These findings offer practical insights for designing hardware-efficient NF-ISAC systems and highlight RSMA’s robust interference management in NF environments.

Abstract

Integrated sensing and communication (ISAC) networks leverage extremely large antenna arrays and high frequencies. This inevitably extends the Rayleigh distance, making near-field (NF) spherical wave propagation dominant. This unlocks numerous spatial degrees of freedom, raising the challenge of optimizing them for communication and sensing tradeoffs. To this end, we propose a rate-splitting multiple access (RSMA)-based NF-ISAC transmit scheme utilizing hybrid analog-digital antennas. RSMA enhances interference management, while a variable number of dedicated sensing beams adds beamforming flexibility. The objective is to maximize the minimum communication rate while ensuring multi-target sensing performance by jointly optimizing receive filters, analog and digital beamformers, common rate allocation, and the sensing beam count. To address uncertainty in sensing beam allocation, a rank-zero solution reconstruction method demonstrates that dedicated sensing beams are unnecessary for NF multi-target detection. A penalty dual decomposition (PDD)-based double-loop algorithm is introduced, employing weighted minimum mean-squared error (WMMSE) and quadratic transforms to reformulate communication and sensing rates. Simulations reveal that the proposed scheme: 1) achieves performance comparable to fully digital beamforming with fewer RF chains, (2) maintains NF multi-target detection without compromising communication rates, and 3) significantly outperforms conventional multiple access schemes and far-field ISAC systems.

Figures (8)

• Figure 1: The considered RSMA-enabled NF-ISAC networks.
• Figure 2: The block diagram of our algorithm, where these three subproblems are iteratively solved until convergence.
• Figure 3: Max-min communication rate versus transmit power.
• Figure 4: Max-min communication rate versus the number of RF chains.
• Figure 5: Max-min communication rate versus the number of users.