Flexible Rate-Splitting Multiple Access for Near-Field Integrated Sensing and Communications

TL;DR

This work tackles near-field ISAC by introducing a flexible rate-splitting multiple access framework that selectively decodes a common stream among a subset of users and reuses preconfigured NF beams for sensing an additional target. The core method minimizes the trace of the Cramér-Rao bound for joint distance and angle estimation by jointly optimizing power allocation, common-rate allocation, and user selection, solved via a quadratic-transform-based surrogate approach and simulated annealing. A key theoretical result shows that no extra probing signal is needed for sensing, simplifying system design. Simulations demonstrate that the proposed flexible RSMA significantly outperforms conventional RSMA and SDMA in NF sensing accuracy, with notable reductions in distance and angle estimation errors. The work offers a practical pathway to enhanced NF ISAC performance and paves the way for future developments in hybrid precoding and robust sensing under imperfect CSI and multi-target scenarios.

Abstract

This letter presents a flexible rate-splitting multiple access (RSMA) framework for near-field (NF) integrated sensing and communications (ISAC). The spatial beams configured to meet the communication rate requirements of NF users are simultaneously leveraged to sense an additional NF target. A key innovation lies in its flexibility to select a subset of users for decoding the common stream, enhancing interference management and system performance. The system is designed by minimizing the Cramér-Rao bound (CRB) for joint distance and angle estimation through optimized power allocation, common rate allocation, and user selection. This leads to a discrete, non-convex optimization problem. Remarkably, we demonstrate that the preconfigured beams are sufficient for target sensing, eliminating the need for additional probing signals. To solve the optimization problem, an iterative algorithm is proposed combining the quadratic transform and simulated annealing. Simulation results indicate that the proposed scheme significantly outperforms conventional RSMA and space division multiple access (SDMA), reducing distance and angle estimation errors by approximately 100\% and 20\%, respectively.

Figures (3)

• Figure 1: Flexible RSMA-enabled NF ISAC.
• Figure 2: RCRB versus the number of users
• Figure 3: RCRB versus the QoS of users