Hybrid Beamfocusing Design for RSMA-Enabled Wideband Near-Field Systems

Wideband near-field communication (NFC) systems are subject to the spatial-wideband effect arising from frequency-dependent array responses, leading to array-gain loss and inter-user interference leakage. To address this challenge, we propose a rate-splitting multiple access (RSMA)-enabled NFC transmit scheme that integrates true-time-delay (TTD)–based hybrid beamfocusing. RSMA enables flexible inter-user interference management, while TTD-based architectures effectively mitigate spatial-wideband effect and significantly reduce radio frequency chain requirements. As a proxy for the performance degradation induced by the spatial-wideband effect, we adopt the minimum user rate as the optimization metric. Specifically, we aim to maximize the minimum rate by jointly optimizing frequency-dependent analog beamfocusing, digital beamfocusing, and common rate allocation. The resulting problem is highly nonconvex. To solve it efficiently, we develop a penalty-based iterative algorithm that partitions the design variables into three blocks and applies block coordinate descent (BCD) to optimize each block in an alternating manner. The proposed framework is further extended to accommodate sub-connected TTD-based hybrid architectures. Comprehensive simulation results demonstrate that the proposed scheme: (i) effectively compensates for the spatial-wideband effect, addressing a critical wideband NFC bottleneck; (ii) achieves performance close to that of full-digital beamfocusing with substantially lower hardware complexity; and (iii) delivers significant performance gains compared to existing benchmark schemes.