Sub-Connected Hybrid Beamfocusing Design for RSMA-Enabled Near-Field Communications with Imperfect CSI and SIC

TL;DR

This paper explores the effectiveness of near-field beam focusing in interference mitigation for RSMA-enabled communications, revealing limitations of beamfocusing alone and demonstrating the advantages of RSMA with a practical hybrid beamforming scheme.

Contribution

It introduces a sub-connected hybrid beamforming design for near-field RSMA communications with imperfect CSI, and develops algorithms for joint analog-digital beamfocusing optimization.

Findings

Beamfocusing alone cannot fully eliminate interference even with perfect CSI.

RSMA outperforms SDMA in interference management under imperfect CSI.

The proposed hybrid architecture achieves near-optimal performance with fewer RF chains.

Abstract

Near-field spherical waves inherently encode both direction and distance information, enabling spotlight-like beam focusing for targeted interference mitigation. However, whether such beam focusing can fully eliminate interference under perfect and imperfect channel state information (CSI), rendering advanced interference management schemes unnecessary, remains an open question. To address this, we investigate rate-splitting multiple access (RSMA)-enabled near-field communications (NFC) under imperfect SCI. Our transmit scheme employs a sub-connected hybrid analog-digital (HAD) architecture to reduce hardware overhead while incorporating imperfect successive interference cancellation (SIC) for practical implementation. A minimum rate maximization problem is formulated by jointly optimizing the analog beamfocuser, the digital beamfocuser, and the common rate allocation. To solve the non-convex problem, we develop a penalty-based block coordinate descent (BCD) algorithm, deriving closed-form expressions for the optimal analog and digital beamfocusers solutions. Furthermore, to reduce computational complexity, we propose a low-complexity algorithm, where analog and digital beamfocusers are designed in two separate stages. Simulation results underscore that: 1) beamfocusing alone is insufficient to fully suppress interference even under perfect CSI; 2) RSMA exhibits superior interference management over SDMA under imperfect CSI and SIC conditions; 3) sub-connected HAD architecture delivers near-optimal digital beamfocusing performance with fewer radio frequency chains.