[Paper Review] Non-Orthogonal Multiple Access For Near-Field Communications

Jiakuo Zuo, Xidong Mu+1 more|arXiv (Cornell University)|Apr 25, 2023

Advanced Wireless Communication Technologies4 citations

TL;DR

This paper proposes non-orthogonal multiple access (NOMA) for near-field communications (NF-NOMA), exploiting beamfocusing in both angular and distance domains via hybrid beamforming. It introduces two frameworks—single-location and multiple-location beamfocusing—achieving higher spectral efficiency than far-field NOMA, especially for high-QoS users located farther away, with interference well mitigated through ZF digital beamforming and optimized power allocation.

ABSTRACT

The novel concept of near-field non-orthogonal multiple access (NF-NOMA) communications is proposed. The near-filed beamfocusing enables NOMA to be carried out in both angular and distance domains. Two novel frameworks are proposed, namely, single-location-beamfocusing NF-NOMA (SLB-NF-NOMA) and multiple-location-beamfocusing NF-NOMA (MLB-NF-NOMA). 1) For SLB-NF-NOMA, two NOMA users in the same angular direction with distinct quality of service (QoS) requirements can be grouped into one cluster. The hybrid beamformer design and power allocation problem is formulated to maximize the sum rate of the users with higher QoS (H-QoS) requirements. To solve this problem, the analog beamformer is first designed to focus the energy on the H-QoS users and the zero-forcing (ZF) digital beamformer is employed. Then, the optimal power allocation is obtained. 2) For MLB-NF-NOMA, the two NOMA users in the same cluster can have different angular directions. The analog beamformer is first designed to focus the energy on both two NOMA users. Then, a singular value decomposition (SVD) based ZF (SVD-ZF) digital beamformer is designed. Furthermore, a novel antenna allocation algorithm is proposed. Finally, a suboptimal power allocation algorithm is proposed. Numerical results demonstrate that the NF-NOMA can achieve a higher spectral efficiency and provide a higher flexibility than conventional far-field NOMA.

Motivation & Objective

• Address the lack of NOMA research in near-field communications (NFC), where unique spherical wavefronts enable new degrees of freedom.
• Overcome limitations of far-field NOMA, which relies on planar wavefronts and cannot exploit distance-domain multiplexing.
• Design hybrid beamforming frameworks that leverage near-field beamfocusing to support multi-user NOMA with improved spectral efficiency and interference management.
• Maximize sum rate for high-quality-of-service (H-QoS) users while satisfying QoS constraints for all users through joint analog/digital beamforming and power allocation.
• Demonstrate that NF-NOMA outperforms both far-field NOMA and near-field orthogonal multiple access in spectral efficiency and interference mitigation.

Proposed method

• Propose single-location-beamfocusing NF-NOMA (SLB-NF-NOMA), where two users in the same angular direction but different distances are served by one analog beamformer focused on a single location.
• Design analog beamformers to concentrate energy on H-QoS users, and apply zero-forcing (ZF) digital beamforming to suppress inter-cluster interference.
• Formulate a sum rate maximization problem for H-QoS users under user rate constraints, solved via optimal power allocation after beamformer design.
• Introduce multiple-location-beamfocusing NF-NOMA (MLB-NF-NOMA), where one analog beamformer focuses on two distinct locations to serve users with different angular directions.
• Use beam-splitting to design multi-focal analog beamformers, and apply SVD-based ZF (SVD-ZF) digital beamforming to mitigate inter-cluster interference.
• Develop an antenna allocation algorithm using many-to-one matching to assign users to beamforming clusters, and propose an iterative fractional programming algorithm for suboptimal power allocation.

Experimental results

Research questions

• RQ1 Can near-field beamfocusing enable NOMA transmission in both angular and distance domains, offering advantages over conventional far-field NOMA?
• RQ2 How can hybrid beamforming be designed in NF-NOMA to support multi-user clustering with distinct angular and distance locations?
• RQ3 What is the performance gain of NF-NOMA over far-field NOMA in terms of spectral efficiency and interference mitigation?
• RQ4 How does beamfocusing in the distance domain improve spectral efficiency, especially for H-QoS users located farther from the base station?
• RQ5 Can interference be effectively suppressed in NF-NOMA using ZF digital beamforming and beam-splitting techniques?

Key findings

• The proposed SLB-NF-NOMA and MLB-NF-NOMA schemes achieve higher spectral efficiency than conventional far-field NOMA, even when H-QoS users are located farther away.
• Beamfocusing in the distance domain enables effective user clustering and interference mitigation, particularly through ZF digital beamforming and optimized power allocation.
• The MLB-NF-NOMA scheme achieves better spectral efficiency than MB-FF-NOMA due to enhanced beamfocusing and interference suppression, despite increased beamwidth from beam-splitting.
• Numerical results confirm that NF-beamfocusing effectively mitigates both intra-cluster and inter-cluster interference, outperforming far-field beamsteering.
• The sum rate of H-QoS users is maximized through joint analog beamformer design and optimal power allocation, with the SVD-ZF digital beamformer effectively suppressing inter-cluster interference.
• The iterative fractional programming algorithm converges to suboptimal power allocation solutions that significantly improve spectral efficiency in NF-NOMA systems.