Key Takeaways & Executive Findings
- •• Proposes an analog-only beamforming (AoBF) architecture to replace hybrid beamforming, simplifying system reconfiguration and eliminating pilot overhead for near-field multiuser MIMO communications. • Formulates AoBF design as maximizing power to the target user while minimizing leakage to other users, solved via beam focusing and beam nulling using the majorization-minimization algorithm. • Develops two AoBF schemes: one with perfect CSI for performance benchmarking and one with imperfect CSI obtained from near-field codebook beam sweeping for practical deployment. • Simulation results show that the proposed AoBF schemes approach HBF sum-rate performance while achieving higher energy efficiency, making them attractive for energy-constrained systems.
Abstract
For near-field multiuser communications based on hybrid beamforming (HBF) architectures, high-quality effective channel estimation is required to obtain the channel state information (CSI) for the design of the digital beamformer. To simplify the system reconfiguration and eliminate the pilot overhead required by effective channel estimation, we consider an analog-only beamforming (AoBF) architecture in this study. AoBF is designed to maximize the sum rate, it is transformed into a problem maximizing the power transmitted to the target user equipment (UE) and meanwhile minimizing the power leaked to the other UEs. To solve this problem, we use beam focusing and beam nulling and propose two AoBF schemes based on the majorization–minimization algorithm. First, the AoBF scheme based on perfect CSI is proposed, with the focus on beamforming performance and regardless of CSI acquisition. Then, the AoBF scheme based on imperfect CSI is proposed, where low-dimensional imperfect CSI is obtained by beam sweeping based on a near-field codebook. Simulation results demonstrate that the two AoBF schemes can approach HBF schemes in terms of the sum rate and outperform HBF schemes in terms of energy efficiency.
1. Introduction
Millimeter wave (mmWave) communications and terahertz (THz) communications are regarded as key technologies to support the very high data rate for future wireless communications. To deal with the serious path loss of signal propagation in these bands, large-scale antenna arrays are equipped at base stations (BSs) to improve the beam gain. Fortunately, the small wavelength of mmWave and THz wave allows the integration of more antennas on a small surface. However, the utilization of large-scale arrays enlarges the Rayleigh distance to dozens of meters, which makes the near-field effect not negligible. Different from far field, wireless propagation in the near field is precisely modeled as spherical waves instead of plane waves (Zhang et al., 2023; Chen et al., 2024). Thus, the near-field beam can be focused on a specific location, which is different from the far-field beam aligned to a certain angle regardless of distance (Wang J et al., 2024). Therefore, conventional far-field multiuser interference suppression methods may not be suitable for near-field multiuser communications (Alkhateeb et al., 2015).
To mitigate multiuser interference in the near field, various beamforming architectures are designed. The most flexible solution is the fully digital beamforming architecture (Zhang et al., 2022). However, this solution causes overwhelming hardware costs for large-scale multiple-input multiple-output (MIMO) systems since each antenna needs to be connected to a radio frequency (RF) chain. To deal with this problem, hybrid beamforming (HBF) architectures are widely used. An HBF scheme for near field is proposed to perform analog beamforming and digital beamforming alternatively until the HBF results approach the solutions of fully digital beamforming (Zhang et al., 2022). However, this scheme is designed based on the perfect knowledge of channel state information (CSI), which is impractical. One method is to estimate the near-field channels (Cui and Dai, 2022). Due to the equipped large-scale antenna arrays, the dimension of the channel matrix is high, and channel estimation is complex and challenging. To address this problem, beam sweeping is performed to obtain low-dimensional imperfect CSI.
Based on the imperfect CSI obtained by beam sweeping, a two-stage HBF scheme is proposed for far-field communications (Alkhateeb et al., 2015). In the first stage of this scheme, analog beamforming is designed according to the code word selected by beam sweeping, and then the effective channel (Sun XY et al., 2019) is estimated. In the second stage, digital beamforming is designed based on the effective channel to mitigate multiuser interference. This two-stage HBF method is extended to the near field (Wu ZD and Dai, 2023). To simplify the system reconfiguration and eliminate the pilot overhead required by effective channel estimation, in this study, we consider using the analog-only beamforming (AoBF) architecture to replace HBF. Similar to the reconfigurable intelligent surface (RIS) composed of passive reflecting elements (Tian et al., 2022, 2024; Liu et al., 2023; Wang XH et al., 2023; Yang et al., 2023), AoBF can flexibly adjust the beam pattern only relying on phase shifters. The AoBF architecture omits the digital beamforming module, and its removal can improve energy efficiency (Cao et al., 2017; Shen et al., 2024).
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Ying Wang, Chenhao Qi (2025). Analog-only beamforming for near-field multiuser MIMO communications. Frontiers of Information Technology & Electronic Engineering. https://doi.org/10.1631/FITEE_2400433
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Frequently Asked Questions
What is analog-only beamforming (AoBF) for near-field multiuser MIMO communications?
AoBF is a beamforming architecture that uses only phase shifters to adjust the beam pattern, eliminating the digital beamforming module and the pilot overhead required for effective channel estimation in hybrid beamforming systems.
How does the proposed AoBF scheme mitigate multiuser interference in the near field?
The AoBF design is formulated to maximize the power transmitted to the target user equipment while minimizing power leaked to other users. This is achieved through beam focusing and beam nulling, solved using the majorization-minimization algorithm.
What CSI requirements do the two AoBF schemes have?
The first scheme assumes perfect CSI to benchmark beamforming performance, while the second scheme uses imperfect CSI obtained via beam sweeping based on a near-field codebook, making it more practical for real-world deployment.
How does AoBF compare with hybrid beamforming in performance?
Simulation results show that the proposed AoBF schemes can approach the sum-rate performance of HBF schemes while achieving higher energy efficiency due to the removal of the digital beamforming module.
Why is near-field beamforming different from far-field beamforming?
In the near field, wireless propagation is modeled as spherical waves instead of plane waves, enabling beams to be focused at specific locations rather than only at angles. This difference makes conventional far-field interference suppression methods unsuitable for near-field multiuser communications.
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