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6G Wireless Communications: From Far-field Beam Steering to Near-field Beam Focusing
Haiyang Zhang, Nir Shlezinger, Francesco Guidi, Davide Dardari, Yonina C. Eldar
TL;DR
Future 6G requirements motivate extremely large-scale arrays that may operate partly in the radiating near-field, where planar-wave channel assumptions no longer apply. The paper surveys near-field channel characteristics, beam focusing, applications, and design challenges, and reports interference mitigation and user discrimination benefits from near-field-aware focusing.
Problem
Future 6G requirements may bring communications into the radiating near-field, where conventional plane-wave assumptions and far-field channel models are no longer valid.
Method
The paper reviews near-field physical features and channel models, explains spherical-wave beam focusing, discusses applications, and examines design challenges and research directions.
Results
Near-field-aware beam focusing can mitigate co-channel interference in both angle and distance domains, distinguish users sharing an angular direction, and increase spectrum efficiency near focal points.
Takeaways & Limitations
Near-field operation offers additional spatial control for multi-user communications and supports applications including localization, sensing, and wireless power transfer.
Abstract
from arXiv · showhide
6G networks will be required to support higher data rates, improved energy efficiency, lower latency, and more diverse users compared with 5G systems. To meet these requirements, electrically extremely large-scale antenna arrays are envisioned to be key physical-layer technologies. As a consequence, it is expected that some portion of future 6G wireless communications may take place in the radiating near-field (Fresnel) region, in addition to the far-field operation as in current wireless technologies. In this article, we discuss the opportunities and challenges that arise in radiating near-field communications. We begin by discussing the key physical characteristics of near-field communications, where the standard plane-wave propagation assumption no longer holds, and clarifying its implication on the modelling of wireless channels. Then, we elaborate on the ability to leverage spherical wavefronts via beam focusing, highlighting its advantages for 6G systems. We point out several appealing application scenarios which, with proper design, can benefit from near-field operation, including interference mitigation in multi-user communications, accurate localization and focused sensing, as well as wireless power transfer with minimal energy pollution. We conclude by discussing some of the design challenges and research directions that are yet to be explored to fully harness the potential of near-field operation.
I. INTRODUCTION
Future 6G systems are expected to combine extremely large-scale arrays and high-frequency bands, bringing some communications into the radiating near-field. The article surveys near-field characteristics, beam focusing opportunities, applications, and unresolved design challenges.
- 6G targets terabit-per-second peak rates, over 1 million connections per square kilometre, high energy efficiency, and latency as low as 1 microsecond.
- Extremely large-scale arrays with hundreds or thousands of antennas and high-frequency bands are envisioned to support these requirements.
- Large high-frequency arrays may place future communications in the radiating near-field, whose boundary is determined by the Fraunhofer distance.
- Near-field spherical wavefronts enable beam focusing at specific spatial locations, unlike far-field beam steering, which points energy toward a direction.
- The article examines near-field communications, localization and sensing, and wireless power transfer, including interference mitigation through angle- and distance-domain degrees of freedom.
- Key open challenges include near-field channel estimation, wideband mis-focusing or beam split, high-rank line-of-sight MIMO, and hardware implementation.
II. RADIATING NEAR-FIELD: PHYSICAL FEATURES
Near-field propagation differs from conventional far-field propagation because spherical wavefronts can no longer be approximated as planar. This difference supports spatially focused radiation and potentially more information-bearing wave behavior.
- Traditional wireless systems approximate spherical electromagnetic wavefronts as planar because propagation distances are large relative to wavelength.
- Spherical wavefronts permit beam focusing at a spatial location, adding depth control beyond the directional control of far-field beam steering.
- In radiating near-field conditions, the received wavefront remains spherical rather than planar.
B. Near-Field Channel Model
Near-field channel models must account for distance-dependent array responses and non-stationary multipath across electrically large apertures. These features make conventional angular sparse and fading models insufficient on their own.
- In the far field, approximately equal element-to-user distances yield common path gains and steering-vector phases linear in antenna index.
- Near-field array steering depends on both angle and user distance, so an angular-only Fourier dictionary is unsuitable for channel representation.
- Large apertures create non-stationary spatial channels in which multipath components may be observed by only some antenna elements.
- Mixed near-field and far-field scatterers require hybrid channel models, while conventional fading models do not describe these non-stationary channels adequately.
III. BEAM FOCUSING
Far-field beam steering controls radiation primarily by direction, whereas near-field beam focusing uses spherical wavefronts to control both direction and depth. This adds distance-domain interference shaping and supports near-field MIMO designs.
- From Beam Steering to Beam Focusing: Far-field beam steering sends signals toward a direction and controls the relative angle of most radiated energy.
- From Beam Steering to Beam Focusing: Near-field beam focusing concentrates radiated energy at a specific spatial location, controlling both angle and depth.
- From Beam Steering to Beam Focusing: Beam focusing precodes each antenna’s spherical-wave signal so contributions combine constructively at the focal point and destructively elsewhere.
- From Beam Steering to Beam Focusing: Beam focusing provides a new degree of freedom for shaping multi-user interference in the distance domain as well as the angle domain.
- From Beam Steering to Beam Focusing: Beam focusing enables capacity-approaching near-field MIMO communications through complex phase profiles approximated with simpler multiple focusing beams.
B. Application Scenarios
Near-field beam focusing offers application benefits by controlling signal energy in both angle and distance. The paper highlights interference mitigation, localization and sensing, and wireless power transfer with reduced energy pollution.
- Near-Field Multi-User Communications: Near-field beam focusing can mitigate co-channel interference between users sharing the same relative direction.Distance-aware focusing supplements angular-domain interference control.
- Near-Field Localization and Sensing: Distance-aware near-field channels can enhance wireless localization and sensing by exploiting position information in spherical wavefronts.Holographic localization may improve positioning without additional triangulation nodes.
- Near-Field Wireless Power Transfer: Near-field beam focusing can jointly support efficient wireless power transfer and minimal energy pollution.Focusing addresses low transfer efficiency and the need to avoid radiating energy at sensitive locations.
IV. NUMERICAL RESULTS
Numerical simulations compare near-field beam focusing with far-field beam steering in single-user and same-angle multi-user settings. The results show focusing improves performance near intended focal points and suppresses interference elsewhere.
- Simulation Setup: The simulation models two same-angle users served by a fully-digital UPA under a 28 GHz near-field line-of-sight channel.The UPA is positioned in the xy-plane, with users placed along the z-axis.
- Single-User Comparison: Beam focusing significantly increases spectrum efficiency near its focal point compared with beam steering.Beam steering suffers performance loss for near-field users because its far-field design mismatches the channel.
- Single-User Comparison: Far from the focal point, beam focusing yields lower spectrum efficiency than beam steering, indicating reduced radiating interference.The focused beam concentrates energy near the intended location rather than uniformly along the steering direction.
- Multi-User Comparison: For two users with the same angular direction, each focused beam peaks around its corresponding focal point, while far-field steering leaves one user with approximately zero spectrum efficiency.The focused beams provide reliable communication with minimal interference degradation at the two focal points.
- Interference Mitigation: Near-field focusing concentrates each user's signal around its focal point and generates negligible co-channel interference to the other same-angle user.The results indicate interference can be controlled in both angle and distance domains.
V. DESIGN CHALLENGES AND RESEARCH DIRECTIONS
Realizing near-field beam-focusing gains requires careful treatment of several design challenges, creating open research opportunities.
- Design Challenges and Research Directions: Near-field beam focusing requires careful solutions to design challenges before its performance gains can be fully realized.The paper frames these challenges as open research opportunities and directions.
- Design Challenges and Research Directions: Beam focusing is attractive for 6G applications because it sends signals to target regions with weak power leakage elsewhere.This property motivates further work on near-field communication designs.
A. Channel Estimation
Near-field channel estimation must account for both user angle and distance because the steering vector depends on both. Accurate channel knowledge is especially important for focused beams, which are sensitive to CSI errors.
- Channel Estimation: Accurate channel knowledge is critical because focused beams concentrate energy around small target regions and are more sensitive to errors than beam steering.The paper identifies accurate estimation and robust focusing under CSI inaccuracy as research directions.
- Channel Estimation: Near-field steering vectors depend on both angle of arrival and user distance, making angular-only Fourier dictionaries inappropriate.Polar-domain modeling uses non-uniform distance sampling to represent near-field channels.
B. Beam Misfocus/Split Issue
Near-field wideband beam focusing requires frequency-selective precoding, but frequency-flat phase-shifter hardware causes beam misfocus or beam split across frequencies.
- Near-field focusing requires each antenna phase to compensate transmission delay based on distance and frequency or wavelength.
- Frequency-flat phase shifters generate beams at different frequencies that focus at different locations, producing beam misfocus or beam split.
- Beam misfocus limits the phased array’s effective bandwidth and degrades wideband communication performance.
- Signal processing or flexible precoding hardware is needed to reduce or eliminate beam misfocus.
C. MIMO Multiplexing Gain
Near-field distance-aware channels provide additional degrees of freedom that can enable MIMO multiplexing gains, while making CSI estimation and hardware design more sensitive to antenna geometry and wavelength.
- Near-field distance awareness adds degrees of freedom that can provide potential MIMO multiplexing gains.
- Unlike far-field line-of-sight MIMO channels, near-field line-of-sight channels are not restricted to rank-one matrices.
- Optimal precoding and decoding are highly sensitive to antenna geometry normalized to the wavelength, making CSI estimation more critical.
- Beam focusing can efficiently approximate optimal precoding and decoding while reducing hardware requirements.
D. Hardware Implementation
Implementing extremely large-scale, high-frequency antenna arrays remains challenging, with power-efficient hardware architectures needed for 6G data rates and near-field operation.
- Realizing extremely large-scale antenna arrays with high-frequency signals remains subject to multiple implementation challenges.
- Power-amplifier efficiency typically decreases as carrier frequency increases, motivating advanced semiconductor technologies for energy-efficient hardware.
- Traditional ADC/DAC architectures consume excessive power at terabit-per-second data rates.
- Analog or RF-domain architectures using high-order modulation are identified as a research opportunity to address transceiver power consumption.
- Near-field wireless communications remain promising for 6G, but further research is needed for realistic evaluation and possible commercialization.