---
title: 'Near-Field Beamfocusing: Principles & Applications'
url: https://www.emergentmind.com/topics/near-field-beamfocusing
type: topic
---

# Near-Field Beamfocusing: Principles & Applications

Searching arXiv for the cited near-field beamfocusing papers to ground the article in current literature.
to=arxiv_search.search  大发快三是什么query":"near-field beamfocusing beam focusing multi-user MIMO 2105.13087 2306.16861 2506.21325 2405.19347 2502.05819 2603.24077 2512.00774 2506.15470 2505.07991 2605.03488 2409.14685 2311.18334 2511.06994","max_results":10}  suliffeքները  手机天天中彩票 to=arxiv_search.search code 娱乐彩票注册  在天天中彩票json
{"results":[{"arxiv_id":"2506.15470","title":"Analyzing URA Geometry for Enhanced Spatial Multiplexing and Extended Near-Field Coverage","authors":["A. Ali","S. Kim","A. Chaaban"],"abstract":"With the deployment of large antenna arrays at high frequency bands, future wireless communication systems are likely to operate in the radiative near-field. Unlike far-field beam steering, near-field beams can be focused within a spatial region of finite depth, enabling spatial multiplexing in both the angular and range dimensions. This paper derives the beamdepth for a generalized uniform rectangular array (URA) and investigates how array geometry influences the near-field beamdepth and the limits where near-field beamfocusing is achievable. To characterize the near-field boundary in terms of beamfocusing and spatial multiplexing gains, we define the effective beamfocusing Rayleigh distance (EBRD) for a generalized URA. Our analysis reveals that while a square URA achieves the narrowest beamdepth, the EBRD is maximized for a wide or tall URA. However, despite its narrow beamdepth, a square URA may experience a reduction in multiuser sum rate due to its severely constrained EBRD. Simulation results confirm that a wide or tall URA achieves a sum rate of 3.5 X more than that of a square URA, benefiting from the extended EBRD and improved spatial multiplexing capabilities.","categories":["eess.SP"],"published":"2025-06-18","pdf_url":"http://arxiv.org/pdf/2506.15470v1"},{"arxiv_id":"2506.21325","title":"Localization-Based Beam Focusing in Near-Field Communications","authors":["M. O. M. Afif","L. Sanguinetti"],"abstract":"Shifting 6G-and-beyond wireless communication systems to higher frequency bands and the utilization of massive multiple-input multiple-output arrays will extend the near-field region, affecting beamforming and user localization schemes. In this paper, we propose a localization-based beam-focusing strategy that leverages the dominant line-of-sight (LoS) propagation arising at mmWave and sub-THz frequencies. To support this approach, we analyze the 2D-MUSIC algorithm for distance estimation by examining its spectrum in simplified, tractable setups with minimal numbers of antennas and users. Lastly, we compare the proposed localization-based beam focusing, with locations estimated via 2D-MUSIC, with zero forcing with pilot-based channel estimation in terms of uplink sum spectral efficiency. Our numerical results show that the proposed method becomes more effective under LoS-dominated propagation, short coherence blocks, and strong noise power arising at high carrier frequencies and with large bandwidths.","categories":["eess.SP"],"published":"2025-06-26","pdf_url":"http://arxiv.org/pdf/2506.21325v1"},{"arxiv_id":"2605.03488","title":"Near-Field Beam Focusing Characterization for 2D Waveguide-Fed Metasurface Antennas","authors":["A. de Jesus Torres","R. C. de Lamare"],"abstract":"Two-dimensional (2D) waveguide-fed metasurfaces enable scalable antenna apertures through guided wave excitation of distributed radiating elements. However, the resulting non-uniform excitation challenges classical interpretations of near-field characteristics. Using a physics-compliant model, this paper analyzes the near-field beam focusing behavior of such architectures. We derive asymptotic scaling laws for the beamforming gain, showcasing that the power-normalized gain scales linearly with the number of radiating elements. Furthermore, we introduce a normalized beam-depth formulation and obtain a compact analytic expression that characterizes the transition to far-field-like behavior. The presented analysis is validated against simulations based on the full electromagnetic model, confirming the accuracy of the derived scaling laws and beam-depth limits.","categories":["physics.optics","physics.app-ph"],"published":"2026-05-05","pdf_url":"http://arxiv.org/pdf/2605.03488v1"},{"arxiv_id":"2512.00774","title":"Hybrid Beamfocusing Design for RSMA-Enhanced Near-Field Secure Communications","authors":["X. Jiang","L. You","J. Wang","X. Qiao","X. Gao"],"abstract":"Near-field spherical wavefronts enable spotlight-like beam focusing to mitigate unintended energy leakage, creating new opportunities for physical-layer security (PLS). However, under hybrid analog-digital (HAD) antenna architectures, beamfocusing alone may not provide foolproof privacy protection due to reduced focusing precision. To address this issue, this paper proposes a rate-splitting multiple access (RSMA)-enhanced secure transmit scheme for near-field communications with fully-connected or sub-connected HAD architectures. In the proposed scheme, the common stream is designed for dual purposes, delivering the desired message for legitimate users while acting as artificial noise to disrupt eavesdropping. The primary objective is to maximize the minimum secrecy rate by jointly optimizing the analog beamfocuser, digital beamfocuser, and common secrecy rate allocation. To solve the formulated non-convex problem, we develop a penalty-based alternating optimization algorithm. Specifically, the variables are partitioned into three blocks, where one block is solved via a surrogate optimization method, while the others are updated in closed form. Simulation results reveal that our transmit scheme: (1) approaches fully digital beamfocusing with substantially fewer radio frequency chains, (2) outperforms conventional beamfocusing-only and far-field security schemes, and (3) preserves secrecy without significantly compromising communication rates.","categories":["eess.SP"],"published":"2025-11-30","pdf_url":"http://arxiv.org/pdf/2512.00774v1"},{"arxiv_id":"2409.14685","title":"Near-field Beam-focusing Pattern under Discrete Phase Shifters","authors":["Z. Wang","L. Liu","S. Jin"],"abstract":"Extremely large-scale arrays (XL-arrays) have emerged as a promising technology for enabling near-field communications in future wireless systems. However, the huge number of antennas deployed pose demanding challenges on the hardware cost and power consumption, especially when the antennas employ high-resolution phase shifters (PSs). To address this issue, in this paper, we consider low-resolution discrete PSs at the XL-array which are practically more energy efficient, and investigate the impact of PS resolution on the near-field beam-focusing effect. To this end, we propose a new Fourier series expansion method to efficiently tackle the difficulty in characterizing the beam pattern properties under phase quantization. Interestingly, we analytically show, for the first time, that 1) discrete PSs introduce additional grating lobes; 2) the main lobe still exhibits the beam-focusing property with its beam power increasing with PS resolution; and 3) there are two types of grating lobes, featured by the beam-focusing and beam-steering properties, respectively. In addition, we provide intuitive understanding for the appearance of grating lobes under discrete PSs from an array-of-subarrays perspective. Finally, numerical results demonstrate that the grating lobes generally degrade communication rate performance. However, a low-resolution of 3-bit PSs can achieve similar beam pattern and rate performance with the continuous PS counterpart, while it attains much higher energy efficiency.","categories":["eess.SP"],"published":"2024-09-23","pdf_url":"http://arxiv.org/pdf/2409.14685v1"},{"arxiv_id":"2401.08651","title":"Towards Near-Field 3D Spot Beamfocusing: Possibilities, Challenges, and Use-cases","authors":["Y. Al-Eryani","A. M. Eltawil"],"abstract":"Spot beamfocusing (SBF) is the process of focusing the signal power in a small spot-like region in the 3D space, which can be either hard-tuned (HT) using traditional tools like lenses and mirrors or electronically reconfigured (ER) using modern large-scale intelligent surface phased arrays. ER-SBF can be a key enabling technology (KET) for the next-generation 6G wireless networks offering benefits to many future wireless application areas such as wireless communication and security, mid-range high-power and safe wireless chargers, medical and health, physics, etc. Although near-field HT-SBF and ER-beamfocusing have been studied in the literature and applied in the industry, there is no comprehensive study of different aspects of ER-SBF and its future applications, especially for nonoptical (mmWave, sub-THz, and THz) electromagnetic waves in the next generation wireless technology, which is the aim of this paper. The theoretical concepts behind ER-SBF, different antenna technologies for implementing ER-SBF, employing machine learning (ML)-based schemes for enabling channel-state-information (CSI)-independent ER-SBF, and different practical application areas that can benefit from ER-SBF will be explored.","categories":["cs.IT","cs.NI","eess.SP"],"published":"2023-12-24","pdf_url":"http://arxiv.org/pdf/2401.08651v2"},{"arxiv_id":"2306.16861","title":"Beamfocusing Optimization for Near-Field Wideband Multi-User Communications","authors":["M. Cui","L. Dai"],"abstract":"A near-field wideband communication system is investigated in which a base station (BS) employs an extra-large scale antenna array (ELAA) to serve multiple users in its near-field region. To facilitate near-field multi-user beamforming and mitigate the spatial wideband effect, the BS employs a hybrid beamforming architecture based on true-time delayers (TTDs). In addition to the conventional fully-connected TTD-based hybrid beamforming architecture, a new sub-connected architecture is proposed to improve energy efficiency and reduce hardware requirements. Two wideband beamforming optimization approaches are proposed to maximize spectral efficiency for both architectures. 1) Fully-digital approximation (FDA) approach: In this method, the TTD-based hybrid beamformer is optimized by the block-coordinate descent and penalty method to approximate the optimal digital beamformer. This approach ensures convergence to the stationary point of the spectral efficiency maximization problem. 2) Heuristic two-stage (HTS) approach: In this approach, the analog and digital beamformers are designed in two stages. In particular, two low-complexity methods are proposed to design the high-dimensional analog beamformers based on approximate and exact line-of-sight channels, respectively. Subsequently, the low-dimensional digital beamformer is optimized based on the low-dimensional equivalent channels, resulting in reduced computational complexity and channel estimation complexity. Our numerical results show that 1) the proposed approach effectively eliminates the spatial wideband effect, and 2) the proposed sub-connected architecture is more energy efficient and has fewer hardware constraints on the TTD and system bandwidth compared to the fully-connected architecture.","categories":["eess.SP"],"published":"2023-06-29","pdf_url":"http://arxiv.org/pdf/2306.16861v2"},{"arxiv_id":"2505.07991","title":"Near-Field Beamfocusing, Localization, and Channel Estimation with Modular Linear Arrays","authors":["M. K. Rasmussen","A. L. F. de Almeida","E. De Carvalho"],"abstract":"This paper investigates how near-field beamfocusing can be achieved using a modular linear array (MLA), composed of multiple widely spaced uniform linear arrays (ULAs). The MLA architecture extends the aperture length of a standard ULA without adding additional antennas, thereby enabling near-field beamfocusing without increasing processing complexity. Unlike conventional far-field beamforming, near-field beamfocusing enables simultaneous data transmission to multiple users at different distances in the same angular interval, offering significant multiplexing gains. We present a detailed mathematical analysis of the beamwidth and beamdepth achievable with the MLA and show that by appropriately selecting the number of antennas in each constituent ULA, ideal near-field beamfocusing can be realized. In addition, we propose a computationally efficient localization method that fuses estimates from each ULA, enabling efficient parametric channel estimation. Simulation results confirm the accuracy of the analytical expressions and that MLAs achieve near-field beamfocusing with a limited number of antennas, making them a promising solution for next-generation wireless systems.","categories":["eess.SP"],"published":"2025-05-12","pdf_url":"http://arxiv.org/pdf/2505.07991v1"},{"arxiv_id":"2511.06994","title":"Experimental Validation of Reflective Near-Field Beamfocusing using a b-bit RIS","authors":["D. T. Demir","P. M. Korpijärvi","G. C. Alexandropoulos","M. Juntti"],"abstract":"This paper presents the first experimental validation of reflective near-field beamfocusing using a reconfigurable intelligent surface (RIS). While beamfocusing has been theoretically established as a key feature of large-aperture RISs, its practical realization has remained unexplored. We derive new analytical expressions for the array gain achieved with a b-bit RIS in near-field line-of-sight scenarios, characterizing both the finite depth and angular width of the focal region. The theoretical results are validated through a series of measurements in an indoor office environment at 28 GHz using a one-bit 1024-element RIS. The experiments confirm that near-field beamfocusing can be dynamically achieved and accurately predicted by the proposed analytical model, despite the presence of hardware imperfections and multipath propagation. These findings demonstrate that near-field beamfocusing is a robust and practically viable feature of RIS-assisted wireless communications.","categories":["eess.SP"],"published":"2025-11-10","pdf_url":"http://arxiv.org/pdf/2511.06994v1"},{"arxiv_id":"2502.05819","title":"Stacked Intelligent Metasurface Enabled Near-Field Multiuser Beamfocusing in the Wave Domain","authors":["M. Li","X. Zhang","L. Dai","Y. Han","Y. Zhang"],"abstract":"Intelligent surfaces represent a breakthrough technology capable of customizing the wireless channel cost-effectively. However, the existing works generally focus on planar wavefront, neglecting near-field spherical wavefront characteristics caused by large array aperture and high operation frequencies in the terahertz (THz). Additionally, the single-layer reconfigurable intelligent surface (RIS) lacks the signal processing ability to mitigate the computational complexity at the base station (BS). To address this issue, we introduce a novel stacked intelligent metasurfaces (SIM) comprised of an array of programmable metasurface layers. The SIM aims to substitute conventional digital baseband architecture to execute computing tasks with ultra-low processing delay, albeit with a reduced number of radio-frequency (RF) chains and low-resolution digital-to-analog converters. In this paper, we present a SIM-aided multiuser multiple-input single-output (MU-MISO) near-field system, where the SIM is integrated into the BS to perform beamfocusing in the wave domain and customize an end-to-end channel with minimized inter-user interference. Finally, the numerical results demonstrate that near-field communication achieves superior spatial gain over the far-field, and the SIM effectively suppresses inter-user interference as the wireless signals propagate through it.","categories":["eess.SP"],"published":"2025-02-09","pdf_url":"http://arxiv.org/pdf/2502.05819v1"}]}
Retrieving additional foundational and specialized papers referenced in the article outline.
to=arxiv_search.search  天天中彩票未code":"{\"query\":\"2105.13087 2311.18334 2405.19347 2603.24077 near-field beamfocusing\", \"max_results\": 10}"} 
to=arxiv_search.search  อาคารจีเอ็มเอ็มարկադրություն  ฝ่ายขายข่าวێjson
{"query":"2105.13087 2311.18334 2405.19347 2603.24077 near-field beamfocusing","max_results":10} 
to=arxiv_search.search  天天送钱彩票json
{"query":"2105.13087 Beam Focusing for Near-Field Multi-User MIMO Communications","max_results":5} 
to=arxiv_search.search  аамҭазы  大发快三是什么query":"2311.18334 Near-Field Beamfocusing with Polarized Antennas","max_results":5} 
Near-field beamfocusing is the synthesis of electromagnetic wavefronts that are constructive at a desired spatial point rather than merely along a desired direction. It arises when large apertures and short wavelengths place practical links in the radiative near-field, so the planar-wave approximation underlying far-field beam steering no longer applies and the array response becomes jointly dependent on angle and range. In this regime, beamforming becomes a polar-domain operation: energy can be concentrated within a finite-depth region in space, enabling user separation not only in angle but also in distance, with direct implications for multi-user MIMO, XL-array communications, RISs, metasurfaces, localization, security, and wave-domain computing [2105.13087].

## 1. Physical basis and propagation regime

The standard geometric boundary used throughout the literature is the Fraunhofer or Rayleigh distance,
$$
d_{\mathrm F}=\frac{2D^2}{\lambda},
$$
where \(D\) is the aperture diameter or length and \(\lambda\) is the wavelength. Beyond this distance, the field can be approximated as planar; below it, spherical-wave curvature across the aperture matters, and near-field beamfocusing becomes physically meaningful [2105.13087]. Survey treatments further distinguish the radiative near-field from the reactive region and emphasize that sharp spot focusing is associated with operation sufficiently deep inside the Fresnel zone rather than near the far-field boundary [2401.08651].

The defining difference from far-field beam steering is spatial selectivity in depth. In the far field, a single aperture can create angular directivity but not a local power maximum at a chosen distance; in the near field, exact element-to-point path differences permit coherent addition at a chosen focal point and weaker addition elsewhere. This is why near-field communication is naturally described in angle–range or polar coordinates rather than angle alone [2105.13087].

A recurring misconception is that near-field beamfocusing is simply “stronger steering.” The literature treats it as a different radiation regime. Steering keeps energy high along a direction, whereas focusing concentrates energy around a point or compact region in space. Work on 3D spot beamfocusing formalizes this distinction through the **beamfocusing radius (BFR)**, the radius of a circle on a reference plane centered at the desired focal point that contains a fraction \(\eta\) of the total radiated power on that plane [2401.08651].

## 2. Spherical-wave channel models and polar-domain patterns

Near-field beamfocusing is rooted in exact element-wise propagation distances. In downlink near-field multi-user MIMO, the received signal at user \(m\) can be written as
$$
r(\mathbf{p}_m)=\mathbf{a}_m^H\mathbf{s}+n_m,
$$
where the channel vector \(\mathbf a_m\) replaces the far-field steering vector and depends on the full user position \((x_m,y_m,z_m)\), not only on direction [2105.13087]. Likewise, uplink localization-based formulations use a near-field steering vector
$$
\mathbf b(f_{\mathrm c},r,\theta),
$$
whose entries are built from exact path lengths
$$
\bar r_n(r,\theta)=\sqrt{r^2+\delta_n^2 d^2-2r\delta_n d\sin\theta},
$$
making the array response explicitly dependent on both \(r\) and \(\theta\) [2506.21325].

Analytical multi-user work in the polar domain makes the same point through Fresnel expansions. The per-antenna distance is approximated as a linear angular phase term plus a quadratic distance-dependent term; the first is the far-field component, while the second is the near-field curvature term that creates finite beam depth and range discrimination [2512.17283]. In this view, the normalized gain
\[
\mathcal{G}(\theta,r;\theta_\kappa,r_\kappa)
\]
is a polar-domain antenna pattern rather than an angle-only beam pattern.

This modeling shift has direct communication consequences. In the far field, users on the same line of sight are hard to distinguish because their channel vectors are nearly aligned. In the near field, the channel depends on exact distance from each element to each user, so users sharing the same angle but occupying different ranges become separable. Sum-rate optimization over these spherical-wave channels therefore yields true focused beams even when the optimization problem itself is written in the same algebraic form as a far-field precoding problem [2105.13087].

## 3. Beamfocusing synthesis and transmitter architectures

The canonical communication-theoretic formulation is downlink sum-rate maximization with architecture-specific constraints. For a fully digital array, the single-user optimum is matched to the near-field channel, while the multi-user problem is handled through weighted-MMSE-based alternating optimization [2105.13087]. In that setting, the main conceptual result is that maximizing sum-rate over spherical-wave channel vectors automatically produces focusing in angle and distance.

Three transmitter architectures have been studied in detail for narrowband multi-user near-field beamfocusing: fully digital arrays, phase-shifter-based hybrid precoders, and dynamic metasurface antennas (DMAs). Fully digital architectures provide the benchmark because each element has an independent RF chain. Hybrid phase-shifter arrays approximate the fully digital solution under constant-modulus analog constraints, and one practical route is to optimize the analog stage on the product manifold of complex unit circles using a Riemannian conjugate gradient method [2105.13087]. DMAs impose stronger hardware structure: waveguide propagation along microstrips, element responses constrained by a Lorentzian phase model, and partially connected analog coupling. In the single-user case, their optimal relaxed focusing law is interpretable as phase conjugation of both free-space propagation and waveguide delay [2105.13087].

Wideband operation introduces an additional difficulty: the spatial wideband effect. In near-field ELAA systems, different subcarriers focus at different spatial points, producing not only angular squint but also range defocusing. True-time-delayer (TTD)-based hybrid beamforming addresses this by making the analog phase linear in frequency. Two design routes have been proposed: a fully-digital approximation (FDA) approach based on block-coordinate descent and penalty methods, and a heuristic two-stage (HTS) approach in which TTDs compensate inter-subarray delay differences while phase shifters realize the intra-subarray near-field phase profile [2306.16861]. Under that framework, the proposed sub-connected architecture is reported to be more energy efficient and to have fewer hardware constraints on the TTD and system bandwidth than the fully-connected architecture [2306.16861].

Near-field beamfocusing also admits nonstandard hardware realizations. Stacked intelligent metasurfaces (SIMs) have been proposed as wave-domain processors integrated into the base station, replacing part of conventional baseband processing with cascaded propagation and programmable diffraction across multiple metasurface layers. In that model, the SIM is optimized to make the end-to-end channel approximate a zero-forcing target in the wave domain, and simulations report a system sum rate \(7.6\%\) higher than a far-field model at the tested comparison point, with the fitting NMSE dropping from \(0.74\) to \(0.002\) as the number of layers increases from \(1\) to \(12\) [2502.05819].

## 4. Localization-assisted and learning-based beamfocusing

A major line of work replaces full channel estimation with geometry estimation. In uplink near-field communications, one approach is to estimate user locations with 2D-MUSIC and form a receive combiner from the corresponding near-field steering vectors,
$$
\mathbf W=\hat{\mathbf B}(\hat{\mathbf B}^{\mathrm H}\hat{\mathbf B})^{-1},
$$
where \(\hat{\mathbf B}=[\mathbf b(\hat r_1,\hat\theta_1),\ldots,\mathbf b(\hat r_K,\hat\theta_K)]\) [2506.21325]. This is zero forcing in the location-induced steering-vector domain rather than in the estimated-channel domain.

The appeal of this approach is specific to LoS-dominated mmWave and sub-THz propagation. If the channel is well approximated by its dominant geometric LoS component, then estimating \((r,\theta)\) can act as a low-overhead surrogate for estimating the whole channel. In the reported simulations, localization-based beam focusing with 2D-MUSIC uses \(\tau_{\mathrm{Loc}}=0.005T\), whereas a pilot-based LS channel-estimation baseline uses \(\tau_{\mathrm{Pil}}=0.2T\), and the localization-based method becomes more effective under LoS-dominated propagation, short coherence blocks, and strong noise power arising at high carrier frequencies and with large bandwidths [2506.21325]. An important limitation is that range estimation is not equally reliable across depth: the distance-estimation error standard deviation is reported to scale approximately as
$$
\zeta(r)\simeq \eta r^2,
$$
so localization quality degrades rapidly with range [2506.21325].

Another direction is CSI-independent beamfocusing using machine learning. For extremely large-scale programmable metasurfaces, distributed TD3-based deep reinforcement learning has been used to learn subarray beamfocusing matrices from power feedback alone. A transfer-learning refinement exploits correlation among subarray phase-distribution images (PDIs) through an effective circular cross-correlation (ECC) criterion, together with subarray policy propagation and quasi-liquid layers (QLLs) for soft fine-tuning [2405.19347]. In the reported simulations, the proposed scheme improves the training speed about 5 times, dynamic desired-focal-point management with policy blending augments convergence rate up to 8-fold, and joint focusing by all subarrays reduces the beamfocusing radius from about \(45\ \mathrm{cm}\) to \(6.3\ \mathrm{cm}\) [2405.19347].

These developments reflect a broader implementation reality: near-field beamfocusing is often limited less by the synthesis law itself than by the acquisition of sufficiently accurate geometric or channel information. Survey work treats exact per-element CSI for 2D extremely large apertures as a central bottleneck and motivates distributed learning, transfer learning, and feedback-based schemes precisely because planar-wave sparsity no longer applies in the Fresnel regime [2401.08651].

## 5. Geometry, quantization, polarization, and scalable apertures

Near-field beamfocusing depends strongly on array geometry. For generalized uniform rectangular arrays (URAs), beamdepth is finite and geometry dependent, and the **effective beamfocusing Rayleigh distance (EBRD)** is defined as the practical boundary within which finite-depth beamfocusing is achievable [2506.15470]. The main design tradeoff is that a square URA achieves the narrowest beamdepth, whereas a wide or tall URA maximizes EBRD. Because EBRD determines the spatial region in which range selectivity is usable, a square URA can underperform in multi-user settings despite its sharper beamdepth; simulations report that a wide or tall URA achieves a sum rate of \(3.5\times\) that of a square URA [2506.15470].

A different scalability route is the modular linear array (MLA), formed by widely spaced ULAs along a common line. This architecture enlarges total aperture without increasing the total antenna count. Its transverse beam pattern is the product of a module-level sinc envelope and an inter-module ripple factor, yielding a 3 dB beamwidth
$$
{\rm BW}_{3\rm dB}\approx \frac{1.77F}{N},
$$
where \(N\) is the number of antennas in each constituent ULA and \(F\) is focal distance [2505.07991]. For the two-ULA case, “ideal beamfocusing” is associated with the clean-focusing criterion
$$
\frac{N\lambda}{D_{\rm array}}\ge 0.62,
$$
which suppresses multiple ripple peaks inside the useful focal region [2505.07991]. The same modularity supports a low-complexity localization pipeline based on per-ULA AoA estimation and geometric fusion, with roughly three orders of magnitude lower complexity than full 2D-MUSIC in the reported channel-estimation study [2505.07991].

Hardware phase resolution creates another important tradeoff. For discrete phase shifters in XL-array near-field beamfocusing, Fourier-series analysis shows that quantization introduces additional grating lobes, while the main lobe retains the beam-focusing property and its beam power increases with phase-shifter resolution [2409.14685]. A central result is that the grating lobes come in two types: one exhibits beam-focusing behavior, the other beam-steering behavior [2409.14685]. Despite this distortion, a low-resolution of 3-bit PSs is reported to achieve similar beam pattern and rate performance to the continuous-PS counterpart while attaining much higher energy efficiency [2409.14685].

Specialized array physics can also increase the available spatial modes. In a simple ULA LoS setting with polarized antennas, near-field propagation supports as many as 3 spatial degrees of freedom when both transmitter and receiver employ three orthogonal linear polarizations, whereas in the far field the maximum number of spatial degrees of freedom tends to be only 2 because the equivalent MIMO channel becomes rank deficient [2311.18334]. This suggests that near-field beamfocusing can interact with polarization not only as a gain mechanism but as a rank-enhancement mechanism.

Finally, waveguide-fed metasurface apertures alter classical aperture intuition because their excitation is intrinsically non-uniform. Even so, asymptotic analysis for 2D waveguide-fed metasurfaces shows that the power-normalized gain still scales linearly with the number of radiating elements, and a normalized beam-depth formulation yields a compact expression for the transition to far-field-like behavior in the radial direction [2605.03488].

## 6. Security, experimental validation, and unresolved issues

The high spatial selectivity of near-field beamfocusing has made it a natural tool for physical-layer security, but recent work also clarifies its limits. Under secure communication with an uncertain eavesdropper location, conventional focal-point beamforming can be highly sensitive to localization error. A physics-informed alternative uses curved caustic beams: the aperture is partitioned into focusing and caustic subarrays so that some rays illuminate the legitimate user directly while others bypass the eavesdropping region [2603.24077]. In the reported simulations, this design achieves up to an \(80\%\) reduction of the worst-case eavesdropping rate for a localization error of \(0.25\) m [2603.24077].

A related line argues that beamfocusing alone is not sufficient under practical hybrid analog–digital architectures because reduced RF-chain budgets limit focusing precision. In RSMA-enhanced secure near-field transmission, the common stream is used both to deliver information and to act as artificial-noise-like interference to the eavesdropper, while analog and digital beamfocusers are jointly optimized through a penalty-based alternating procedure [2512.00774]. The reported outcome is that the scheme approaches fully digital beamfocusing with substantially fewer RF chains and outperforms beamfocusing-only and far-field security baselines [2512.00774].

The strongest evidence that near-field beamfocusing is not only a modeling construct comes from measurement. Reflective near-field beamfocusing with a \(b\)-bit RIS has been validated experimentally at 28 GHz using a one-bit 1024-element RIS, in what is described as the first experimental validation of reflective near-field beamfocusing with a dynamic RIS [2511.06994]. The measurements confirm finite depth and finite angular width of the focal region, with about \(1\) m half-power beamdepth in one setup and about \(3^\circ\) half-power beamwidth in azimuth in another, despite hardware imperfections and multipath propagation [2511.06994].

Several open issues recur across the literature. Many formulations assume accurate user positions or exact near-field CSI at the base station, often under free-space or LoS-dominant propagation, single-antenna users, and narrowband operation [2105.13087]. Survey and systems papers identify practical obstacles including CSI acquisition for 2D extremely large apertures, mobility, calibration, quantization, grating-lobe control, and the extension of beamfocusing theory to richer scattering, wideband operation, and safety-constrained applications such as wireless power transfer and biomedical systems [2401.08651]. A plausible implication is that the next stage of the field will be defined less by whether near-field beamfocusing is possible and more by how robustly it can be integrated with localization, low-resolution hardware, metasurface physics, and multi-user resource allocation.

Source: https://www.emergentmind.com/topics/near-field-beamfocusing