Beam Squint Research Articles

HINOTORI and Maser Observations

AbstractHINOTORI (Hybrid Integration Project in Nobeyama, Triple-band Oriented) has constructed a higher sensitivity 22/43 GHz and a 22/43/86 GHz simultaneous observation systems in the Nobeyama 45-m telescope by introducing new frequency separation filters in the telescope’s quasi-optics. The performance of the observation systems, such as the beam squint, the aperture efficiency, the system noise temperature when inserting the filters, and the phase stability of the signal path have been evaluated. It is indicated that the established systems have sufficient performance for single-dish and VLBI observations. The single-dish observation demonstrations using the triple-band system were successfully conducted in acquiring scientific data including multiple maser lines.

Wideband mmWave MIMO: One Prior-Aided LAMP-Based Network Combining with Residual Learning for Beamspace Channel Estimation

Existing channel estimation schemes for wideband systems generally estimate the channel matrix by pre-estimating the common support, which has a limited validity owing to the effect of beam squint. It is worth noting that the learned approximate message passing (LAMP) network need not pre-estimate the support and has obtained a relatively reliable estimation quality in the narrowband systems. In order to degrade the performance penalty caused by inaccurate estimation of the support in the wideband millimeter-wave MIMO systems, a prior-aided Gaussian mixture LAMP (GM-LAMP) network combining with residual learning is presented. Specifically, the multicarrier Gaussian mixture threshold shrinkage function is constructed for the GM-LAMP network, which can directly estimate the wideband beamspace channel while avoiding pre-estimating the support; then, considering the impact of channel noise and the coarse estimation error by the GM-LAMP, a residual network (ResNet) is designed to improve the estimation performance. Simulation results validate the efficiency of the proposed network (referred to as GLAMP-ResNet) with the lower computational complexity compared with the existing schemes.

Hybrid Beamforming for Terahertz Wireless Communications with Beam Squint: A Survey

Benefiting from the ultra-wide bandwidth, terahertz (THz) communication is considered one of the core technologies of the 6G mobile communication. However, high path loss is a severe issue for THz band, which needs to be solved by the ultra-massive multiple-input multiple-output (MIMO) technology. For the sake of reducing the hardware cost and power comsumption, the hybrid beamforming technology is required, which has been adopted in the 5G communication. In the hybrid beamforming for the THz wideband system, the enormous antenna and ultra-wide bandwidth cause the spatial- and frequency-wideband effect, resulting in acute beam squint. Eliminating beam squint is the key to the engineering realization of ultra massive MIMO for the THz band. To circumvent this issue, this paper presents a comprehensive overview of the hybrid beamforming technology including the channel model, the traditional phase shifter based and the true time delay based architectures, the 3D hybrid beamforming for the uniform planar array. Finally, multiple significant open issues and potential challenges are pointed out and elaborated.

Design of a Low-Cost, Low-Sidelobe-Level, Differential-Fed SIW Slot Array Antenna with Zero Beam Squint

This paper presents a low-cost, low-sidelobe-level, differential-fed, substrate-integrated waveguide (SIW)-based slot array antenna with zero beam squint. The antenna consists of two identical six-way unequal power dividers (PDs) and a 6 × 16 slot array and is realized on a single-layer substrate. The six-way unequal PD provides tapered amplitude and in-phase excitation for six SIWs, and each of them has 16 radiating slots. The 1 × 16 linear slot array on each SIW is excited using a differential feed to avoid undesired beam squinting across its operating band. A two-way hybrid waveguide (WG)-to-SIW E-plane PD is developed to provide equal amplitude and out-of-phase excitation for two six-way unequal power dividers. Moreover, metallic decoupling walls are implemented between two adjacent linear slot arrays to reduce E-plane external mutual coupling. An antenna prototype is fabricated and experimentally verified. The fabricated antenna shows that the measured −10 dB reflection bandwidth is 7.15% (9.57–10.28 GHz), with the achieved gain ranging from 20.30 to 21.92 dBi. A stable boresight beam is observed over the entire operating band. Furthermore, at the designed frequency of 10 GHz, peak SLLs of −29.1 dB and −29.4 dB are achieved in the E- and H-plane, respectively.

A metasurface enabled dual band antenna exhibiting dual polarizations with reduced beam squint and cross polarization

A compact, dual-band metasurface-based microstrip patch antenna with both linear and circular polarization (CP) for multiband communication is reported in this work. The ingredients in the antenna design consist of a combination of metasurface and metamaterial inspired structures that are demonstrated as an effective technique to correct the beam squint problem and reduce the cross-polarized radiation, usually experienced by thick substrate-based broadband CP antennas. A fractal boundary metasurface is introduced to achieve CP over a small frequency band. Eight Electric-LC resonators are symmetrically placed in the same plane of the radiating patch to widen the CP bandwidth, correct the beam squint, reduce the cross-polarized radiation, and realize a subwavelength resonance of λ0/5.15 order. The measured impedance bandwidths (|S11| < −10 dB) of the proposed antenna are 7.8% (3.7–4.0 GHz) and 27.58% (5.05–6.65 GHz) in the lower and upper-frequency band, respectively. The measured 3-dB axial ratio bandwidth (ARBW) is 24.8% (5070–6510 MHz) at 5.8 GHz. The fabricated prototype has an overall size of 55 × 55 × 4.08 mm (0.696λ0 × 0.696λ0 × 0.051λ0 at 3.8 GHz or 1.063λ0 × 1.063λ0 × 0.078λ0 at 5.8 GHz) with a realized peak gain of 4 dBi in the lower band and 7.1 dBic in the upper band. The measured results are consistent with the simulation results.

Miniaturized reconfigurable dielectric resonator antenna array with reduced beam squinting

In this study, a compact reconfigurable dielectric resonator antenna (DRA) array is proposed to operate in the band IEEE 802.11 WLAN. The main goal of this design is to propose an array feeding topology to avoid radiated beam squinting due to frequency alteration and acquire some gain enhancement with miniaturized array size. For that, the parallel–series geometry is used to layout a microstrip feeder with four output ports on the front plane. Each output port coupled with a rectangular slot (RS) at the bottom plane, and a rectangular dielectric resonator (RDR) mounted above the RS. The frequency reconfigurability is attained by employing PIN diodes that are integrated with the four output ports to operate as switches. Moreover, an impedance equivalent circuit model of the proposed array is postulated for studying the resonance behavior to ensure the validity of the proposed design prior to simulation and fabrication. The prototype 64 × 18 mm2 achieves frequency agility in the range of 5.8 to 5.17 GHz. Also, the realized gain ranges from 12.2 to 10. 5 dBi. The array specifications make it a decent potential candidate to be used for WLAN wireless systems.

Port isolation improvement of full‐duplex single‐patch antenna by loading full‐wave resonators

In this article, a pair of full-wave microstrip resonators is loaded at the nonradiative edges of the patch to improve port isolation of two-port copolarized single-patch antennas (SPAs) for full-duplex applications. First, the decoupling mechanism and its resulting radiation characteristics are exhaustively analyzed, which indicates that more similar radiation patterns and lower crosspolarization can be achieved based on the directly suppressing port coupling. Then, a transversal-section-like topology composed of an additional interference path and the SPA is proposed to realize this purpose. To clearly illustrate the operation principle in practical design, the phase shifts of both paths, that is, the interference paths from Port 1 to Port 2 via the patch and the full-wave resonators, are exhaustively investigated. Attributing to the realization of a stable 180° out of phase between the two paths, the resultant signal is highly and stably attenuated by the interfering signal, and the desired port isolation is satisfactorily attained by manipulating its amplitude readily. The measured results show a dramatic enhancement in isolation between the two ports from 4 to 55 dB with good impedance matching at the two ports. Most importantly, benefiting from the idea of direct port isolation, the radiation performance is hardly deteriorated, with a maximum gain of 8 dBi, beam squint angle of only 10°, and low crosspolarization in the upper half-space.

A Substrate Integrated Slot Leaky-Wave Antenna for Point-to-Point Communication

In this communication, we present a longitudinally slotted substrate integrated slot (SIS) leaky-wave antenna (LWA) for point-to-point communication. To mitigate the dispersion of traditional substrate integrated waveguide (SIW) LWAs, an SIS is proposed by etching a long straight slot along the SIW. The loaded slot mitigates the dispersion of SIW due to the special electric field distribution. Anti-TE10 mode is excited inside the SIS. Then, longitudinal slots are symmetrically loaded on both sides of the long slot. The prototype of SIS LWA is validated by the simulated and experimental results. A frequency bandwidth of 10.3% is realized with the beam squinting within 7° and realized gain of 13.9 ± 0.7 dB at a fixed angle of −7°. The proposed antenna is intended for point-to-point communication.

Energy-Efficient Dynamic-Subarray With Fixed True-Time-Delay Design for Terahertz Wideband Hybrid Beamforming

Hybrid beamforming for Terahertz (THz) ultra-massive multiple-input multiple-output (UM-MIMO) systems is a promising technology for 6G space-air-ground integrated networks, which can overcome huge propagation loss and offer unprecedented data rates. With ultra-wide bandwidth and ultra-large-scale antennas array in THz band, the beam squint becomes one of the critical problems which could reduce the array gain and degrade the data rate substantially. However, the traditional phase-shifters-based hybrid beamforming architectures cannot tackle this issue due to the frequency-flat property of the phase shifters. In this paper, to combat the beam squint while keeping high energy efficiency, a novel dynamic-subarray with fixed true-time-delay (DS-FTTD) architecture is proposed. Compared to the existing studies which use the complicated adjustable TTDs, the DS-FTTD architecture has lower power consumption and hardware complexity, thanks to the low-cost FTTDs. Furthermore, a low-complexity row-decomposition (RD) algorithm is proposed to design hybrid beamforming matrices for the DS-FTTD architecture. Extensive simulation results show that, by using the RD algorithm, the DS-FTTD architecture achieves near-optimal array gain and significantly higher energy efficiency than the existing architectures. Moreover, the spectral efficiency of DS-FTTD architecture with the RD algorithm is robust to the imperfect channel state information.

Beam Squint-Aware Integrated Sensing and Communications for Hybrid Massive MIMO LEO Satellite Systems

The space-air-ground-sea integrated network (SAGSIN) plays an important role in offering global coverage. To improve the efficient utilization of spectral and hardware resources in the SAGSIN, integrated sensing and communications (ISAC) has drawn extensive attention. Most existing ISAC works focus on terrestrial networks and cannot be straightforwardly applied in satellite systems due to the significantly different electromagnetic wave propagation properties. In this work, we investigate the application of ISAC in massive multiple-input multiple-output (MIMO) low earth orbit (LEO) satellite systems. We first characterize the statistical wave propagation properties by considering beam squint effects. Based on this analysis, we propose a beam squint-aware ISAC technique for hybrid analog/digital massive MIMO LEO satellite systems exploiting statistical channel state information. Simulation results demonstrate that the proposed scheme can operate both the wireless communications and the target sensing simultaneously with satisfactory performance, and the beam-squint effects can be efficiently mitigated with the proposed method in typical LEO satellite systems.

Generalized Sequential Rotation Arrays With Full Control of Dual-Circularly-Polarized Aperture-Field Distribution Based on Elliptically-Polarized Elements

In this article, the theory, design, and experimental validation of wideband generalized sequential rotation arrays (GSRAs) are reported. In particular, the GSRA has the ability to provide dual-circularly polarized (dual-CP) beams with different handedness using a single feeding network, whose aperture amplitude and phase distributions can both be independently controlled. Such functionality is achieved by jointly utilizing the dynamic phase and Berry phase, as well as exploiting elliptically polarized (EP) array elements with different axial ratio (AR) values. The proposed technique and the design methodology are verified by two proof-of-concept microstrip GSRAs consisting of an <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$8\times8$ </tex-math></inline-formula> array of EP stacked-patch elements operating in the K-band. The first prototype exhibits a 3 dB gain difference between the two CP beams, while the other offers a 6 dB gain difference and different sidelobe levels (SLLs) for the generated dual-CP beams. Both arrays are fabricated and measured, experimentally achieving a joint <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$S_{11} &lt; -10$ </tex-math></inline-formula> dB, AR < 3 dB, and 3 dB gain bandwidth of more than 28.4% and 20.1% with a beam squinting of less than 3°. In addition, the two GSRAs enable a bandwidth of about 19% and 3% within which the gain difference between the two CP beams of different handedness deviates from the targeted value by less than 1 dB. With the large degree of freedom in aperture field control, the proposed GSRAs could be potential candidates for use in satellite communications, point-to-multipoint communications, and so on.

Time-Delay Unit Based Beam Squint Mitigation for RIS-Aided Communications

Reconfigurable intelligent surface (RIS) is considered as a critical technology for future wireless communication networks. In this letter, we show that in a RIS-aided wideband THz communication system, the radiated beams steer away from the desired direction, which is known as the beam squint. Aiming to mitigate the beam squint, we utilize the element-grouping strategy and equip each sub-surface of RIS with a time-delay unit. Simulation results show that the beam squint becomes serious with the increase of the RIS’s size, and that the time-delay unit could effectively mitigate the beam squint and significantly improve the power gain.

A Hybrid True-Time and Phase-Delayed Approach for Millimeter-Wave Beam Steering

In this research study, a novel hybrid approach to beam steering based on the combination of true time delay and phase shift to enable continuous beam steering with minimal beam squint in wideband mm-wave transceivers is investigated. With modern high-speed communication links in the mm-wave spectrum requiring ever higher bandwidths, and losses increasing with frequency, there is a need for steerable arrays in mobile applications. This steering capability is commonly realized by phase shifters, which enable continuous beam direction control, but produce significant beam squint at large bandwidths or steering directions. An alternative is to use a time-delayed control, which resolves the squint issue but covers only a limited number of beam directions, producing considerable loss and being large in size. This research work presents the first 2-bit time delay stage operating above 200 GHz. In addition, a novel approach is described to enable broadband continuous beam control with minimal beam squint; this is achieved by combining the time delay circuit with a 90° vector-sum phase shifter. To prove the concept, the circuit is realized in a 130-nm SiGe Bipolar Complementary Metal Oxide Semiconductor (CMOS) (BiCMOS) technology. At 4.33 mW of dc power consumption, the small-signal gain is −8.6 dB at a 3-dB band of 220–250 GHz, resulting in 12.8% of relative bandwidth. The time delay stage provides coarse beam direction control with a maximum steering angle of 43.9° resulting from a maximum delay of 1.47 ps with a resolution of 0.39 ps. Across all tuning states, the root-mean-square delay error is less than 23.7 fs. The following phase shifter enables fine direction control while also compensating ±1.5 dB of gain variation resulting from the time delay. This is achieved while reducing the area consumption to half of what is required for comparable true time delay circuits. In a minimal <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2\times 1$ </tex-math></inline-formula> antenna array, this enables continuous beam direction control between ±54.6° while limiting the beam squint to less than 2.5°, equating to a more than 75% reduction in comparison to the common phase shift approach.

Broadband Fixed-Beam Leaky-Wave Antenna With Consistent and High Gain Based on Ridge Gap Waveguide

In most communication scenarios, antennas are often required to achieve broadband and directional radiation, and beam squint with frequency is unfavorable for broadband signal transmission. The traditional leaky-wave antennas (LWAs) have beam squint with frequency. In this letter, an LWA with fixed beam direction in a broadband is designed with consistent gain over the band. A ridge gap waveguide (RGW) is adopted to construct a nondispersive structure, and one lateral side is connected to a parallel plate filled with dielectric slab. Thus, leakage radiation is produced at the interface between the RGW and the parallel plate, generating a beam with fixed direction and high gain. The novelty is that in theory the fixed beam can be realized in infinite band owing to the nondispersive waveguide. Finally, a broad band of up to 58% (from 22 to 40 GHz) is achieved due to the limitations of the stopband structure and feeding port. The measured beam is fixed at 41° over the whole band, with a slight beam deviation of 2.6°. The realized gain is consistent over the band with a peak gain of 19.3 dBi. The designing method provides a simple and efficient way to realize the broadband fixed-beam LWAs.

A LTCC-Based Ku-Band 8-Channel T/R Module Integrated with Drive Amplification and 7-Bit True-Time-Delay.

Ku-band drive amplification and a 7-bit true-time-delay (TTD) function were realized as a part of a LTCC-based T/R module to increase integration. The 8-channel T/R module was fabricated and its key characteristics were measured, including a 3-bit (1/2/4 λ) TTD, 4-bit (0.25/0.5/1/2 λ) TTD, receive gain, noise figure and output power. The 8-channel T/R module can be further adopted to increase bandwidth and scanning angle of phased arrays without beam squint.

Wideband Cell-Free mmWave Massive MIMO-OFDM: Beam Squint-Aware Channel Covariance-Based Hybrid Beamforming

This paper considers the design of beam squint-aware channel covariance-based hybrid beamformers for wideband cell-free millimeter-wave (mmWave) massive multiple-input multiple-output (MIMO)-orthogonal frequency division multiplexing (OFDM) systems. Single and double phase shifter-based analog radio frequency (RF) precoder/combiner strategies are proposed that are based on dissimilar beam squint awareness assumptions. The digital precoding/combining stage is implemented using centralized designs maximizing the signal-to-interference-plus-noise-ratio (SINR) and relying on the uplink (UL)/downlink (DL) duality enabled by the time division duplex (TDD) protocol and the use of either phase shift-aware minimum mean square error (MMSE) or phase shift-unaware linear MMSE (LMMSE) channel estimators. The performance of the proposed beam squint-aware hybrid beamforming strategies is assessed through extensive numerical simulations under different scenarios. Numerical results show that beam squint-aware designs outperform the beam squint-unaware strategies, specially in typical wideband cell free mmWave massive MIMO-OFDM scenarios where a combination of a very high carrier frequency, a large system bandwidth, and a large-scale antenna array causes the spatial-wideband effect.

Evolutionary Trends in True Time Delay Line Technologies for Timed Array Radars

Timed array technology is rapidly evolving in multiple areas such as high resolution imaging radar, automotive, medical, high data rate communication applications etc. Timed arrays by utilising True Time Delay (TTD) lines in place of phase shifters mitigate beam squint and pulse dispersion issues associated with wide instantaneous bandwidth arrays. This paper presents on review of evolutionary trends in TTD line architectures starting from coaxial cable to photonic integrated circuit. The paper also reports on critical parameters of TTD lines, their importance and implication in design of typical X-band imaging radar. Comparison of different TTD line architectures in terms of configuration, implementation, merits and demerits are discussed in detail for wideband array application. The paper also brings out the integration aspects of TTD lines as part of T/R modules and proposes suitable design schemes towards performance optimization and realisation of timed arrays.

Design of Channel Estimation for Hybrid Beamforming Millimeter-Wave Systems in the Presence of Beam Squint

In this article, channel estimation for millimeter-wave massive multiple-input–multiple-output (MIMO) systems with hybrid beamforming is considered. With a large number of antenna elements, a progressive time delay across antenna elements incurs a change in the beam direction as a function of subcarriers in an orthogonal frequency-division multiplexing system. Considering the difference in delay across the entire array, a new channel estimation framework is proposed by using a maximum likelihood criterion as a function of the analog training beam and pilot subcarrier assignment. The resulting design achieves asymptotic orthogonality among the channel paths and minimizes the mean squared error. An extension of the proposed design to a uniform planar array is discussed. The Cramér–Rao lower bound for joint channel estimation with hybrid beamforming is derived to verify the effectiveness of the proposed design.

Efficient Channel Estimation for Wideband Millimeter Wave Massive MIMO Systems With Beam Squint

Massive multiple-input-multiple-output (MIMO) and millimeter wave have been adopted as the enabling technologies for the 5G and beyond 5G (B5G) systems. A challenging problem introduced by the use of large antenna size and wide bandwidth is beam squint, i.e., spatial-wideband effect. Beam squint can significantly degrade the channel estimation performance for conventional channel estimators. Research effort on channel estimation under beam squint conditions has been very limited. For the few available work that attempts to address this problem, they require either all subcarriers or multiple symbols used as pilot for channel estimation, so large overhead becomes inevitable. Therefore, in this paper, we propose an efficient channel estimation method that only requires a small number of subcarriers. The channel estimation problem is formulated as a nonlinear least squares optimization problem. Initial parameter estimation is critical, which will affect the efficiency and convergence of the proposed algorithm. Using a densely-spaced antenna structure and consecutive subcarriers assignment approach, we can effectively avoid the aliasing effect and reduce the ambiguity during the initialization phase. A subcarrier assignment criterion is proposed to achieve the optimal performance. Closed-form expressions of the Cramér-Rao lower bound (CRLB) and the achievable rate are derived to evaluate the performance. Both simulation results and theoretical analysis show that even with a small number of subcarriers, the estimation error closely approaches the CRLB, and its effect is negligible compared with the noise when evaluating the signal-to-noise ratio with a simple linear detector. Furthermore, the number of pilot subcarriers has little impact on the achievable rate.

Corrections to “RF Beamforming and Subcarrier Allocation Using Beam Squint in mmWave Systems”

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