Inter-antenna Spacing Research Articles

Integrated Sensing and Communications With Joint Beam-Squint and Beam-Split for mmWave/THz Massive MIMO

Integrated sensing and communications (ISAC) has attracted tremendous attention for the future 6G wireless communications systems. To improve the transmission rates and sensing accuracy, massive multi-input multi-output (MIMO) technique is leveraged with large transmission bandwidth in millimeter wave (mmWave)/terahertz (THz) band. However, the growing size of antenna array and transmission bandwidth results in the beam-squint effect, which hampers the performance of communications. Moreover, the time overhead of the traditional sensing algorithm is prohibitively high for practical systems. In this paper, instead of alleviating the beam-squint effect, we take advantage of joint beam-squint and beam-split effect and propose a novel integrated sensing and communications scheme for massive MIMO system. Specifically, with the beam-squint effect, the base station (BS) utilizes the true-time-delay (TTD) lines to steer the beams of different OFDM subcarriers towards distributive directions simultaneously. Different users then feedback their respective subcarrier frequency with the maximum array gain to BS, based on which BS could calculate the directions of the users. Moreover, by selecting sub-array with the inter-antenna spacing larger than half-wavelength, the beam-split effect can be introduced and exploited to expand the sensing range. The proposed sensing method operates over frequency-domain, and the intended sensing range is covered by all the subcarriers simultaneously, which significantly reduces the time overhead compared to the conventional sensing scheme. Simulation results have demonstrated the effectiveness as well as the superior performance of the proposed ISAC scheme.

Optimal Design of Interleaved Subconnected Hybrid Structure for mmWave Massive MIMO Systems

Massive multiple-input multiple-output (MIMO) systems based on millimeter-wave (mmWave) technology commonly utilize a combination of hybrid analog and digital signal processing to reduce the number of radiofrequency (RF) chains, cutting down the hardware costs and power consumption. The subconnected structure is one of the architectures that realize hybrid analog and digital processing, in which each RF chain is connected to a subset of antennas through phase shifters so as to reduce the implementation complexity. In this paper, we investigate the best subconnected structure design for mmWave massive MIMO systems. First, we formulate the optimization problem based on a simplified connection model. Next, we derive a suboptimal solution in closed form by leveraging the mmWave channel characteristics and discuss the impact of different connection parameters on the spectral efficiency. The spectral efficiency is closely related to the structure of the analog processing and the interantenna spacing. When the interantenna spacing is an even a multiple of half wavelength, the localized structure, i.e., i = 1 achieves the maximum spectral efficiency. When the interantenna spacing is an odd multiple of half wavelength and the analog processing is accomplished by i = 2 interleaved structure, the best performance is obtained. Numerical results demonstrate the correctness of the theoretical analysis.

Nested inter-antenna spacing constrained sparse arrays for direction finding

In the design of a sparse array with large uniform degree of freedom (uDOF) for direction finding, the minimum inter-antenna spacing is usually constrained to be no more than half-wavelength. This is not attractive for reducing antenna mutual coupling especially when the antennas are dense. To alleviate antenna mutual coupling, we herein propose a new sparse array structure whose inter-antenna spacing is larger than half-wavelength. This three-level nested inter-antenna spacing constrained (termed NISC) array can be obtained by implicitly imposing the inter-antenna spacing constraint on three sparse uniform linear arrays (ULAs). The NISC difference co-array has holes due to the one-wavelength inter-antenna spacing constraint; however, these holes can be easily filled by adding just one antenna into NISC (termed NISC-A). Both the spatial smoothing subspace schemes and the compressive sensing schemes can be directly applied to the proposed NISC and NISC-A for direction finding. Simulation results are provided to illustrate the behaviors of NISC and NISC-A, in terms of antenna mutual coupling reduction, uDOF increase, and direction finding accuracy.

Massive MIMO Channels With Inter-User Angle Correlation: Open-Access Dataset, Analysis and Measurement-Based Validation

In practical propagation environments, different massive MIMO users can have correlated angles in spatial paths. In this paper, we study the effect of angle correlation on inter-user channel correlation via a combination of measurement and analysis. We show three key results. First, we collect a massive MIMO channel dataset for examining the inter-user channel correlation in a real-world propagation environment; the dataset is now open-access. We observed channel correlation higher than 0.48 for <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">all</i> close-by users. Additionally, over 30% of far-away users, even when they are tens of wavelengths apart, have inter-user channel correlation that is at least twice higher than the correlation in the i.i.d. Rayleigh fading channel. Second, we compute the inter-user channel correlation in closed-form as a function of inter-user angle correlation, the number of base-station antennas, and base-station inter-antenna spacing. Our analysis shows that inter-user angle correlation increases the inter-user channel correlation. Inter-user channel correlation reduces with a larger base-station array aperture, i.e., more antennas and larger inter-antenna spacing. Third, we explain the measurements with numerical experiments to show that inter-user angle correlation can result in significant inter-user channel correlation in practical massive MIMO channels.

DOA Estimation With Non-Uniform Linear Arrays: A Phase-Difference Projection Approach

Phase wrapping is a major problem in direction-of-arrival (DOA) estimation using phase-difference observations. For an antenna pair with an inter-antenna spacing greater than half of the wavelength ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\lambda /2$ </tex-math></inline-formula> ) of the signal, phase wrapping occurs at certain DOA angles leading to phase-difference ambiguities. Existing phase unwrapping methods exploit either frequency or spatial diversity. These techniques work by imposing restrictions on the utilized frequencies or the receiver array geometry. In addition to these restrictions, sensitivity to noise and calibration errors is another limitation of these methods. We propose a grid-less <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">phase-difference projection</i> (PDP) DOA algorithm to overcome these issues. The concept of <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">wrapped phased-difference pattern</i> (WPDP) is introduced, which allows us to compute most of the parameters required for DOA estimation in an offline manner. This results in a superior computational speed in real-time compared to methods with similar DOA estimation performance. Simulation results demonstrate the excellent performance of the proposed algorithm, both in terms of accuracy and speed.

Inter-User Interference Reduction in Massive MIMO for Linear and Planar Arrays

The huge demand for wireless communications has brought the study of new techniques, such as the use of a very large number of antennas at a base station serving multiple users terminals with a single antenna each one, known as massive MIMO. This technique exploits the “favorable propagation paradigm.” However, in constrained spaces, it is limited by interference among users. In this work, the inter-user interference reduction factor in massive MIMO is evaluated for linear and planar arrays as a function of the inter-antenna spacing. Furthermore, exact expressions for sub-6 GHz frequencies or mmWave are obtained, showing that inter-user interference does not vanish in massive MIMO with constrained space. Finally, by measuring the array edge length, the behavior of linear and planar antenna arrays is analyzed, evidencing array compaction and the inter-user interference trade off.

Spectral efficiency of multi-user millimeter wave systems under single path with uniform rectangular arrays

In this paper, we investigate the downlink achievable ergodic spectral efficiency (SE) of a single-cell multi-user millimeter wave system, in which a uniform rectangular array is used at the base station (BS) to serve multiple single-antenna users. We adopt a three-dimensional channel model by considering both the azimuth and elevation dimensions under single-path propagation. We derive the achievable ergodic SE for this system in with maximum ratio transmission precoding. This analytical expression enables the accurate and quantitative evaluation of the effect of the number of BS antennas, signal-to-noise ratio (SNR), and the crosstalk (squared inner product between different steering vectors) which is a function of the angles of departure (AoD) of users and the inter-antenna spacing. Results show that the achievable ergodic SE logarithmically increases with the number of BS antennas and converges to a value in the high SNR regime. To improve the achievable ergodic SE, we also propose a user scheduling scheme based on feedback of users’ AoD information and obtain the maximum achievable ergodic SE. Furthermore, we consider a dense user scenario where every user’s AoD becomes nearly identical and then derive the system’s minimum achievable SE.

Design and characterisation of a phased antenna array for intact breast hyperthermia

Purpose: Currently available hyperthermia technology is not well suited to treating cancer malignancies in the intact breast. This study investigates a microwave applicator incorporating multiple patch antennas, with the goal of facilitating controllable power deposition profiles for treating lesions at diverse locations within the intact breast.Materials and methods: A 3D-computational model was implemented to assess power deposition profiles with 915 MHz applicators incorporating a hemispheric groundplane and configurations of 2, 4, 8, 12, 16 and 20 antennas. Hemispheric breast models of 90 mm and 150 mm diameter were considered, where cuboid target volumes of 10 mm edge length (1 cm3) and 30 mm edge length (27 cm3) were positioned at the centre of the breast, and also located 15 mm from the chest wall. The average power absorption (αPA) ratio expressed as the ratio of the PA in the target volume and in the full breast was evaluated. A 4-antenna proof-of-concept array was fabricated and experimentally evaluated.Results: Computational models identified an optimal inter-antenna spacing of 22.5° along the applicator circumference. Applicators with 8 and 12 antennas excited with constant phase presented the highest αPA at centrally located and deep-seated targets, respectively. Experimental measurements with a 4-antenna proof-of-concept array illustrated the potential for electrically steering power deposition profiles by adjusting the relative phase of the signal at antenna inputs.Conclusions: Computational models and experimental results suggest that the proposed applicator may have potential for delivering conformal thermal therapy in the intact breast.

Coherent sources direction finding and polarization estimation with various compositions of spatially spread polarized antenna arrays

Various compositions of sparsely polarized antenna arrays are proposed in this paper to estimate the direction-of-arrivals (DOAs) and polarizations of multiple coherent sources. These polarized antenna arrays are composed of one of the following five sparsely spread sub-array geometries: (1) four spatially spread dipoles with three orthogonal orientations, (2) four spatially spread loops with three orthogonal orientations, (3) three spatially spread dipoles and three spatially spread loops with orthogonal orientations, (4) three collocated dipole–loop pairs with orthogonal orientations, and (5) a collocated dipole–triad and a collocated loop–triad. All the dipoles/loops/pairs/triads in each sub-array can also be sparsely spaced with the inter-antenna spacing far larger than a half-wavelength. Only one dimensional spatial-smoothing is used in the proposed algorithm to derive the two-dimensional DOAs and polarizations of multiple cross-correlated signals. From the simulation results, the sparse array composed of dipole–triads and loop–triads is recommended to construct a large aperture array, while the sparse arrays composed of only dipoles or only loops are recommended to efficiently reduce the mutual coupling across the antennas. Practical applications include distributed arrays and passive radar systems.

Tri-polarized MIMO systems in real-world channels: Channel investigation and performance analysis

Polarized multi-antenna systems are an effective solution for reducing inter-antenna spacing while still maintaining low inter-antenna correlation. Traditionally, only dual-polarized antenna systems are used for polarized transceivers. In this paper, tri-polarized antenna systems are investigated. Starting from the polarization mechanisms in the wireless propagation channel, it is shown that dual-polarized MIMO systems show high sensitivity to the transmitter and receiver orientation, which may be very critical in practical applications. Tri-polarized MIMO systems are introduced as a solution to obtain a robust MIMO performances, which are independent of the transmitter and receiver orientation. The performances of dual- and tri-polarized MIMO systems are evaluated on real-world measured channels, and the limits of each of these systems is highlighted.

Isolation Improvement Technique for Two Closely Spaced Loop Antennas Using MTM Absorber Cells

This paper presents a new method to improve isolation between two loop antennas with absorber cells exhibiting negative permittivity and permeability at the aimed frequency of 2.54 GHz. The metamaterial (MTM) unit cell was constructed by two open complementary split ring resonator (OCSRR) and a split ring resonator (SRR) arrangement. The proposed antenna structure consists of two loop antennas occupying a space with the dimensions of51 mm×24 mm×2 mmand three absorber cells. The size of each absorber unit cell is7.4 mm×7 mm×2 mm. The isolation of more than 20 dB was achieved with three absorber cells loaded between two loop antennas; interantenna spacing (center to center) is 0.23λ0at 2.54 GHz. The results show that port-to-port isolation between two antennas can be improved with absorber.

Mutual coupling suppression in closely spaced antennas

A technique to significantly reduce the mutual coupling between closely spaced antennas operating at the same frequency band is presented. The method comprises the use of two ground-plane side walls that are erected vertically next to adjacent antennas. The side walls are defected with a lattice pattern of slots to form a defected wall structure (DWS). In addition, the adjacent antennas sharing a common ground plane are separated with a pair of slits in the ground plane. This technique implemented on closely packed antenna suppresses surface waves in the antenna&apos;s operation band and can provide isolation of 56 dB, which was achieved with a DWS loaded with mushroom-shape slots separating two microstrip patch antennas. In this case, the inter-antenna spacing (centre to centre) is 0.272λo and ground-plane size is 0.606×0.370λo2. Also investigated is the number of DWS slot loadings on isolation between adjacent antennas. Furthermore, the proposed technique is shown to substantially reduce mutual coupling between planar inverted-F antennas (PIFAs) and monopoles. The measured results validate the proposed technique and vindicate the simulation results.

Reducing Mutual Coupling of Closely Space Microstrip Antennas for MIMO Application at 5.8 GHZ

An efficient mutual coupling reduction method is introduced in this paper for an extremely closely-spaced dual-element microstrip antennas positioned on a finite sized ground plane for MIMO application at 5.8 GHz. High isolation can be achieved through a simple transmission connection line (TNL) between the microstrip antennas. The position, length, and line width of the transmission connection line need optimize designing to maximize the isolation. It is found that isolations of more than 48 dB can be achieved between two parallel microstrip antennas sharing a common ground plane, with inter-antenna spacing of 0.038λ 0 and ground plane size 0.85λ 0 × 0.735λ 0. All along this paper, several prototypes are fabricated and their performances measured to validate the obtained IE3D moment method-based simulation results.

A Polarized Clustered Channel Model for Indoor Multiantenna Systems at 3.6 GHz

Multiple-input-multiple-output (MIMO) technologies allow high data rates to be obtained, but they suffer from interantenna correlation caused by the limits in interantenna spacing. Polarized MIMO systems resolve this problem by using colocated perpendicularly polarized antennas that have low interantenna correlation. In this paper, a polarized single-directional channel model for 2× <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</i> MIMO systems at 3.6 GHz in an indoor environment is presented. The wireless channel is modeled as a sum of clusters, where each cluster has specular and diffuse components. The polarization of the specular component of the clusters is included by considering a per-path polarization. The diffuse component of the clusters is modeled with a Fisher-Bingham (FB5) spectrum in the azimuth-coelevation domain and with an exponential power delay profile. Polarization is analyzed by introducing the cross-polar discrimination of the exponential power delay profile parameters. All of the parameters in the model are extracted from an experimental measurement campaign performed in an indoor environment at 3.6 GHz. Individual paths are extracted from the measurements with the space-alternating generalized expectation-maximization (SAGE) algorithm. These paths are grouped in clusters within the azimuth of arrival-elevation of arrival-delay domains at the receiver side using automatic clustering algorithms. The specular component properties of the clusters are then determined. Finally, the diffuse components of the clusters are investigated and parameterized by applying a beamforming algorithm on the diffuse part of the impulse response.

Capacity Enhancement Using MIMO Antenna Arrays in Realistic Macro-Cellular Urban Environment

In MIMO systems the antenna array configuration in the BS and MS has a large influence on the available channel capacity. In this paper, we first introduce a new Frequency Selective (FS) MIMO framework for macro-cells in a realistic urban environment. The MIMO channel is built over a previously developed directional channel model, which considers the terrain and clutter information in the cluster, line-of-sight and link loss calculations. Next, MIMO configuration characteristics are investigated in order to maximize capacity, mainly the number of antennas, inter-antenna spacing and SNR impact. Channel and capacity simulation results are presented for the city of Lisbon, Portugal, using different antenna configurations. Two power allocations schemes are considered, uniform distribution and FS spatial water-filling. The results suggest optimized MIMO configurations, considering the antenna array size limitations, specially at the MS side.

Isolation Enhancement Between Two Closely Packed Antennas

This paper introduces a coupling element to enhance the isolation between two closely packed antennas operating at the same frequency band. The proposed structure consists of two antenna elements and a coupling element which is located in between the two antenna elements. The idea is to use field cancellation to enhance isolation by putting a coupling element which artificially creates an additional coupling path between the antenna elements. To validate the idea, a design for a USB dongle MIMO antenna for the 2.4 GHz WLAN band is presented. In this design, the antenna elements are etched on a compact low-cost FR4 PCB board with dimensions of 20times40times1.6 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> . According to our measurement results, we can achieve more than 30 dB isolation between the antenna elements even though the two parallel individual planar inverted F antenna (PIFA) in the design share a solid ground plane with inter-antenna spacing (Center to Center) of less than 0.095 lambda <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">o</sub> or edge to edge separations of just 3.6 mm (0.0294 lambda <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">o</sub> ). Both simulation and measurement results are used to confirm the antenna isolation and performance. The method can also be applied to different types of antennas such as non-planar antennas. Parametric studies and current distribution for the design are also included to show how to tune the structure and control the isolation.

Reduction of Mutual Coupling Between Closely-Packed Antenna Elements

A simple ground plane structure that can reduce mutual coupling between closely-packed antenna elements is proposed and studied. The structure consists of a slitted pattern, without via's, etched onto a single ground plane and it is therefore low cost and straightforward to fabricate. It is found that isolations of more than -20 dB can be achieved between two parallel individual planar inverted-F antennas (PIFAs) sharing a common ground plane, with inter-antenna spacing (center to center) of 0.116 lambda <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">o</sub> and ground plane size 0.331lambda <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">o</sub> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . At 2.31 GHz it is demonstrated that this translates into an edge to edge separation between antennas of just 10 mm. Similarly the structure can be applied to reduce mutual coupling between three or four radiating elements. In addition the mutual coupling between half wavelength patches and monopoles can also be reduced with the aid of the proposed ground plane structure. Results of parametric studies are also given in this paper. Both simulation and measurement results are used to confirm the suppression of mutual coupling between closely-packed antenna elements with our slitted ground plane.

Spatial-correlation-coefficient at the basestation, in closed-form explicit analytic expression, due to heterogeneously Poisson distributed scatterers

This letter analytically derives, for stochastically located scatterers, closed-form expressions of the uplink received-signal's spatial-correlation-coefficient across the basestation antenna array's spatial aperture, explicitly as a function of 1) the basestation's interantenna spacing, and 2) each scatterer-cluster's distance and spatial angle from the basestation. This rigorous analytical derivation is based on idealized geometric relationships among the mobile transmitter, the basestation's receiving antennas, and the scatterers (whose spatial distribution is modeled as Poisson-distributed with a multi-Gaussian heterogeneous intensity).