MIMO Array Research Articles

A 28-GHz Beam-Space MIMO RX With Spatial Filtering and Frequency-Division Multiplexing-Based Single-Wire IF Interface

The evolution of mm-wave phased array receivers (RX) to MIMO RX promises multi-beamforming and improved capacity. Although digital beamforming (DBF) provides the highest flexibility, digitization in the beam space as opposed to element space enables ADC digitization scalability with number of beams ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\le $ </tex-math></inline-formula> 4) and spatial filtering prior to the ADC. mm-Wave MIMO arrays must also address the challenge of increased IO routing while supporting dense fill-factors with <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> antenna spacing. In this work, an MIMO beam-space RX array architecture with simultaneous spatial filtering and single wire frequency-domain multiplexing (FDM) is presented. The reconfigurable weighted multi-phase mixing based beam-space RX scheme achieves both: 1) multi-beam formation/spatial filtering and 2) FDM of multiple beam-space outputs, preserving full MIMO field-of-view while ensuring a single IF interface. A 28-GHz four-element RX prototype demonstrates the proposed architecture in 65-nm CMOS. The IC occupies only 3.4 mm <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\times $ </tex-math></inline-formula> 3.1 mm for a four-element MIMO 28-GHz array and can form four independent beams with >400-MHz 3-dB BW and FDM on to a single IF interface. The array consumes 450 mW and achieves >27-dB beam-to-beam isolation over 150-MHz BW, while consuming 28.1 mW/stream/beam for 28 GHz. Concurrent MIMO measurements with two wireless 28-GHz beams are shown at 400 Mb/s (100 Msps, 16QAM) data rate demonstrating mm-wave MIMO operation.

Compact dual‐mode wideband MIMO filtering antenna array with high selectivity and improved isolation

A novel compact MIMO dual-mode filtering antenna array with wide bandwidth, high selectivity and good isolation is proposed. The filtering antenna unit consists of a patch radiator at the top layer coupled with a high-order dual-mode filter at the bottom layer through a rectangular slot in the middle ground layer. The fourth-order filter adopts two E-shape dual-mode resonators to achieve wide bandwidth and high selectivity. A pair of filtering antenna units form a MIMO array with a distance of 0.2λg by using three interdigital lines in the middle to enhance the port isolation. Finally, a 1 × 2 MIMO filtering antenna array operating at 2.45 GHz is designed and fabricated, which shows wide fractional bandwidth of 24.5%, sharp skirt slope with a pair of radiation nulls, smooth realized gain of 4.5 dBi as well as high isolation more than 20 dB.

On the Design and Analysis of Compact Super-Wideband Quad Element Chiral MIMO Array for High Data Rate Applications

This paper presents a compact, bouquet-inspired, four-element MIMO array for super wideband (SWB) applications. The proposed unit element monopole antenna has compact geometry, and it is deployed by the fusion of an elliptical and circular-shaped radiator. The convoluted geometry and semi-elliptical ground plane, along with the narrow rectangular slit defected ground structure, provides a wide impedance bandwidth. The designed unit cell has the dimensions of 32 mm × 20 mm × 0.8 mm, operates from 2.9 to 30 GHz (S11 ≤ −10 dB) and provides a bandwidth dimension ratio (BDR) of 2894. The proposed low-profile diversity array without any decoupling structures consists of four orthogonally placed, uncorrelated antennas with an inter element spacing of 0.05 λ0, occupies an area of 57 mm × 57 mm and provides dual polarization. The performance metrics of the diversity array were validated for frequencies over ultra-wideband, using mutual coupling characteristics, envelope correlation coefficient (ECC) by far-field radiation, diversity gain (DG), total active reflection coefficient (TARC), channel capacity loss (CCL) and cumulative distribution function (CDF) analysis. The measured mutual coupling over the operating band was less than −18 dB, the ECC was less than 0.004 and the TARC was less than −15 dB, and a better CCL of ˂0.28 bits/s/Hz was achieved by the fabricated antenna.

On the Application of K-User MIMO for 6G Enhanced Mobile Broadband.

This paper presents a high-throughput wireless access framework for future 6G networks. This framework, known as K-User MIMO, facilitates all-to-all communication between K access points and K mobile devices. For such a network, we illustrate the demodulation of independent data streams through a new interference cancellation beamforming algorithm that improves spectral efficiency compared to massive MIMO. The paper derives a multi-user Shannon Capacity formula for K-User MIMO when K is greater than or equal to 3. We define an Orthogonal Frequency Division Multiplexing (OFDM) frame structure that demonstrates the allocation of time-frequency resources to pilot signals for channel estimation. The capacity formula is then refined to include realistic pilot overheads. We determine a practical upper-bound for MIMO array sizes that balances estimation overhead and throughput. Lastly, simulation results show the practical capacity in small cell geometries under Rayleigh Fading conditions, with both perfect and realistic channel estimation.

EARPC – Energy Aware Routing Protocol for Cooperative MIMO Scheme in WSNs

Wireless sensor networks are typically operated on batteries. Therefore, in order to prolong network lifetime, an energy efficient routing algorithm is required. In this paper, an energy-aware routing protocol for the co-operative MIMO scheme in WSNs (EARPC) is presented. It is based on an improved cluster head selection method that considers the remaining energy level of a node and recent energy consumption of all nodes. This means that sensor nodes with lower energy levels are less likely to be chosen as cluster heads. Next, based on the cooperative node selection in each cluster, a virtual MIMO array is created, reducing uneven distribution of clusters. Simulation results show that the proposed routing protocol may reduce energy consumption and improve network lifetime compared with the LEACH protocol

Reconfigurable Intelligent Surfaces for Wireless Communications: Principles, Challenges, and Opportunities

Recently there has been a flurry of research on the use of reconfigurable intelligent surfaces (RIS) in wireless networks to create smart radio environments. In a smart radio environment, surfaces are capable of manipulating the propagation of incident electromagnetic waves in a programmable manner to actively alter the channel realization, which turns the wireless channel into a controllable system block that can be optimized to improve overall system performance. In this article, we provide a tutorial overview of reconfigurable intelligent surfaces (RIS) for wireless communications. We describe the working principles of reconfigurable intelligent surfaces (RIS) and elaborate on different candidate implementations using metasurfaces and reflectarrays. We discuss the channel models suitable for both implementations and examine the feasibility of obtaining accurate channel estimates. Furthermore, we discuss the aspects that differentiate RIS optimization from precoding for traditional MIMO arrays highlighting both the arising challenges and the potential opportunities associated with this emerging technology. Finally, we present numerical results to illustrate the power of an RIS in shaping the key properties of a MIMO channel.

Efficient Frequency Scaling Algorithm for Short-Range 3-D Holographic Imaging Based on a Scanning MIMO Array

Millimeter-wave (MMW) holographic imaging technology is widely used in plenty of short-range applications like security and medical diagnosis. When combining with multiple-input-multiple-output (MIMO) array, such a technology can acquire precise reconstruction with wider field of view and higher dynamic range. However, to focus the higher dimensional data set obtained from MIMO architecture, the complicated iteration or interpolation employed by the previous state-of-the-art focusing techniques prevents the real-time operation of such an imaging system under a general computation power. It is more economical to increase the operational speed by improving the algorithm efficiency. Hence, a novel fast imaging algorithm that uses multistatic frequency scaling technique is proposed in this article for achieving real-time 3-D imaging on a 1-D MIMO scanning system. Only fast Fourier transform (FFT)/inverse FFT (IFFT) and multiplications are employed in the algorithm, which can be easily implemented. Compared with the previous state-of-the-art techniques, the proposed algorithm has the lower computation complexity. Practical experiments with self-developed MMW MIMO scanning radar prove the accuracy and efficiency of the algorithm. On a common laptop without any acceleration technology, the proposed algorithm requires less than one tenth of the time as required by the previous state-of-the-art techniques.

Mutual coupling reduction using coupling matrix based band stop filter

The capability of the coupling matrix-based band stop filter in mutual coupling reduction between the MIMO arrays is presented. Initially, a microstrip band-stop filter with 3 GHz center frequency is designed using coupling matrix method. This filter structure is then placed between 2×1 radiating patches separated by 0.22 λ in a reference MIMO antenna designed for 3 GHz WiMAX applications. Both array antennas with/without band stop filters are fabricated and the scattering parameters are obtained separately and plotted comparatively. A good agreement is achieved between measurement and simulation results, and about 28 dB isolation is observed at operating frequency after the proposed filter application. Besides, the Envelope Correlation Coefficient (ECC) is computed and surface current distributions are simulated for both cases. It is observed that ECC has dropped to almost zero and the surface currents induced in the second patch due to the first patch excitation are extinguished with the implementation of the proposed isolation wall. Finally, to clarify the advantages of our approach, a few related studies are summarized in a comparative table.

3-D Short-Range Imaging With Irregular MIMO Arrays Using NUFFT-Based Range Migration Algorithm

3-D imaging with irregular planar multiple-input-multiple-output (MIMO) arrays is discussed. Due to signal acquisition on irregular spatial sampling grids by using these antenna arrays, the fast Fourier transform (FFT)-based imaging algorithms cannot readily be used for image formation. To avoid the application of computationally intensive coherent summation algorithms such as filtered backprojection or Kirchhoff migration, we propose a nonuniform FFT (NUFFT)-based MIMO Range Migration Algorithm (i.e., NUFFT-based MIMO-RMA) for efficient microwave imaging. The algorithm exploits NUFFT to reconstruct the wavenumber–domain spectra related to each Fourier frequency. It is generic and applicable to 3-D imaging with irregular planar MIMO arrays. The effects of irregular spatial sampling and signal bandwidth on the imaging performance and computational efficiency of the proposed algorithm are analyzed. Finally, some numerical simulations and experimental results are presented to demonstrate the performance of the proposed imaging algorithm.

An Optimization Method of MIMO Radar Array Based on Genetic Algorithm

Since the traditional radar imaging takes too much time, it is necessary to find new imaging radar which can image fast and conserve resources. MIMO radar has waveform diversity or spatial diversity. So it can realize high resolution snapshot imaging under the same condition. However, the imaging performance of MIMO radar is tightly related to the antenna array design. This paper proposes a MIMO array optimization method based on the genetic algorithm (GA). The GA is used to directly calculate the position of the transceiver element and at meantime the fitness function based on MIMO array design is proposed. Experiments show that this method not only can reduce the number of array elements, but also has certain optimization ability in beam pattern performance

Compact broadband slot‐based MIMO antenna array for vehicular environment

AbstractThis article presents the design, fabrication, and testing of a broadband directional slot antenna and its application in a vehicular environment. The proposed antenna consists of a slot antenna fed by a simple microstrip line. Broadband impedance matching is achieved by replacing the conventional microstrip line with a modified microstrip line. The modified microstrip line consists of a microstrip line terminated with an impedance transformer and a square patch. The proposed antenna offers 76.9% bandwidth centered around 4.8 GHz covering most of the communication applications between 3 and 6.75 GHz. Furthermore, the antenna's radiation pattern is shaped to obtain the desired directional characteristics using a simple reflector and array of via holes. The typical gain enhancement is 2.45 dBi at 5 GHz. This gain enhanced slot antenna is further used to construct the four‐element MIMO antenna. The proposed MIMO antenna configuration offers omnidirectional coverage with realized antenna gain greater than 6 dBi making the antenna more suitable for the vehicular environment. The prototype antenna and the MIMO array are fabricated and measurement results are presented. The measurement results are in good agreement with the simulation results.

Image reconstruction algorithm based on frequency-wavenumber decoupling for three-dimensional MIMO-SAR imaging.

In this paper, a frequency-wavenumber decoupling algorithm with high-efficiency and high-precise for three-dimensional (3-D) multiple-input-multiple-output synthetic aperture radar (MIMO-SAR) imaging is proposed. Based on one-dimensional (1-D) MIMO array combined with synthetic aperture scan along another dimension, MIMO-SAR imaging scheme allows the number of array elements to be greatly reduced compared with the two-dimensional (2-D) MIMO arrays. By multi-dimensional Fourier transforming and Method of Stationary Phase (MSP), analytical expression of the object function in the frequency-wavenumber domain was derived. By further expanding the range Fourier transform factor to its Taylor series form, the range compression can be realized by a simple fast Fourier transform (FFT) without multi-dimensional interpolation. After that, a decoupling factor was multiplied to compensate for the cross-range and range coupling in frequency domain. Finally, 2-D IFFT is carried out after rearrangement in the MIMO spatial frequency to get a fully focused 3-D image. Simulation and experimental results demonstrated that the algorithm can obtain the same high-precision images as back projection (BP) algorithm, and has the same high efficiency as range migration algorithm (RMA) while avoiding cumbersome multi-dimensional interpolation. A bistatic prototype imaging system in 0.1 THz band was designed for the proof-of-principle experiments. The 3-D reconstruction results of different targets were presented to verify the theoretical results and effectiveness of the proposed algorithm for MIMO-SAR imaging.

Non-Reciprocity View of the MIMO Antenna Arrays in Transmitting and Receiving Modes Using the Maximized Unique Receiving Pattern Theory Resulted by Angle-Wise Array Factor

A new approach to employ MIMO array behavior in system design calculations by distinguishing the transmitting and receiving modes' characteristics is presented. It shows that the conventional array formulation complies merely with the transmitting mode behavior of MIMO arrays, while the newly proposed formulation eases the estimation of the receiving mode behavior by introducing the Angle Wise Array Factor (AWAF). A novel theory of “Non-reciprocity view of transmitting and receiving modes in MIMO arrays” is being discussed along with “Pattern non-uniqueness in transmitting mode” and “Pattern uniqueness in receiving mode” theories. The grating lobe definition is also re-introduced, proposing that this definition is supposed to be reserved merely for transmitting mode.

A Metasurface-Based Low-Profile Array Decoupling Technology to Enhance Isolation in MIMO Antenna Systems

In this paper, a metasurface-based coupling reduction method is presented with the advantage of low profile for the MIMO antenna array. Unlike other metasurface-based decoupling technologies, this proposed metasurface for decoupling is loaded in the same layer as the coupled two-element patch antennas. Hence, the profile of the proposed decoupled array is greatly reduced compared to that of other published metasurface-based decoupled arrays. In this design, the two patch antennas with a short edge-to-edge distance of approximately 0.06 λ are surrounded by the period split ring resonator (SRR) elements. Next, two prototypes are fabricated; the test results show that the isolation performance for the decoupled array is better than that of a coupled array without SRRs, and the value of IS21I improves from -8 dB to -25 dB in a wide band of 5.0-6.0 GHz with IS21I <; -25 dB. The impedance matching bandwidths for two arrays without and with SRRs are approximately 700 MHz (12.7%) and 1500 MHz (27%), respectively, with IS11I <; -10 dB. In other words, a high-isolation MIMO array with the advantages of a low profile and a compact size is realized. It is exciting that the peak gains for the proposed decoupled array are improved by approximately 2 dB in the entire working band range. In addition, the efficiencies of the decoupled array increase by 15% compared to those of the coupled array, and the envelope correlation coefficient (ECC) also greatly improves. Therefore, the signs suggest that this proposed metasurface-based decoupling method is an efficient measure for MIMO array applications in the future.

Antenna Preprocessing and Element-Pattern Shaping for Multi-Band mmWave Arrays: Multi-Port Receivers and Antennas

Classical MIMO arrays employ a series of identical transceiver elements with periodically spaced identical and static antenna elements at a given frequency. The fixed antenna spacing limits the frequency of operation of the array due to the appearance of severe grating lobes at frequencies where the spacing approaches one wavelength. In this article, we propose element-level programmability, where each element in an array can be independently reconfigured to synthesize a set of element patterns. Through the control of element lobe and notch, new array patterns can be synthesized with strict control of side and grating lobes over a wide spectral range for multi-band arrays. In addition, we present the co-design methodology between the unit transmitter element and the integrated electromagnetic (EM) interface that allows broadband operation and unique antenna-level processing of signals that are distinct from classical arrays. This includes signal combination through the antenna, thus creating reconfigurable element patterns, dual beams for supporting wide angles of arrival and departure, and processing of more than one independent data stream in a single radiating surface (and reciprocal properties in a receiver). As a proof of concept, we present a four-port transmitter and EM interface implemented in a 65-nm bulk CMOS process. The chip demonstrates a wide effective isotropic radiated power (EIRP) bandwidth approximately across 40–70 GHz at the broadside and a peak EIRP of 25.6 dBm at 60 GHz with the ability to create a wide array of transmitter element patterns in a single aperture. The multi-functional EM interface and the broadband transmitters can enable future efficient and compact MIMO arrays for reliable links exploiting frequency and pattern diversities.

Study of the Extended Phase Shift Migration for Three-Dimensional MIMO-SAR Imaging in Terahertz Band

In this paper, the extended phase shift migration (PSM) for three-dimensional (3-D) multi-input-multi-output synthetic aperture radar (MIMO-SAR) imaging in terahertz (THz) band was studied. Based on one-dimensional MIMO arrays combined with synthetic aperture scan along another dimension, MIMO-SAR imaging scheme allows the number of array elements to be greatly reduced compared with the two-dimensional (2-D) MIMO arrays. By analyzing the derived analytical expression of the scattered waves in the frequency-wavenumber domain, the MIMO-SAR data in a certain frequency is mapped to another frequency of the `explode fields' of the monostatic form in accordance with a newly defined 3-D dispersion relations. By multiplying the modified phase shift terms in each frequency of `explode fields', the `explode fields' at different range planes can be reconstructed. Finally, the `explode fields' at time t = 0 can be successfully derived to realize fast imaging reconstruction for MIMO-SAR, with great reduction on the time consuming as compared with the BP algorithm and better accuracy compared with the Stolt migration. Additionally, due to its iterative nature in the range direction, the proposed algorithm is more flexible in treating more complicated scenarios, such as the image reconstruction in the multi-layer medium. A bistatic prototype imager was designed for the proof-of-principle experiments in THz band. The 3-D imaging results of different targets and computational complexity were also given to demonstrate the good performance of the proposed algorithm for THz MIMO-SAR imaging.

A Meta-Surface Antenna Array Decoupling (MAAD) Design to Improve the Isolation Performance in a MIMO System

In this paper, a design to reduce the mutual coupling between two closely MIMO antennas is presented. A metasurface superstrate consisting of periodic circular split ring resonator (CSRR) elements is placed above the coupled two-element patch antennas to improve the isolation performance. The propagation of the electromagnetic wave from antenna 1 to antenna 2 causing mutual coupling will be blocked since the loading of metasurface, which can be considered as a negative-permeability medium. First, the size of the CSRR element is determined by properly designing the range of negative permeability in the desire band. Thereafter, a decoupled MIMO array with a high isolation performance is realized with an intolerable matching curve. Then, some measures are employed on each patch to enhance the matching performance, such as etching slots and loading short probes on the patches. Next, two prototypes for the coupled and decoupled arrays are fabricated and measured to verify the improvement of the isolation performance. Measured results show that the value of |S21| for the decoupled array has been greatly improved from -8 dB to less than -20 dB within the entire working band of 5.58-6.0 GHz. Meanwhile, the match bandwidth of |S11| <; -15 dB for the decoupled array is approximately 420 MHz, which has also been improved in the desire band compared with the coupled array. In addition, the measured gains for the decoupled array are improved by approximately 1.8 dB as compared to the coupled array, whereas the efficiencies are increased by 16% which is higher than the coupled array. Moreover, the envelope correlation coefficient (ECC) for the decoupled array is reduced from 0.17 to 0.06 within the entire band of interest. Therefore, all measured results demonstrate that the proposed MAAD design is a good candidate for the improvement of MIMO antenna arrays.

A Wideband and Low-Loss Spatial Power Combining Module for mm-Wave High-Power Amplifiers

We present a low-loss power combiner, providing a highly integrated interface from an array of mm-wave power amplifiers (PAs) to a single standard rectangular waveguide (WG). The PAs are connected to an array of parallel and strongly coupled microstrip lines that excite a substrate integrated waveguide (SIW) based cavity. The spatially distributed modes then couple from the cavity to the rectangular WG mode through an etched aperture and two stepped ridges embedded in the WG flange. A new co-design procedure for the PA-integrated power combining module is presented that targets optimal system-level performance: output power, efficiency, linearity. A commercial SiGe quad-channel configurable transmitter and a standard gain horn antenna were interfaced to both ends of this module to experimentally demonstrate the proposed power combining concept. Since the combiner input ports are non-isolated, we have investigated the effects of mutual coupling on the transmitter performance by using a realistic PA model. This study has shown acceptable relative phase and amplitude differences between the PAs, i.e. within ±15° and ±1 dB. The increase of generated output power with respect to a single PA at the 1-dB compression point remains virtually constant (5.5 dB) over a 42% bandwidth. The performed statistical active load variation indicates that the interaction between the PAs through the combiner has negligible effect on the overall linearity. Furthermore, the antenna pattern measured with this combiner shows negligible deformation due to non-identical PAs. This represents experimental prove-of-concept of the proposed spatial power combining module, which can be suitable for applications in MIMO array transmitters with potentially coupled array channels.

Integration of 5G Rectangular MIMO Antenna Array and GSM Antenna for Dual-Band Base Station Applications

In this paper, a dual-band dual-polarized antenna array is presented for 5G base station application. The developed antenna array consists of a $4\times 4$ planar MIMO array operating at 3.3-5.0 GHz band (upper band UB) and a single antenna element working at 0.69-0.96 GHz band (lower band LB). The dual-band operation is based on our previously proposed antenna topology. The novelty of this paper is that the UB antenna array is a large-scale rectangular lattice array with 16 antenna elements for practical MIMO applications. Meanwhile, both the cross-band and in-band mutual coupling among the LB and UB antennas are furtherly suppressed by employing three decoupling techniques including rectangular ring resonator, ferrite chock ring, and novel baffle structure. With these decoupling technologies, the UB antenna array and the LB antenna are successfully integrated within a compact volume of $0.93\lambda _{L} \times 0.93\lambda _{L} \times 0.17\lambda _{L}$ ( $\lambda _{L}$ is the wavelength at 0.82 GHz). Experimental results show that the proposed dual-band antenna array offers high cross-band port isolation (>30 dB). Stable radiation patterns are also achieved for both the LB and UB antennas with averaged gain of 8.6 dBi and 7.3 dBi, respectively. The radiation efficiency is higher than 90% across the entire operation bands. It is the first time to realize a large scale sub-6 GHz MIMO antenna array shared-aperture working with a GSM band antenna element in a such compact volume.

Bandwidth Enhancement Technique for Broadside Tri-Modal Patch Antenna

A technique for enhancing the bandwidth of a broadside tri-modal patch antenna is described. The key idea of the technique is to incorporate a dual-resonance structure into the broadside tri-modal patch geometry. By increasing one edge of the tri-modal patch while decreasing its size at the opposite edge, the resulting structure can be viewed as two superimposed Y-shaped structures of different resonant frequencies. This intuition is confirmed using characteristic mode analysis (CMA). Furthermore, guided by CMA, further modifications enable two sets of resonant modes to be tuned for increasing the bandwidth of the tri-modal patch antenna. Importantly, the proposed bandwidth enhancement technique does not affect the desired broadside radiation patterns significantly. Therefore, it can be utilized to modify the tri-modal patch antenna without degrading its potential for massive MIMO array application. Measurement results show that the technique enhances the 10 dB impedance bandwidth from 4.3% to 19.7% with the largest antenna dimension of $0.48{\lambda _{c}}$ , where ${\lambda _{c}}$ is the wavelength in air at the center frequency. The design example of the proposed technique is able to cover widely used 3 GHz bands in 5G communication systems and its potential usage in massive MIMO arrays is demonstrated.