MIMO Research Articles

Prescribed‐Time Fault‐Tolerant Control of Linear Time‐Varying Systems by Linear Time‐Varying Feedback

ABSTRACTIn this paper, the prescribed‐time fault‐tolerant control for multiple input multiple output (MIMO) linear time‐varying (LTV) systems is investigated. Both lexicographically fixed and non‐lexicographically fixed LTV systems have been considered. By leveraging the special structure of the controllable canonical form and employing the properties of the parametric Lyapunov equation, a global prescribed‐time fault‐tolerant controller is designed. With the aid of a suitable Lyapunov‐like function, it is demonstrated that the state can converge to zero within the prescribed time. The convergence time is independent of the initial condition. In addition, the designed controller is continuous, bounded, and even maintains a linear and concise form. Ultimately, the effectiveness of the controller is verified through the simulation of the elliptical orbital rendezvous system.

High-Gain Miniaturized Multi-Band MIMO SSPP LWA for Vehicular Communications

This paper introduces a novel miniaturized, four-mode, semi-flexible leaky wave Multiple-Input Multiple-Output (MIMO) antenna specifically designed to advance vehicular communication systems. The proposed antenna addresses key challenges in 5G low- and high-frequency bands, including millimeter-wave communication, by integrating innovative features such as a periodic Spoof Surface Plasmon Polariton Transmission Line (SSPP-TL) and logarithmic-spiral-like semi-circular strip patches parasitically fed via orthogonal ports. These design elements facilitate stable impedance matching and wide impedance bandwidths across operating bands, which is essential for vehicular networks. The hybrid combination of leaky wave and SSPP structures, along with a defected wide-slot ground structure and backside meander lines, enhances radiation characteristics by reducing back and bidirectional radiation. Additionally, a naturalization network incorporating chamfered-edge meander lines minimizes mutual coupling and introduces a fourth radiation mode at 80 GHz. Compact in size (14 × 12 × 0.25 mm3), the antenna achieves high-performance metrics, including S11 < −18.34 dB, dual-polarization, peak directive gains of 11.6 dBi (free space) and 14.6 dBi (on vehicles), isolation > 27 dB, Channel Capacity Loss (CCL) < 3, Envelope Correlation Coefficient (ECC) < 0.001, axial ratio < 2.25, and diversity gain (DG) > 9.85 dB. Extensive testing across various vehicular scenarios confirms the antenna’s robustness for Vehicle-to-Vehicle (V2V), Vehicle-to-Pedestrian (V2P), and Vehicle-to-Infrastructure (V2I) communication. Its exceptional performance ensures seamless connectivity with mobile networks and enhances safety through Specific Absorption Rate (SAR) compliance. This compact, high-performance antenna is a transformative solution for connected and autonomous vehicles, addressing critical challenges in modern automotive communication networks and paving the way for reliable and efficient vehicular communication systems.

Performance prediction and optimization of a high-efficiency tessellated diamond fractal MIMO antenna for terahertz 6G communication using machine learning approaches.

This paper introduces the design and exploration of a compact, high-performance multiple-input multiple-output (MIMO) antenna for 6G applications operating in the terahertz (THz) frequency range. Leveraging a meta learner-based stacked generalization ensemble strategy, this study integrates classical machine learning techniques with an optimized multi-feature stacked ensemble to predict antenna properties with greater accuracy. Specifically, a neural network is applied as a base learner for predicting antenna parameters, resulting in increased predictive performance, achieving R², EVS, MSE, RMSE, and MAE values of 0.96, 0.998, 0.00842, 0.00453, and 0.00999, respectively. Utilizing regression-based machine learning, antenna parameters are optimized to attain dual-band resonance with bandwidths of 3.34 THz and 1 THz across two bands, ensuring robust data throughput and communication stability. The antenna, designed with dimensions of 70 × 280μm², demonstrates a maximum gain of 15.82 dB, excellent isolation exceeding - 32.9 dB, and remarkable efficiency of 99.8%, underscoring its suitability for high-density, high-speed 6G environments. The design methodology integrates CST simulations and an RLC equivalent circuit model, substantiated by ADS simulations, with comparable reflection coefficients validating the accuracy of the models. With its compact footprint, broad bandwidth, and optimized isolation and efficiency, the proposed MIMO antenna is positioned as an ideal candidate for future 6G communication applications.

Low-order MIMO controller for turbomachinery supported by active magnetic bearings

In industry, the operation of turbomachinery supported by active magnetic bearings (AMBs) requires a robust and simple controller to meet increased performance requirements and stringent regulations. This study proposes an innovative low-order Multiple-Input Multiple-Output (MIMO) controller. Its structure is derived from the decentralized augmented Proportional-Integral-Derivative (PID) controller, enhanced with fixed terms in the skew-diagonals of the controller matrix. The resulting controller couples the information acquired by the AMB sensors on the same control axis to enhance the overall performance. The parameters of the new MIMO structure are tuned using a model-based procedure that exploits a non-smooth optimization algorithm, with the rotor model adjusted on experimental measurements to represent the real dynamics of the system. The novel controller performance is evaluated through two case studies: first, an expander-compressor system; second, a centrifugal turbo compressor designed for oil and gas applications, facing challenges associated with the observability and controllability of the second bending mode. The performance is then compared with that obtained using a decentralized augmented PID controller whose parameters have been tuned with the same non-smooth optimization algorithm. The new controller demonstrates a significant reduction in vibration caused by rotor bending modes. For example, in the analyzed case studies, unbalance responses were reduced by up to a factor of 10. Moreover, the proposed controller increases robustness compared to the decentralized case, as demonstrated by the second case study where controllability and observability issues are addressed.

Detection of Beyond 5G Signal Detection Using Hybrid Method

ABSTRACTNonorthogonal Multiple Access (NOMA) is a promising fifth‐generation (5G) radio system candidate. NOMA's performance is further increased when integrated with Multiple inputs and Multiple Outputs (MIMO). The complexity of detecting NOMA signals also increases the utilization of numerous antennas, which upsurges the system's latency. In this article, we projected a hybrid signal detection algorithm combining zero forcing equalizer (ZFE) and minimum mean square error (MMSE) for the NOMA‐MIMO structure with the Rician and Rayleigh channel. For 16 × 16 and 64 × 64 MIMO‐NOMA, the Bit error rate (BER) is evaluated and compared for ZFE, MMSE, and Maximum likelihood (ML) detection and proposed ZFE‐MMSE algorithms. The simulation outcomes reveal that the projected ZFE‐MMSE detects the signal at low BER, outperforming the contemporary algorithms at complexity similar to MMSE and ZFE.

Enhancing 5G Performance: A Study of an MIMO Antenna with Elliptical Ring Polarization

This paper discusses the design and evaluation of a four-element Multiple-Input, Multiple-Output (MIMO) antenna array with Circular Polarization (CP), tailored explicitly for sub-6 GHz 5G wireless systems. The proposed antenna utilizes an elliptical ring slot with a variable width in the ground plane and an asymmetric feed line layout to achieve orthogonal CP radiation in both the forward and backward directions. Three interconnected strip lines are incorporated into the ground plane for enhanced performance, increasing isolation and ensuring effective linkage between the antenna components and their respective grounds. The antenna exhibits strong isolation, a low Envelope Correlation Coefficient (ECC), and minimal Channel Capacity Loss (CCL), making it ideal for MIMO applications. The analysis confirms the antenna's stable CP gain and operational efficiency throughout the sub-6 GHz band, meeting the rigorous standards of contemporary 5G networks.

Performance Analysis of a Swastika shaped MIMO Antenna for Wireless Communication Applications

A novel wideband 4-port Multiple-Input Multiple-Output (MIMO) circular patch antenna is presented for operation in a resonant frequency band from 1.76 to 9.06 GHz used for PCs, Wireless Local Area Networks (WLANs) and satellite applications. The structure of the design consists of microstrip fed slot antenna considered on the four corners of the FR-4 substrate. To reduce mutual coupling and enhance the parameters of the MIMO antenna, a swastika-shaped ground is embedded. The simulated and experimental results show that the design achieves good performance, isolation >15 dB and radiation efficiency greater than 85%. The radiation patterns, surface currents, gain and channel capacity are also investigated. This circular shaped patch antenna is applicable to WLAN and satellite communication applications. Each circular patch on the four corners is connected to a 50 Ω microstrip feedline. A ring slot is etched on the circular patch and a small rectangular slot is embedded to the etched circular patch. The design provides good radiation patterns and the operating bands are obtained at 1.76 GHz, 2.4 GHz, 3.5 GHz and 9.06 GHz attaining reflection loss of -16 dB, -22 dB, -24 dB, -15 dB. The structure of the proposed design is simulated and evaluated in High-Frequency Structure Simulator (HFSS) software. The MIMO structure is fabricated on the FR-4 substrate. The minimum proximity between simulated and measured results is observed.

Optimal Beamforming for Covert Communication in MIMO Relay Systems

ABSTRACTThis paper investigates the optimal beamforming design for covert communication in a multiple‐input multiple‐output (MIMO) relay system consisting of source, destination, relay and warden. For both scenarios when each transmitting node knows the perfect or partial warden's channel state information (CSI), we first develop theoretical models to depict the inherent relationship between the beamforming matrices at source/relay and end‐to‐end covert performance of the system regard the minimum detection error probability and covert capacity. Based on these models, we then formulate the optimal beamforming for covert capacity maximization under perfect and partial CSI as linear non‐convex (LNC) and non‐linear non‐convex (NLNC) optimization problems, respectively. We then apply Lagrangian and Hadamard inequality methods to solve the LNC optimization problem to determine the closed‐form expressions for the optimal beamforming matrices at source/relay, based on which a stochastic gradient descent algorithm is developed to evaluate the maximum covert capacity. For the complicated NLNC problem, we propose an efficient search algorithm based on S‐procedure lemma and interior point methods to identify the optimal beamforming matrices to reach the maximum covert capacity. Finally, we provide extensive numerical results to illustrate the covert performance enhancement from adopting the optimal beamforming in MIMO relay systems.

ST-TNet: An spatio-temporal joint transformer network for CSI feedback in FDD-MIMO systems

In recent years, deep learning methods have been shown to have strong potential and superiority in reducing channel state information (CSI) feedback overhead and further improving feedback accuracy to maximize the performance benefits of massive Multiple-Input Multiple-Output (MIMO) in frequency division duplex (FDD) mode. As the CSI matrices are transformed into sequences for input to the Transformer model, the rearrangement leads to the loss of the original physical location relationships. Based on this problem, this paper proposes a transformer decoder based on spatio-temporal joint (ST-T). We employ a spatial attention mechanism to compensate for this information loss and focus on key spatial features more accurately, further exploiting the potential of single- and two-layer transformers in reconstructing CSI matrices. The results are validated by simulations based on DCRNet and CLNet encoders, which show that higher performance can be achieved with lower computational load compared to other lightweight models.

A Miniaturized Eight-Port MIMO Antenna for 5G Ultra-Slim Smartphones

This paper presents the design of a miniaturized eight-port multiple-input multiple-output (MIMO) antenna. Each proposed antenna element occupies only 6.2 × 7 × 0.8 mm³, making it ideal for integration into ultra-slim 5G smartphones. The proposed MIMO antenna offers a −10 dB impedance bandwidth of 1.7 GHz (3.3 to 5.0 GHz), covering the N77 (3.3–4.2 GHz), N78 (3.3–3.8 GHz), and N79 (4.8–5.0 GHz) bands. The isolation between the antenna elements is larger than 16 dB. The envelope correlation coefficient (ECC) of the designed antenna is lower than 0.09. Meanwhile, the peak gain exceeds 2.2 dBi, the efficiency surpasses 70%, and the diversity gain (DG) is higher than 9.95. This innovative design addresses the demands of ultra-slim mobile devices while maintaining high performance.

A Hybrid Heuristic‐Aided Algorithm of Serial Cascaded Autoencoder and ALSTM for Channel Estimation in Millimeter‐Wave Massive MIMO Communication System

A Hybrid Heuristic‐Aided Algorithm of Serial Cascaded Autoencoder and ALSTM for Channel Estimation in Millimeter‐Wave Massive MIMO Communication System

Compact wide band quad element MIMO antenna with inbuilt isolator for 5G applications

Abstract A compact quad-element multiple-input multiple-output (MIMO) antenna with self-isolation and pattern diversity features for 5G applications has designed in this article. In the proposed MIMO antenna design, drop-shaped slots have been cut from the ground plane to attain wide bandwidth of 1 GHz in the operating band 3.3-4.3 GHz of near radio (NR-77) band for 5G applications. Further, a criss-cross shaped isolator is formed after integrating four single wideband antenna elements with different orientations in the ground plane. This shape is responsible for high self-isolation of 25 dB in the desired band of 5G. Further, the 2-D radiation pattern of the MIMO antenna has pattern diversity characteristics with a high peak gain of 6 dB in the working band, which validates the proposed work for the 5G MIMO application. The diversity performance of the proposed MIMO antenna has been validated with measured envelope correlation coefficient of 0.0002, diversity gain of 10 and channel capacity loss is below 0.5 bits/sec/Hz across the entire frequency band. Furthermore, simulation studies have been demonstrated for different practical applications such as housing, automotive and on body for practical utility of proposed prototype.

Four-port flexible UWB-MIMO antenna with triple band-notched for wearable IoT applications

Abstract A four-port ultra-wideband (UWB) multiple-input multiple-output (MIMO) antenna with three notch bands is proposed in this work. The antenna uses ultra-thin flexible material liquid crystal polymer (LCP) as the substrate. Four identical monopole radiators are designed in this proposed antenna system. The notch bands of the antenna are generated by adding complementary split ring resonator (CSRR) and ring branch. A cross-shaped stub is set in the center of the four antenna units to enhance the isolation. The measured bandwidth of the antenna is 2.54–10.69 GHz, filtering out three notch frequency bands of 2.81–3.85, 5.11–5.98, and 7.34–8.69 GHz. The isolation in the entire working frequency is better than 22 dB. The bent performances of the MIMO system and the specific absorption rate (SAR) value are analyzed. The low SAR values, low envelope correlation coefficient (<0.05), high diversity gain (>9.999), and stable gain of the proposed antenna indicate that in UWB-MIMO systems and wearable Internet of Things applications, it can be widely used.

Angle and Range Unambiguous Estimation with Nested Frequency Diverse Array MIMO Radars

This paper proposes an unambiguous method for joint angle and range estimation in colocated multiple-input multiple-output (MIMO) radar using the nested frequency diverse array (NFDA). Unlike a conventional phased array (PA), the transmission beampattern of FDA-MIMO radar depends not only on angle but also on range, which enables the precise identification of ambiguous regions in the two-dimensional frequency space. As a result, we can simultaneously estimate the angle and range of targets using FDA-MIMO radar, even when range ambiguity exists. By employing a nested array configuration, the degrees of freedom (DOFs) of the FDA are expanded. This expansion leads to improved accuracy in parameter estimation and enables a greater number of identifiable targets. In addition, the Cramér–Rao lower bound (CRLB) and the algorithm complexity are obtained to facilitate performance analysis. The simulation outcomes are presented to showcase the superior performance of the suggested approach.

Dual-channel near-field holographic MIMO communications based on programmable digital coding metasurface and electromagnetic theory

Holographic multiple-input multiple-output (MIMO) method leverages spatial diversity to enhance the performance of wireless communications and is expected to be a key technology enabling for high-speed data services in the forthcoming sixth generation (6G) networks. However, the antenna array commonly used in the traditional massive MIMO cannot meet the requirements of low cost, low complexity and high spatial resolution simultaneously, especially in higher frequency bands. Hence it is important to achieve a feasible hardware platform to support theoretical study of the holographic MIMO communications. Here, we propose a near-field holographic MIMO communication architecture based on programmable digital coding metasurface (PDCM) and electromagnetic theory. The orthogonal holographic patterns on the transmitting and receiving apertures are firstly obtained using the Hilbert-Schmidt decomposition of the radiation operator. Then the information to be transmitted is pre-encoded on PDCM following the principle of direct digital modulations. A PDCM-based holographic MIMO prototype is designed and experimentally verified in microwave frequencies. The measured results of constellations show that the prototype can realize dual-channel signal transmissions under quadrature-phase shift keying scheme. The proposed paradigm features low complexity, low cost and low power consumption, and may become a valuable technique in beyond fifth generation and 6G wireless communications.

Optimal tracking performance of communication constrained systems with fading channel and quantization constraint

AbstractThis paper studies the optimal tracking performance (OTP) of multiple‐input multiple‐output (MIMO) discrete communication constrained systems (CCSs) with fading channel and quantization constraints. An equivalent average channel is used to replace the fading channel with colored noise, and the gain of the output ports of the two channels is consistent in the first and second moments. Combined with spectral decomposition, co‐prime decomposition, partial fractional decomposition and other methods, the quantitative explicit expression of OTP is eventually derived. The result shows that the OTP is related to nonminimum phase (NMP) zeros and unstable poles and their directions of controlled plant, and is also affected by quantization error and colored noise. At last, a numerical example is utilized to verify the theory.

Wireless power transfer empowered by reconfigurable intelligent surfaces

ABSTRACT Reconfigurable Intelligent Surfaces (RIS) represent a significant advancement in hardware technology, enhancing both wireless energy and spectral efficiency within wireless communication systems. Comprising a programmable metasurface, RIS can adeptly manipulate the wavefronts of incident signals – encompassing parameters such as phase, amplitude, frequency, and polarisation – without necessitating complex signal processing techniques. The integration of RIS into wireless communication networks enables the strategic manipulation of radio waves, thereby mitigating the adverse effects associated with natural wireless propagation. This study examines a multi-user (MU) multiple-input multiple-output (MIMO) wireless power transfer (WPT) system augmented by multiple RISs, wherein the transmitter is equipped with a constant-envelope beamformer. The optimisation of the phase shifts of the RIS and the beamformer of the transmitter is conducted jointly to maximise the total power received by users. An alternating optimisation approach is proposed, utilising the semidefinite relaxation (SDR) method. Additionally, a problem is formulated to maximise the total received power while adhering to constraints on the minimum power received by users, thereby addressing issues of user fairness. To resolve this problem, an iterative algorithm based on the Alternating Direction Method of Multipliers (ADMM) is introduced. Analysis and simulation results indicate a significant enhancement in total received power when employing the proposed methodology compared to existing algorithms documented in the literature. Furthermore, in scenarios involving multiple users, a detailed analysis of the system’s transmission performance, facilitated by RIS, is conducted. The numerical results substantiate the validity of the analysis and demonstrate the efficacy of the proposed algorithms.

Inexpensive SAR testbed for 3D mmWave imaging applications

AbstractIn this paper, the authors investigate and develop a low‐cost synthetic aperture radar (SAR) testbed and utilise it to capture and reconstruct high‐resolution 3D mmWave SAR images. Despite much attention from both industrial and academic scholars, cost restraints and complexity have hindered the deployment of SAR systems, particularly in the mmWave domain. This paper outlines the major components and systems of the 3D SAR testbed. The authors build a testbed with low‐cost, commercially available hardware components and include open‐source software that is highly reconfigurable and simple to reproduce. The multiple‐input multiple‐output (MIMO) radar sensor uses a frequency‐modulated continuous wave (FMCW) chirp at the V‐band (40–75 GHz) and synthesises a rectilinear two‐dimensional planar aperture. The authors presented several reconstructed 3D images imitating real‐world scenarios using the testbed. A non‐linear sampling technique that has significantly decreased the amount of time required for data acquisition was implemented. In addition, the authors employed multiple image quality metrics to quantify and compare the images produced by the testbed. The testbed can be used to test and validate new techniques and algorithms.

An Improved Power Allocation Scheme for Downlink NOMA‐Based MIMO Visible Light Communication Systems

ABSTRACTNon‐orthogonal multiple access (NOMA), as a multiple access scheme, is foreseen as an emerging technology in enhancing the achievable sum rate of indoor downlink multiple‐input multiple‐output (MIMO) visible light communication (VLC) networks. Also, power allocation scheme plays a vital role in achieving these enhanced achievable sum rates. In literature, there are multiple power allocation schemes available such as gain ratio power allocation (GRPA), normalized gain difference power allocation (NGDPA), normalized logarithmic gain ratio power allocation (NLGRPA), and so forth. In this paper, an improved power allocation scheme has been proposed, which is based on the ratio of logarithmic sum channel gains of users as well as on the user index in the decoding order. In the proposed power allocation scheme, multiple transmitting light emitting diodes (LEDs) and multiple users are considered along with their optical channel information required for data communication. The transmitting LEDs are fixed on the room ceiling at optimal positions. The optimal positions of LEDs are selected by using particle swarm optimization (PSO) algorithm. The simulation results depict that the proposed power allocation scheme with optimal LED positions can achieve a performance gain of 5% or higher in the achievable sum rate over NLGRPA scheme when the number of users present in the room is more than five, and even when the user location is far from the LED. Thus, the proposed power allocation scheme with optimal LED positions achieves significant performance improvement when compared with existing power allocation schemes for NOMA‐based MIMO‐VLC networks for randomly located users in indoor scenario.

Improvement of Internet of Things (IoT) Interference Based on Pre-Coding Techniques over 5G Networks

The advent of 5G technology has revolutionized wireless communication, offering unprecedented data rates, reduced latency, and enhanced connectivity. A critical component driving these advancements is Multiple-Input Multiple-Output (MIMO) technology. MIMO utilizes multiple antennas at both the transmitter and receiver ends to improve communication performance. In the context of the Internet of Things (IoT), MIMO plays a pivotal role in enhancing network efficiency, reliability, and capacity and can improve system capacity and reduce interference between different users. By leveraging MIMO, IoT devices can achieve higher data throughput and better signal quality, even in challenging environments. This is particularly important for IoT applications that require real-time data transmission and low latency, such as smart cities, autonomous vehicles, and industrial automation. Additionally, MIMO technology helps in mitigating interference and improving spectrum utilization, making it an essential enabler for the massive connectivity demands of IoT networks in the 5G era. However, due to the high channel dimension, complex channel estimation and precoding algorithms in the system, the system hardware and software overhead will increase. The precoding algorithms of massive MIMO systems are divided into three types: digital, analog and hybrid. The three types of precoding algorithms are summarized and compared, and the advantages and disadvantages of different precoding algorithms and applicable scenarios are summarized. The channel estimation schemes are divided into training estimation and blind estimation, and the advantages and disadvantages of the two types of schemes are summarized. It is pointed out that the reasonable use of the channel sparsity of massive MIMO can improve the quality of channel estimation and reduce the estimation overhead.