Search Results (270)

The rapid expansion of smart city infrastructures and Internet of Things (IoT) networks has led to extremely dense wireless deployments, driving unsustainable energy consumption and exacerbating environmental concerns. To improve sustainability in the long term, future wireless systems must fundamentally prioritize energy-efficient and autonomous operation. Integrated sensing and communication (ISAC) is emerging as a key enabler for next-generation systems by jointly supporting sensing and communication through shared spectrum, hardware, and signal processing resources. In IoT systems, sensing of target parameters, e.g., range, angle, velocity and identity, etc., form the basis of autonomous and environment-aware applications. However, this integration increases overall power consumption due to the added coordination overhead and the workload placed on shared hardware components. To this end, backscatter communication provides a low-power alternative that enables passive data transmission through energy harvesting and sharply reduces the need for active radio circuits. However, the coexistence of sensing and backscatter functions introduces mutual interference, which often requires large multiple-input multiple-output (MIMO) arrays for effective mitigation. Furthermore, sensing performance depends heavily on line-of-sight conditions, while backscatter links operate only over short ranges. Although increasing array size or transmit power can extend coverage, it imposes substantial energy and hardware costs and undermines sustainability goals. To address these limitations, cell-free MIMO is emerging as a promising candidate technology for next-generation systems. Cell-free MIMO relies on a dense deployment of distributed access points that cooperate to serve devices across a wide area. This cooperation enables effective beamforming and interference management, providing spatial diversity comparable to large, centralized antenna arrays without incurring their associated hardware or power costs. They also enable aggregation of weak double-hop reflections, reduced effective-illumination distances, multi-view sensing, and robustness to blockage, which is invaluable to backscatter communication. This perspective article introduces the foundations, challenges, and architectural considerations of cell-free backscatter-aided integrated sensing and communication (CF-BISAC) systems. By leveraging the advantages of battery-less backscatter IoT devices and the distributed nature of cell-free MIMO, CF-ISABC aims to maximize sensing and communication performance under strict energy constraints, contributing toward energy-aware ISAC systems capable of supporting high-density, low-power wireless applications. Full article

This paper presents a new beamforming algorithm for Multi-User Multiple-Input Multiple-Output (MU-MIMO) systems using Multi-Agent Reinforcement Learning (MARL). The proposed approach is shown to significantly enhance the efficiency and performance of future wireless communication systems. The system comprises two base stations, each equipped with a Uniform Rectangular Array (URA) of directional antennas. Each base station has RL algorithms that use beamforming to provide the optimal Signal-to-Interference-Plus-Noise Ratio (SINR) for each user. These algorithms also work with the other base stations to prevent user interference and ensure efficient resource use. Simulation results demonstrate that the potential of the proposed method has the potential for dynamically adapting beam patterns and maintaining high SINR across the network, resulting in more than a 2-fold improvement in throughput and a 5453% improvement in SINR. Full article

Non-contact monitoring of human vital signs using microwave radar has attracted increasing attention due to its capability to operate unobtrusively and through clothing or light obstacles. In vector network analyzer (VNA)-based radar systems, vital signs can be extracted from phase variations in the forward transmission coefficient $S_{21}$, whose sensitivity strongly depends on the electromagnetic performance of the antenna system. This work presents the design, optimization, fabrication, and experimental validation of a high-gain 12 GHz 4 × 4 microstrip patch antenna array specifically developed for phase-based vital signs monitoring. The antenna array was progressively optimized through coaxial feeding, slot-based impedance control, stepped transmission line matching, and mitered bends, achieving a simulated gain of 17.8 dBi, a measured gain of 17.06 dBi, a reflection coefficient of −26 dB at 12 GHz, and a total efficiency close to 74%. The antenna performance was experimentally validated in an anechoic chamber and subsequently integrated into a continuous-wave VNA-based radar system. Comparative measurements were conducted against a commercial biconical antenna, a single patch radiator, and an MIMO antenna under identical conditions. Results demonstrate that while respiration can be detected with moderate-gain antennas, reliable heartbeat detection requires high-gain, narrow-beam antennas to enhance phase sensitivity and suppress environmental clutter. The proposed array significantly improves pulse detectability in the (1–1.5) Hz band without relying on advanced signal processing. These findings highlight the critical role of antenna design in $S_{21}$-based biomedical radar systems and provide practical design guidelines for high-sensitivity non-contact vital signs monitoring. Full article

This paper presents a dual four-port circularly polarized (CP) MIMO antenna based on substrate integrated waveguide (SIW) technology for sub-6 GHz applications. The design consists of two identical four-port SIW-based CP-MIMO antennas arranged in a mirror-symmetric configuration with an air gap of 15 mm. Each antenna employs four symmetrically arranged cross-shaped SIW patches excited by coaxial probes. Bidirectional radiation is achieved by applying a 180° phase difference between corresponding ports of the mirror symmetric configuration, referred to as the Backward-Radiating Unit (BRU) and the Forward-Radiating Unit (FRU). The bidirectional radiation mechanism is supported by array-factor-based theoretical modelling, which explains the constructive and destructive interference under phase-controlled excitation. To ensure high isolation and stable polarization performance, the antenna design incorporates defected ground structures, inter-element decoupling strips, and vertical metallic vias. Simulations indicate an operating band from 5.1 to 5.4 GHz. Measurements show a −10 dB bandwidth from 5.25 to 5.55 GHz, with the frequency shift attributed to fabrication tolerances and measurement uncertainties. The antenna achieves inter-port isolation better than −15 dB. A 3 dB axial-ratio bandwidth is maintained across the operating band. Measured axial-ratio values remain below 3 dB from 5.25 to 5.55 GHz, while simulations predict a corresponding range from 5.1 to 5.4 GHz. The proposed configuration achieves a peak gain exceeding 4 dBi and maintains an envelope correlation coefficient below 0.05. These results confirm its suitability for CP-MIMO systems with controlled spatial coverage. With a physical size of 0.733λ₀ × 0.733λ₀ per array, the proposed antenna is well-suited for vehicular and space-constrained wireless systems requiring bidirectional CP-MIMO coverage. Full article

Wireless Power Transfer (WPT) has become a pivotal technology, enabling the battery-free operation of Internet of Things (IoT) and biomedical devices while supporting environmental sustainability. This review provides a comprehensive analysis of microstrip patch antennas (MPAs) operating at the 5.8 GHz Industrial, Scientific, and Medical (ISM) band, emphasizing their advantages over the more commonly used 2.4 GHz band. A detailed and systematic classification framework for MPA architectures is introduced, covering single-element, multi-band, ultra-wideband, array, MIMO, wearable, and rectenna systems. The review examines advanced optimization methodologies, including Defected Ground Structures (DGS), Electromagnetic Bandgap (EBG) structures, Metamaterials (MTM), Machine Learning (ML), and nanomaterials, each contributing to improvements in gain, bandwidth, efficiency, and device miniaturization. Unlike previous surveys, this work offers a performance-benchmarked classification specifically for 5.8 GHz MPAs and provides a quantitative assessment of key trade-offs, such as efficiency versus substrate cost. The review also advocates for a shift toward Power Conversion Efficiency (PCE)-centric co-design strategies. The analysis identifies critical research gaps, particularly the ongoing disparity between simulated and experimental performance. The review concludes by recommending multi-objective optimization, integrated antenna-rectifier co-design to maximize PCE, and the use of advanced materials and computational intelligence to advance next-generation, high-efficiency 5.8 GHz WPT systems. Full article

An avionic weather radar antenna should be able to operate in multiple modes to cope with the change in resolution and elevation coverage as an aircraft approaches a storm cell that could expand 10 km in elevation. To solve this problem, we propose the addition of four auxiliary antenna (AuxAnt) arrays based on the phased-MIMO antenna structure to the existing avionic weather radar for future field data collection missions. Two types of signals are employed: the Type I signal transmitted by AuxAnt 1 and 2 is designed based on a non-overlapping subarray configuration, with Subarray 1 and 2 dedicated to the transmission of long and short pulses, respectively, so that the near-range blind zone is mitigated. Leveraging the waveform design and beamforming flexibility provided by the phased-MIMO antenna, pulse compressions based on frequency modulation and phase-coding are employed for wide and narrow main beams, respectively. To suppress the range sidelobes, adaptive pulse compression is used at the receiver end in substitute of the conventional matched filter. In contrast, the Type II signal transmitted by AuxAnt 3 and 4 is designed based on the contextual information so that the transmitted beampatterns have specific sidelobe levels at certain directions for interference suppression. The advantages of the proposed signaling strategy are verified with a series of ingeniously devised experiments based on real weather data. Full article

This paper presents a compact magneto-electric dipole (MED) antenna optimized for wideband circularly polarized (CP) radiation for 5G applications. It incorporates a staircase-shaped electric dipole with trimmed corners to excite orthogonal modes for enhanced CP performance. The proposed single-layer MED antenna achieves a $20.6\%$ wide-impedance bandwidth ($|S_{11}| <-10$ dB, $22.97$–$28.12$ GHz) and $21.9\%$ CP bandwidth ($AR<3$ dB, $22.23$–$27.83$ GHz) with a compact footprint of $15\times 15\times 1.6{\mathrm{mm}}^3$. There is a symmetrical radiation pattern with a co-to-cross polarization ratio $>23$ dB and a stable gain of $8.8$ dBi. An equivalent circuit model is optimized via particle swarm optimization (PSO). The optimized MED antenna is utilized to investigate various CP-MIMO configurations and wideband sequential arrays. Next, a $1\times 2$ CP-MIMO antenna system is developed, employing polarization diversity in parallel and mirror configurations. Isolation is improved by etching a ground slot between the MED elements, yielding isolation levels of below $-20$ dB and $-23$ dB, respectively. Further, a $2\times 2$ CP-MIMO configuration is designed and evaluated. This arrangement demonstrates an envelope correlation coefficient (ECC) of $1\times {10}^{-3}$ and a diversity gain of approximately 10 dB across the operating bandwidth. Finally, a sequential array is designed that applies a ${90}^{\circ }$ sequential rotation and phase excitation to MED elements for high-gain CP 5G communications. Here, various array sizes are evaluated, with an $8\times 8$ MED array providing CP radiation ($AR\le 1$ dB) from 20 to 30 GHz with enhanced impedance and axial ratio bandwidths and stable gain with a peak value of $27.47$ dBi. Full article

Unmanned aerial vehicles (UAVs) equipped with antenna arrays can deliver high-capacity, high-throughput, and low-latency communication services. Considering a UAV-assisted mmWave multi-input and multi-output (MIMO) system, a two-stage beamforming scheme based on a budgeted combinatorial multi-armed bandit (BC-MAB) is proposed to improve the system’s spectral efficiency (SE). The pre-beamformer design problem is initially formulated as a BC-MAB problem. In this framework, the reward is the received energy, while the cost corresponds to the energy consumed by each RF chain and the budget is represented by the residual energy of the UAV. To achieve a favorable trade-off between the number of communication slots and the energy acquired per slot, a pre-beamforming scheme based on the bang-per-buck ratio is introduced to optimize the number of activated RF chains, therefore maximizing the cumulative reward. The second stage utilizes the reduced-dimensional instantaneous channel state information to design and optimize the beamformer to achieve maximum system SE. The proposed scheme achieves more than 7.1% improvement in SE compared to the benchmark schemes. Simulations validate the superiority of the proposed scheme. Full article

The compact and wideband patch antennas applied to WiFi 7 multiple-input multiple-output (MIMO) antenna systems are presented. The MIMO antenna structure consists of four multi-branch radiating patches fed by coupled microstrip lines, which occupies a size of $32\times 32\times 1 {\mathrm{m}\mathrm{m}}^3$. By exploiting multiple resonant modes, an impedance bandwidth of 37% (5.07–7.37 GHz) achieves a reflection coefficient of less than −10 dB and fully encompasses both WiFi 7 high-frequency ranges. To alleviate mutual coupling, two decoupling structures, named complementary split-ring resonators (CSRRs), are employed between the MIMO elements to interact with the undesirable surface current; furthermore, the proposed orthogonal placement of four elements further minimizes radiation coupling. Consequently, the proposed array achieves measured isolations greater than 14.5 dB and 11 dB at 5 GHz and 6 GHz bands, respectively. The prototype of the proposed MIMO antenna has been manufactured. It has also been measured and the results show similarity with the simulations. The measured radiation pattern and the diversity performance, including the envelope correlation coefficient (ECC), diversity gain (DG), and multiplexing efficiency, are calculated, and they verify the outstanding diversity characteristics of the proposed MIMO antenna. This makes it a promising solution for emerging WiFi 7 wideband applications. Full article

This work presents the design and experimental validation of a high-performance 6-port MIMO antenna array designed for radar applications in the 24 GHz Industrial, Scientific, and Medical (ISM) band. The proposed design, configured as a 1 × 10 series-fed microstrip patch array on an RT/Duroid 5880 substrate, is engineered to meet the demanding requirements of automotive and industrial radar systems, where high resolution and target discrimination are critical. A key challenge in such dense MIMO arrays is mutual coupling, which was addressed by integrating novel metamaterial structures between the radiating elements. These structures effectively suppress surface waves, resulting in exceptional inter-port isolation exceeding 46 dB at 24.3 GHz. The antenna achieves a peak gain of 17.4 dBi, ensuring sufficient range for sensing applications. Furthermore, the radiation pattern exhibits a simulated low side lobe level (SLL) of −24.6 dB in the E-plane, and −15.8 dB in the H-plane, a critical parameter for minimizing false detections and enhancing accuracy in cluttered environments. With an operational bandwidth of 0.71 GHz, the proposed design demonstrates comprehensive performance metrics—high gain, outstanding isolation, and low SLL—that surpass many existing MIMO solutions. The results confirm the antenna’s strong potential for advanced MIMO radar systems operating in the 24 GHz-ISM band, paving the way for reliable high-resolution sensing. Full article

Massive Multiple-Input Multiple-Output (MIMO) systems with hundreds or thousands of antenna elements are fundamental to next-generation wireless networks, promising unprecedented spectral efficiency through spatial multiplexing and beamforming. However, the computational burden of channel state information (CSI) acquisition and processing scales dramatically with array size, creating a critical bottleneck for practical deployments. While previous works demonstrated that Fast Fourier Transform (FFT)-based beamspace processing can exploit the inherent angular sparsity of wireless channels to compress CSI feedback, the digital implementation requires intensive computations that become prohibitive for ultra-large arrays. This paper presents an analog alternative using Butler matrices—passive beamforming networks that realize the Discrete Fourier Transform in hardware—combined with RF switching circuits to select only dominant angular components. We provide a comprehensive analysis of Butler matrix architectures for arrays up to 32 × 32 elements, characterizing insertion losses across different technologies (microstrip, substrate-integrated waveguide, and waveguide) and operating frequencies (10–30 GHz). The proposed system incorporates parallel power sensing with Winner-Take-All circuits for sub-microsecond beam selection, drastically reducing the number of active RF chains. Full-wave simulations and capacity evaluations at 12 and 30 GHz demonstrate that the Butler-based approach achieves comparable performance to FFT methods while offering significant advantages in power consumption and processing latency. For a 256 × 256 array, FFT computation requires 0.36 ms compared to near-instantaneous analog processing, making Butler matrices particularly attractive for real-time massive MIMO systems. These findings establish Butler matrix front-ends as a practical pathway toward scalable, energy-efficient beamspace processing in 6G networks. Full article

To mitigate potential interference in a coexisting system, an ultra-wideband (UWB) antenna with triple-band-notched characteristics is proposed. Based on transmission line theory, three notched bands are achieved by utilizing the open- or short-circuited properties of microstrip line resonators and slot resonators. Each antenna element consists of a patch etched with three half-wavelength slots and a one-wavelength strip. Measurement results demonstrate that the antenna exhibits excellent rejection performance at the three designated frequency bands. Furthermore, the effects of array configuration and element deflection angle on mutual coupling are investigated using a 2 × 1 face-to-face multiple-in, multiple-out (MIMO) array. Finally, a two-element MIMO array with high isolation was fabricated and measured. Experimental results show that an isolation level better than 24.6 dB is maintained across the operating band. Full article

In this paper, the extrapolation of a 2D multiple-input multiple-output (MIMO) array is proposed using the Burg algorithm to achieve higher angular resolution beyond that of the corresponding 2D MIMO virtual array. The main advantage of such an approach is that it allows us to dramatically decrease both the physical size and the number of antenna elements of the MIMO array. The performance and limitations of the Burg algorithm are examined through both simulation and experimentation at 77 GHz. The experimental methodology used to acquire 3D data of range, azimuth and elevation information with the 1D MIMO off-the-shelf radar is described. Using this method, the performance of the proposed array can be tested experimentally, especially at frequencies where it is desired to assess the antenna response prior to fabricating the antenna. Full article

This article presents a novel design method for realizing wideband operation and excellent isolation in fifth-generation (5G) multiple-input multiple-output (MIMO) systems. The proposed MIMO antenna employs a ground-plane slot in the shape of the Chinese character “工” and maintains an unbroken metal frame, thereby avoiding slot openings on the rim. The theory of characteristic modes (TCM) is applied to determine appropriate feeding structures and locations for two functional antenna modules. This design achieves wide bandwidth and high isolation without requiring additional decoupling structures, simplifying the overall system. Two prototype arrays, consisting of four and eight antenna elements, were implemented for 5G operation in the 3.4–5.0 GHz band. The measured results confirm isolation levels above 21.6 dB and 16.2 dB for the four- and eight-element arrays, respectively, with envelope correlation coefficients (ECCs) below 0.16. These results indicate that the proposed design is a promising solution for integration into 5G smartphones. Full article

This paper presents the design and implementation of an ultra-wideband MIMO antenna for sub-6 GHz 5G metal-frame smartphones. The proposed antenna array includes four pairs, each comprising a slotted patch element and an open-slot structure on the metallic rim. The design achieves compactness by sharing the same aperture, critical for overcoming metal-frame smartphone constraints. It minimizes the required ground clearance to 40 × 0.7 mm² to fit the limited space of metallic bezels while maintaining high inter-element isolation. Specifically, one element operates at 2.5–3.8 GHz and 4.8–7.0 GHz, while the other provides continuous coverage from 2.5 to 6.5 GHz, supporting all global sub-6 GHz 5G frequency bands. Specifically, one element operates at 2.5–3.8 GHz and 4.8–7.0 GHz, while the other offers continuous coverage from 2.5 to 6.5 GHz, supporting all sub-6 GHz 5G frequency bands. The open-slot configuration enlarges the operational bandwidth and improves isolation, achieving more than 12.6 dB isolation between elements. A prototype was fabricated and experimentally tested. Measured results indicate that the antenna array maintains a total efficiency above 56% and an envelope correlation coefficient below 0.18 across the target bands. The measured and simulated results are in good agreement, confirming the effectiveness of the proposed design. The proposed antenna is a strong candidate for next-generation 5G smartphone applications due to its wideband performance, high isolation, and compact integration. Full article