novel wideband antenna with five reconfigurable polarization states is presented for intelligent IoT applications, where polarization agility can be exploited by a higher-layer controller to enhance link reliability in dynamic multipath environments. To achieve wideband performance across all polarization states, coupling mechanisms are strategically introduced into the radiation structure, feeding structure, and feeding network simultaneously. Firstly, through equivalent circuit analysis, shorting pins, metal strips, and slots are integrated into the radiation structure. The coupling mechanisms excite first- and second-order frequency responses, resulting in significant gain improvement and overlapping bandwidth enhancement. Secondly, a co-planar capacitive feeding structure is implemented into the feeding network to improve impedance matching. Subsequently, weakly coupled transmission lines are deployed within both differential and full-polarization feeding networks. Such a configuration ensures wide overlapping bandwidth performance throughout all five polarization states by suppressing phase shift errors. Finally, by controlling the ON/OFF states of PIN diodes, the feeding network generates desired phase differences, enabling the proposed antenna to be reconfigured between five polarization states, i.e., linearly polarization (LP) with three orientations, left-hand circularly polarization (LHCP), and right-hand circularly polarization (RHCP). Experimental results demonstrate that the antenna achieves an operating bandwidth of 8–13 GHz (47.6%), with axial ratio (AR) < 3.0 dB and in-band gain fluctuation < 3.0 dB. Characterized by wide bandwidth, full-polarization agility, and standard fabrication, the design presents a viable solution for demanding multi-polarization applications in industrial Internet of Things (IoT) scenarios.

Rotatable antennas (RAs) expand the spatial degrees of freedom of multiple-input multiple-output (MIMO) systems beyond conventional fixed-panel arrays. However, existing evaluations often model rotation merely as a scalar gain adjustment and overlook three hardware-critical effects: 1) the phase carried by the antenna pattern, 2) rotation-induced changes in the polarization, and 3) orientation-dependent mutual coupling variations. In this context, we develop a unified modeling-evaluation-optimization framework that explicitly incorporates phase, polarization, and mutual coupling effects through a rotation-explicit complex vector element pattern (CVEP). Polarization coupling and mismatch are captured by embedding the rotated CVEPs into a$2\times 2$per-ray polarization structure, while mutual coupling variation is modeled via power-consistent interpolation of embedded radiation patterns under joint orientation states. Additionally, a compact RA base-station (RA-BS) element and a$6\times 3$RA-BS prototype supporting ±60° mechanical rotation with a high front-to-back ratio are designed, assembled, and measured. The simulation and measurement results validate that the proposed electromagnetic (EM)-consistent RA system model accurately characterizes rotation-induced variations in gain, polarization, and radiation patterns. Furthermore, an alternating feasible-direction optimization algorithm, tailored to the jointly programmable BS- and subarray-level rotations, is developed to maximize the downlink sum-rate in multi-user MIMO deployments. Our simulations demonstrate that the proposed rotation-aware RA-BS architecture significantly outperforms conventional gain-only and mutual-coupling-neglected baselines. These findings highlight the necessity of incorporating phase, polarization, and mutual coupling effects into RA modeling and optimization to ensure robust performance under uniform user distributions and maximize achievable throughput in hotspot deployments.

This paper investigates optimal antenna array configurations for multiple-input-multiple-output (MIMO) systems. The array configuration, particularly the element spacing, significantly affects the correlation between elements, which can lead to channel rank deficiency or deterioration in condition number. In practical sparse channels, deploying a massive number of antenna elements can become redundant. Therefore, exploring optimal array configurations is crucial for enhancing MIMO communication system. Although substantial theoretical analyses have been conducted for antenna configurations in line-of-sight (LOS) environments, analyses in non-line-of-sight (NLOS) environments remain limited. Through theoretical analysis, this paper demonstrates that in NLOS environments, the optimal condition number of MIMO channels is achieved when the antenna array configuration renders the antenna array matrix orthogonal, with the optimal condition number being determined by the wireless propagation environment. Building upon this finding, we show that there always exists an antenna spacing that makes the antenna array matrix orthogonal in a two-cluster environment, and we provide a closed-form solution for this spacing. For three-cluster environments, we derive the conditions that cluster angles must satisfy to achieve antenna array matrix orthogonality and provide a closed-form solution for optimal antenna spacing under these conditions. Furthermore, we propose a systematic solution based on Newton descent algorithm to calculate optimal antenna spacing, aimed at reducing analytical complexity in environments with more clusters. Finally, numerical simulations validate the theoretical analysis and provide recommendations for optimizing antenna placement in IoT devices operating in sparse channel environments.

Holographic multiple-input and multiple-output (MIMO) communications introduce innovative antenna array configurations, such as dense and volumetric arrays, which offer notable advantages over conventional planar arrays with half-wavelength element spacing. However, accurately assessing the performance of these new holographic MIMO systems necessitates careful consideration of channel matrix normalization, as it is influenced by array gain, which, in turn, depends on the array topology. Traditional normalization methods may be insufficient for assessing these advanced array topologies, potentially resulting in misleading or inaccurate evaluations. In this study, we propose electromagnetic normalization approaches for the channel matrix that accommodate arbitrary array topologies, drawing on the array gains from analytical, physical, and full-wave methods. Additionally, we introduce a normalization method for near-field MIMO channels based on a rigorous dyadic Green’s function approach, which accounts for potential losses of gain at near field. Finally, we perform capacity analyses under quasi-static, ergodic, and near-field conditions, through adopting the proposed normalization techniques. Our findings indicate that channel matrix normalization should reflect the realized gains of the antenna array along target directions. Failing to accurately normalize the channel matrix can result in errors when evaluating the performance limits and benefits of unconventional holographic array topologies, potentially compromising the optimal design of holographic MIMO systems.

It is well known that there is inherent radiation pattern distortion for the commercial base station antenna array, which usually needs three antenna sectors to cover all space. To eliminate pattern distortion and further enhance beamforming performance, we propose an electromagnetic hybrid beamforming (EHB) algorithm based on 3D superdirective holographic antenna arrays. Specifically, EHB consists of antenna excitation current vectors (analog beamforming) and digital precoding matrices, where the implementation of analog beamforming involves real-time adjustments to the radiation pattern to adapt to the wireless environment. Meanwhile, the digital beamforming is optimized based on the channel characteristics of analog beam-forming to further improve the achievable rate of communication systems. An electromagnetic channel model incorporating array radiation pattern and coupling effect is also developed to evaluate the benefits of our proposed scheme. Simulation results show that the proposed scheme achieves a sum rate gain of over 150 % compared to traditional beamforming algorithms.

In this article, a quasi-isotropic antenna with system ground plane is proposed for Internet of Things (IoT)-based smart home applications. The proposed antenna consists of a system ground plane and a modified$\lambda $/4 U-shaped radiator with vertical branch passing through the ground plane but not contacting it. The antenna operates in balanced mode and does not excite the system ground plane mode (i.e., similar to the chassis mode in the terminal antenna), which is essential to obtain quasi-isotropic radiation pattern. The dimensions of the designed antenna are$0.13\,\,\lambda \times 0.14\,\,\lambda \times 0.12\,\,\lambda $, which does not take into account the size of the system ground plane. The detailed analysis and evolution process of the designed quasi-isotropic antenna are presented. In particular, the principle that a modified$\lambda $/4 U-shaped radiator without a system ground plane can produce a quasi-isotropic radiation pattern is first stated; then the principle that the modified$\lambda $/4 U-shaped radiator can produce a quasi-isotropic radiation pattern when the system ground plane is present is stated, and at the same time, the method for impedance matching and bandwidth improvement of the antenna is explained; and finally, a parametric analysis is performed to determine the influence of important antenna structure parameters on antenna performance. To demonstrate the performance of the proposed antenna, the antenna prototype was fabricated and measured. The simulated (measured) bandwidth (BW), gain variation (GV), realized gain, and efficiency at 2.45 GHz are 110 MHz (120 MHz), 6.3 dB (7.7 dB), 1.6 dBi (1.36 dBi), and 98% (82%), respectively. Compared with the state-of-the-art quasi-isotropic antennas, the proposed antenna can still generate a quasi-isotropic radiation pattern with a small GV in the presence of a system ground plane, making it more suitable for practical engineering application scenarios, which is a promising antenna candidate for routers in IoT-based smart homes.

It is well known that there is inherent radiation pattern distortion for the commercial base station antenna array, which usually needs three antenna sectors to cover the whole space. To eliminate pattern distortion and further enhance beamforming performance, we propose an electromagnetic hybrid beamforming (EHB) scheme based on a three-dimensional (3D) superdirective holographic antenna array. Specifically, EHB consists of antenna excitation current vectors (analog beamforming) and digital precoding matrices, where the implementation of analog beamforming involves the real-time adjustment of the radiation pattern to adapt it to the dynamic wireless environment. Meanwhile, the digital beamforming is optimized based on the channel characteristics of analog beamforming to further improve the achievable rate of communication systems. An electromagnetic channel model incorporating array radiation patterns and the mutual coupling effect is also developed to evaluate the benefits of our proposed scheme. Simulation results demonstrate that our proposed EHB scheme with a 3D holographic array achieves a relatively flat superdirective beamforming gain and allows for programmable focusing directions throughout the entire spatial domain. Furthermore, they also verify that the proposed scheme achieves a sum rate gain of over 150% compared to traditional beamforming algorithms.

Multibeam antenna (MBA) radiations provide linkages to multiple nodes and can bring enhanced communication quality with reduced power consumption for Internet of Things (IoT) applications. In this article, a novel method providing a low-cost and flexible MBA solution is proposed. Multiple orbital angular momentum (OAM) modes are used as internal excitations of the antenna, and the generated multiple beams are pencil beams with no OAM mode, which is convenient for IoT nodes connection. First, aperture phase distributions that convert incident vortex beams with different and orthogonal topological orders into designated directional pencil beams are acquired and superimposed to form a metasurface aperture that, under excitations of vortex beams with multiple orders, produces multibeam each with a predefined direction. The proposed multibeam method is then illustrated with a transmissive metasurface design, with four beams pointing at arbitrary half-space directions. Techniques, such as OAM modes spacing and aperture partitioning are utilized for reducing sidelobe levels (SLLs). Furthermore, a similar multibeam metasurface antenna with the additional linear-to-circular polarization conversion function is demonstrated, which opens the possibility of utilizing different polarizations on top of the OAM-based multibeam. This article successfully explores and achieves a novel antenna system with multiple radiations suited for multiple IoT nodes accessing with low cost and low energy consumption, and also provides a new degree-of-freedom for OAM applications.

Nowadays, there is no good scheme for simultaneously achieving super-resolution imaging within a coherent beamwidth and beam scanning in microwave radar. In this paper, a synthetic beam scanning method based on a randomly excited antenna array, which possesses the ability of super-resolution coincidence imaging, is proposed. The beamwidth of the randomly excited array is comparable to that of the conventional phased array with the same size, and its steering angle can be adjusted by modulating the excitation signals. Firstly, the optimized covariance matrix of the proposed array to uniformly bunch the spatial radiation energy in a specific angle range is obtained by means of the sequential quadratic programming (SQP) algorithm. Accordingly, the partially correlated excitation signals restricted by the optimized covariance matrix are determined. Then, a secondary weighted modulation method is proposed to steer the main beam to the direction of interest. The excitation signals are weighted by the conjugated radiation fields in the direction of interest. Furthermore, the super-resolution ability of the proposed coincidence imaging system is analyzed based on the first-order statistical characteristics of the radiation fields. Simulations and experiments demonstrate that synthetic beam scanning can be realized by controlling the excitation signals. Besides, a 10 times super-resolution coincidence image can be reconstructed within a coherent beamwidth (3dB-beamwidth) when the signal-to-noise (SNR) is greater than 15 dB, assisted by the fast Bayesian learning (FBL) algorithm.

The resolution of the microwave radar coincidence imaging (MRCI) can break the diffraction limit, which has been validated by experiments. However, there is still no theoretical analysis. In this article, the resolution of the MRCI is theoretically analyzed using the orthogonal subspace projection algorithm based on the space spanned by the discrete reference radiation mode. First, a target location estimate (TLE) method using the data from the nonfocusing radar array of the MRCI system is proposed to estimate the target position assisted by the equivalent detection method. The estimation precision of the TLE method is approximately equal to the 3-dB beamwidth of the coherent transmitting radar array with the same aperture; hence, the imaging plane can be obtained. Then, the equivalent internal noise (generated by the error of the target distance estimation, i.e., the location of the imaging plane) is theoretically analyzed. Finally, the imaging resolution threshold of the MRCI system is theoretically analyzed. The relationship between the resolution threshold and the factors (such as the deployment of the transmitting radar array, the distance between the target and the radar array, and the signal-to-noise ratio of the imaging system) is summarized. The proposed estimation method and the analyses of the MRCI system are validated through a set of simulations and experiments.

The mutual coupling between antenna elements significantly deteriorates the performance of antenna arrays. Many decoupling techniques have been developed over the years. A key step of decoupling processing is to obtain the decoupling matrix (or mutual impedance matrix). However, the accurate decoupling matrix is unavailable in practice because mutual coupling depends not only on the geometry of the array, but also on the excitations of the elements. Unfortunately the decoupling errors can also lead to severe performance degradation of antenna arrays, especially for the adaptive beamformers. In this paper we propose a decoupling error reduction method based on the reduced- rank multistage Wiener filter (MWF). The reduced-rank MWF projects the weight vector of beamformer onto the Krylov subspace, which has a regularization effect that can reduce the performance degradation caused by random errors in the array. Besides having a simple implementation structure, the proposed method is computationally efficient. Moreover, simulation results show that the proposed method considerably improves the signal-to-interference-plus-noise ratio (SINR) of the beamformer when there exist decoupling errors in the array.

The sectored multi-probe anechoic chamber (MPAC) is regarded as a promising millimeter-wave (mmWave) over-the-air (OTA) testing solution. However, there are still debates in the literature about which channel emulation method, i.e., pre-faded synthesis (PFS) or plane wave synthesis (PWS), should be used for the sectored MPAC testing system. In this paper, a thorough comparison between the two channel emulation methods is conducted. It is found that for sectored MPAC (where the active probes are unlikely to be uniformly distributed over the probe panel) the PFS is more accurate than the PWS. Explanations are given.

The effects of phase noises (PNs), power imbalances, and correlations on multiuser orthogonal frequency division multiplexing (OFDM) multiple‐input multiple‐output (MIMO) systems are studied. It is assumed that each user is equipped with a single antenna, whereas the base station (BS) has multiple antennas and use zero‐forcing (ZF) decoder for multiuser detection. Since each user has an independent oscillator, the received uplink (UL) signal at each BS antenna is corrupted by all of these independent PNs. Furthermore, there may be power imbalances and correlations (due to common scatterers) between different users. These impairments are jointly analyzed in this work. A closed‐form expression of the mean square error (MSE) performance of the multiuser MIMO‐OFDM system is derived. The analytical results are verified by simulations.

Frequencies above 6 GHz are being considered by mobile communication industry for the deployment of future 5G networks. However in the higher carrier frequencies, especially the millimeter-wave frequencies (above 30 GHz), there can be severe degradations in the transmitted and received signals due to Phase Noise (PN) introduced by the local oscillators. In this paper, the effect of PN has been investigated for Orthogonal Frequency Division Multiplexing (OFDM), Offset QAM Filter-Bank Multi-Carrier (OQAM-FBMC) and QAM Filter-Bank Multi-Carrier (QAM-FBMC). The sources of degradation in these waveforms are quantified and closed-form expressions are derived for Signal-to-Interference Ratio (SIR). Evaluations are performed in terms of SIR and Symbol Error Rate (SER) for mm-wave frequencies using mmMAGIC PN model. The results reveal that OFDM outperforms OQAM-FBMC and QAM-FBMC and is a promising candidate for mm-wave communication.

Frequency bands above 6GHz are receiving great attention for 5G mobile radio communications due to the potential availability of enormous channel bandwidths. Various multi-carrier and single-carrier waveforms are being considered as potential candidates for 5G radio interface. When evaluating the waveform performance at high carrier frequency and large bandwidths, the inclusion of realistic hardware impairments is of great importance. In this paper, various 5G candidate waveforms (OFDM, DFTS-OFDM, W/P-OFDM, UF-OFDM, FBMC) are evaluated under hardware impairments (i.e., oscillator phase noise and non-linear amplifier) for above 6GHz mobile communications. It is observed that low out-of-band emission advantage of frequency localized waveforms (W/P-OFDM, UF-OFDM, FBMC) over OFDM and DFTS-OFDM may degrade or even vanish when hardware impairments are considered.

A super-resolution computational imaging method, called post random modulation radar imaging, based on single-input multiple-output radar detection system is proposed in this letter. In the proposed method, the target is detected in coherent mode using the multilinear frequency modulation signal. The echoes received by the radar elements of the receiving array are recorded, respectively. Then, random modulations of the directional pattern factor of the receiving radar array, which are nonlinear processes, are optimized according to the position and the size of the target that are estimated roughly using the traditional method. Finally, the super-resolution radar imaging is obtained to solve the equation group formed by the data from the nonlinear post random modulation process. The resolution of the proposed imaging method can break the diffraction limit corresponding to the aperture of the receiving radar array. The anti-interference ability of the proposed approach is improved significantly compared with the incoherent imaging method using a multiple-input single-output structure. Experiments are carried out to validate the proposed approach.

The effects of oscillator phase noises (PNs) on multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) systems are studied. It is shown that PNs of common oscillators at the transmitter and at the receiver have the same influence on the performance of (single-stream) beamforming MIMO-OFDM systems, yet different influences on spatial multiplexing MIMO-OFDM systems with singular value decomposition (SVD) based precoding/decoding. When each antenna is equipped with an independent oscillator, the PNs at the transmitter and at the receiver have different influences on beamforming MIMO-OFDM systems as well as spatial multiplexing MIMO-OFDM systems. Specifically, the PN effect on the transmitter (receiver) can be alleviated by having more transmit (receive) antennas for the case of independent oscillators. It is found that the independent oscillator case outperforms the common oscillator case in terms of error vector magnitude (EVM).

In this paper, a carrier recovery scheme for phase noise mitigation (in the presence of imperfect synchronization and channel estimation) is proposed for orthogonal frequency division multiplexing (OFDM) based wireless backhaul. This scheme models the phase noise as a piece-wise linear function and compensate the phase noise within each OFDM symbol. It is shown that the carrier recovery scheme can be applied to both OFDM and orthogonal frequency-division multiple access (OFDMA). Since the proposed scheme does not require detailed knowledge of statistical properties of the phase noise, it can be easily applied to practical OFDM/OFDMA systems. Hence the proposed scheme is applicable to both point-to-point and point-to-multipoint backhauls.

In this paper, we study the performances of cyclic prefix orthogonal frequency division multiplexing (CP-OFDM) and universal filtered orthogonal frequency division multiplexing (UF-OFDM) in the presence of phase noises. A phase noise mitigation scheme was proposed for CP-OFDM in a previous work. In this work, we extend the scheme to UF-OFDM. Phase noise estimation parameters are studied extensively in order to improve the performance of the scheme with a respect of the performance-complexity trade-off. The proposed scheme does not require detailed knowledge of statistics of the phase noise and can effectively compensate the phase noise within each multicarrier symbol with reasonable complexity.