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This work presents the systematic design and optimization of a compact quad-element MIMO antenna system in a roof cavity for the expected channel behavior at a 700 MHz LTE band. Beginning with a standard low-profile antenna element, the desired radiation pattern is synthesized by structural modification. The configuration for a quad-element design is then optimized. The results show that the proposed hidden solution even outperforms a non-hidden reference antenna of four quarterwave-monopoles in the cavity, making it attractive for real application.

Over the past decade, characteristic mode analysis (CMA) research has grown from a niche topic to a mainstream topic, warranting a tutorial-style special issue to survey the significant progress that has been made in this field. In thisintroductory article (PAPER 1), the focus is on providing the big picture. We start with a simple description of characteristic modes. Next, we examine the trends in this field, followed by providing further insights into CMA’s historical development. We will also address common myths surrounding the subject. Then, leaving the detailed coverage of major topics to the following papers, we summarize recent applications of CMA in scattering and other emerging topics. Finally, we conclude with some future perspectives on this field.

A low-profile wideband tri-polarized MIMO antenna is proposed in this paper. Based on characteristic mode analysis of a circular patch, four slots are etched symmetrically on the patch to transform the high-order monopole-like modes into broadside modes with lower resonant frequencies. Four sets of shorting pins are also applied to simultaneously adjust the resonant frequencies of the monopole-like modes and broadside fundamental modes of the patch. The effects of the slots and the shorting pins on these modes are analyzed in detail. Three dual-resonance ports with high isolation are achieved by exciting three pairs of orthogonal modes of the patch. The proposed antenna is wideband with respect to its profile (bandwidth/height = 216), as facilitated by the dual-resonance property of each port. Experimental results show that the proposed antenna features high port isolation (> 35 dB) and high gain (> 5.58 dB). The proposed low-profile wideband tri-port antenna can be applied in highly integrated MIMO systems.

The objective of this Special Issue is to showcase a unified vision for the applications of AI in radio propagation for communications and other relevant aspects. More specifically, the initial announcement encouraged emphasis in, but not limited to, the following areas: novel AI or AI-enabled techniques for radio propagation characterization and analysis; AI-enabled data analysis and parameter extractions of wireless channels; clustering analysis for radio channel characterization and modeling; AI-enabled channel modeling and communication system simulation; AI algorithm design in radio propagation for the applications in communications.

In order to provide higher data rates, as well as better coverage, cost efficiency, security, adaptability, and scalability, the 5G and beyond 5G network are development with various artificial intelligence techniques. In this twopart paper, we investigates the application of artificial intelligence (AI) and in particular machine learning (ML) to the study of wireless propagation channels. It firstly provides a comprehensive overview of ML for channel characterization and ML-based antenna-channel optimization, and then gives state-of-the-art literature review of channel scenario identification and channel modeling in Part II. Fundamental results and key concepts of ML for communication networks are presented, and widely used ML methods for channel data processing, propagation channel estimation, and characterization are analyzed and compared. A discussion of challenges and future research directions for ML enabled next generation networks rounds off the paper.

This two-part paper investigates the application of artificial intelligence (AI) and, in particular, machine learning (ML) to the study of wireless propagation channels. In Part I of this article, we introduced AI and ML and provided a comprehensive survey on ML-enabled channel characterization and antenna-channel optimization, and in this part (Part II), we review the state-of-the-art literature on scenario identification and channel modeling here. In particular, the key ideas of ML for scenario identification and channel modeling/prediction are presented, and the widely used ML methods for propagation scenario identification and channel modeling and prediction are analyzed and compared. Based on the state of the art, the future challenges of AI-/ML-based channel data processing techniques are given as well.

This letter proposes a co-designed millimeter-wave (mm-wave) and sub-6GHz antenna system. The antenna system consists of four 28GHz mm-wave arrays with reconfigurable radiation patterns and two sub-6GHz antennas fed with two corner capacitive coupling elements (CCEs). Each corner CCE is formed by the connected ground planes of two mm-wave arrays in the shared-aperture configuration. The two CCEs are separately matched to cover two sub-6GHz bands. Each mm-wave array consists of an active patch element and two parasitic patch elements loaded with PIN diodes, realizing 90-degree beam scanning range with two states of the PIN diodes. The measured results of the fabricated prototype show good agreement with the simulated ones. The prototyped mm-wave arrays cover the band 27.5-28.35GHz, and each achieves 90-degree beam scanning at 28GHz, with measured peak realized gain of 7.9 dBi. The CCE ports cover the two sub-6GHz bands of 0.79-0.96 GHz and 1.7-5GHz, with measured isolation of above 17dB and 20dB, respectively. The mm-wave band isolation is above 26dB.

A wideband compact series-fed patch subarray is proposed for 79 GHz multiple-input multiple-output (MIMO) radar. The proposed subarray is, for the first time, loaded with stacked micro-vias (SMVs). Two sets of patches in the subarray are fed 180-degrees out-of-phase. A low-cost, high-resolution multilayer PCB technology, called “Any-Layer PCB”, is used to implement the SMVs, which cannot be done by standard HDI PCB technology. Moreover, the technology supports small SMVs on different layers, hence thick machine-drilled vias can be avoided. For comparison, a series-fed patch subarray with no SMV loading is designed. It is shown that SMV loading facilitates a larger overlap of the impedance and gain bandwidths, leading to a larger operating bandwidth. The measured 10 dB impedance and 3 dB gain bandwidths are 15% and 9.6%, respectively. The sidelobe suppression is above 12.9 dB, and the maximum array gain is 12.67 dBi at 79.1 GHz. This subarray is suitable for MIMO radar, as its width is around 1/2-wavelength and the SMVs around it mitigate the crosstalk between the transmitting and receiving arrays.

Autonomous vehicle is being developed for widespread deployment. Its reliability and safety are critically dependent on advanced wireless technologies, e.g., vehicle-to-everything (V2X) communication. The frontend of a V2X system needs an antenna module that enables the vehicle to reliably connect to all other networks. Designing V2X antenna is challenging due to the complex in-vehicle environment, trend for hidden antenna solution, long simulation time and need for omnidirectional coverage. In this article, we survey these challenges as well as existing V2X antenna solutions. In view of the drawbacks in the existing solutions, we propose an efficient design methodology for V2X antennas to provide the desired coverage. The method utilizes a simple geometrical model of the vehicle that captures the shadowing effects of the vehicle body to obtain candidate antenna locations that offer the best coverage via multi-antenna diversity. Hence, complex full-wave simulation can be avoided. The approach is validated through comprehensive full-wave simulations and pattern measurements on two car models. The results confirm that, at 5.9GHz, line-of-sight shadowing has more dominant effect on the received power than multipath propagation due to the car body. In cases of strong diffraction and surface waves, a simple rule-of-thumb can be devised to improve the accuracy of the method.

A co-designed (3+3)-port antenna for dual-band operation that requires no feeding or decoupling network is proposed and verified. The antenna consists of six ports which are divided into two groups that resonate in two different frequency ranges. The basic radiating element is a tri-modal patch in a folded snowflake-shaped structure with the largest antenna dimension of 0.48λ1 or 0.68λ2 , where λ1 and λ2 are the wavelengths in air at the center frequencies of the lower and upper operating bands, respectively. Measurement results show that the 10 dB impedance bandwidths of the two respective bands are 19.7% and 14.4%. The proposed antenna exhibits compact, multiport, multiband and broadside radiation characteristics which are not only suitable for dual-band MIMO applications, but also for energy harvesting systems with spatial and frequency diversities, or dual-function wireless systems with simultaneous information and power transfer.

A co-designed (3+3)-port antenna for dual-band operation that requires no feeding or decoupling network is proposed and verified. The antenna consists of six ports which are divided into two groups that resonate in two differentfrequency ranges. The basic radiating element is a tri-modal patch in a folded snowflake-shaped structure with the largest antenna dimension of 0.48λ1 or 0.68λ2, where λ1 and λ2 are the wavelengths in air at the center frequencies of the lower and upper operating bands, respectively. Measurement results showthat the 10 dB impedance bandwidths of the two respective bands are 19.7% and 14.4%. The proposed antenna exhibits compact, multiport, multiband and broadside radiation characteristics which are not only suitable for dual-band MIMO applications, but also for energy harvesting systems with spatial and frequency diversities, or dual-function wireless systems with simultaneousinformation and power transfer.

In recent years, the screen-to-body ratio of mobile handsets has been increasing. Today, the screen nearly fills up the entire front side. Conventionally, the screen is mainly seen as a metallic object that adversely affects antenna performance. In this paper, the large screen is used for the first time to facilitate an additional uncorrelated MIMO port in a tri-port design, for several LTE bands below 1 GHz. To this end, the screen and the terminal chassis are modeled as two metal plates and their characteristic modes are analyzed. Four modes are then tuned to resonance and selectively excited to yield three uncorrelated MIMO ports. Simulation and measurement results are in good agreement. The measured bandwidths are 23%, 17% and 21%. Within the operating band, the measured isolation is above 13 dB, envelope correlation coefficient below 0.16 and average total efficiency above 72%.

This letter proposes a two-port MIMO smartphone antenna for frequency bands below 1 GHz, which is robust to user effects. The design is achieved by first analyzing the characteristic modes of a chassis that includes the large screen. Two modes predicted to be less affected by the user than other commonly used modes are selected. The modal currents and near-fields of the two desired modes then guide the design: The monopole-like mode introduced by the screen is tuned to resonance using shorting pins and selectively excited using the center feed location. The non-resonant loop mode is selectively excited for the first time by four inductive feeds added along the longer sides of the chassis, with proper phase shifts provided by a feeding network. The proposed antenna features isolation of above 19 dB and envelope correlation coefficient of below 0.12 in the considered scenarios. The measured bandwidth is above 15% for both ports, and the average radiation efficiency is 2 dB and 4.57 dB higher for two user scenarios with respect to a reference design. Moreover, no adaptive matching is needed as the impedance matching is robust to the user hand/head.

In this paper, we present a systematic approach to design a compact dual-mode monopole-like patch antenna using characteristic mode analysis (CMA). The modal analysis of a slotted circular patch structure incorporating a new shorting pin loading technique is presented. To achieve a compact monopole-like antenna with wideband operation, it is demonstrated that the first two significant modes with monopole-like patterns are the most suitable ones for dual-mode excitation. Based on the analysis of the modal currents and electric fields, four groups of shorting pins and four slots are introduced to individually tune the two modes, which facilitates the optimization. The effects of these slots and shorting pins on the resonant frequencies of the two modes are analyzed in detail. Finally, a CPW T-junction power divider is applied to simultaneously excite these two modes and suppress the undesired modes. Apart from a more compact form factor and higher gain than existing work, it also features a competitive gain-bandwidth per volume ratio.

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 super-imposed Y-shaped structures of different resonant frequencies. This intuition is confirmed using characteristic mode analysis (CMA). Furthermore, guided by CMA, further modificationsenable 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 withoutdegrading 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 0.48λc, where λc 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.

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 0.48λc, where λc 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.

The design of two antenna ports for a compact Y-shaped patch on top of a square ground plane is considered. Specifically, characteristic mode analysis reveals candidate regions to locate two direct probe feeding ports to jointly excite two significant modes of the Y-shaped patch with low mutual coupling and correlation. The feed design procedure, based on both the amplitude and phase distributions of the characteristic electric near-fields, is validated by full-wave simulation with two example realizations of two-port designs.

The design of two antenna ports for a compact Y-shaped patch on top of a square ground plane is considered in this work. Specifically, characteristic mode analysis reveals candidate regions to locate two direct probe feeding ports to jointly excite two significant modes of the Y-shaped patch with low mutual coupling and correlation. The feed design procedure, based on both the amplitude and phase distributions of the characteristic electric near-fields, is validated in full-wave simulation with two example realizations of two-port designs.

Wireless connectivity is becoming an important feature in cars, which together with recent developments in car design point to the need to accurately predict the performance of real antennas in simulation, to speed up the design cycle. However, it is challenging to accurately represent structurally complex real cars in simulation. This paper proposes a car model simplification approach for designing vehicular antennas, exemplified using three progressively simplified models of a hatchback car. For validation, a monopole and a PIFA operating at 800 MHz and 2.4 GHz, respectively, were mounted and simulated at two locations on these prototypes. The proposed scheme reduced the computational time to less than a third, while maintaining similar simulated antenna performance. Specifically, the antenna patterns of the simplest prototype and original one are correlated by 84% and 59% at 800 MHz and 2.4 GHz, respectively. Therefore, the proposed scheme is promising for real application.

Multiple-input multiple-out (MIMO) is a mature technology in modern wireless communications. However, it is challenging to implement multi-antennas for MIMO operation in compact mobile terminals, due to high mutual coupling and correlation among closely spaced antenna elements. Moreover, a conventional terminal chassis only offers one resonant mode below 1 GHz, complicating multi-antenna design. Recently, it has been shown that the chassis can be modified to facilitate a few resonant modes below 1 GHz. However, attempts to excite these modes selectively using a single coupling feed per antenna port result in limited bandwidth and isolation. In this work, we propose dual-feed antenna ports to improve selective feeding of the resonant modes of an existing two-port MIMO terminal antenna below 1 GHz. Simulation results reveal significantly enhanced bandwidths of 30% and 15% for the two ports, as well as high isolation of over 32 dB.

Massive multiple-input multiple-output (MIMO) arrays are becoming critical elements in cellular base station infrastructure. We utilize the theory of characteristic modes (TCM) to design a novel tri-modal broadside antenna element which is suitable for the construction of compact massive MIMOarrays. The proposed antenna element consists of three ports which is formed by reviewing an existing compact two-port Y-shaped patch antenna from a TCM perspective. Using this perspective the Y-shaped antenna is modified into a snowflake-shaped patch antenna and excited by three ports via capacitive coupling. The advantage of the design approach is that the size of the three-port antenna is approximately the same as a conventional dual-polarized patch antenna, hence allowing 50% more antenna elements for the same array aperture. By cutting the ground plane into a hexagonal shape, multiple of the proposed three-port canonical antennas are concatenated together to form 21- and 102-port massive MIMO arrays with all radiation patterns pointing in the broadside direction. Key performance characteristics of the massive MIMO arrays, such asthe Hermitian product of the simulated massive MIMO channel and mutual coupling, validate the effectiveness of the compact design.

Multiple-input multiple-output (MIMO) is a key enabler for high data rates in mobile communications. However, it is challenging to design MIMO terminal antennas for LTE bands below 1 GHz, due to the conventional chassis offering only one resonant characteristic mode (CM). Recently, it was shown that minor structural changes can yield up to two additional resonant modes for designing two-port MIMO antennas. Nonetheless, the resulting bandwidth for the second port is relatively small. To simultaneously meet bandwidth and other practical requirements (including low profile and no off-ground clearance), a step-by-step approach for structural changes and feed design is applied in this work to exemplify the use of physical insights from CM analysis to achieve a competitive wideband two-port solution. The main novelty is that an entirely new mode is identified and appropriately tuned by structural modification for creating an additional resonance below 1 GHz. Moreover, two simple probe-feed ports are designed to jointly excite different subsets of four modes over frequency. In addition, far-field pattern orthogonality is guaranteed by the different phase shifts of the characteristic electric fields at the port locations. Furthermore, bulkier self-resonant antenna elements are avoided. To show design flexibility, a three-port version is also demonstrated.