Abstract: With the rapid development of the fifthgeneration wireless communication systems, a profound revolution in terms of transmission capacity, energy efficiency, reliability, latency, and connectivity is highly expected to support a new batch of industries and applications. To achieve this goal, wireless networks are becoming extremely dynamic, heterogeneous, and complex. The modeling and optimization for the performance of realworld wireless networks are extremely challenging due to the difficulty to predict the network performance as a function of network parameters, and the prohibitively huge number of parameters to optimize. The conventional network modeling and optimization approaches rely on drive test, trial-and-error, and engineering experience, which are labor intensive, error-prone, and far from optimal. On the other hand, while the research community has spent significant efforts in understanding the fundamental limits of radio channels and developing physical layer techniques to operate close to it, very little is known about the performance limits of wireless networks, where millions of radio channels interact with one another in complex manners. This paper reviews the very recent mathematical and learning based techniques for modeling and optimizing the performance of real-world wireless networks in five aspects, including channel modeling, user demand and traffic modeling, throughput modeling and prediction, network parameter optimization, and IRS empowered performance optimization, and also presents the corresponding notable performance gains.
Abstract: With the demand for Internet connectivity in remote areas, the space-air-ground integrated network (SAGIN) was proposed to achieve ubiquitous coverage and enhance service capabilities of extant terrestrial networks. The paradigm of virtual network element placement (NEP) is applied into SAGIN. It can save energy and operating costs by placing specific network elements (NEs) as software instances. In addition, it helps to provide services in end-to-end networks with its ability to allocate and manage resources flexibly. However, NEP faces some challenges in SAGIN. The network topologies can be dynamic, and links such as the satellite-to-ground and air-to-ground ones are prone to fail. These will make NEP management more complicated. Moreover, the static NEP schemes are hard to accommodate the time-varying traffic. In this context, this work explains the NEP problem in SAGIN from three aspects, i.e., the SAGIN radio access network (RAN), the SAGIN core network (CN), and the SAGIN barrier network (BN). First, the physical and networking architectures of SAGIN are introduced. Then the status of the network element placement and corresponding challenges are described from these two aspects. Finally, this paper discusses future research directions and key technical challenges.
Abstract: Thinking space came into being with the emergence of human civilization. With the emergence and development of cyberspace, the interaction between those two spaces began to take place. In the collision of thinking and technology, new changes have taken place in both thinking space and cyberspace. To this end, this paper divides the current integration and development of thinking space and cyberspace into three stages, namely Internet of brain (IoB), Internet of thought (IoTh), and Internet of thinking (IoTk). At each stage, the contents and technologies to achieve convergence and connection of spaces are discussed. Besides, the Internet of creation (IoC) is proposed to represent the future development of thinking space and cyberspace. Finally, a series of open issues are raised, and they will become thorny factors in the development of the IoC stage.
Abstract: Benefiting from the ultra-wide bandwidth, terahertz (THz) communication is considered one of the core technologies of the 6G mobile communication. However, high path loss is a severe issue for THz band, which needs to be solved by the ultra-massive multiple-input multiple-output (MIMO) technology. For the sake of reducing the hardware cost and power comsumption, the hybrid beamforming technology is required, which has been adopted in the 5G communication. In the hybrid beamforming for the THz wideband system, the enormous antenna and ultra-wide bandwidth cause the spatial- and frequency-wideband effect, resulting in acute beam squint. Eliminating beam squint is the key to the engineering realization of ultra massive MIMO for the THz band. To circumvent this issue, this paper presents a comprehensive overview of the hybrid beamforming technology including the channel model, the traditional phase shifter based and the true time delay based architectures, the 3D hybrid beamforming for the uniform planar array. Finally, multiple significant open issues and potential challenges are pointed out and elaborated.
Abstract: Carrier behavior in halide perovskite is a critical factor impacting on the properties of material, and finally determines the performance of perovskite photovoltaic and luminescent devices. It is necessary to clarify the mechanism of carrier relaxation and migration at extremely microscopic time scale. Time-resolved spectroscopy provides a powerful means for the detection of ultrafast processes, which has been an indispensable technique in research on perovskites. In this review, we will elaborate the basic principle and system implementation of time-resolved spectroscopy, and introduce the applications for carrier dynamics in perovskite.
Abstract: Deep forest is a tree-based deep model made up of non-differentiable modules that are trained without backpropagation. Despite the fact that deep forests have achieved considerable success in a variety of tasks, feature concatenation, as the ingredient for forest representation learning, still lacks theoretical understanding. In this paper, we aim to understand the influence of feature concatenation on predictive performance. To enable such theoretical studies, we present the first mathematical formula of feature concatenation based on the two-stage structure, which regards the splits along new features and raw features as a region selector and a region classifier respectively. Furthermore, we prove a region-based generalization bound for feature concatenation, which reveals the trade-off between Rademacher complexities of the two-stage structure and the fraction of instances that are correctly classified in the selected region. As a consequence, we show that compared with the prediction-based feature concatenation (PFC), the advantage of interaction-based feature concatenation (IFC) is that it obtains more abundant regions through distributed representation and alleviates the overfitting risk in local regions. Experiments confirm the correctness of our theoretical results.
Abstract: With the rapid growth of multimedia data, cross-media hashing has become an important technology for fast cross-media retrieval. Because the manual annotations are difficult to obtain in real-world application, unsupervised cross-media hashing is studied to address the hashing learning without manual annotations. Existing unsupervised cross-media hashing methods generally focus on calculating the similarities through the features of multimedia data, while the learned hashing code cannot reflect the semantic relationship among the multimedia data, which hinders the accuracy in the cross-media retrieval. When humans try to understand multimedia data, the knowledge of concept relations in our brain plays an important role in obtaining high-level semantic. Inspired by this, we propose a knowledge guided unsupervised cross-media hashing (KGUCH) approach, which applies the knowledge graph to construct high-level semantic correlations for unsupervised cross-media hash learning. Our contributions in this paper can be summarized as follows: 1) The knowledge graph is introduced as auxiliary knowledge to construct the semantic graph for the concepts in each image and text instance, which can bridge the multimedia data with high-level semantic correlations to improve the accuracy of learned hash codes for cross-media retrieval. 2) The proposed KGUCH approach constructs correlation of the multimedia data from both the semantic and the feature aspects, which can exploit complementary information to promote the unsupervised cross-media hash learning. The experiments are conducted on three widely-used datasets, which verify the effectiveness of our proposed KGUCH approach.
Abstract: This paper presents a Ku-Band fully differential 4-element phased-array transceiver using a standard 180-nm CMOS process. Each transceiver is integrated with a 5-bit phase shifter and 4-bit attenuator for high-resolution radiation manipulation. The front-end system adopts time-division mode, and hence two low-loss T/R switches are included in each channel. At room temperature, the measured root-mean-square (RMS) phase error is less than 5.5°. Furthermore, the temperature influence on passive switched phase shifters is analyzed. Meanwhile, an extra phase-shifting cell is developed to calibrate phase error varying with the operating temperatures. With the calibration, the RMS phase error is reduced by 7° at −45 ℃, and 5.4° at 85 ℃. The RMS amplitude error is less than 0.92 dB at 15−18 GHz. In the RX mode, the tested gain is 9.6±1.1 dB at 16.5 GHz with a noise figure of 10.9 dB, and the input P1dB is −15 dBm, while the single-channel’s gain and output P1dB in the TX mode are 11.3±0.4 dB and 9.4 dBm at 16.1 GHz, respectively. The whole chip occupies an area of 5×4.2 mm2 and the measured isolation between each two adjacent channels is lower than −23.1 dB.
Abstract: Visible light communication (VLC) has emerged as a promising communication method in 6G, particularly in the domain of underwater communication. However, because of its intimate coupling to the light-emitting diodes (LED), the transmission rate is limited by the modulation bandwidth and the nonlinearity of the LED. With current device development, LED bandwidth is substantially lower than that of other optical communication devices, necessitating improved spectral efficiency and nonlinearity robustness to achieve high-speed transmission. In this paper, we propose a hexagonal constellation-based geometrically shaped (GS) 32QAM with carrierless amplitude and phase (CAP) modulation for the underwater VLC system. A comprehensive performance comparison of four GS modulation methods, including our proposed hexagonal-32QAM, normal 32QAM, square-32QAM and 32APSK is investigated in both theory and experiment. The hexagonal-32QAM with a modified binary switching coding (BSC) can obtain a Q factor gain of 0.44 dB, and a data rate improvement of 25 Mb/s compared with Gary coding based normal 32QAM. The experimentally results illustrate the feasibility of geometric shaping 32QAM in underwater visible light communication.
Abstract: Resonance is a common physical phenomenon in real world, and the modal analysis is a useful tool. Within the regime of electromagnetics, characteristic mode theory is established in frequency domain (FD), and it is doubtful that whether we can find similar modes in time domain (TD). In this paper, resonating modes of a vibrating string are briefly reviewed. Special attentions are paid to the time-domain behaviors of the resonating modes, by following which a temporal modal analysis is proposed. Additionally, a narrow plate is selected as an example since it has a similar structure as the vibrating string. Temporal modal behaviors of the plate are presented and discussed. To further demonstrate this concept, a rigorous analysis of a sphere is provided. A frequency-independent condition is discussed and verified for small objects, and it results in a band-limited constraint. In addition, the temporal modal analysis with excitations is presented, and its potential applications are discussed with emphasis on the analyzing and optimizing transient behaviors of antennas. This work expands largely the field of characteristic mode and may find applications for scattering and antennas.
Abstract: In dual-band shared-aperture base station antennas, the secondary radiation (SR) field caused by cross-band couplings always leads to radiation pattern distortion. In this work, the SR field caused by the coupling is attributed to the excitation of the target high-order modes from the perspective of the characteristic mode. To effectively suppress the target modes, L-shaped slots are proposed to load the arms of the radiator and altering the current paths. As compared with conventional dipoles without L-shaped slots, the proposed antenna generates less SR field. Therefore, the radiation pattern of the high-band antennas keeps stable in a dual-band shared-aperture antenna array environment. Prototype of a dual-band antenna array consisting of the proposed low-band (LB) antenna and a 4 × 4 high-band (HB) antenna array is fabricated. Simulated and measured results show that the designed dual-band array operates at both 0.74–0.82 GHz and 3.30–3.80 GHz. The experimental results verify that the cross-band decoupling has been achieved with the high-order mode suppression.
Abstract: In this paper, a characteristic mode design of dual-band reconfigurable frequency selective surface (RFSS) with transmission/reflection band is presented. Through the visualization of modal behavior, the theory of characteristic modes (TCM) is used to guide design from a physical perspective. At first, the dominant modes of two basic structures that have a major impact on reflection are clarified by observing the modal behaviors. The appropriate combination of the two structures forms an initial element with bandpass properties. Then, the positive-intrinsic-negative (PIN) diode is placed in the middle of the microstrip line and as the state shifts from ON to OFF, the dominant mode resonant frequency of the microstrip line shifts upward. Such a feature can be used to design the transmission/reflection RFSS. Experimentally, a dual-band RFSS operating at 8.8 GHz and 10.4 GHz is designed and fabricated. The measured results that agree with the full-wave simulation validate the proposed TCM-based design.
Abstract: The demand for a high transmission data rate in 5G leads to the wide application of the multi-input, multi-output (MIMO) technique. However, the narrow design space in mobile phone causes severe unexpected coupling between antenna units and deteriorate the system performance dramatically. In this paper, we focus on the antenna coupling within the MIMO system. Some decoupling techniques are summarized, such as neutralization line, decoupling network, common/differential mode cancellation, etc. Finally, new decoupling research developments of modal currents cancellation based on the theory of characteristic mode are elaborated. Two design examples are shown to validate the benefit of the proposed decoupling method. The −6 dB impedance bandwidth of the head-to-head antenna pair can cover 3.4–3.6 GHz. The isolation level is improved from more than 3.7 dB to more than 13 dB. The tail-to-tail antenna pair can cover 3.3–5 GHz and the isolation level is improved from more than 5 dB to more than 10 dB in the whole operating bandwidth. Both antenna pairs can achieve an envelop correlation coefficient of less than 0.2 and the antenna efficiencies are more than 40%.
Abstract: Vortex electromagnetic waves can carry orbital angular momentum (OAM) modes of different order number, which are mutually orthogonal. Communication technologies multiplexing different OAM topological charges have attracted extensive attention in improving spectrum utilization. In this regard, the efficient generation of OAM modes becomes a key issue. However, it is difficult to generate high-order OAM modes by using a microstrip antenna with a simple structure, and the mode purity is usually not high. Methodologically, the OAM wave of a microstrip antenna is mostly achieved by synthesizing degenerate eigenmodes. In this paper, combined with eigenmode analysis, the construction of high-order and high-purity OAM mode microstrip antennas is carried out from the perspectives of radiating element, ground plane, and structural symmetry. For a regular hexagonal patch antenna of the 4th-order OAM mode, the mode purity is improved from about 40% to over 80% by removing the inner conductor of the patch layer, using a circular ground plane, and adding lug patches on the outside. This method of improving mode purity through the analysis of mode significance and characteristic current distribution also has guiding significance for the design of other high-order OAM mode patch antennas.
Abstract: The solution of large matrix eigenvalues and complex linear equations limits the Fourier modal method (FMM) application in ultrathin metallic gratings (UMG) analysis. This paper proposes an efficient and explicit FMM method for analyzing UMG. The proposed method avoids solving complex linear equations and eigenvalues of eigenmatrix in the conventional method by simplifying the implementation equations. Two numerical examples then verify the reliability of the proposed method compared with CST simulations and the conventional method. The proposed method is proven efficient in decreasing the CPU time by over 80% and demanding significantly less memory.
Abstract: The reduction of mutual coupling between antenna elements has been a research hotspot in the design of multi-input multi-output (MIMO) systems. In this paper, a review of the existing theory of characteristic mode (TCM) based decoupling methods is introduced, and they are summarized and divided into two categories according to mechanisms. The first category consists of the use of the modal properties of the radiator whereas the second one is based on the ground plane to enhance isolation. Moreover, a wideband self-decoupling method is implemented for the MIMO microstrip patch antenna (MPA) array using the characteristic modes (CMs) of the ground plane. It is found that, by adjusting the dimension of the ground plane, their CMs along with two or three null-field regions, can be excited by the active MPA. When the feeding position of the adjacent passive MPA is arranged in the null-field region, the energy cannot be coupled with the receiving port, thereby achieving a good isolation level. For verification, a prototype of the proposed MIMO MPA array has been fabricated and tested. The results show that a wide usable bandwidth of 24.2% has been achieved.
Abstract: Multiple antennas with low correlations are highly in demand in the mobile handsets for higher data rate. At the same time, the total pattern provided by the antennas should be as round as possible for reliable links. In this paper, by analysing the characteristic modes of the mobile chassis loaded by ultra-thin metal frames, a four-antenna system with a round total pattern is designed at 2.4 GHz. The gain variations are only 3.4 dB and 3.6 dB on two dominant planes. The isolations between antennas are above 15 dB, and the correlations are below 0.01, with intact ground plane and small ground clearance of 2 mm maintained. Afterwards, an eight-port antenna system is designed by taking advantage of the ground clearance and exciting modes with different phases. Low correlations of below 0.07 are obtained. The antenna system is fabricated, with the measured results agreeing well with the simulated ones.
Abstract: When the dielectric sheet is electrically thin in the normal direction, the conventional volume integral equation (VIE) can be approximately simplified to the surface one. On this basis, a novel surface integral equation-based formulation is presented for analyzing characteristic mode (CM) of thin dielectric sheets. The resultant CMs are expressed with tangential and normal components of electric volume currents, which are more intuitive than the conventional VIE-based one. Due to the application of volume equivalence principle, the proposed formulation is immune to non-physical modes. The CMs of typical thin dielectric bodies, including the dielectric substrate and radome, are analyzed to show that the proposed formulation is computationally efficient with encouraging accuracy.