Metamaterials and metasurfaces have attracted much attention due to their powerful ability to control electromagnetic (EM) waves. In this paper, we review the recent developments in the field of EM metamaterials, starting from their exotic physics to their applications in novel information systems. First, we show the fundamental understanding on traditional metamaterials based on the effective medium theory and related applications, such as invisibility cloaks and meta-lenses. Second, we review the two-dimensional versions of metamaterials, i.e., metasurfaces, for controlling spatial waves and surface waves and thereafter present their typical designs. In particular, we briefly introduce spoof surface plasmon polaritons and their applications in microwave frequencies. Following the above approach, we emphatically present the concepts of digital coding metamaterials, programmable metamaterials, and information metamaterials. By extending the principles of information science to metamaterial designs, several functional devices and information systems are presented, which enable digital and EM-wave manipulations simultaneously. Finally, we give a brief summary of the development prospects for microwave metamaterials.
A metasurface (MTS) can be characterized in terms of dispersion properties of guided waves and surface waves. By engineering the rich dispersion relations, setting particular boundary conditions, and selecting proper excitation schemes, multiple adjacent resonance modes can be excited to realize the wideband operation of low-profile MTS antennas. We introduce the op-erating principles of typical dispersion-engineered MTS antennas, and review the recent progress in dispersion-engineered MTS antenna technology. The miniaturization, circular polarization, beam-scanning, and other functionalities of MTS antennas are discussed. The recent development of MTS antennas has not only provided promising solutions to the wideband and low-profile antenna design but also proven great potential of MTS in developing innovative antenna technologies.
With the commercialization of fifth generation networks worldwide, research into sixth generation (6G) networks has been launched to meet the demands for high data rates and low latency for future services. A wireless propagation channel is the transmission medium to transfer information between the transmitter and the receiver. Moreover, channel properties determine the ultimate performance limit of wireless communication systems. Thus, conducting channel research is a prerequisite to designing 6G wireless communication systems. In this paper, we first introduce several emerging technologies and applications for 6G, such as terahertz communication, industrial Internet of Things, space-air-ground integrated network, and machine learning, and point out the developing trends of 6G channel models. Then, we give a review of channel measurements and models for the technologies and applications. Finally, the outlook for 6G channel measurements and models is discussed.
Multiple-input multiple-output (MIMO) technique is a key technique for communication in the future. It can effectively enhance channel capacity. For future fifth-generation (5G) terminals, it is still a challenging task to realize desirable isolation within a compact size. To achieve an acceptable isolation level, many decoupling methods have been developed. We review the most recent research on decoupling methods, including the employment of external decoupling structures, orthogonal modes, and reduction of ground effect, and discuss the development trends of the MIMO array in 5G smartphones.
Highly efficient power amplifiers (PAs) and associated linearization techniques have been developed to accommodate the explosive growth in the data transmission rate and application of massive multiple input multiple output (mMIMO) systems. In this paper, energy-efficient integrated Doherty PA monolithic microwave integrated circuits (MMICs) and linearization techniques are reviewed for both the sub-6 GHz and millimeter-wave (mm-Wave) fifth-generation (5G) mMIMO systems; different semi-conductor processes and architectures are compared and analyzed. Since the 5G protocols have not yet been finalized and PA specifications for mMIMO are still under consideration, it is worth investigating novel design methods to further improve their efficiency and linearity performance. Digital predistortion techniques need to evolve to be adapted in mMIMO systems, and some creative linearity enhancement techniques are needed to simultaneously improve the compensation accuracy and reduce the power consumption.
With a lot of millimeter-wave (mm-Wave) applications being issued, wideband circuits and systems have attracted much attention because of their strong applicability and versatility. In this paper, four transformer-based ultra-wideband mm-Wave circuits demonstrated in CMOS technologies are reviewed from theoretical analysis, implementation, to performance. First, we introduce a mm-Wave low-noise amplifier with transformer-based Gm-boosting and pole-tuning techniques. It achieves wide operating bandwidth, low noise figure, and good gain performance. Second, we review an injection-current-boosting tech-nique which can significantly increase the locking range of mm-Wave injection-locked frequency triplers. Based on the injection- locked principle, we also discuss an ultra-wideband mm-Wave divider with the transformer-based high-order resonator. Finally, an E-band up-conversion mixer is presented; using the two-path transconductance stage and transformer-based load, it obtains good linearity and a large operating band.
A filtering antenna is a device with both filtering and radiating capabilities. It can be used to reduce the cross-band mutual coupling between the closely spaced elements operating at different frequency bands. We review the authors’ work on filtering antenna designs and three related dual-band base-station antenna arrays as application examples. The filtering antenna designs include single- and dual-polarized filtering patch antennas, a single-polarized omni-directional filtering dipole antenna, and a dual-polarized filtering dipole antenna for the base station. The filtering antennas in this paper feature an innovative concept of eliminating extra filtering circuits, unlike other available antennas. For each design, the filtering structure is finely integrated with the radiators or feeding lines. As a result, the proposed designs have the advantages of compact size, simple structure, good in-band radiation performance, and low levels of loss, and do not contain complicated filtering circuits. Based on the proposed filtering antennas, single- and dual-polarized dual-band antenna arrays were developed. Separate antenna elements at different frequency bands were used to achieve the dual-band performance. The cross-band mutual couplings between the elements at different bands were reduced substantially using the antenna inherent filtering performance. The dual-band arrays exhibited better performance as compared to typical industrial products. Some of the proposed technologies have been transferred into the industry.
A type of millimeter-wave antenna array with flexible design is proposed for a variety of applications at 60 GHz. The antenna array can be adjusted to be linearly or circularly polarized by simply changing the radiation part of the antenna array. High gain, wideband, and high radiation efficiency characteristics can be achieved by adopting a low insertion loss feeding network and broadband antenna elements. For the linearly polarized antenna array, simulation results show that the impedance bandwidth of the 2×2 antenna subarray reaches 21.6%, while the maximum gain achieves 15.1 dBi and has a fluctuation of less than 0.4 dBi within the working bandwidth. Simulation results of the 8×8 linearly polarized antenna array show a bandwidth of 21.6% and a gain of (26.1±1) dBi with an antenna efficiency of more than 80%. For the 8×8 circularly polarized antenna array, simulation results show that an impedance bandwidth of 18.2% and an axial ratio (AR) bandwidth of 13.3% are obtained. Gain and efficiency of up to 27.6 dBi and 80% are achieved, respectively. A prototype of antenna array is fabricated, and results are compared and analyzed.
Rotman lens is a type of beamforming network with many advantages, such as true-time delay characteristic, multibeam capability, and wide bandwidth. Rotman lens has been used in a wide range of applications in today’s wireless communication systems. However, the size of a conventional Rotman lens is considerably large. So, difficulties may arise with respect to its integration with base station antennas in wireless communication systems. In this study, three techniques for the miniaturization of a Rotman lens, i.e., Chebyshev impedance transformers, power dividers, and truncated ports with energy distribution slots, are introduced to design the Rotman lens to reduce the size of the ports and hence the total area occupied by the Rotman lens. Simulation and measurement results indicate that good impedance matching between the lens body and its feed lines can be achieved. Using the proposed truncated ports with energy distribution slots, the size of the Rotman lens can be greatly reduced without performance degradation or production cost increment. Moreover, two possible applications of the proposed miniaturized Rotman lens to wireless communication systems are investigated. Rotman lens can not only provide multiple phase difference signals along the array ports to realize multibeams, but also generate high-performance formed beams such as flat-topped radiation pattern.
A large dual-polarization microstrip reflectarray with China-coverage patterns in two operating bands is designed. To sufficiently compensate for the spatial phase delay differences in two operating bands separately, a three-layer rectangular patch element is addressed, which is suitable for the large dual-polarization reflectarray. Due to the complexly shaped areas and high gain requirements, there are more than 25 000 elements in the reflectarray, making it difficult to design, due to more than 150 000 optimization variables. First, the discrete fast Fourier transform (DFFT) and the inverse DFFT are used to establish a one-to-one relationship between the aperture distribution and the far field, which lays a foundation for optimizing the shaped-beam reflectarray. The intersection approach, based on the alternating projection, is used to obtain the desired reflection phases of all the elements at some sample frequencies, and a new method for producing a suitable initial solution is proposed to avoid undesired local minima. To validate the design method, a dual-polarization shaped-beam reflectarray with 7569 elements is fabricated and measured. The measurement results are in reasonable agreement with the simulation ones. Then, for the large broadband reflectarray with the minimum differential spatial phase delays in the operating band, an approach for determining the optimal position of the feed is discussed. To simultaneously find optimal dimensions of each element in two orthogonal directions, we establish a new optimization model, which is solved by the regular polyhedron method. Finally, a dual-band dual-polarization microstrip reflectarray with 25 305 elements is designed to cover the continent of China. Simulation results show that patterns of the reflectarray meet the China-coverage requirements in two operating bands, and that the proposed optimization method for designing large reflectarrays with complexly shaped patterns is reliable and efficient.
We propose a miniaturized wideband metasurface antenna for 60-GHz antenna-in-package applications. With the glass integrated passive device manufacturing technology, we introduce a coplanar-waveguide-fed (CPW-fed) ring resonator to char-acterize the material properties of the glass substrate. The proposed antenna is designed on a high dielectric constant glass substrate to achieve antenna miniaturization. Because of the existence of gaps between patch units compared with the conventional rec-tangular patch in the TM10 mode, the radiation aperture of this proposed antenna is reduced. Located right above the center feeding CPW-fed bow-tie slot, the metasurface patch is realized, supporting the TM10 mode and antiphase TM20 mode simultaneously to improve the bandwidth performance. Using a probe-based antenna measurement setup, the antenna prototype is measured, demonstrating a 10-dB impedance bandwidth from 53.3 to 67 GHz. At 60 GHz, the antenna gain measured is about 5 dBi in the boresight direction with a compact radiation aperture of 0.31λ0×0.31λ0 and a thickness of 0.06λ0.
We propose a dual-module multiple-input multiple-output (MIMO) antenna for portable terminals. The operating bands of the handheld terminal antenna are 5G (3.4–3.8 GHz) and WLAN (5.150–5.925 GHz). Antenna elements of 5G and WLAN are spaced to reduce coupling between antenna elements in the same module. The return loss of all antenna elements is larger than 6 dB. The isolation between all elements is larger than 14 dB. The radiation efficiency of the high-frequency antenna is greater than 50%, and the radiation efficiency of the low-frequency antenna is greater than 40%. The far-field gain of all elements is greater than 2.2 dBi.
We introduce the basic concept, background, and development of mobile communication systems from the first generation (1G) to the fifth generation (5G) including their antenna systems. We also describe the requirements for 5G networking and optimization of antenna systems, and present the basic principle of three-dimensional array antennas. Weight optimization methods of massive multiple-input multiple-output (MIMO) antennas are proposed and verified. Finally, several ideas are given to solve the problem of power consumption of 5G antenna systems.