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We present a C-band amplifying reconfigurable intelligent surface (ARIS) integrated with power amplifier (PA) to enhance the reflected electromagnetic (EM) signals. An aperture-coupled patch element with two orthogonal hourglass-shaped slots is elaborately constructed to ensure a broad operation bandwidth. A power combining and dividing network is also introduced to combine the 2 $\times2$ elements into a subarray to reduce the number of PAs and the system cost. The simulation results indicate that the achieved gain of ARIS is 7.7–12.2 dB from 5.0 to 6.0 GHz. A prototype of the proposed ARIS is designed, fabricated, and experimentally verified. The measured results agree well with the simulation. The proposed ARIS is promising for applications in future wireless communication systems for signal enhancement and coverage extension. [ABSTRACT FROM AUTHOR]

Reconfigurable intelligent surfaces (RISs) offer an entirely new route to alter the propagation properties of electromagnetic waves and thus control their reflection, refraction, and scattering features in arbitrary manners. Such physical attributes are perceived to bring about fundamental influence on the modern wireless communication system due to the possibilities to establish artificial and controllable propagation environments for radio signals, which no longer rely on the complex encoding, decoding, and other signal processing techniques. Recent studies reveal that the wave manipulation is not the only skill of the RISs. With the rapid developments of space–time digital metasurface and information metasurface, there has been increasing attention focused on the information manipulation via these artificial surfaces. In this article, we provide an overview of the theoretical models of the space–time digital metasurface and information metasurface, the mechanisms of wavefront shaping, and the signal modulations in space and time domains during the wave–matter interactions. We will also address some practical issues during implementations of the reconfigurable intelligent metasurfaces and the associated hardware architectures at microwave frequencies to realize simplified radio frequency transmitters. Several modulation schemes and the corresponding demonstration systems are introduced to illustrate the powerful abilities of the reconfigurable intelligent metasurfaces. Potential research directions of this technique are briefly discussed for their potential applications in future wireless networks. [ABSTRACT FROM AUTHOR]

Reconfigurable intelligent surfaces (RISs) provide an interface between the electromagnetic world of wireless propagation environments and the digital world of information science. Simple yet sufficiently accurate path loss models for RISs are an important basis for theoretical analysis and optimization of RIS-assisted wireless communication systems. In this paper, we refine our previously proposed free-space path loss model for RISs to make it simpler, more applicable, and easier to use. The impact of the antenna’s directivity of the transmitter, receiver, and the unit cells of the RIS on the path loss is explicitly formulated as an angle-dependent loss factor. The refined model gives more accurate estimates of the path loss of RISs comprised of unit cells with a deep sub-wavelength size. Based on the proposed model, the properties of a single unit cell are evaluated in terms of scattering performance, power consumption, and area, which allows us to unveil fundamental considerations for deploying RISs in high frequency bands. Two fabricated RISs operating in the millimeter-wave (mmWave) band are utilized to carry out a measurement campaign. The measurement results are shown to be in good agreement with the proposed path loss model. In addition, the experimental results suggest an effective form to characterize the power radiation pattern of the unit cell for path loss modeling. [ABSTRACT FROM AUTHOR]

The smart radio environment has become an eye-catching topic and is believed to own great potential in future wireless networks. Its essential demand lies in sensing and manipulating the electromagnetic (EM) fields accurately and simultaneously with the same aperture, which is hard to achieve using conventional methods due to the difficulty in integrating different systems. We propose a millimeter-wave space–time-coding digital metasurface (STCM) to realize simultaneous in situ direction finding and field manipulation. An analytic method of direction finding is first developed using the STCM modulated by a space–time-coding (STC) matrix while maintaining its original capability of EM field manipulations. In addition, three STC matrices corresponding to different EM functions are applied to the STCM to verify the feasibility and robustness of direction findings experimentally. Finally, a proof-of-concept prototype is built to exhibit different EM functions according to the incident angle detected simultaneously. The proposed concept sets the stage for the integrated implementation of the perception and response of the radio environment, which may find promising applications in future intelligent wireless networks and self-adaptive systems. [ABSTRACT FROM AUTHOR]

This article presents a reconfigurable multifunctional metasurface (RMM) with independent control of orthogonally polarized waves. Each meta-atom of RMM has four varactor diodes (two sets of orthogonal parallel diodes) connected and voltage biased. The voltage difference of the two sets of parallel varactor diodes is a constant. Thus, the orthogonally polarized reflected wave can be tuned independently. Unlike in other reported voltage-controlled RMMs, in the developed RMM, biasing voltages can be applied in one direction only. A tunable range from 7.2 to 7.8 GHz can be achieved. Two functionalities of the proposed RMM, namely, beam steering and polarization conversion, are demonstrated numerically and experimentally. Beam steering can be independently achieved in the ${x}-/y$ -directions when the incident plane wave is an $x-/y$ -polarized plane wave. For an arbitrary linear polarized incident plane wave, the decomposed orthogonally polarized waves can also be reflected toward two predefined directions based on the dual-polarized independent tunability of the proposed design. As for polarization conversion, the reflected waves can be flexibly switched among linear-to-orthogonal (LP-to-XP), linear-to-circular, and linear-to-elliptical polarizations in the operating frequencies depending on the phase requirement. The polarization conversion ratio is up to 90% for LP-to-XP polarization and the axial ratio is smaller than 3 dB for linear-to-circular polarizations. [ABSTRACT FROM AUTHOR]

Channel reciprocity greatly facilitates downlink precoding in time-division duplexing (TDD) multiple-input multiple-output communications without the need for channel state information feedback. Recently, reconfigurable intelligent surfaces (RISs) have emerged as a promising technology to enhance the performance of future wireless networks. However, since the artificial electromagnetic characteristics of RISs do not strictly follow the normal laws of nature, it raises a question: does the channel reciprocity hold in RIS-assisted TDD wireless networks? After briefly reviewing the reciprocity theorem, in this article, we show that there is still channel reciprocity for RIS-assisted wireless networks satisfying certain conditions. We also experimentally demonstrate the reciprocity at the sub-6 GHz and millimeter-wave frequency bands by using two fabricated RISs. Furthermore, we introduce several RIS-assisted approaches to realizing nonreciprocal channels. Finally, potential opportunities brought by reciprocal/nonreciprocal RISs and future research directions are outlined. [ABSTRACT FROM AUTHOR]

Satellite stereo mapping, together with laser altimetry, can be used to obtain three-dimensional geospatial information. Spaceborne laser altimeter can provide high-accuracy elevation information; however, due to the lack of detailed intensity information, its planimetric accuracy is usually worse than the ranging accuracy. The Chinese Gaofen-7 (GF-7) satellite, which was designed for civilian mapping application, was launched on November 3, 2019. The GF-7 satellite’s main payloads are a laser altimeter system (with footprint camera) and a dual-linear charge-coupled device (CCD) mapping camera. According to the pixel coordinate of the laser footprint in the stereo image, the laser altimeter together with the footprint camera can provide planimetric geodetic coordinates for the control points of a higher accuracy than the other traditional satellite laser altimeters, and represents a new technology for satellite mapping. In this article, a laser footprint planimetric location method for the GF-7 satellite is proposed. The method is designed based on the main payload characteristics of GF-7 and the working modes of the laser altimeter by the combined use of subpixel phase correlation image matching and four types of laser spot center detection methods. The planimetric positioning accuracies of the laser spots in urban, suburban, farmland, forest, mountainous, and ice sheet areas were also analyzed. The experimental results show that the accuracy of planimetric location relative to stereo image for the laser footprint is 0.3–1.0 m (except for ice sheets ~12 m) when the footprint camera works under the synchronous mode, and 0.2–0.4 m when the footprint camera works under asynchronous mode (AM). [ABSTRACT FROM AUTHOR]

The V-shaped electrothermal actuator has gained significant popularity as it was demonstrated to be a compact, stable and high rectilinear displacement device. However, existing electrothermal actuators usually require the driven voltage as high as several tens of volts to realize large displacements, imposing restrictions on the actuator’s integration in miniature mobile mechatronic systems. The paper presents a V-shaped metal-silicon actuator with low voltage, low power consumption and large displacement. The electro-thermal conversion and heat conduction mechanism of the actuator are improved by optimizing the architecture design of the actuator. An innovative method utilizing the double-sided inductively coupled plasma etching technique is developed for the fabrication. The motion of the actuator is characterized with a microscope-based dynamic test system. Finite element analysis is conducted to verify the device design and experimental results. The transient dynamic behaviour of the actuator is modelled for future control strategy. A rectilinear displacement as high as 80.7 μm is achieved at a voltage of 4.0 V and power of 1.12 W. The displacement per unit length to voltage ratio of the metal-silicon actuator is the largest among existing silicon actuators. The displacement per unit length to power of the actuator is also comparative to the highest value of existing actuators, demonstrating that the metal-silicon actuator can achieve large displacement with low voltage and power consumption. The proposed actuator has great potential for the applications in the miniature mechatronic systems such as cell phone, camera, safety and arming device. [2021-0048] [ABSTRACT FROM AUTHOR]

Metasurface has great capabilities in tailoring the electromagnetic (EM) waves in a controllable way owing to the carefully designed and arranged subwavelength meta-atoms. In this review paper, the digital version of metasurface, information metasurface (IM), is systematically introduced with its application in wireless communications. IM extends the research scope of metasurface from physics to the information perspective, in which the meta-atoms are represented by using digital codes “0” and “1” to characterize the EM responses. It can achieve programmable controls of EM waves in real time by integrating with the field-programmable gate array (FPGA). On the other hand, the digital coding strategy bridges the physical world and digital world, and further make the metasurface realize direct information processing. With the outstanding advantages, IMs have shown great potentials to bring revolutionary applications in wireless communications. In this article, the basic concepts and principles of IMs and their programmable features are presented first. Then it focuses on the IM-based new architecture transmitters, as well as the IM-integrated wireless networks in more detail. Finally, the current progress and the future directions of IMs in the wireless communications are respectively summarized. [ABSTRACT FROM AUTHOR]

Reconfigurable intelligent surfaces (RISs) comprised of tunable unit cells have recently drawn significant attention due to their superior capability in manipulating electromagnetic waves. In particular, RIS-assisted wireless communications have the great potential to achieve significant performance improvement and coverage enhancement in a cost-effective and energy-efficient manner, by properly programming the reflection coefficients of the unit cells of RISs. In this article, free-space path loss models for RIS-assisted wireless communications are developed for different scenarios by studying the physics and electromagnetic nature of RISs. The proposed models, which are first validated through extensive simulation results, reveal the relationships between the free-space path loss of RIS-assisted wireless communications and the distances from the transmitter/receiver to the RIS, the size of the RIS, the near-field/far-field effects of the RIS, and the radiation patterns of antennas and unit cells. In addition, three fabricated RISs (metasurfaces) are utilized to further corroborate the theoretical findings through experimental measurements conducted in a microwave anechoic chamber. The measurement results match well with the modeling results, thus validating the proposed free-space path loss models for RISs, which may pave the way for further theoretical studies and practical applications in this field. [ABSTRACT FROM AUTHOR]