Simultaneous Conversion of Polarization and Frequency via Time‐Division‐Multiplexing Metasurfaces

AbstractMetasurfaces are artificially engineered two‐dimensional materials composed of sub‐wavelength meta‐atoms, which have shown unprecedented capabilities in manipulating the amplitude, phase, frequency, and polarization states of electromagnetic waves. Specifically, polarization control can be attained via suitable anisotropic, linear, and time‐invariant designs, while frequency conversion is realized via nonlinear or time‐varying platforms. Simultaneous manipulations of polarization and frequency would be of considerable practical interest in many application scenarios, but remain unattainable with current approaches. Here, a time‐division‐multiplexing metasurface is proposed to realize the simultaneous conversion of polarization and frequency. The platform relies on time‐modulated polarization switches and, by varying the duty cycle and time delays of the polarization channels, can arbitrarily rotate the polarization at the central frequency of operation, and synthesize various polarization states at selected harmonic frequencies. Theoretical predictions are validated via measurements on a prototype operating at microwave frequencies, providing the first experimental evidence of simultaneous polarization and frequency conversions via time‐division‐multiplexing metasurfaces. The outcomes open a new pathway in manipulating the electromagnetic waves via time‐varying metasurfaces, and may be of interest for a broad variety of applications in scenarios ranging from polarization imaging to quantum optics

On Channel Reciprocity in Reconfigurable Intelligent Surface Assisted Wireless Network

Channel reciprocity greatly facilitates downlink precoding in time-division duplexing (TDD) multiple-input multiple-output (MIMO) communications without the need for channel state information (CSI) feedback. Recently, reconfigurable intelligent surfaces (RISs) emerge as a promising technology to enhance the performance of future wireless networks. However, since the artificial electromagnetic characteristics of RISs are not from the nature, it brings up 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 still exists channel reciprocity for RIS-assisted wireless networks satisfying certain conditions. We also experimentally demonstrate the reciprocity at the sub-6 GHz and the 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.Comment: In general, when the control signals applied to the unit cells remain unchanged, commonly designed and fabricated RISs inherently obey the reciprocity theorem. Nevertheless, there are several RIS-assisted approaches to realizing nonreciprocal channels. Potential opportunities brought by reciprocal/nonreciprocal RISs and future research directions are outline

Path Loss Modeling and Measurements for Reconfigurable Intelligent Surfaces in the Millimeter-Wave Frequency Band

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.Comment: Model refinements are introduced to previously proposed free-space path loss model for RISs in order to make it simpler and easier to use. The properties of a single unit cell are evaluated in terms of scattering performance, power, and area, as it is the basic element of an RIS. We report the world's first measurement campaign in the mmWave frequency band to validate the path loss model for RIS

MIMO Transmission through Reconfigurable Intelligent Surface: System Design, Analysis, and Implementation

Reconfigurable intelligent surface (RIS) is a new paradigm that has great potential to achieve cost-effective, energy-efficient information modulation for wireless transmission, by the ability to change the reflection coefficients of the unit cells of a programmable metasurface. Nevertheless, the electromagnetic responses of the RISs are usually only phase-adjustable, which considerably limits the achievable rate of RIS-based transmitters. In this paper, we propose an RIS architecture to achieve amplitude-and-phase-varying modulation, which facilitates the design of multiple-input multiple-output (MIMO) quadrature amplitude modulation (QAM) transmission. The hardware constraints of the RIS and their impacts on the system design are discussed and analyzed. Furthermore, the proposed approach is evaluated using our prototype which implements the RIS-based MIMO-QAM transmission over the air in real time.Comment: This paper aims to investigate the feasibility of using RIS for MIMO wireless transmission for higher-order modulation by presenting an analytical modeling of the RIS-based system and providing experimental results from a prototype which has been buil

Genome-wide gene phylogeny of CIPK family in cassava and expression analysis of partial drought-induced genes

Cassava is an important food and potential biofuel crop that is tolerant to multiple abiotic stressors. The mechanisms underlying these tolerances are currently less known. CBL-interacting protein kinases (CIPKs) have been shown to play crucial roles in plant developmental processes, hormone signaling transduction, and in the response to abiotic stress. However, no data is currently available about the CPK family in cassava. In this study, a total of 25 CIPK genes were identified from cassava genome based on our previous genome sequencing data. Phylogenetic analysis suggested that 25 MeCIPKs could be classified into four subfamilies, which was supported by exon-intron organizations and the architectures of conserved protein motifs. Transcriptomic analysis of a wild subspecies and two cultivated varieties showed that most MeCIPKs had different expression patterns between wild subspecies and cultivatars in different tissues or in response to drought stress. Some orthologous genes involved in CIPK interaction networks were identified between Arabidopsis and cassava. The interaction networks and co-expression patterns of these orthologous genes revealed that the crucial pathways controlled by CIPK networks may be involved in the differential response to drought stress in different accessions of cassava. Nine MeCIPK genes were selected to investigate their transcriptional response to various stimuli and the results showed the comprehensive response of the tested MeCIPK genes to osmotic, salt, cold, oxidative stressors, and ABA signaling. The identification and expression analysis of CIPK family suggested that CIPK genes are important components of development and multiple signal transduction pathways in cassava. The findings of this study will help lay a foundation for the functional characterization of the CIPK gene family and provide an improved understanding of abiotic stress responses and signaling transduction in cassava