Space-time Modulation Research Articles

Efficient Secure Communication in 4-D Antenna Arrays Through Joint Space–Time Modulation

Traditional directional modulation techniques always sacrifice the effective radiated power in order to produce the effect of directional modulation, thus resulting in low radiation efficiency and leading to low channel capacity or short transmission distance. Generally, a signal is first modulated at the baseband and then modulated again at the antenna. The two modulations are always regarded as two separate steps. Therefore, these directional modulation techniques fail to maximize their modulation efficiency, resulting in possible performance degradation. Aimed at this problem, this article proposed a novel secure transmission approach named joint space–time modulation (JSTM) for improving the transmission efficiency of secure communication by using 4-D antenna arrays. By selecting the appropriate modulation strategy, the space–time degrees of freedom are integrated and the signal modulation can be completed with high efficiency, thus being able to reduce the attenuation in the signal modulation process. The proposed approach is able to improve the overall gain of the 4-D arrays close to that of the phased arrays while ensuring good security performance. Numerical and experimental results demonstrate the effectiveness of the proposed approach.

Space-Time Modulation in Lossy Dispersive Media and the Implications for Amplification and Shielding

We investigate the dispersion, attenuation, amplification, and the area of solutions of electromagnetic (EM) waves in a lossy progressively disturbed medium. Approximate, rigorous, and numerical methods are fully developed for a lossy environment, and the differences, merits, and drawbacks are discussed. A new representation of the Floquet theorem accounts for the loss exposed to both the pumped wave and the signal wave. The second-order small perturbation approximation is employed to yield closed-form solutions for transverse EM waves’ dispersion relation. It is proven to be valid in small-perturbed lossy space–time-modulated (STM) media with nonsuperluminal modulation. Analyzing the dispersion relation, an elaborated sufficiency condition is proposed. Moreover, the nonunique solutions and abnormal effects created by the loss factor are analyzed thoroughly. The special harmonic amplification and shielding properties experienced in a lossy STM media are brought to attention throughout the process. The developed approximate and rigorous analytical results are finally compared to finite-difference time-domain (FDTD) simulations in a realistic test, where a signal is going through upconversion/downconversion in an STM medium. Finally, a set of useful conclusions, implications, and applications has been raised to give more insight into how amplification and shielding might be affected in a lossy STM environment.

Drifting Electrons: Nonreciprocal Plasmonics and Thermal Photonics

Light propagates symmetrically in opposite directions in most materials and structures. This fact -- a consequence of the Lorentz reciprocity principle -- has tremendous implications for science and technology across the electromagnetic spectrum. Here, we investigate an emerging approach to break reciprocity that does not rely on magneto-optical effects or spacetime modulations, but is instead based on biasing a plasmonic material with a direct electric current. Using a 3D Green function formalism and microscopic considerations, we elucidate the propagation properties of surface plasmon-polaritons (SPPs) supported by a generic nonreciprocal platform of this type, revealing some previously overlooked, anomalous, wave-propagation effects. We show that SPPs can propagate in the form of steerable, slow-light, unidirectional beams associated with inflexion points in the modal dispersion. We also clarify the impact of dissipation (due to collisions and Landau damping) on nonreciprocal effects and shed light on the connections between inflexion points, exceptional points at band edges, and modal transitions in leaky-wave structures. We then apply these concepts to the important area of thermal photonics, and provide the first theoretical demonstration of drift-induced nonreciprocal radiative heat transfer between two planar bodies. Our findings may open new opportunities toward the development of nonreciprocal magnet-free devices that combine the benefits of plasmonics and nonreciprocal photonics for wave-guiding and energy applications.

Lightweight Low‐Noise Linear Isolator Integrating Phase‐ and Amplitude‐Engineered Temporal Loops

AbstractThe quest for efficient and versatile microwave and optical isolators has recently led to spawn space–time‐modulated isolator structures. However, such space‐time isolators suffer from a large profile and complex architecture that is required for a progressive nonreciprocal space–time coupling. To overcome these limitations, here a nonmagnetic phase‐engineered temporal loop‐based isolator featuring large isolation levels, suppressed undesired time harmonics while exhibiting a low profile is proposed. The proposed isolator is composed of two temporal loops that provide desired constructive and destructive interferences of different time harmonics. Furthermore, these two loops are designed in a way to assure that the circulation and reflection of different time harmonics strengthen a unidirectional signal transmission with low insertion loss. An experimental demonstration of the proposed time‐modulated isolator is provided at microwave frequencies, featuring strong unidirectional wave transmission through the isolator with more than 27 dB contrast between the forward and backward waves, across a fractional bandwidth of 14.3%. The proposed technique outperforms alternative approaches, that is, space–time modulation, ferrite magnets, nonlinearity, and HBT/CMOS transistors. It features a highly linear response with OP1dB of higher than 31 dBm, high power rating of more than 47 dBm, and a low noise figure of 3.4 dB.

Efficient Computation of Spatially Discrete Traveling-Wave Modulated Structures

Traveling-wave modulation is a form of space-time modulation which has been shown to enable unique electromagnetic phenomena such as non-reciprocity, beam-steering, frequency conversion, and amplification. In practice, traveling-wave modulation is achieved by applying a staggered time-modulation signal to a spatially-discrete array of unit cells. Therefore, the capability to accurately simulate spatially-discrete traveling-wave modulated structures is critical to design. However, simulating these structures is challenging due to the complex space-time dependence of the constituent unit cells. In this paper, a field relation (referred to as the interpath relation) is derived for spatially-discrete traveling-wave modulated structures. The interpath relation reveals that the field within a single time-modulated unit cell (rather than an entire spatial period) is sufficient to determine the field solution throughout space. It will be shown that the interpath relation can be incorporated into existing periodic method of moments solvers simply by modifying the source basis functions. As a result, the computational domain is reduced from an entire spatial period to a single time-modulated unit cell, dramatically reducing the number of unknowns. In the context of traveling-wave modulation, this enables researchers to efficiently simulate both complex structures with patterned unit cells in addition to continuous structures with infinitesimal unit cells.

Space–time wave localization in electromechanical metamaterial beams with programmable defects

A relevant feature for various applications of phononic crystals and metamaterials is the wave localization and guiding using defects introduced into a periodic array. In this context, we present a flexible localization of elastic waves in piezoelectric metamaterial beams with space–time-modulated defects. The vibration energy, originally confined in a point defect, is gradually manipulated in space and time as the inductances of the shunt circuits of subsequent unit cells are properly controlled, perfectly inducing energy localization at the final defect position. This wave localization by programmable defects is also applied to a two-dimensional triangular lattice composed by piezoelectric metamaterial beams, which illustrates the potential and versatility of the proposed strategy in more complex systems. The space–time modulation of point defects presented in this work opens new possibilities for programmable wave localization, and hence, manipulation of vibration energy or information using smart metamaterials, with applications that may involve guiding, sensing, monitoring and energy harvesting.

Wave Analysis and Homogenization of a Spatiotemporally Modulated Wire Medium

In this paper we develop homogenization theory for spatiotemporally modulated wire medium. We first solve for the modal waves that are supported by this composite medium, we show peculiar properties such as extraordinary waves that propagate at frequencies below the cut-off frequency of the corresponding stationary medium. We explain how these unique solutions give rise to an extreme Fresnel drag that exists already with weak and slow spatiotemporal modulation. Next, we turn to derive the effective material permittivity that corresponds to each of the first few supported modes, and write the average fields and Poynting's vector. Nonlocality, nonreciprocity, and anisotropy due to the spatiotemporal modulation direction, are three inherent properties of this medium, and are clearly seen in the effective material parameters. As a figure of merit, we also derive the effective permittivity of a plasma medium with spatiotemporally modulated plasma frequency. This comparison is interesting since the plasma medium can be considered as the effective medium that is obtained by a stationary wire medium. We validate that homogenization and spatiotemporal variation are not necessarily interchangeable operations. And indeed, in certain parameter regimes the homogenization should be performed directly on spatiotemporally modulated composite medium, rather than first homogenize the stationary medium and then phenomenologically introduce the effect of the space-time modulation.

Millimeter-Wave Angle Estimation of Multiple Targets Using Space–Time Modulation and Interferometric Antenna Arrays

A new method of angle estimation of multiple targets using a distributed interferometric antenna array and wideband space-time modulation is presented in this work. Interferometric array measurements of angle of arrival are generally ambiguous in the presence of one or more targets. We propose a new method of mitigating ambiguities in interferometric measurements by multiplying the angle pseudo-spectra from multiple antenna baselines, resulting in detections at only the angles of the targets. Using a single linear frequency modulated (LFM) transmitter and a receive interferometric array with $N$ elements we show a simple and computationally efficient technique to estimate the angle of up to $\mathcal{O}(N^2)$ targets. We describe the theory behind the technique, present detailed simulations, and an experimental verification using a three-element millimeter-wave measurement system.

Multi-user Space-Time Modulation Scheme for Ultra Reliable Low Delay Communication

5G (The Fifth Generation) mobile networks are expected to achieve significant improvements in several key performance metrics, where low latency is urgently needed by many emerging IoT businesses, such as remote surgery, augmented reality, haptic Internet and IoV. In this paper, a multi-user space - time modulation scheme for super - reliable low - delay communication is studied. Based on 5 g technology support in the new wireless frame structure of the coexistence of a variety of business, for business and eMBB URLLC business coexist in the data transmission of conduct the thorough research to the downlink scheduling mechanism, this mechanism allows URLLC business data to grab some data are transmitted eMBB wireless resources, occupied in eMBB business to transmit data on punch to URLLC business can be scheduled in time, to meet the requirements of URLLC business ultra low delay.

Spacetime modulation in floating thin elastic plates

We consider the propagation of flexural-gravity waves in thin elastic plates floating atop nonviscous fluids, e.g., seawater, which are governed by a partial differential equation with Laplacian and tri-Laplacian terms. We investigate the effect of time modulation as well as spacetime modulation on thin floating elastic plates and show the peculiarity of the phenomena of the $k$-band gap and the rotated $k$-band gap in the context of flexural-gravity waves. This makes possible floating plates with nonreciprocal features and behaving as elastodynamic analogs of luminal electromagnetic metamaterials, with exotic applications in enhanced control of ocean waves, such as filtering devices, unidirectional acoustic propagation, and isolation effects and energy harvesting in maritime engineering.

Homogenization Theory of Space-Time Metamaterials

We present a general framework for the homogenisation theory of space-time metamaterials. By mapping to a frame co-moving with the space-time modulation, we derive analytical formulae for the effective material parameters for travelling wave modulations in the low frequency limit: electric permittivity, magnetic permeability and magnetoelectric coupling. Remarkably, we show that the theory is exact at all frequencies in the absence of back-reflections, and exact at low frequencies when that condition is relaxed. This allows us to derive exact formulae for the Fresnel drag experienced by light travelling through travelling-wave modulations of electromagnetic media.

Improved Pair-Wise Detections of Differential Quasi-Orthogonal Space-Time Modulation with Four Transmit Antennas

In this paper, we propose new complex and real pair-wise detection for conventional differential space–time modulations based on quasi-orthogonal design with four transmit antennas for general QAM. Since the new complex and real pair-wise detections allow the independent joint ML detection of two complex and real symbol pairs, respectively, the decoding complexity is the same as or lower than conventional differential detections. Simulation results show that the proposed detections exhibit almost identical performance with an optimum maximum-likelihood receiver, as well as improved performance compared with conventional pair-wise detections, especially for higher modulation order.

Floquet Analysis of Space-Time Modulated Metasurfaces With Lorentz Dispersion

A rigorous semi-analytical Floquet analysis is proposed for a zero-thickness space-time modulated Huygens' metasurface to model and determine the strengths of the new harmonic components of the scattered fields. The proposed method is based on Generalized Sheet Transition Conditions (GSTCs) treating a metasurface as a spatial discontinuity. The metasurface is described in terms of Lorentzian electric and magnetic surface susceptibilities, $\chi_\text{e}$ and $\chi_\text{m}$, respectively, with parameters (e.g. resonant frequency) that are periodically modulated in both space and time. The unknown scattered fields are expressed in terms of Floquet harmonics, for which the amplitudes can be found by numerically solving a set of linear equations, leading to the total scattered fields. Using existing computational techniques, the method is validated using several examples of pure-space and pure-time modulation with different modulation strengths and pumping frequencies. Finally, two cases of spacetime modulation (standing wave perturbation and a traveling wave perturbation) are presented to demonstrate the breaking of Lorentz reciprocity. The proposed method is simple and versatile and able to determine the steady-state response of a space-time modulated Huygen's metasurface that is excited with an oblique plane wave, or a general incident field such as a Gaussian beam.

Breaking reciprocity and preserving-frequency using linear acoustic metamaterials

In this paper, we introduce a linear waveguide implemented by cascading acoustic black holes (ABHs). The asymmetric wave propagation, up to 46 dB, is observed and verified in simulation and experiment. It is shown that, in comparison with the previous nonlinear acoustic diodes, our waveguide can rectify the sound without shifting the impinging sound frequency. The device is simple and easy-to-fabricate without using complex nonlinear materials and space–time modulation. This feature could open a new route for designing acoustic waveguide devices that preserve the key information.

Past, present and future of seismic metamaterials: experiments on soil dynamics, cloaking, large scale analogue computer and space–time modulations

Past, present and future of seismic metamaterials: experiments on soil dynamics, cloaking, large scale analogue computer and space–time modulations

Human-like stereo sensors using plasmonic antenna embedded MZI with space–time modulation control [Invited

A micro stereo sensor system is proposed for human sensors, where eyes, ears, tongue, nose, body, and brain are applied by six panda rings embedded in a Mach–Zehnder interferometer (MZI). The input power is applied to the upper branch of MZI and propagates within the system. The six antennas (sensors) are formed by the whispering gallery modes of the panda rings. The space–time modulation signal is applied to the MZI lower branch. The modulated stereo signals can be configured as the plasmon (electron) spin orientations, which can be identified and applied for quantum codes and quantum consciousness.

Space-Time Medium Functions as a Perfect Antenna-Mixer-Amplifier Transceiver

We show that a space-time-varying medium can function as a front-end transceiver, i.e., an antenna-mixer-amplifier. Such a unique functionality is endowed by space-time surface waves associated with complex space-time wave vectors in a subluminal space-time medium. The proposed structure introduces pure frequency up- and down-conversions and with very weak undesired time harmonics. In contrast to other recently proposed space-time mixers, a large frequency up-/down conversion ratio, associated with gain is achievable. Furthermore, as the structure does not operate based on progressive energy transition between the space-time modulation and the incident wave, it possesses a subwavelength thickness (metasurface). Such a multi-functional, highly efficient and compact medium is expected to find various applications in modern wireless telecommunication systems.

Современные устройства, антенны и отражатели с невзаимными свойствами (обзор).

The review deals with microwave antennas, devices (including reflectors), materials and media with nonreciprocal properties, which, due to their uniqueness and promising application, have become the topic of a large number of scientific studies and publications. It is noted that amplitude nonreciprocity can be realized not only with the help of magnetized ferrites and semiconductor amplifiers, but also based on parametric and nonlinear structures using space-time modulation. Therefore, polarization nonreciprocity can also be achieved on magnet-free components, including varactors, and other parametric elements. When analyzing reciprocal reflectors made both on "thin" surface structures and on bulk waveguides, it is appropriate to consider reciprocal cross-polarizing (depolarizing) "invisible"-reflectors, converting the initial polarization of the incident wave into the orthogonal polarization of the reflected wave, which can be built with using only reciprocal passive components without the use of non-reciprocal ones (ferrite circulators, gyrators, etc.). The combination of polarization and nonreciprocal properties in radar reflectors, RFID tags, etc., is an additional degree of freedom in the design of new generation electronic systems.

Saturation phenomenon of swirling spray flames at pressure antinodes of a transverse acoustic field

This paper describes an experimental investigation of a linearly-arranged multiple-injection system of three swirling spray flames, simulating an unfolded sector of an annular combustor. Here we focus on the central flame placed in the basin of a pressure-antinode (PAN) of a standing transverse wave. Its response, quantified by means of the rms CH* amplitude filtered at the forcing frequency fr, is studied as a function of the acoustic pressure rms amplitude reduced by the bulk aerodynamic pressure, Π, and parameterized by the flame power P. As Π increases, three zones are identified: a linear growth, a transition zone and a saturation zone. Data in the linear growth and transition zone reduced by an adequate parameter merge into a unique self-similar curve whatever P. In the saturation zone, the time-frequency analysis applied to photomultiplier signals shows that the frequency signal at fr degrades in intensity level and uniformity, showing a loss of robustness in the flame response to the acoustic forcing over time. The gas phase displays strongly oscillating aerodynamics that can disrupt the central recirculation zone and destructure the mixing process. The liquid-phase shows a severe space-time modulation in droplet distribution and properties at high Π. Droplets are mainly found in an annular domain, characterized by a large time-dependence and inhomogeneity: a few quite big and slow fuel droplets are detected when the acoustic pressure P′(t) is positive, while a large population of small and rapid droplets is found when P′(t) is negative. In contrast, a little populated central core widens at positive P′(t) and fills up with big and slow droplets at negative P′(t). Such a periodic clustering might thus slow down the evaporation process.

Symmetry-breaking waves and space-time modulation mechanisms in two-dimensional plane Poiseuille flow

Two distinct scenarios of spatial modulation in two-dimensional plane Poiseuille flow have been studied. The first one is based on the identification of a new family of asymmetric Tollmien-Schlichting waves (TSW) breaking the reflectional symmetry about the channel midplane. The second follows the fate of a branch of time-periodic space-modulated waves that exclusively bridge upper-branch TSW-trains of different number of replicas by means of a codimension-2 bifurcation point. These modulated waves may therefore play a relevant role in the strange saddle governing domain-filling turbulent dynamics at high Reynolds numbers.