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Abstract
Conventional radar jamming and deception systems typically necessitate the custom design of complex circuits and algorithms to transmit an additional radio signal toward a detector. Consequently, they are often cumbersome, energy-intensive, and difficult to operate in broadband electromagnetic environment. With the ongoing trend of miniaturization of various devices and the improvement of radar system performance, traditional techniques no longer meet the requirements for broadband, seamless integration, and energy efficiency. Time-varying metasurfaces, capable of manipulating electromagnetic parameters in both temporal and spatial domains, have thus inspired many contemporary research studies to revisit established fields. In this paper, we introduce a time-varying metasurface driven radar jamming and deception system (TVM-RJD), which can perfectly overcome the aforementioned intrinsic challenges. Leveraging a programmable bias voltage, the TVM-RJD can alter the spectrum distribution of incident waves, thereby deceiving radar into making erroneous judgments about the target's location. Experimental outcomes affirm that the accuracy deviation of the TVM-RJD system is less than 0.368 meters, while achieving a remarkable frequency conversion efficiency of up to 96.67%. The TVM-RJD heralds the expansion into a wider application of electromagnetic spatiotemporal manipulation, paving the way for advancements in electromagnetic illusion, radar invisibility, etc.
© 2024 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Electromagnetic (EM) metamaterials, composed of periodically arranged sub-wavelength meta-atoms, are capable of generating novel physical phenomena and unique EM functionalities unattainable with natural materials [1]. Metasurfaces are 2-D representations of EM metamaterials [2], and have been widely researched owing to their easy processing and unique EM manipulation capabilities [3–9]. Capable of controlling the phase, amplitude, and polarization of EM waves [10,11], metasurfaces have broad applications in various scenarios, such as target invisibility [12–16], beam steering [17–23], and focusing [24–28]. However, the EM properties of traditional metasurfaces are fixed once they are fabricated, limiting their applications. Reconfigurable metasurfaces can change their EM responses via external excitation, allowing them to realize various EM manipulations flexibly. This capability facilitates their close integration with electronic information systems, contributing to their status as a research hotspot in the field [29–33]. For example, tunable metasurfaces loaded with active elements such as varactor diodes have been extensively studied in EM illusion and cloaking devices. Due to the reconfigurable characteristics, such devices can achieve flexible false scattering or invisibility responsive to the surrounding environment under the real-time control of programmable bias voltage [34,35]. Benefiting from the extraordinary capabilities of metasurface in controlling spatial EM scattering and the relentless efforts of researchers, the long-standing dream of cloaking appears to be gradually materializing. However, most existing works on EM invisibility and illusion focus solely on the spatial domain, with limited exploration in the temporal domain.
Time-varying metasurface (TVM) is a reconfigurable metasurface controlled by a time-modulated voltage waveform, typically exhibiting periodic variations in its reflection or transmission coefficient over time. Based on the Fourier transform principles, EM waves subjected to periodic time modulation will exhibit responses in the frequency domain [36]. Due to their unique EM performance in both temporal and spatial domains, TVMs have been extensively researched in fields such as radar jamming and deception (RJD) [37–40], invisibility cloaks [41,42], and communication systems [43–46]. Recent studies indicate that TVMs can control the spectrum distribution of scattered signals, envisioning their application in Doppler velocity cloak [47,48]. Despite the promising results of these studies, their applicability is limited to monochromatic signal incidences. In practical scenarios, radar detection typically employs broadband signals, such as frequency-modulated continuous wave (FMCW) signals. Moreover, in real-world scenarios, it is crucial to consider not only velocity invisibility but also range deception. In conventional RJD techniques, the jammers function by transmitting an additional radio signal towards a detector. However, complex signal processing and large system architectures increase the reliance on circuits and algorithms. As a result, such devices are often cumbersome, energy-intensive, and costly.
In this work, we propose an efficient broadband radar range jamming strategy that overcomes the aforementioned intrinsic limitation using an elaborate TVM. Drawing inspiration from the Fourier transform principle, the programmable metasurface is capable of manipulating detection signals in the frequency domain at will under the control of time-varying voltage. Additionally, to achieve a more convincing deception effect, modulation voltage waveforms are tailored in various frequency bands, ensuring wideband and efficient frequency conversion. Simulations and experiments in FMCW radar range jamming scenarios validate the feasibility of the proposed broadband time-varying metasurface driven radar jamming and deception system (TVM-RJD), with results basically aligning with theoretical derivations. This approach can induce radar operators to misjudge by displaying a false target at predetermined locations as desired. The designed TVM is lightweight, easily integrable, and energy-efficient, and the jamming device can cope with detectors in different frequency bands by changing the modulation voltage waveform in a programmable way. Consequently, this method is compatible with compact systems like unmanned aerial vehicles (UAVs), potentially unlocking new possibilities such as real-time adaptability and flexibility with the support of algorithms. The integration of TVM and RJD significantly reduces the signal processing complexity and simplifies the architecture of the RJD system. This approach opens a feasible way for advancements in EM illusion, anti-identification, and radar invisibility.
2. Principle
To illustrate the function and working principle of the designed TVM-RJD, a potential application scenario, as depicted in Fig. 1, is presented. In this scenario, a UAV in the sky, located at an actual distance R from the detector, is perceived by the detection source to be at a distance $R + \Delta R$. Consequently, the UAV can obscure its actual distance, causing the detector to erroneously perceive it as being at a more distant or closer location. The success of this deception against the detector is attributed to the TVM-RJD installed on the UAV, comprising a programmable TVM and a time-variant control system. The TVM, consisting of an array of active meta-atoms with varactor diodes, can be attached to the target surface without modifying the original shape of the object due to its ultra-thin nature. The time-variant control system, composed of a computer and an arbitrary waveform generator (AWG), generates the required arbitrary waveform files, as calculated by MATLAB software on the computer, and then downloads them to the AWG for TVM control.
Fig. 1. Schematic of the proposed broadband TVM-RJD and its application in radar range jamming. Under the control of the AWG, the TVM can perform frequency conversion on incident FMCW signals. When the TVM mounted at the bottom of the UAV is operational, a target at the actual distance R will exhibit a distance deviation $\Delta R$ in radar observations.
Recent research has shown that when the reflection phase of the metasurface varies continuously and linearly over a period ${T_m}$, the incident wave frequency ${f_i}$ is efficiently converted into a new reflected wave frequency ${f_r}$, thereby generating an artificial frequency shift ${f_m} = |{f_r} - {f_i}|$ [49]. The linear variation in the phase of the reflection coefficient over time follows the expression:
wherein ${f_m}$ can be artificially controlled by modulating the voltage applied across the varactor diode, satisfying ${f_m} = 1/{T_m}$. When the phase of the metasurface is linearly increased over time due to the applied modulation signal, the reflected wave frequency will be blue-shifted, relative to the incident wave frequency, i.e., ${f_r} = {f_i} + {f_m}$. Conversely, the frequency of the incident wave will be red-shifted, i.e., ${f_r} = {f_i} - {f_m}$.In FMCW radar, a continuous wave of varying frequency is emitted during the sweep cycle. The echo, reflected by an object, exhibits a certain frequency difference compared to the transmitted signal. This frequency difference is measurable and provides information regarding the distance between the target and the radar. For a sawtooth wave-modulated FMCW radar, the formula for calculating the detection distance is [37]:
wherein $\Delta {f_{total}}$ represents the total frequency difference between transmitted and reflected waves, ${K_r}$ is the frequency modulation slope, and c is the speed of light. Utilizing the metasurface frequency conversion principle, the frequency difference $\Delta {f_{total}}$ can be controlled by altering the frequency of the echo, which in turn changes the distance R. In conjunction with Fig. 2, we can offer a clearer analysis of the principles behind the proposed range jamming method. The radar emits signals and the signals will be modulated by the metasurface on the target, which can change the frequency of the reflected wave. This change results in false range calculation of radar, as the total frequency difference $\Delta {f_{total}}$ is influenced by both the distance-induced difference $\Delta {f_R}$ and the time modulation-induced frequency conversion ${f_m}$, i.e., $\Delta {f_{total}} = |\Delta {f_R} \pm {f_m}|$. By adjusting the modulation frequency ${f_m}$, the total frequency difference $\Delta {f_{total}}$ can be altered, allowing for deception to be achieved. Notably, applying a down-conversion voltage signal increases the detected distance, while an up-conversion signal decreases it.Fig. 2. The principle of FMCW radar range jamming. ${f_m}$ and $\Delta {f_R}$ represent the frequency differences caused by time modulation and target distance, respectively. $\Delta {f_{total}}$ denotes the total frequency difference that is known as the intermediate frequency in radar systems and can be obtained from the mixer. ${f_0}$ is the initial frequency of the chirp signal sweep.
3. Simulation and meta-atom design
Simulation verification of the TVM-based RJD was carried out in MATLAB using the FMCW radar ranging simulation algorithm. The radar was set to operate at 5 GHz, with a sweep bandwidth of 200 MHz and a period of 10 ms, aiming at a target distance of 3 m. By means of performing periodic linear phase modulation on the incoming wave signal and changing the modulation frequency, the results of multiple simulations as shown in Fig. 3(a) were obtained. With modulation frequencies set at 2, 4, 6, 8, and 10 KHz, the original 3 m peak is shifted to 18, 33, 48, 63, and 78 m correspondingly. In simple terms, a 2 KHz increase in the modulation frequency results in a 15 m change in distance, which is consistent with calculations from Eq. (2).
Fig. 3. (a) Simulation results of periodic linear phase modulation interference in radar range jamming. (b) Structure of the proposed unit cell. Each unit consists of an interdigital pattern and a solid metallic ground, located at the top and bottom of a substrate with a relative permittivity of 2.65. The periodicity along the x and y directions is $P = 15{\; }mm.$ and the thickness of substrate is $T = 2.5{\; }mm.$ The top-layer interdigital pattern's structural parameters include $W1 = 0.3{\; }mm,{\; }W2 = W3 = 0.5{\; }mm,{\; }L1 = 1.2{\; }mm,{\; }L2 = 1.5{\; }mm,\textrm{\; and\; }M = 1.5{\; }mm.$ A varactor diode is centrally positioned to modify the unit's EM characteristics by adjusting the reverse bias voltage. (c) and (d) Curves show the frequency-dependent relationship of the metasurface's reflection coefficient corresponding to different capacitance values.
Based on the theoretical analysis and simulation verification, our initial step was to design a meta-atom with the range of phase change covering as close to 360° as possible. To achieve this, we carefully designed a tunable meta-atom structure, as shown in Fig. 3(b). The unit cell comprises a top-layer interdigital structure and a bottom metallic ground, separated by a low-loss F4B substrate. A varactor diode is positioned at the central gap of the top layer pattern, with its capacitance modulated by applying a DC bias voltage across the diode through bias lines. To ensure broadband tunability and high reflectivity, we selected the MAVR-000120-14110P varactor diode produced by MACOM, which offers a high capacitance ratio and low parasitic resistance. The junction capacitance of the varactor varies from 0.14 pF to 1.15 pF in response to changes in the applied DC bias voltage.
To attain large reflection phase differences and reflectivity, structural parameters were simulated and optimized in CST Studio Suite 2020 (CST). The final dimensions of the interdigital structure are $W1 = 0.3\; mm,\; W2 = W3 = 0.5\; mm,\; L1 = 1.2\; mm,\; L2 = 1.5mm,\; \textrm{and}\; M = 1.5mm.$ This coupled structure enhances the capacitance between the two metal sheets. The substrate's thickness, set at $T = 2.5\; mm,$ features a relative permittivity of 2.65 and a loss tangent of 0.001. In CST simulations, an $RLC$ lumped element was used to model the MAVR-000120-14110P varactor diode. Figure 3(c) illustrates that within the 4.20 - 6.36 GHz, the unit achieves over 300° phase difference, up to 335°. Figure 3(d) shows that the resonant frequency shifts from 4.05 to 8.76 GHz as the capacitance ${C_T}$ varies from 1.15 to 0.14 pF, and the reflection amplitude remains above −2.11 dB across this band. These results indicate that the designed tunable unit cell exhibits superior performance, encompassing a broad range of phase coverage and high reflectivity.
4. Experimental results and discussion
To validate the proposed TVM-RJD method, a reconfigurable metasurface comprising 32 × 29 meta-atoms was fabricated on an F4B substrate $({\mathrm{\varepsilon } = 2.65} )$ using economical PCB technology, as shown in Fig. 4(a). The fabricated sample measures 500 × 490 mm2 in total area with an actual thickness of 2.58 mm. Using surface mount technology (SMT), 928 MAVR-000120-14110P varactor diodes were soldered at the center of each unit, with all units sharing the same reverse bias voltage via an external interface. For achieving a reflection phase change over a modulation period, closely resembling the linear variation shown in Fig. 4(b) and thereby enhancing frequency conversion efficiency, the reflection phase under reverse biases ranging from 0-10 V was tested. Figure 4(c) presents the measured variation curves of the reflection phase with bias voltage at several operating frequencies, normalized between −180° and 180°. Although the actual maximum phase coverage did not reach the simulated 335° due to manufacturing errors, the measured 321° phase range sufficiently validates our approach.
The aforementioned modulating signal can make the reflection phase of the metasurface vary linearly with time. If the reflection phase of the metasurface exhibited a linear relationship with the bias voltage, it would be feasible to control the varactor diodes with the simple voltage signals that linearly increase with time. However, as illustrated in Fig. 4(c), such a relationship is strongly nonlinear. It is mainly due to the nonlinear response of the metasurface to the effective capacitance of the loaded varactor diodes and the inherent nonlinear response of the varactor diode itself. To achieve linear phase variation with time, it is necessary to drive the diodes with nonlinear voltage signals. Figure 4(d) provides an example periodic modulation voltage waveform at 5 GHz, calculated using MATLAB. The control system, comprising a computer and an AWG, generates the required modulation voltage waveform for the TVM. Based on the measured relationship between the reflection phase and bias voltage, the modulation voltage waveform file is calculated using MATLAB on the computer and downloaded to the AWG for output. Altering the modulation signal frequency can achieve varying levels of displacement in radar detection distance. Notably, the TVM made as a validation operates under y-polarization excitation, but our strategy can still be extended to full-polarization.
Fig. 4. (a) The physical model of the TVM, composed of 32 × 29 units. Each unit connects to an external interface via bias lines, sharing a common reverse bias voltage. (b) and (d) Curves depict periodic linear phase changes required for frequency conversion by the TVM and the time-modulated voltage waveform at a 5 GHz operating frequency. The illustrated modulation frequency is 3 KHz, with the blue line representing frequency up-conversion and the red line indicating frequency down-conversion. (c) Measured curves of the reflection phase at various operating frequencies as a function of reverse bias voltage.
To achieve the anticipated deception effect, the TVM should possess an optimally efficient frequency conversion rate. As an illustration, Fig. 5 presents the measured spectral power distributions of the reflected waves from the TVM under vertical irradiation by a 5 GHz monochromatic signal at various down-conversion modulation frequencies. When the modulation signal is applied, the original main peak shifts to the left by ${f_m}$. Although numerous harmonics are inevitably generated, the frequency component at the main peak remains significantly larger than those of other harmonic components. The designed TVM, controlled by customized modulation waveforms, achieves a maximum frequency conversion efficiency of 96.67%, endowing it with exceptional detection interference capabilities.
Fig. 5. Measured spectral power distributions of the reflected waves from the TVM under vertical irradiation by a 5 GHz signal at various down-conversion modulation frequencies.
For this experiment, an FMCW radar module operating in the 4-6 GHz frequency band was selected. The radar signal was configured with a sweep bandwidth of 200 MHz and a period of 10 ms. As depicted in Fig. 6(a), the antenna was positioned 3 m from the center of the TVM and directed vertically to incident chirp signals. A computer, connected to an AWG, functioned as the controller, downloading pre-calculated arbitrary waveform files to the AWG for controlling the TVM. Additionally, this computer was connected to the radar module, acting as a data collector to gather distance data. To eliminate external interference, the experiment was conducted entirely within an anechoic chamber.
Fig. 6. (a) Experiment setup for TVM-RJD, the polarization direction of the electric field is consistent with the feed direction of the horn. (b) Measured results without modulation, TVM is illuminated by the chirp signal in the 4.9-5.1 GHz, and the real target is located at 2.744 m. (c) Measured results under the modulation signal with a frequency of 2 KHz, TVM is illuminated by the chirp signal in the 4.6-4.8 GHz, and the false target is located at 17.836 m. (d) Measured results under the modulation signal with a frequency of 4 KHz, TVM is illuminated by the chirp signal in the 4.9-5.1 GHz, and the false target is located at 32.928 m. (e) Measured results under the modulation signal with a frequency of 6 KHz, TVM is illuminated by the chirp signal in the 5.2-5.4 GHz, and the false target is located at 48.02 m. (f) Measured results under the modulation signal with a frequency of 8 KHz, TVM is illuminated by the chirp signal in the 5.5-5.7 GHz, and the false target is located at 63.112 m. (g) Measured results under the modulation signal with a frequency of 10 KHz, TVM is illuminated by the chirp signal in the 5.8-6.0 GHz, and the false target is located at 77.518 m.
For illustration, the jamming results are demonstrated for a radar operating at a center frequency of 5 GHz. When the TVM was not modulated, the radar-detected waveform, as shown in Fig. 6(b), corresponded to a distance of 2.744 m, with the discrepancy from the actual distance mainly due to radar resolution. Theoretically, applying modulation voltages varying in 2 KHz steps from 2 KHz to 10 KHz, the radar range offsets, as per Eq. (2), should have been 15, 30, 45, 60, and 75 m, respectively. The measured radar detection waveform, as shown in Figs. 6(c)-(g), exhibits wave crests offset by 15.092, 30.184, 45.276, 60.368, and 74.774 m, respectively. The experimental results were within ±0.368 m of the theoretical calculations, possibly due to radar resolution and the metasurface's phase not covering a full 360°. Furthermore, the amplitude difference introduced from the phase modulation will reduce frequency conversion efficiency and promote the generation of higher-order harmonic components, consequently degrading the jamming performance. Compared to Fig. 6(b), Figs. 6(c) and (d) show residual peaks at the actual target location, and a smaller false target appears at a position further than anticipated. Therefore, to enhance the persuasiveness of the range jamming, it is necessary to improve the reflectivity of the metasurface. It is noteworthy that, as demonstrated in the insets of Figs. 6(c)-(g), the modulation voltage waveform controlling the TVM changes in response to the irradiation of chirp signals with different center frequencies. Due to the radar module's range limitations, peak shifts exceeded the observation range when the modulation signal frequency was increased beyond a specified value. Consequently, the actual range offset capability of the proposed radar deception system is greater than 80 m.
5. Conclusion
In conclusion, we have experimentally demonstrated a broadband TVM-RJD strategy, which significantly reduces the reliance on complicated architecture as well as device size and cost of the systems. A reconfigurable TVM, composing a 32 × 29 array of active units equipped with varactor diodes, can alter the spectrum distribution of EM waves under the control of programmable bias voltage signals. The results indicate that the proposed deception approach can convincingly confuse radar detection in different frequency bands by altering modulation signal characteristics in a programmable way. Experimental results align with simulations and theoretical derivations, achieving a deviation capability of at least 80 m with an error of less than 0.368 m and a frequency conversion efficiency of up to 96.67%. The proposed TVM-RJD provides a lightweight and energy-efficient hardware solution for various EM terminals. This technique holds promise for applications in EM illusion and radar invisibility.
Funding
National Natural Science Foundation of China (62071423); Natural Science Foundation of Zhejiang Province (LQ21F050002, LR23F010004); Top-Notch Young Talent of Zhejiang Province; Key Research and Development Program of Zhejiang Province (2022C01036, 2024C01160); Fellowship Program of China Postdoctoral Science Foundation (GZB20230654).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data can be obtained from the authors upon reasonable request.
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