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Space-time-modulated metasurfaces can manipulate electromagnetic waves in space and frequency domains simultaneously. In this article, an analytical design of space-time-modulated metasurfaces with modulation elements composed of ...
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Space-time-modulated metasurfaces can manipulate electromagnetic waves in space and frequency domains simultaneously. In this article, an analytical design of space-time-modulated metasurfaces with modulation elements composed of two paths, In-phase (I) and Quadrature (Q), is proposed. The model is derived analytically, the space-/frequency-domain manipulations are achieved by designing the dimension and time sequence of I and Q paths. In the specular reflection direction, an objective frequency shift of the reflected first-order harmonic can be obtained. While, in other directions, the opposite first-order harmonic can be easily controlled by changing the dimension of I/Q paths, and the objective first-order harmonic remains unchanged. Furthermore, with a small dimension of I/Q paths, the first-order harmonic can be used for beam scanning by predesigning the start time of the modulation element. To realize the space-time-modulated metasurface with the required periodically time-varying responses, 2 bit unit cells loaded with dynamically switchable pin diodes are used as I/Q modulation. Both the analytical and numerical results demonstrate that space- and frequency-domain manipulations of the reflected fields by the first-order harmonics can be simultaneously obtained. The proposed designs have potential applications in wireless communications, radar camouflaging, and cloaking.
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This paper gives a spectrum expression which is simple and has a clear physical conception for pulse frequency modulation signal , and has been confirmed by an experiment simultaneously.
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Unintentional modulation on pulse (UMOP) is the basis for specific emitter identification and has been studied for decades. However, the isolated distribution of UMOP in frequency domain has not been investigated. This Letter firs...
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Unintentional modulation on pulse (UMOP) is the basis for specific emitter identification and has been studied for decades. However, the isolated distribution of UMOP in frequency domain has not been investigated. This Letter first gives the signal model of UMOP, and then evaluates its frequency-domain distribution based on sinc-function fitting. Experiments on real-world data validate the effectiveness of the proposed method, and further indicate that UMOP mainly distributes in low frequencies with dramatically limited bandwidth. Furthermore, the proposed distribution density can also serve as a practical UMOP feature which exhibits better recognition performance than conventional spectrum features.
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The increasingly complex radio environment may cause the received low probability of intercept (LPI) radar signals to overlap in time–frequency domains. Analyzing overlapping LPI radar signals requires identifying the modulation ...
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The increasingly complex radio environment may cause the received low probability of intercept (LPI) radar signals to overlap in time–frequency domains. Analyzing overlapping LPI radar signals requires identifying the modulation type and estimating the parameters of each component. Prior research performs overlapping signal analysis as a multistage task, where each stage is designed to perform a part of the task. The multistage system will increase the calculation burden and cannot be optimized as a whole. Instead, this article proposes a novel framework for analyzing overlapping signals in a single stage. Specifically, we develop a joint semantic learning deep convolutional neural network (JSLCNN) that jointly learns three tasks, i.e., feature restoration, modulation classification, and parameter regression. Since the whole cognitive pipeline is a single network, it can be optimized end-to-end directly on cognitive performance. To verify the validity of the proposed JSLCNN, numerous comparative experiments are carried out in terms of modulation recognition and parameter estimation of overlapping signals. Experimental results demonstrate that the JSLCNN has desirable extensibility for identifying unseen signal combinations and robustness against unknown jamming. Furthermore, we show that the JSLCNN outperforms other existing approaches in generic real-time parameter estimation for LPI radar signals.
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The recognition and estimation of frequency-modulated continuous-wave (FMCW) radar signals are critical for both military electronic countermeasures and civilian autonomous driving. However, the increasingly complex radio environm...
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The recognition and estimation of frequency-modulated continuous-wave (FMCW) radar signals are critical for both military electronic countermeasures and civilian autonomous driving. However, the increasingly complex radio environment poses two new challenges for detection equipment. First, multiple FMCW radars can share the same frequency band, resulting in an overlap of received FMCW signals in the time–frequency domain. Second, unexpected signals in unknown spectrum environments can affect the cognitive performance of FMCW signals. This article proposes a semantic-based learning network (SLN) that simultaneously learns modulation classification and parameter regression of FMCW signals. By integrating recognition and estimation into a single network, the system can be optimized end-to-end as a whole. Additionally, instance-level semantic learning facilitates the parallel analysis of multiple components in overlapping signals. Finally, contrastive clustering in SLN achieves suppression of unexpected signals. Numerous comparative experimental results demonstrate that SLN has the desirable ability to simultaneously recognize and estimate FMCW signals in real-time, even in unknown spectrum environments.
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Different modulation techniques need to be combined in medium-voltage high-power variable-speed drives as the fundamental output frequency changes. Due to this fact, transitions between different modulation techniques are required...
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Different modulation techniques need to be combined in medium-voltage high-power variable-speed drives as the fundamental output frequency changes. Due to this fact, transitions between different modulation techniques are required in a real-time converter operation. However, these transitions may generate phase overcurrent due to changes in the switching patterns. This article presents a method that analyzes and models the modulation transitions. It also identifies the optimal fundamental period angle interval where the transition should take place to minimize or eliminate any overcurrent. The proposed method is generic and can be extrapolated to any converter topology and modulation technique. It is demonstrated for transitions between selective harmonic elimination pulsewidth modulation and space vector modulation on a three-level neutral-point-clamped converter through simulations and experimentally validated in a 45-kW scaled-down converter and 6.5-MW full-scale drive.
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A novel approach, named multiple time modulation frequency, is proposed for the effective suppression of the sideband radiations in 4-D antenna arrays. The time modulation frequency, different for each array element, is exploited ...
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A novel approach, named multiple time modulation frequency, is proposed for the effective suppression of the sideband radiations in 4-D antenna arrays. The time modulation frequency, different for each array element, is exploited as additional design parameter to avoid the superposition of the sideband frequencies generated by the array elements. A customized optimization strategy including the analysis of the mutual coupling effects among the array elements is proposed for the 4-D array synthesis. Selected numerical and experimental results are provided to show that the sideband level of a 4-D antenna array can be significantly reduced by properly designing the switch-on time intervals of each element, even with uniform amplitude excitations.
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This paper investigates the critical parameter design technique for a cascaded H-Bridge (CHB) with selective harmonic elimination (SHE)/compensation (SHC) for single-phase systems. The critical parameters include the filter induct...
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This paper investigates the critical parameter design technique for a cascaded H-Bridge (CHB) with selective harmonic elimination (SHE)/compensation (SHC) for single-phase systems. The critical parameters include the filter inductance, the number of H-bridge cells, the dc-bus voltage and the switching frequency. This paper derives the relationship of electrical parameters, topologies, and modulation techniques to the voltage/current harmonic envelopes, and compares the filter inductor design for an H-Bridge with sinusoidal pulsewidth modulation (SPWM), an H-Bridge (HB) with SHE/SHC and a CHB with SHE/SHC. Furthermore, a harmonic-envelope-based approach is developed to determine the optimum design parameters. The developed approach designs the harmonic envelope with two proposed criteria: the first one is the capacity criterion, which guarantees that the fundamental and controllable components are within the applicable solution range to avoid overmodulation; the other one is the attenuation criterion, which ensures that the uncontrollable harmonic complies with the grid limit. Simulations and experiments verified that a CHB designed with the proposed technique can simultaneously fulfill the compensation objectives and meet the harmonic limits.
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In this letter, a novel method for synthesizing power pattern in time-modulated linear array is proposed. First, through a study and analysis, it is investigated that both maximum sideband radiation ( $\mathrm{SR_{max}}$ ) and the...
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In this letter, a novel method for synthesizing power pattern in time-modulated linear array is proposed. First, through a study and analysis, it is investigated that both maximum sideband radiation ( $\mathrm{SR_{max}}$ ) and the total amount of sideband power ( $P_\mathrm{{SR}}$ ) of the time-modulated array are decreased when the maximum power of the main beam ( ${P_{0}}_\mathrm{{max}}$ ) at center frequency is increased toward its maximum value. Then, to synthesize the desired center frequency pattern with suppressed sideband radiations (SRs), unlike the conventional method of dealing $\mathrm{SR_{\rm {max}}}$ and $P_\mathrm{{SR}}$ other than the desired radiation parameters of the intended pattern, only ${P_{0}}_\mathrm{{max}}$ is invoked in formulating the cost function. Finally, the computational proficiency and the potential viability of the proposed method to realize the desired pattern by suppressing the SR has been illustrated through representative numerical results.
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Communication systems using orthogonal time functions for the transmission of information over radio links are characterized by the following properties: 1. The same amount of information may be transmitted per unit (pu) bandwidth...
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Communication systems using orthogonal time functions for the transmission of information over radio links are characterized by the following properties: 1. The same amount of information may be transmitted per unit (pu) bandwidth and pu time with double-sideband modulation as with single-sideband modulation. 2. No phase lock between the carriers produced at the transmitter and at the receiver is necessary for detection by correlation. Hence, communication is less affected by rapid phase changes of the carrier than in systems requiring phase lock. 3. It is possible to operate under conditions of large Doppler shift without changing the frequency of the carrier produced at the receiver, provided the Doppler shift varies sufficiently slowly.
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