Multiple-input multiple-output (MIMO) synthetic aperture radar (SAR) has received much attention due to its interesting application potentials, but effective waveform diversity design is still a ...technical challenge. In a MIMO SAR, each antenna should transmit a unique waveform, orthogonal to the waveforms transmitted by other antennas. The waveforms should have a large time-bandwidth product, low cross-correlation interferences, and a low peak-average ratio. To reach these aims, this paper proposes an orthogonal frequency division multiplexing (OFDM) chirp waveform with random matrix modulation. The designed waveforms are time-delay and frequency-shift decorrelated. Referring to MIMO SAR high-resolution imaging, the proposed OFDM chirp waveform parameters are optimally designed, and their performances are analyzed through the ambiguity function and range-Doppler-based MIMO SAR imaging algorithm. Extensive and comparative simulation results show that the waveforms have the superiorities of high range resolution, constant time domain and almost constant frequency-domain modulus, large time-bandwidth product, low peak-average ratio, and low time-delay and frequency-shift correlation peaks. More importantly, this scheme can easily generate over three orthogonal waveforms with a large time-bandwidth product.
Recently, nested and coprime arrays have attracted considerable interest due to their capability of providing increased array aperture, enhanced degrees of freedom (DOFs), and reduced mutual coupling ...effect compared to uniform linear arrays (ULAs). These features are critical to improving the performance of direction-of-arrival estimation and adaptive beamforming. In this paper, a new sparse array configuration based on the maximum inter-element spacing constraint (MISC) is proposed. The MISC array configuration generally consists of three sparse ULAs plus two separate sensors that are appropriately placed. The MISC array configurations are designed based on the inter-element spacing set, which, for a given number of sensors, is uniquely determined by a closed-form expression. We also derive closed-form expressions for the number of uniform DOFs of the MISC arrays with any number of sensors. Compared with the existing sparse arrays, the MISC array enjoys two important advantages, namely, providing a higher number of DOFs and reducing the mutual coupling effects. Numerical simulations are conducted to verify the superiority of the MISC array over other sparse arrays.
Time and frequency modulated arrays have numerous application areas including radar, navigation, and communications. Specifically, a time modulated array can create a beampattern with low sidelobes ...via connecting and disconnecting the antenna elements from the feed network, while the frequency modulated frequency diverse array produces a range-dependent pattern. In this paper, we aim to introduce these advanced arrays to the signal processing community so that more investigations in terms of theory, methods, and applications, can be facilitated. The research progress of time/frequency modulated array studies is reviewed and the most recent advances are discussed. Moreover, potential applications in radar and communications are presented, along with their technical challenges, especially in signal processing aspects.
Different from phased array providing only angle-dependent transmit beampattern, frequency diverse array (FDA) employs a small frequency increment across its array elements to provide ...range–angle-dependent transmit beampattern. This enables the array beam to scan without the need of phase shifters or mechanical steering. Since FDA has received much attention in antenna and radar signal processing societies, it is necessary to make an overview on this interesting topic. This study introduces what FDA is and why it could be exploited for radar and navigation applications from a top-level system description and appeal to the radar signal processing and system engineering communities for more investigations on this promising array technique. The status of FDA studies is overviewed and the most recent advances of FDA radar are discussed. The basic FDA system architectures are introduced, along with performance compared to a conventional phased-array. Next, guidelines for choosing good system parameters and typical implementation schemes are provided. Finally, potential applications in range and angle estimation of targets, cognitive FDA radar and low probability of identification FDA radar are discussed, along with several technical challenges.
Phased-array is widely used in communication and radar systems, but the beam steering is fixed in an angle for all the ranges. In this paper, we propose a range-angle dependent beampattern synthesis ...scheme for linear frequency diverse array (FDA) using the discrete spheroidal sequence, with an aim to focus the transmit energy in a desired two-dimensional spatial section. Different from conventional phased-arrays, FDA employs a small frequency increment, compared to the carrier frequency across the array elements. The range-angle dependent beampattern synthesis method allows the FDA to transmit energy over a desired range or angle sector. This provides a potential to suppress range-dependent clutter and interference, which is not accessible for conventional phased-arrays. The system performance of the proposed FDA is evaluated by the output signal-to-interference-plus-noise ratio (SINR). The effectiveness is verified by comprehensive numerical simulation results.
Inspired by a cognitive radar in exploiting its environment to update current operation parameters and frequency diverse array (FDA) in offering a range-dependent beampattern due to the employment of ...frequency increments across the elements, this paper proposes a moving-target tracking approach to achieve cognitive radio frequency stealth using an FDA antenna for surveillance applications. Since surveillance systems are highly visible to intercept receivers, a traditional high-gain phased-array antenna beam is replaced by a series of low-gain FDA beams with spoiled frequency increments to reduce the system visibility, without degrading the surveillance system performance. Moreover, a cognitive closed-loop update scheme is presented to update the operation parameters in real time for improved moving-target tracking performance. All of the proposed methods are verified by simulation results.
Zinc–air batteries offer a possible solution for large‐scale energy storage due to their superhigh theoretical energy density, reliable safety, low cost, and long durability. However, their ...widespread application is hindered by low power density. Herein, a multiscale structural engineering of Ni‐doped CoO nanosheets (NSs) for zinc–air batteries with superior high power density/energy density and durability is reported for the first time. In micro‐ and nanoscale, robust 2D architecture together with numerous nanopores inside the nanosheets provides an advantageous micro/nanostructured surface for O2 diffusion and a high electrocatalytic active surface area. In atomic scale, Ni doping significantly enhances the intrinsic oxygen reduction reaction activity per active site. As a result of controlled multiscale structure, the primary zinc–air battery with engineered Ni‐doped CoO NSs electrode shows excellent performance with a record‐high discharge peak power density of 377 mW cm−2, and works stable for >400 h at 5 mA cm−2. Rechargeable zinc–air battery based on Ni‐doped CoO NSs affords an unprecedented small charge–discharge voltage of 0.63 V, outperforming state‐of‐the‐art Pt/C catalyst‐based device. Moreover, it is shown that Ni‐doped CoO NSs assembled into all‐solid‐state coin cells can power 17 light‐emitting diodes and charge an iPhone 7 mobile phone.
A multiscale structure engineering of Ni‐doped CoO nanosheets from micro‐ through nano‐ to atomic scale for high‐power‐density zinc–air batteries is demonstrated. The engineered zinc–air battery based on Ni‐doped CoO nanosheets realizes sufficient mass transport, abundant catalysts active sites, and excellent intrinsic activity simultaneously, affording a record‐high discharge peak power density of 377 mW cm−2.
Boredom is a psychological phenomenon that is defined as a state of hatred or incompatibility with any kind of repetitive experience in situations where liberation from instability is not possible ...and has several consequences. Boredom is one of the important causes of decreased motivation in EFL learners, and it is necessary to identify the factors affecting it. Therefore, this literature review addresses the state of boredom in relationship with teachers' immediacy and professional commitment. Reviewing the literature has revealed that while teachers' immediacy and professional commitments affect EFL learners' boredom, other factors such as individual differences and environmental factors are in action. However, many studies confirmed that teachers play the most important role in decreasing or increasing learners' state of boredom. The findings are significant for teacher educators to design more appropriate teacher education programs.
Multiple-input multiple-output (MIMO) radar enjoys the advantage of increased degrees-of-freedom and spatial diversity gain, but it cannot effectively resolves the targets closely spaced in the same ...angle cell (but different range cells). Frequency diverse array (FDA)-MIMO radar can handle this problem by exploiting its range-dependent beampattern. FDA-MIMO radar was, thus, suggested for range-angle estimation of targets. Nevertheless, it is necessary to provide theoretical performance analysis for such a relatively new radar technique. Since multiple signal classification (MUSIC) algorithm is widely adopted in most of the FDA-MIMO literature, this paper derives the Cramér-Rao lower bound and mean square error expressions in MUSIC-based range-angle estimation algorithms for a general FDA-MIMO radar. Furthermore, the corresponding range and angle resolution thresholds in target detection and localization are also derived. Numerical results verify that the FDA-MIMO indeed outperforms conventional MIMO radar in both range-angle estimation and resolution threshold performance.
Simultaneous wireless information and power transfer (WIPT) has so far mainly relied on a phased-array antenna, but only angle-focusing capability without range focusing can be achieved for ...phased-array antennas. In this paper, we depart from the phased-array antenna and propose the use of retrodirective frequency diverse array (FDA) focusing in base station for long-range WIPT applications. In the retrodirective FDA focusing, an additional small frequency offset is added across the phase-conjugating mixers in the retrodirective array to produce both range-dimension and angle-dimension focused beamforming for energy-efficient WIPT. The FDA beampattern is decoupled in range-angle dimension and can be independently controlled through properly designing the transmit weighting vector or frequency offsets. More importantly, the proposed WIPT scheme allows self-tracking the direction-of-arrival of the pilot signals without requirement of any a priori knowledge. A maximum efficiency optimization approach based on discrete prolate spheroidal sequence is also proposed for the beampattern synthesis. In addition, robust physical-layer security can be obtained by utilizing FDA angle-range-time-dependent beampattern characteristics. The harvested energy, achievable bit rate, and achievable secrecy rate at the user receivers are derived for the WIPT, along with extensive numerical results. Both the theoretical analysis and numerical results verify the superiority of FDA over phased array in energy efficiency and security for WIPT applications.
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