High Sidelobes Research Articles

Multi-Phase-Center Sidelobe Suppression Method for Circular GBSAR Based on Sparse Spectrum

Circular ground-based synthetic aperture radar (GBSAR) is a new type of GBSAR with three-dimensional (3D) imaging capability. Its relatively low bandwidth-to-frequency ratio and donut shaped spectrum cause high sidelobes problem, which seriously affects the quality of SAR images. To solve this problem, a multi-phase-center sidelobe suppression method for circular GBSAR based on sparse spectrum is proposed in this paper. This method optimizes the sparse spectrum distribution of multi-phase-center circular GBSAR to suppress the originally high sidelobes. In this paper, the signal model of multi-phase-center circular GBSAR is established, the point spread function (PSF) is derived, and the relationship between the sparse spectrum distribution and the sidelobe level is also analyzed. Moreover, we provide the optimal distribution of the sparse spectrum, thus the optimal distribution of multiple phase centers can be calculated with given imaging geometry, providing the best sidelobe performance. The proposed method is verified via simulation experiments.

Frequency Scanning Single-Ridge Serpentine Dual-Slot-Waveguide Array Antenna

This paper presents a novel one-dimensional (1D) frequency scanning dual-slot-waveguide array antenna with versatile advantages such as large scan volume, high frequency sensitivity, low cross-polarization and low sidelobe. Electromagnetic waves radiate through the leaky-wave dual slots, machined on the side chambers of the single-ridge serpentine waveguide. By properly designing the serpentine waveguide, which plays the role of delay line, such 1D frequency scanning array antenna can achieve 39° scanning over a frequency range from 9.7 GHz to 10.3 GHz, and high frequency sensitivity of 65°/GHz. The dual slots with all the adjacent monomers inclined in same direction are designed to acquire low cross-polarization and avoid high-order mode radiation. The cross-polarization is 45 dB lower than the corresponding co-polarization over the whole working band. Taylor aperture distribution is employed to achieve a low sidelobe (−21 dB). The nonresonance VSWR is below 1.2, and meanwhile the resonance VSWR remains a low level, around 2.5. Furthermore, our proposed 1D array as a building block can compose a novel 2D electric scanning array, with frequency scanning in one dimension and phase scanning in the orthogonal dimension. A 2D array consisting of three waveguide elements is simulated to predict the phase scanning performance, and the results indicate that a 120° scanning performance can be obtained. At last, a 1D serpentine dual-slot-waveguide array antenna is fabricated, whose measurements show good agreement with simulations.

A Dielectric Resonator Fed Wideband Metasurface Antenna With Radiation Pattern Restoration Under its High Order Modes

In this article, a dielectric resonator (DR) excited wideband metasurface (MTS) antenna with differential feeding is proposed, which uses two modes of the MTS and the fundamental mode of the DR to achieve multimode operation and obtain wide bandwidth. Initially, based on the characteristic mode theory, relevant modes of the MTS are analyzed. Then, a DR operating at the fundamental mode ( ${\mathrm {TE}}_{\delta 11}^{\mathrm {x}}$ ) is introduced as the magnetic current source to feed the MTS, exciting the dominant mode and high order mode (HOM) of the MTS simultaneously. However, this HOM has a weak gain at the broadside direction and large side lobes. According to the superposition principle of radiation patterns, the maximum at the broadside generated by the DR is utilized to enhance the boresight gain of the HOM, thus transforming the top-weak radiation pattern of this HOM into a broadside one and improving the gain flatness. Additionally, the differential feeding technique is introduced to improve the asymmetry of radiation pattern. Furthermore, by analyzing the out-of-phase currents and the periodicity of the MTS patches, three measures —loading transverse open-ended slots, shortening periodicity and loading longitudinal slots— are introduced to reduce the high side lobe level of the HOM while preserving radiation patterns undistorted and improving the gain stability. As a result, this HOM and the dominant mode of the MTS are combined with the fundamental mode of the DR, achieving multimode operation. Finally, the proposed antenna was fabricated and measured. The measured results agree well with the simulated ones, indicating an impedance bandwidth of 51.4% (4.38-7.41 GHz), a 5–9 dBi in-band gain and a 15 dB front-to-back ratio (FBR) within the operating band.

On companding techniques to mitigate PAPR in SC-FDMA systems

Single Carrier Frequency Division Multiple Access (SC-FDMA) is being used successfully in the uplink communications of Long Term Evolution (LTE) standard. Owing to smaller envelope changes in SC-FDMA signal, it has a lower Peak to Average Power Ratio (PAPR) than Orthogonal Frequency Division Multiple Access (OFDMA) signal. But for higher order modulations, PAPR is still high and there is a requirement to mitigate PAPR in localised SC-FDMA systems. This paper discusses error function, exponential, rooting and logarithmic companding techniques that can be utilised to mitigate PAPR in SC-FDMA systems. Simulation results show that exponential companding technique provides better PAPR and Bit Error Rate (BER) performances at the cost of higher side lobes, and error function companding transform provides good PAPR and Power Spectral Density (PSD) performances at the cost of higher BER without increasing average signal power.

Millimetre-Wave Planar Phased Patch Array with Sidelobe Suppression for High Data-Rate Transmission

A design of 38 GHz planar phased patch array with sidelobe suppression for data-rate enhancement is proposed in the paper. The proposed array is formed of three 24-element subarrays of patches. Each patch has its own transmit/receive modules (TRM) consisting of a digitally controlled attenuator and phase shifter. In order to achieve high data-rate communications, the noise, especially due to the undesired signals received from the sidelobes, should be reduced with high sidelobe suppression of subarray. The sidelobe suppression of the proposed subarray is first improved to 17.92 dB with a diamond-shaped aperture and then better than 35 dB with a tapered radiation power distribution. The excellent sidelobe suppression of the antenna array is essential for the beam-division multiplexing applications when the signal sources are close to each other. The proposed design is validated experimentally, including the data-rate measurements showing that the 7 Gbps data transmission can be achieved with sufficient sidelobe suppression of the proposed design.

Multiple Delay and Sum With Enveloping Beamforming Algorithm for Photoacoustic Imaging.

Delay and Sum (DAS) is one of the most common beamforming algorithms for photoacoustic imaging (PAI) reconstruction. Based on calculating beamformed signal with simple delaying and summing, DAS can function in a quick response and is quite suitable for real-time PAI. However, high sidelobes and intense artifacts may appear when using DAS due to summing with unnecessary data. In this paper, a beamforming algorithm called Multiple Delay and Sum with Enveloping (multi-DASE) is introduced to solve this problem. Compared to DAS, the multi-DASE algorithm calculates not only the initial value of the beamformed signal but also the complete N-shaped photoacoustic signal for each pixel. Through computer simulation, a phantom experiment and experiment on human finger joint, the multi-DASE algorithm is compared with other beamforming methods in removing artifacts by evaluating the quality of the reconstructed images. Furthermore, by rearranging the calculation sequences, the multi-DASE algorithm can be computing in parallel using GPU acceleration to meet the needs of real-time clinical application.

Hybrid Turbo Coding PTS with Enhanced Switching Algorithm Employing DE to Carry Out Reduction in PAPR in AUL-Based MIMO-OFDM

High peak-to-average power ratio (PAPR) is considered as a critical issue in modern multiple-input multiple-output (MIMO) orthogonal frequency-division multiplexing (OFDM) systems. This underlying drawback can be overcome by employing partial transmit sequence (PTS) method. The complexity of computation by this method is quite high since phase factors are searched comprehensively to optimally reduce PAPR. Enhanced switching differential (ESD) algorithm is incorporated into PTS scheme to solve the problem of high search complexity. In this work, adaptive update lifting (AUL)-based wavelet transform method is proposed that enhanced the system performance by decreasing high side lobes, while enhancing implementation flexibility and sensitivity to time and frequency synchronization. Lastly, a combination of turbo coding and ESD-based PTS scheme is presented for improvement in the reduction in PAPR, bit error rate (BER) and computational complexity in AUL-based MIMO-OFDM system. Turbo decoder allows error correction and phase factor searching function in an efficient way. Analysis of the theoretical performance of the suggested scheme is performed with relevant formulas, and comparative studies with the existing techniques are also presented here. MATLAB simulation outcomes confirm that the suggested hybrid technique is able to reduce PAPR and improve BER to a significant extent with low search computational complexity compared to others.

Energy Efficient Collaborative Beamforming for Reducing Sidelobe in Wireless Sensor Networks

Collaborative beamforming (CB) in wireless sensor networks (WSNs) based on a virtual node antenna array (VNAA) can increase the transmission distance and enhance the energy efficiency of sensor nodes. However, a VNAA cannot be pre-designed like the conventional antenna arrays due to the randomly deployed sensor nodes, thereby causing a high sidelobe level (SLL) which increases the interferences. In this article, we formulate a hybrid discrete and continuous optimization problem (HDCOP) for reducing the maximum SLL. HDCOP requires to solve both the discrete and the continuous problems simultaneously, and we propose both centralized and consensus-based distributed CB strategies for solving HDCOP. For the centralized strategy, we convert HDCOP into two sub-optimization problems, and propose a discrete cuckoo search (CS) algorithm for the node location selection optimization and a continuous CS algorithm to optimize the excitation current weights of the selected nodes. For the distributed strategy, we propose a parallel distributed CS algorithm to solve the discrete and continuous parts of HDCOP simultaneously. Moreover, we propose two operating mechanisms based on these two algorithms. Simulation results verify the effectiveness of the proposed strategies for reducing the maximum SLL of CB in WSNs. Moreover, the proposed CB strategies have better performance in terms of the energy efficiency compared with other approaches such as the cross-entropy optimization-based method.

Range-Doppler Sidelobe Suppression for Pulse-Diverse Waveforms

Waveform diversity, which can be used to obtain synthetic bandwidth or mitigate range ambiguity, interference, and folded clutters, has attracted a great deal of attention in recent years. However, the range sidelobe modulation effect between agile pulses yields high range-Doppler sidelobes in pulse-Doppler (PD) radar applications, because of which traditional window functions or mismatched filters (MMFs) do not apply. In this article, a Doppler-tuned matched filter is used to equivalently obtain the PD result. Then, to mitigate the high range-Doppler sidelobes, a two-dimensional MMF (2-D MMF) is proposed, and the relevant signal-to-noise ratio (SNR) loss is deduced. A cyclic algorithm is also designed to calculate the 2-D MMF for suppressing sidelobes over desired range-Doppler regions with constrained SNR loss and mainlobe width. Several simulations and an experiment based on real measured data of an airborne platform are conducted to illustrate the effectiveness of the proposed 2-D MMF.

A Wideband High-Gain Planar Integrated Antenna Array for $E$ -Band Backhaul Applications

A novel wideband high-gain planar antenna array for $E$ -band backhaul applications is presented in this article. The antenna element is composed of a substrate integrated waveguide (SIW) cavity-backed patch and a stepped waveguide horn. The SIW cavity-backed patch is worked as wideband waveform and impedance transformer to excite the stepped waveguide horn on the upper layer. The stepped waveguide horn is employed to enhance the gain of the antenna element by increasing the radiation aperture. Based on the proposed antenna element, a $2 \times 2$ subarray with element spacing larger than a wavelength is designed. A groove, which works as secondary radiation source, is etched between the elements to improve the gain and suppress the high sidelobes of the $2 \times 2$ subarray. By taking the advantage of the groove, the $2 \times 2$ subarray achieves a maximum boresight gain of 17 dBi and an −10 dB impedance bandwidth of 21.8% from 70 to 87 GHz. For further enhancing the gain, a prototype including $16 \times 16$ elements is manufactured and measured. The experimental results confirm that the proposed antenna achieves the −10 dB impedance bandwidth of 22.4% from 71 to 88.5 GHz and a peak gain of 28.8–30.9 dBi. With the merits of a wider operating bandwidth, high gain, low profile, and low cross-polarization, the proposed antenna array is suitable for application in the $E$ -band wireless backhaul systems.

Tunable and reconfigurable microwave photonic filter based on Fourier domain optical processor and cascaded fiber rings

A tunable and reconfigurable microwave photonic filter (MPF) based on a Fourier domain optical processor (FD-OP) and cascaded fiber rings is proposed and demonstrated. A stable and multiple wavelengths source can be realized by using a broadband source and a waveshaper. The multiple wavelengths source with different wavelength spacings is achieved by programming the waveshaper. Two fiber rings of the same structure are cascaded together to achieve a high main sidelobe suppression ratio (MSSR) and a high Q factor. The FD-OP is induced to add additional phase shifts to achieve tunability. When the number of taps is 60, by introducing cascaded two-fiber rings and the FD-OP, the MSSR is 59.356 dB, the maximum Q factor is 470.895 and the 3dB bandwidth of the filter is 0.019 GHz. The simulating results show that the center frequency continuously tuned from 4.47–13.41 GHz. The tunability and reconfigurability of the MPF are well realized.

Performance analysis of sparse and small arrays

Horizontal arrays are often used to detect/separate a weak signal and estimate its direction of arrival among many loud interfering sources and ambient noise. Conventional beamforming (CBF) is robust but suffers from fat beams and high level sidelobes. To improve performance, either high resolution beamforming is used, which has its own problems, or one needs to increase the array aperture, hence requiring more elements on the array. To reduce the cost, sparse and/or nested arrays have been proposed, such as the coprime arrays, to achieve approximately the same aperture, and hence the same beam resolution with a less number of elements. For sonar arrays, deconvolution is shown to yield an unique solution as opposed to, say, image processing. For coprime array, deconvolution algorithm is shown to yield a narrower beam width, lower sidelobe levels and higher array gain than the product or min processing, Deconvolution is shown to yield superdirectivity and supergain for a small array. This raises new research issues about optimal array configuration design.

High-Performance E-Band Continuous Transverse Stub Array Antenna With a 45° Linear Polarizer

This letter presents a continuous transverse stub (CTS) array antenna with high aperture efficiency, low sidelobes, and cross-polarization characteristics over an E-band. The antenna consists of a full-corporate-feed 32-slot CTS array and a 45° linear polarizer. A multiple-aperture excited parallel-plate waveguide structure works as a linear source generator to generate the quasi-TEM wave with a uniform amplitude distribution over the whole operating band from 71 to 86 GHz. The 45° linear polarizer contains two layers of slots, shifting the linear polarization from the principal E-plane (φ = 90°) to the intended E-plane (φ = 135°). The array antenna achieves the low cross polarization and sidelobe level (SLL) while maintaining high aperture efficiency. Measurement results show that the antenna has an aperture efficiency of over 74% with the peak gains of better than 37.9 dBi over the whole E-band, while the far-field SLLs in the intended E- and H-planes approach the ETSI class IV envelope standard.

Sidelobe Leakage Reduction in Random Phase Diversity Radar Using Coherent CLEAN

Random pulse-to-pulse phase diversity provides the means to suppress range-ambiguous interference in medium-to-high pulse repetition frequency radar applications. However, Doppler filters used in random phase diversity processing tend to suffer from high average sidelobe levels. This paper shows how this problem can be addressed using Coherent CLEAN, a deconvolution algorithm originally developed to address the problem of sidelobe interference in sparse antenna arrays. In the context of random phase diversity, Coherent CLEAN can remove sidelobe leakage from the output of a random phase diversity Doppler filter bank or be applied more directly to disentangle returns from multiple range ambiguities. The algorithm is proposed as an effective and computationally efficient feasible alternative to iterative filter refinement or “worst-case” filter shaping for suppression of unexpected returns. Multiple modes of applying Coherent CLEAN are described and a new variation of the algorithm, simultaneous Coherent CLEAN, is introduced.

Range sidelobe suppression for OFDM‐integrated radar and communication signal

To improve the RadCom system of performing both data transmission and radar-sensing functions, here a waveform design method for orthogonal frequency division multiplexing (OFDM) signal is proposed to solve the problem of high range sidelobe level in radar detection process while maintaining a reasonable channel capacity. The method only transmits data in the sub-channels with preferable channel quality. Then, calculate the weight of the chosen subcarriers through the convex optimisation method to traditionally minimise the peak-to-sidelobe (PSL) ratio. Simulation results prove that, compared with traditional range sidelobe suppression method, this algorithm is more suitable for RadCom system.

Modified real‐time sub‐aperture processing algorithm for airborne high‐resolution SAR

The problem of real-time imaging in synthetic aperture radar (SAR) system is addressed here. Sub-aperture processing algorithm is usually used in the engineering applications. However, high azimuth sidelobes and edge aliasing problem usually occur in conventional sub-aperture algorithm, which degenerates the quality of the SAR image. To solve this problem, the reason for high azimuth sidelobes and edge aliasing problem is analysed, and a modified sub-aperture processing algorithm is proposed. The proposed algorithm minimises the effect of the phase discontinuity to improve the SAR image quality. Therefore, the azimuth sidelobes can be improved, and the edge aliasing phenomenon will be eliminated. Real measured airborne SAR datasets demonstrate the effectiveness of the proposed algorithm.

Spectral‐factorisation Root‐MUSIC algorithm for super‐resolution ISAR imaging

Inverse synthetic aperture radar (ISAR) is widely used in both military and civil fields recently. The fast Fourier transformation is the conventional method to obtain high-resolution ISAR images, but it is Rayleigh limited and has high sidelobes. Root-MUSIC served as a super-resolution method can break the Rayleigh limit. Whereas Root-MUSIC has computational redundancy as its roots are presented as conjugated pairs. Here, a spectral factorisation Root-MUSIC (SF-Root-MUSIC) is proposed for super-resolution ISAR imaging. SF-Root-MUSIC efficiently reduces the order of rooting-polynomial of Root-MUSIC into half and has a similar resolution ability as Root-MUSIC. The computational complexity of SF-Root-MUSIC is significantly reduced compared to Root-MUSIC. Simulated signal, MIG-25 data, and real measured Yak-42 data are employed to validate the validity of the proposal.

A Super-Resolution Sparse Aperture ISAR Sensors Imaging Algorithm via the MUSIC Technique

The conventional range-Doppler (RD) technique uses fast Fourier transformation (FFT) to generate focused images. However, the spectrum of FFT has high sidelobes and wide main lobes and the resolution of RD is limited by the radar parameters so that the RD method cannot generate super-resolution images especially with sparse aperture (SA) data. A super-resolution SA inverse synthetic aperture radar (SA-ISAR) imaging algorithm via the multiple signal classification (MUSIC) technique is proposed in this paper. The proposal uses the MUSIC algorithm to estimate the location and employs the least-squares technique to calculate the intensity of each scatterer. Then, the scatterers can be precisely depicted in the images without the interference of sidelobes and wide main lobes. The resolution of the proposal depends less on the radar parameters, and it can be further improved by using a smaller search step. Experiments obtained by processing of simulated and raw data demonstrate that the proposal can efficiently generate super-resolution images with full aperture (FA) or SA data.

Effect Analysis and Spectral Weighting Optimization of Sidelobe Reduction on SAR Image Understanding

This paper focuses on effect analysis and spectral weighting optimization of sidelobe reduction on synthetic aperture radar (SAR) image understanding. First, a statistical model for each pixel in complex-amplitude distribution of the SAR image is derived to investigate how the sidelobes of the nearby scatterers change the speckle noise. Second, the phase error is analyzed for interferometry applications when high sidelobe is in the presence. The potential benefit of using a sidelobe reduction method for improving interferometric phase accuracy is also investigated. Third, to better understand how the sidelobes affect the quality of SAR images at different sidelobe levels, with the same imaging resolution, one novel type of weights is optimized to improve the performance of spectral weighting for SAR sidelobe reduction by using one bioinspired method. Intensive sidelobe reduction experiments are carried out, and the results are verified on the probability density function (PDF), the level of speckle noise, and interferometric phase accuracy of the SAR images. The experimental results indicate that the sidelobe reduction methods can change the speckle noise and the statistical distributions for the underlying SAR imagery. One of the interesting findings is that a better equivalent number of looks (ENL) and interferometric phase accuracy can be obtained by using the optimized spectral weighting. However, the spatially variant apodization damages the PDFs of the SAR data, decreases the ENL of the SAR images, and leads to significant phase distortion for SAR interferometry.

Energy-efficient collaborative transmission algorithm based on potential game theory for beamforming

A group of collaborative nodes can efficiently complete spatial long-distance transmission tasks using beamforming technology. However, a high sidelobe level interferes with communication quality, and uneven energy consumption of nodes affects network lifetime. This paper proposes an energy-efficient collaborative transmission algorithm based on potential game theory for beamforming. First, the minimum number of cooperative nodes is determined in accordance with the energy consumption and spacing limitation condition. A group of nodes satisfying the node spacing condition is selected as cooperative nodes based on the ring array to minimize communication interference among nodes. Second, a potential game model is proposed as a joint method for optimizing the collaborative parameters of the cooperative nodes and their energy consumption balancing features. Finally, the game process is continuously executed until the Nash equilibrium is reached. According to simulation results, the sidelobe level caused by the cooperative nodes is reduced and the transmission conflicts are lessened. Thus, the quality of communication links in between nodes in the network is improved. Energy efficiency is also promoted because a balancing of energy consumption is involved in the proposed potential game model, and network lifetime is effectively prolonged accordingly.