Low Sidelobe Level Research Articles

Supervised Reciprocal Filter for OFDM Radar Signal Processing

In this paper, we address the problem of the range-Doppler map evaluation in continuous wave radar exploiting OFDM signals. This stage is usually implemented by resorting to a suboptimal batches algorithm and a typical choice is to fragment the signal in batches with length equal to the OFDM symbol length and to apply at each batch an appropriate range compression strategy: typically, either Matched Filter (MF) or Reciprocal Filter (RF). The former provides the best performance against noise-limited scenarios, whereas the latter against clutter-limited scenarios, thanks to its high Peak SideLobe Level (PSL). Using “OFDM fragmentation” requires symbol synchronization and sets constraints on the coherent processing chain; moreover, we show that it provides a Signal-to-Noise Ratio (SNR) loss both when using MF and RF. Therefore, we investigate the case of “non-OFDM fragmentation”, which does not require synchronization and avoids setting constraints on the processing chain. Specifically, we address the case of batch lengths longer than a single OFDM symbol that can potentially reduce the SNR loss at long ranges. We find that this is effective for the MF, but causes an even higher SNR loss for the direct application of the RF filter, which still provides a low level of sidelobes. Aiming at preserving the potential benefits of the RF over the MF against the clutter-limited scenarios, we propose some modified versions of the RF for the non-OFDM fragmentation case, which are shown to offer a trade-off between SNR losses and sidelobes level control. The effectiveness of the proposed approaches is demonstrated both by providing theoretical performance prediction expressions and by using simulated analyses. To this purpose, a case study is considered for a passive radar exploiting DVB-T transmissions

Low-Sidelobe Microstrip Patch Filtenna Array Fed by Higher Order Mode SIW Cavity

In this letter, a 1 × 3 microstrip patch (MP) filtenna array with low sidelobe level fed by TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">410</sub> -mode substrate integrated waveguide (SIW) cavity is investigated. Benefiting from the fact that the magnetic field amplitude at the adjacent region of the two central quasi-TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">110</sub> modes of the TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">410</sub> -mode SIW cavity is twice that of the single quasi-TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">110</sub> mode on either side of the edge, it enables the filtenna array to obtain a binomial power distribution and then achieve low sidelobe level. In the fusion design of filtering power divider (FPD), the SIW cavity and the patch radiators are regarded as first and second resonators of filter respectively, meanwhile, the power dividing function is fulfilled by the coupling process between them. For demonstration, a filtenna array prototype centered at 13.2 GHz is designed, fabricated and measured. The measured results show that a peak gain of 11.6 dBi and a maximum efficiency of 85%. The measured sidelobe levels in <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">E</i> -planes are both below than -19.1 dB at 13.06 GHz and 13.32 GHz. Good agreement between the simulation and measurement can be observed.

High-resolution frequency-difference beamforming for a short linear array.

Conventional beamforming (CBF) is a commonly employed approach for detecting and estimating the direction-of-arrival (DOA) of acoustic signals in underwater environments. However, CBF becomes ambiguous due to spatial aliasing when the received signal's half wavelength is smaller than the array spacing. Frequency-difference beamforming (FDB) allows for processing data in the lower frequency Δf without encountering spatial aliasing by utilizing the product of array data at frequency f with its complex conjugate at frequency f+Δf. However, lower frequency results in a wider mainlobe, which can lead to poorer DOA performance for short arrays. In this paper, a fourth-order cumulants FDB method and a conjugate augmented FDB method are proposed to extend an M-element uniform linear array to 2M-1 and 4M-3 elements. The proposed methods generate narrower beams and lower sidelobe levels than the original FDB for short arrays with large spacing. And by setting the signal subspace dimension reasonably, the proposed methods can improve the weak target detection ability under strong interference compared with FDB. Last, we verify the excellent performance of the proposed methods through simulations and experimental data.

Miniaturized Broadband Dual-Polarized MIMO Antenna Array With Stable Radiation Pattern for Base-Station Application

A wideband dual-polarized MIMO antenna array having small size and stable radiation performance for base station applications is introduced. The antenna element, which is formed by four folded dipole antennas arranged in a small square aperture, is introduced to obtain excellent impedance matching and steady radiation pattern within a broad bandwidth of 36.7% (0.69-1 GHz) with low profile. Based on the high-performance element, a compact broadband dual-polarized MIMO array for mobile communications is then designed. To stabilize the radiation pattern of the MIMO array, a new method of element-shared scheme and 90° phase compensation are introduced to design the MIMO array to obtain stable half power beamwidth (HPBW) of around 64° for desired 120° sector coverage of cellular mobile communications. A special feed network with good performance is designed to excite the MIMO array. The MIMO array introduces a broad bandwidth of 43.7% (0.68-1.06 GHz) for reflection coefficient < -15 dB, as well as good isolation of more than 25 dB between all the ports. Steady radiation pattern with HPBW of 64±7° as well as low sidelobe level of below -15 dB are observed, which is desirable for modern mobile communications.

Search and Study of Marked Code Structures for a Spatially Distributed System of Small-Sized Airborne Radars.

When forming the radar situation of a terrain, in order to increase its information content and to extract useful information, multi-position spatially distributed systems for integrating multi-angle radar data established by small-sized UAV-based airborne radars are used. In this case, each radar station belonging to a multi-position system as a probing signal must have its own unique marked signal. Such a setup will allow the signals reflected from ground objects and zones to be "attached" to specific receiving-transmitting positions of the multi-position system. This requirement results from the fact that each transceiver position emits one probing signal, and then receives all the echo signals reflected from the underlying surface and previously emitted by other radar devices of the multi-position system. Such a setup of multi-position systems requires the researcher to look for and investigate specialized systems of marked code structures used to modulate the probing signals in order to identify them in a joint radar channel. Thus, the problem at hand is how to look for and investigate novel marked code structures used to generate a system of probing signals, the use of which will allow it to be "attached" to a specific receiving-transmitting position of a multi-position onboard system and to identify them in a joint radar channel in the course of the remote sensing of the underlying surface. The purpose of this work is to conduct a study on the subject of the squeak and choice of a system of code structures that have a low level of side lobes of autocorrelation functions and uniformly distributed values of the levels of the cross-correlation function. To achieve this goal, the relevance of the study is substantiated in the introduction. The second section analyzes the level of side lobes for classical and modified Barker codes with an asymmetric alphabet. On the basis of this analysis, it was concluded that it is expedient to apply this approach for codes longer than Barker codes. Therefore, in the third section, the features of the generation of M-sequences are considered. The fourth section presents a technique for searching for new marked code structures, taking into account their mutual correlation properties for modifying M-sequences in order to implement multi-positional systems. The fifth section presents computer experiments on the search for marked code structures based on the modifications of M-sequences and presents the numerical characteristics of the correlation properties of the considered marked codes. And finally, in the sixth section, the final conclusions of the study are presented and recommendations are given for their further practical application. The practical significance of this study lies in the fact that the proposed new marked code structures are necessary for the synthesis of probing signals in the implementation of spatially distributed systems that function for the high-probability detection and high-precision determination of the coordinates of physical objects and are also necessary for the formation of radar images in a multi-position mode.

Linear Antenna Array Sectorized Beam Scanning Approaches Using Element Position Perturbation in the Azimuth Plane.

In this paper, two sector beam scanning approaches (BSAs) based on element position perturbations (EPPs) in the azimuth plane are introduced. In EPP-BSA, the elements' excitations are kept constant and the elements' positions in the direction normal to the array line are changed according to a predetermined EPP pattern. The magnitude and repetition rate of the selected EPP pattern determines the steering angle of the main beam. However, EPP-BSA results in a wide scanning range with a significant increase in the side lobe level (SLL). To mitigate this drawback, a reduction in the SLL of the array pattern is firstly performed using the single convolution/genetic algorithm (SC/GA) technique and then perturbing the elements' positions in the azimuth plane. This combination between SLL reduction and EPP-BSA (SLL/EPP-BSA) results in a smaller scanning range with a relatively constant half power beamwidth (HPBW) and a much lower SLL. In addition, keeping the synthesized excitation coefficients constant without adding progressive phase shifters facilitates the manufacturing process and reduces the cost of the feeding network. Furthermore, a planar antenna array thinning approach is proposed to realize the EPP-BSA. The results are realized using the computer simulation technology (CST) microwave studio software package, which provides users with an optimized modeling environment and results in realizable and realistic designs.

Low-profile dual-polarized differential-fed filtering antenna and its novel array with low sidelobe level

This paper presents a single-layer dual-polarized filtering antenna with good out-of-band suppression. By using inherent characteristic of the patch’s eigenmode and meanwhile merging four filtering strategies, i.e., parasitic patches, slot on the driving patch, and defected groundstructure (DGS), a low-profile filtering antenna design with five radiation nulls has been realized. With the designed filtering antenna, a conventional 1×2 array with a spacing of 0.91λ is first developed. In order to reduce the sidelobe level of the array, a sharing patching scheme is introduced to decrease the interspace between the elements as 0.44λ, with the same physical size as the conventional array. In addition, a concept to measure total isolation between two kinds of the polarized ports in the array, i.e., SddVH, has been derived for the first time. The prototype of the proposed array is fabricated with a profile of 0.038λ. Measured results indicate that the designed novel array has an operational band of 3.53–4.06 GHz, SddVH better than 41 dB, a sidelobe level below −18 dB, and a suppression level above 20 dB with the stopband up to 0.5 GHz and 5 GHz.

Alternating Direction Method of Multipliers-Based Constant Modulus Waveform Design for Dual-Function Radar-Communication Systems.

In this paper, we design constant modulus waveforms for dual-function radar-communication (DFRC) systems based on a multi-input multi-output (MIMO) configuration of sensors for a far-field scenario. At first, we formulate a non-convex optimization problem subject to waveform synthesis for minimizing the interference power while maintaining a constant modulus constraint. Next, we solve this non-convex problem, iteratively, using the alternating direction method of multipliers (ADMM) algorithm. Importantly, the designed waveforms approximate a desired beampattern in terms of a high-gain radar beam and a slightly high gain communication beam while maintaining a desired low sidelobe level. The designed waveforms ensure an improved detection probability and an improved bit error rate (BER) for radar and communications parts, respectively. Finally, we demonstrate the effectiveness of the proposed method through simulation results.

THz quad-beam holographic antenna with independent beam control and low sidelobe levels

THz quad-beam holographic antenna with independent beam control and low sidelobe levels

Low-Profile Conical Beam Array Antenna Based on Concentric Annular Planar Inverted-F Antenna Elements

A planar conical beam array antenna with low profile, high gain, and low side lobe level (SLL) is proposed in this letter. The array is composed of three concentric annular planar inverted-F antenna (APIFA) elements, each of which is excited by uniform distributed feeding points with the same excitation amplitude and phase and thus obtain an axisymmetric conical beam. Shorting pins and cambered slots are employed on the APIFA element to realize impedance matching for a given number of feeding points. Synthesis of conical beam for the array can be flexibly realized by adjusting the excitation amplitudes and phases of the elements. A prototype with a low profile of 0.11 <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">λ</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> is designed, fabricated, and measured. Measured results show that the prototype achieves 11.2% (4.71 ∼ 5.27 GHz) -10-dB impedance bandwidth. Within the impedance bandwidth, a stable gain of 11.3 to 12.7 dBi, a stable beam-pointing angle (BPA) of about 15°, and a low SLL of less than -12.5dB is obtained, indicating good conical beam performance.

Focal Plane Array-Fed Shaped-Beam Transmitarray With Small-Gain Ripple and Low Sidelobe Level

A novel shaped-beam transmitarray antenna fed by a focal plane array is proposed. It features the small gain ripple and low sidelobe level. Unlike the traditional shaped-beam transmitarray designed by phase only optimization, the proposed one is put forward based on the superposition principle. An array is used as the feed. When the feed elements are excited, orthogonal beams pointing at different angles are generated. Then the expected beam can be shaped by adding up these orthogonal beams with appropriate weights. Specifically, the selection criteria for the number, the angles and the weights of these orthogonal beams are provided. Several squared-flattop beams and a flattop-cosecant squared beam are designed for validation.

Array Antenna Pattern Synthesis using Improved Particle Swarm Optimization (IPSO) Algorithm

Antenna arrays are used in many different systems, including radar, military systems, and wireless communications. The design of the antenna array has a significant impact on how well the communication system performs. The large number of pieces and the large sidelobe levels provide the biggest design hurdles for such arrays. The antenna arrays have recently been heavily thinned using optimization approaches that take advantage of evolutionary algorithms in order to lower power consumption and enhance the radiation pattern by lowering sidelobe levels. A global optimum for this kind of algorithm is not guaranteed, though, because of the stochastic nature of the resolution techniques. This work characterizes the optimal pattern synthesis of a linear array antenna using the Improved Particle Swarm Optimization (IPSO) algorithm. The main aim is to obtain a low Side Lobe Level (SLL) that avoids interference and a narrow beam width for acquiring high directivity to obtain the optimal solution established on the action of the swarm that adopts the fitness function. To achieve these targets, we analyze the optimization of the excitation amplitude and inter-element spacing of the array. In this article, we have presented the optimal power pattern obtained by two different types of excitation amplitude distributions for both uniformly spaced linear arrays and non-uniformly spaced linear arrays. In the first case of amplitude distribution, namely, non-uniform distribution of excitation amplitude, synthesis of the array pattern for three different values of inter-element spacing as well as optimized spacing are presented for different array sizes. In the second case, optimal thinning of a uniformly spaced array as well as a non-uniformly spaced (optimized) array has been presented. The IPSO algorithm provides a radiation pattern that is used to determine the set of antenna array parameters. The design of an antenna array using the IPSO algorithm gives significant enhancements when compared with a uniformly excited and uniformly spaced array. The flexibility as well as ease of implementation of the IPSO algorithm are evident from this analysis, showing the algorithm’s usefulness in electromagnetic optimization problems.

Directional modulation design for multi-beam multiplexing based on hybrid antenna array structures

For integrated sensing and communication, one important research direction is to employ various beamforming techniques to avoid interference between the two functions. In this work, based on a hybrid beamforming antenna array structure, a physical layer security technique called directional modulation (DM) is studied for multi-beam multiplexing applications. The proposed design can form a more effective directional transmission through both beamforming and DM, while multiplexing multiple user beams through a common set of analog coefficients. In this hybrid beamforming structure, only one digital-to-analog converter (DAC) is connected to each subarray, and finite-precision phase shifters are further considered. Design examples for dual-beam multiplexing with an interleaved subarray structure and a localized subarray structure, respectively, are provided, which show that the interleaved subarray structure can form narrower mainlobe and a lower sidelobe level than the localized structure and has an overall better performance.

Enhancement of Feed Source through Three Dimensional Printing.

The three-dimensional printed wideband prototype (WBP) was proposed, which is able to enhance the horn feed source by generating a more uniform phase distribution that is obtained after correcting aperture phase values. The noted phase variation obtained without the WBP was 163.65∘ for the horn source only, which was decreased to 19.68∘, obtained after the placement of the WBP at a λ/2 distance above the feed horn aperture. The corrected phase value was observed at 6.25 mm (0.25λ) above the top face of the WBP. The use of a five-layer cubic structure is able to generate the proposed WBP with dimensions of 105 mm × 105 mm × 37.5 mm (4.2λ× 4.2λ× 1.5λ), which can improve directivity and gain by 2.5 dB throughout the operating frequency range with a lower side lobe level. The overall dimension of the 3D printed horn was 98.5 mm × 75.6 mm × 192.6 mm (3.94λ× 3.02λ× 7.71λ), where the 100 % infill value was maintained. The horn was painted with a double layer of copper throughout its surface. In a design frequency of 12 GHz, the computed directivity, gain, side lobe level in H- and E- planes were 20.5 dB, 20.5 dB, -26.5 dB, and -12.4 dB with only a 3D printed horn case and, with the proposed prototype placed above this feed source, these values improved to 22.1 dB, 21.9 dB, -15.5 dB, and -17.5 dB, respectively. The realized WBP was 294 g and the overall system was 448 g in weight, which signifies a light weight condition. The measured return loss values were less than 2, which supports that the WBP has matching behavior over the operating frequency range.

A Sparse Shared Aperture Design for Simultaneous Transmit and Receive Arrays with Beam Constraints

The utilization of efficient digital self-interference cancellation technology enables the simultaneous transmit and receive (STAR) phased array system to meet most application requirements through STAR capabilities. However, the development of application scenario requirements makes array configuration technology for STAR phased arrays increasingly important. Thus, this paper proposes a sparse shared aperture STAR reconfigurable phased array design based on beam constraints which are achieved by a genetic algorithm. Firstly, a design scheme for transmit and receive arrays with symmetrical shared apertures is adopted to improve the aperture efficiency of both transmit and receive arrays. Then, on the basis of shared aperture, sparse array design is introduced to further reduce system complexity and hardware costs. Finally, the shape of the transmit and receive arrays is determined by constraining the side lobe level (SLL), main lobe gain, and beam width. The simulated results indicate that the SLL of the transmit and receive patterns under beam-constrained design have been reduced by 4.1 dBi and 7.1 dBi, respectively. The cost of SLL improvement is a reduction in transmit gain, receive gain, and EII of 1.9 dBi, 2.1 dBi, and 3.9 dB, respectively. When the sparsity ratio is greater than 0.78, the SLL suppression effect is also significant, and the attenuation of EII, transmit, and receive gains do not exceed 3 dB and 2 dB, respectively. Overall, the results demonstrate the effectiveness of a sparse shared aperture design based on beam constraints in producing high gain, low SLL, and low-cost transmit and receive arrays.

МЕТОД ПОДАВЛЕНИЯ ИМПУЛЬСНЫХ ПОМЕХ В АДАПТИВНОЙ АНТЕННОЙ РЕШЕТКЕ С ЛОГИЧЕСКОЙ ОБРАБОТКОЙ СИГНАЛОВ

A method for suppressing pulse interference in adaptive antenna arrays is proposed, characterized by the use of a procedure for logical signal processing in the main and compensation channels to form a covariance interference matrix. To form the directional pattern of the main channel, weight coefficients are used to ensure a low level of side lobes. The radiation pattern of the compensation channel is obtained by subtracting the radiation patterns of orthogonal linear antenna arrays highlighted in the opening of the antenna array. Logical processing is used to determine the time domain, which does not contain a signal component, but contains external interference. It is shown that the use of logical processing of the main and compensation channels ensures the exclusion of the signal component from the covariance interference matrix and increases the noise immunity of the antenna.

Circularly Polarized Dual-Frequency MIMO Antenna With Cosecant-Squared Radiation Pattern for Ku-Band Applications

In this paper, the realization of a circularly polarized multiple-input, multiple-output (MIMO) array antenna with a cosecant squared pattern, is presented. The proposed antenna structure consists of microstrip patches with rectangular slots etched from the top. The desired radiation pattern is generated using amplitude tapering obtained by modifying patch widths and slot lengths, based upon the amplitude coefficients of the Woodward-Lawson method for pattern synthesis. A MIMO arrangement with circular polarization is formed by connecting two such arrays excited by two ports. A <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Z</i> -shaped slot is used to minimize the mutual coupling. Using two shorting pins, the isolation is enhanced and kept above 30 dB. This arrangement offers a small size (52 mm ×24 mm × 0.8 mm), wide bandwidth, and high gain antenna operating at 13.2 and 16.6 GHz. The antenna operates in a wide bandwidth, from 12.6 GHz to 14.25 GHz, and from 16 to 17.61 GHz, with admirable directional characteristics. The achieved gain is 16.9 dB and 16.6 dB with a cosecant pattern and low side lobe level. The analysis of the, VSWR, gain, axial ratio, group delay, efficiency, and MIMO performance metrics are also investigated, both in simulation and in experiment.

Phase-Coded FMCW for Coherent MIMO Radar

The phase-coded linear-frequency-modulated continuous-wave (PC-FMCW) waveform with a low sampling processing strategy is studied for coherent multiple-input multiple-output (MIMO) radar. The PC-FMCW MIMO structure, which jointly uses both fast-time and slow-time coding, is proposed to reduce sidelobe levels while preserving high range resolution, unambiguous velocity, good Doppler tolerance, and low sampling needs. The sensing performance and practical aspects of the introduced PC-FMCW MIMO structure are evaluated theoretically and verified experimentally. The numerical simulations and experiments demonstrate that the proposed MIMO keeps the advantages of the linear-frequency-modulated continuous-wave (LFMCW) waveform, including computational efficiency and low sampling demands, while having the ability to provide low sidelobe levels with simultaneous transmission.

A Wideband 1-bit Reconfigurable Electromagnetic Surface for Monopulse Radar Applications

We present a wideband 1-bit reconfigurable electromagnetic surface (REMS) for monopulse radar applications at X-band. The proposed REMS element integrates a 1-bit reflection-type phase shifter controlled by a PIN diode. By altering the working states of the PIN diode, well-defined beam scanning is obtained. Furthermore, selecting different feeding states can realize the sum (Σ) and difference (Δ) patterns. The key design issues, including element modeling, coding design, and feed optimization, are studied to obtain good Σ and Δ beam performance. Experimental results show that REMS achieves a high gain of 23.3 dBi, a low sidelobe level of -15 dB, and a low null depth of -30 dB. The wide-angle monopulse beam-scanning up to ±60° is obtained. The gain bandwidth of -3 dB ranges from 9.35 to 11.75 GHz, with a relative bandwidth of 22.75%. Thus, the proposed multi-feed REMS brings new freedom for the Σ and Δ beam scanning, which provides wide application prospects for the monopulse radar system.

Polarization-mismatching transmissive metasurface for independent amplitude and phase control of circular polarization.

This work presents a strategy for independent control of the amplitude and phase of transmissive circular-polarization (CP) waves. The designed meta-atom consists of an elliptical-polarization receiver and a CP transmitter. By changing the axial ratio (AR) and polarization of the receiver, amplitude modulation can be realized based on polarization mismatching theory, with negligible cumbrous components. While by rotating the element, a full phase coverage enabled by the geometric phase is achieved. Subsequently, a CP transmitarray antenna (TA) with high gain and low side-lobe level (SLL) is implemented to experimentally validate our strategy, and the tested results match well with the simulated ones. During the operating band from 9.6 to 10.4 GHz, the proposed TA obtains an average SLL of -24.5 dB, a lowest SLL of -27.7 dB at 9.9 GHz, and a maximum gain of 19 dBi at 10.3 GHz, with the measured AR lower than 1 dB, which mainly benefits from high polarization purity (HPP) of the proposed elements. The proposed strategy for full amplitude-phase manipulation of CP waves together with HPP paves a way for complicated field manipulations and indicates a promising candidate in antenna applications, such as anti-jamming systems and wireless communications.