Average Sidelobe Research Articles

Calculation algorithm of optimal values of weight coefficients for coherent integration of reflections

Aformula for computing the optimal values of weight coefficients of coherent integration of reflections has been proposed for minimizing the sidelobe level of amplitude-phase characteristics (APC) of Doppler filters applicable at an arbitrary number of such filters. The AFC plots of Doppler filters were calculated and presented for the case of coherent integrator for 8, 12, 16, 24, 32 and 36 filters. The estimated average sidelobe level indicated the validity of specified proposals.

Efficient Technique for Sidelobe Suppression and PAPR Reduction in OFDM-Based Cognitive Radios

Orthogonal Frequency Division Multiplexing (OFDM) is a transceiver technology able to achieve spectrally efficient and high data rate wireless transmissions. It is also able to transmit in a non-contiguous (NC) fashion by utilizing several separate spectral whitespaces. Cognitive radio (CR) is an efficient solution for solving the spectrum scarcity problem, while OFDM based CR system, i.e. NC OFDM system is used as modulation scheme. NC-OFDM has two significant problems, high peak to average power ratio (PAPR) and high sidelobe power, which cause out of band radiation and interference with primary users (PU). Many solutions were proposed for solving either of these problems while in this paper, an efficient technique for reducing both PAPR and sidelobe power is proposed. This technique is based on using the Advanced Cancelation Carriers combined with signal set expansion and gives an efficient reduction for both PAPR and sidelobe power. A modified version is also proposed which reduces the transmitted power and decreases the complexity but gives a little reduction for PAPR. Simulation results cover the PAPR and sidelobe reduction for both the proposed technique and its modified version for comparison purposes.

Performance optimization of apodized FBG-based temperature sensors in single and quasi-distributed DWDM systems with new and different apodization profiles

In this work, different FBG temperature sensors are designed and evaluated with various apodization profiles. Evaluation is done under a wide range of controlling design parameters like sensor length and refractive index modulation amplitude, targeting a remarkable temperature sensing performance. New judgment techniques are introduced such as apodization window roll-off rate, asymptotic sidelobe (SL) decay level, number of SLs, and average SL level (SLav). Evaluation techniques like reflectivity, Full width at Half Maximum (FWHM), and Sidelobe Suppression Ratio (SLSR) are also used. A “New” apodization function is proposed, which achieves better performance like asymptotic decay of 18.4 dB/nm, high SLSR of 60 dB, high channel isolation of 57.9 dB, and narrow FWHM less than 0.15 nm. For a single accurate temperature sensor measurement in extensive noisy environment, optimum results are obtained by the Nuttall apodization profile and the new apodization function, which have remarkable SLSR. For a quasi-distributed FBG temperature sensor the Barthann and the new apodization profiles obtain optimum results. Barthann achieves a high asymptotic decay of 40 dB/nm, a narrow FWHM (less than 25 GHZ), a very low SLav of −45.3 dB, high isolation of 44.6 dB, and a high SLSR of 35 dB. The new apodization function achieves narrow FWHM of 0.177 nm, very low SL of −60.1, very low SLav of −63.6 dB, and very high SLSR of −57.7 dB. A study is performed on including an unapodized sensor among apodized sensors in a quasi-distributed sensing system. Finally, an isolation examination is performed on all the discussed apodizations and a linear relation between temperature and the Bragg wavelength shift is observed experimentally and matched with the simulated results.

Evaluation and performance enhancement for accurate FBG temperature sensor measurement with different apodization profiles in single and quasi-distributed DWDM systems

In this work, different FBG temperature sensors are designed and evaluated with various apodization profiles. Evaluation is done under a wide range of controlling design parameters like sensor length and refractive index modulation amplitude, targeting a remarkable temperature sensing performance. New judgment techniques are introduced such as apodization window roll-off rate, asymptotic sidelobe decay level, number of sidelobes, and average sidelobe level. Evaluation techniques like reflectivity, Full Width at Half Maximum (FWHM), and Sidelobe Suppression Ratio (SLSR) are also used. For a single accurate temperature sensor measurement in extensive noisy environment, optimum results are obtained by the Nuttall apodization profile with a length of 19mm and a refractive index modulation amplitude of 4×10−4. The Nuttall apodization profile achieves a high reflectivity of 97.97%, a narrow FWHM of 0.2135nm, a very low sidelobe-strength of −89.71dB, a very low sidelobe average of −90.79dB, and finally a very high SLSR of 89.62dB. For a quasi-distributed FBG temperature sensor in extensive noisy environment, optimum results are obtained by the Nuttall apodization profile with a length of 10mm and refractive index modulation amplitude of 2.6×10−4. It achieves an acceptable reflectivity of 51.43%, a narrow FWHM of 0.1985nm (less than 25GHZ), a very low sidelobe strength of −100.94dB, a very low sidelobe average of −121.74dB, a very high SLSR of 98dB and a high mainlobe slope of about 73.5dB/nm. These satisfactory results of both cases came on the expense of roll-off rate that is turned out to be low. Nuttall apodization is used to construct an optimum and reliable quasi-distributed temperature sensing system that has an adjacent channel isolation of more than 17dB at 0.4nm channel spacing, which is suitable for DWDM sensing systems.

Analysis of CDMA MIMO Beamforming Multicell Deployment Scenarios using Effective Radiation Patterns

This paper presents approximate formulas for the signal to interference ratio (SIR) gain and the corresponding bit error rate (BER) performance, when the simple concept of an effective radiation pattern of the produced ‘real world’ Multiple-Input-Multiple-Output (MIMO) beamforming radiation pattern, is considered for CDMA multi-cell/tier deployments. These simple and practical formulas can be easily used to produce initial results for a range of effective values corresponding to different operational scenarios, and hence, provide a fast and straightforward approach to evaluate performance aspects of MIMO beamforming multicell deployments. Results show that for macrocellular operational scenarios with $$10^{\circ }$$ effective beamwidth and $$-$$ 10 dB effective average sidelobe level, 11 dB SIR gain and 3.5 orders of magnitude BER improvement (compared to the omnidirectional scenario) can be achieved with an aggressive (MIMO beamforming in all cells) deployment strategy.

Sidelobe reduction in a random spherical volumetric array using frequency diversity

This paper presents a technique for reducing the average sidelobe level of a random spherical volume array using frequency diversity. The technique combines data obtained at several discrete measurement frequencies over a wide bandwidth. By using appropriate signal processing the measured data can be combined to produce an array response with an average sidelobe level that is significantly lower than that obtained at a single frequency. It is applicable to broadband blind-source location estimation and can be used to enhance holographic imaging. Experimental holographic imaging results are presented to validate the method which provides superior image resolution by reducing noise from unwanted directions. Furthermore, this technique can be used in addition to other sidelobe suppression methods such as amplitude tapers.

Mutual Coupling Effects and Their Compensation in a Time-Delay Scanned Array of Irregular Subarrays

A well-known array broadbanding technique employs rectangular subarrays to introduce time delay into phase scanned arrays. This practice results in large quantization lobes that grow with scan and frequency offset. Previous studies have shown that in the absence of mutual coupling, the use of irregular subarrays can eliminate these quantization lobes without significantly increasing the average sidelobe level. However, there has remained some concern that the phase discontinuity between subarrays would, through mutual coupling, further distort the aperture distribution and increase array sidelobes and reflections. This letter addresses the effect of mutual coupling in arrays of time-delayed subarrays and shows that the pattern distortion caused by mutual coupling in bandwidth systems is relatively small compared to the quantization lobes and can be completely eliminated at any one frequency (for “single mode” radiating elements) by selecting the proper incident voltage to compensate for the coupling. After compensation, any remaining sidelobes are due to the subarray phase discontinuity and are identical to those with mutual coupling neglected.

Interference Suppression in Uniform Linear Arrays Through a Dynamic Thinning Strategy

Thinned arrays are designed to have low average sidelobe levels. The randomness in selecting the elements that are turned on/off to achieve the low sidelobes implies that there are several arrangements of the thinned aperture that have the same or nearly the same average sidelobe level. If the same number of elements are always turned on, but the elements that are turned off change, then the array directivity does not change, but the nulls and sidelobes do. This paper presents a technique for dynamically altering the thinning configuration of a linear array in order place low sidelobe and nulls in desired directions. A set of representative results is reported and discussed to show the effectiveness of the proposed approach.

Waveguide-Slot Array Antenna Designs for Low-Average-Sidelobe Specifications

This paper discusses the design, analysis, and development of waveguide-fed planar slot arrays to achieve low-average-sidelobe specifications, as may arise in radiometer applications. Such antennas may be required to meet strict average sidelobe levels in different angular regions, and low average return loss over a specified bandwidth. In addition to Elliott's design technique, we used a Moment-Method analysis program, Ansoft's HFSS code, and results of tolerance studies using Monte Carlo simulations to meet the design objectives. Comparisons of simulated results and experimental results are also presented.

Examination of the Effect of Array Weighting Function on Radar Target Detectability

A methodology is described to assess the detectability of targets by an airborne fire control radar (FCR) operating in a medium pulse-repetition frequency (PRF) mode in the presence of strong ground clutter as a function of the transmitting and receiving antenna array weighting functions and proportion of failed array elements. It describes the radar, antenna, and clutter modelling processes and the method by which target detectability is quantified. The detectability of targets in clutter is described using a detectability map, which provides a useful means of comparing target detectability as clutter conditions change. It concludes that the best target detectability is to be achieved using those weighting functions on transmit and receive which result in the lowest average sidelobe levels but that the margins between the more highly tapered weighting functions were small. Furthermore, it concludes that target detectability degrades as the proportion of failed elements increases. A failure of 5% of the elements gave modest, though meaningful, degradations in target detectability and would therefore form a suitable upper limit.

Collaborative Null-Steering Beamforming for Uniformly Distributed Wireless Sensor Networks

Null-steering transmit beamformers aim to maximize the received signal power in the direction of the intended receiver while substantially reducing the power impinging on the unintended receivers located in other directions. The existing null-steering beamformers may not be directly applied in wireless sensor networks (WSNs) as they do not conform with the decentralized nature of WSNs and require every node to be aware of the locations of all other nodes in the network. This manuscript presents a novel collaborative null-steering beamformer that can be implemented in uniformly distributed WSNs in which each node is oblivious of other nodes' locations. The average beampattern expression of the proposed beamformer is derived and it is shown that the beampattern associated with any arbitrary realization of the nodes' locations converges with probability one to the so-obtained average beampattern as the number of collaborating nodes grows large. Properties of the average beampattern are analytically studied. In particular, it is proven that the average gain of the proposed beamformer is inversely proportional to the number of collaborating nodes in the directions of unintended receivers and further, if a mild condition is satisfied, it is approximately equal to that of the collaborative conventional beamformer in the directions with far angular distance from any unintended receiver. It is argued that if virtual unintended receivers are assumed at proper directions, then the proposed collaborative null-steering beamformer can form an average beampattern with sidelobe peaks substantially smaller than those of the average beampattern of the collaborative conventional beamformer. To substantiate this argument, the optimal direction of a virtual unintended receiver is obtained such that its associated collaborative null-steering beamformer forms an average beam-pattern with minimal largest side-lobe peak. Depending on the number of collaborating nodes, it is further shown that the largest average sidelobe peak of the latter beamformer is up to 6.6 (dB) less than that of the collaborative conventional beamformer.

A Comparative Study of Optimization Techniques in Adaptive Antenna Array Processing: The Bacteria-Foraging Algorithm and Particle-Swarm Optimization

This paper presents a comparative study between the Bacteria-Foraging (BF) and Particle-Swarm Optimization (PSO) algorithms, and their application to the antenna-array optimization problem. The performance of the Bacteria-Foraging Algorithm is studied by varying its different parameters in beamforming and null-steering problems. Null steering along with sidelobe suppression is also done for various degrees of complexity. A generic cost function is developed, the parameters of which can be controlled to meet the requirements of the particular application. The two algorithms are compared for null depth, average sidelobe level, and rate of convergence for different numbers of interference signals. The performance of these methods are compared for output noise power for the same noise inputs. Results are shown for a linear dipole antenna-array system.

PATTERN SYNTHESIS OF CONFORMAL ANTENNA ARRAY BY THE HYBRID GENETIC ALGORITHM

Desired far-field radiation patterns of 5 × 11 conformal antenna array are synthesized using a hybrid genetic algorithm (HGA), which combines the simplified quadratic interpolation (SQI) method and the real-coded genetic algorithm (RCGA). This hybrid genetic algorithm is shown to outperform standard genetic algorithm (GA) when used to synthesize amplitude weights of the elements to satisfy specified deep notches, nulls and average sidelobe level constraints. The HGA procedure appears to be a high effective means to compensate the mutual coupling effects on the individual element patterns for the conformal antenna array.

Transcranial ultrasound focus reconstruction with phase and amplitude correction

Therapeutic and diagnostic ultrasound procedures performed noninvasively through the skull require a reliable method for maintaining acoustic focus integrity after transmission through layered bone structures. This study used a multiple-element, phased-array transducer to reconstruct ultrasound foci through the human skull by amplitude and phase correction. It was previously demonstrated that adaptive phase correction using a multiple-element, focused transducer array yields a significant correction to an acoustic field that has been distorted by the heterogeneities of the skull bone. The introduction of amplitude correction, in a regime in which acoustic pressures from individual transducer array elements are adjusted to be normalized at the focus, has demonstrated a 6% (-0.27 dB) average decrease in acoustic sidelobe acoustic intensity relative to the focal intensity and a 2% (-0.09 dB) average decrease in the full-width-at-half-maximum (FWHM) of the acoustic intensity profile at the focus. These improvements come at the expense of significant ultrasound intensity loss--as much as 30% lower (-1.55 dB)--at the focus because the amplitude correction method requires that, at constant power, a larger proportion of energy is absorbed or reflected by regions of the skull that transmit less energy. In contrast, a second correction method that distributes pressure amplitudes such that the sections of the skull which transmit more ultrasound energy are exposed with higher ultrasound intensities has demonstrated an average sidelobe intensity decrease of 3% (-0.13 dB) with no change in the FWHM at the focus. On average, there was a 2% (0.09 dB) increase in the acoustic intensity at the focus for this inverse amplitude correction method. These results indicate that amplitude correction according to the transmission properties of various segments of the skull have a clear effect on ultrasound energy throughput into a target site within the brain parenchyma.

The electromagnetic properties of aperture-synthesis telescopes shaped as Reuleaux triangles

The electromagnetic properties of aperture-synthesis telescopes in the form of an array of identical antennas sited on the sides of an equilateral Reuleaux triangle are studied in the limit that the number of antennas becomes arbitrarily large. The density of baselines in the uv-plane is derived exactly and is found to be highly irregular: it has seven singularities and 13 finite discontinuities, and is not even approximately circularly symmetric. The response pattern of the synthesized beam is computed numerically and is also found not to be circularly symmetric, although only to a moderate extent. The properties are compared with those of the circular ring array and found to be very similar: the main differences are that the synthesized beam of the Reuleaux triangle is 4 per cent broader than that of the circle with the same diameter whereas the average sidelobe level is 10 per cent lower. The Reuleaux triangle came to attention in a search for the shape of array that gives the most uniform coverage of baselines in the uv-plane, and a discussion is presented whether it really is optimal in this regard.

A low sidelobe asymmetric beam antenna for high altitude platform communications

In a communications network served by a high altitude platform, the antenna beams illuminating each cell require minimized sidelobe powers. Asymmetric beams are advantageous so that cell footprints remain circular. At millimeter wavelengths a lens antenna can have the desired properties. We have chosen a 6 km diameter cell at 32/spl deg/ elevation angle and shown how the required beam asymmetry can be implemented using an optimized polynomial for describing the lens profile. The measured average sidelobe level is below -42 dB.

Effect of aberration on the acoustic field in tissue harmonic imaging (THI)

A numerical simulation was used to study the impact of an aberrating layer on the generation of the fundamental and second-harmonic (SH) field in a tissue harmonic imaging scenario. The simulation used a three-dimensional time-domain code for solving the KZK equation and accounted for arbitrary spatial variations in all acoustic properties. The aberration effect was modeled by assuming that the tissue consisted of two layers where the interface has a spatial variation C that acted like an effective phase screen. Initial experiments were carried out with sinusoidal-shaped interfaces. The sinusoidal interface produced grating lobes which were at least 6 dB larger for the fundamental signal than the SH. The energy outside of the main lobe was found to increase linearly as the amplitude of the interface variation increased. The location of the grating lobes was affected by the spatial period on the interface variation. The inhomogeneous nature of tissue was modeled with an interface with a random spatial variation. With the random interface the average sidelobe level for the fundamental was −30 dB whereas the SH had an average sidelobe level of −36 dB. [Work supported by the NSF through the Center for Subsurface Sensing and Imaging Systems.]

Binary code imbalance and two measures of average sidelobe level

The relationship between binary code imbalance and each of two measures of average autocorrelation sidelobe level is examined. For the first one, the arithmetic mean of signed sidelobes, an especially simple relationship is derived. It is found that the size of this average is minimized when imbalance is as dose as possible to the square root of the code length. The second is the square root of the average mean square sidelobe, where the average is taken over codes having the same imbalance. For this second measure, it is found that for any code length N /spl ges/ 4, there are three extreme points: a local maximum at zero imbalance and two global minima at /spl plusmn/U/sub 0/ for some U/sub 0/ > 0 that depends on N. Formulae are derived for U/sub 0/ in terms of N valid for any integer N /spl ges/ 4. These formulae behave like (3/2)/spl radic/N asymptotically as N increases.

Sparse array technology for 3-D sonar imaging systems

A three-dimensional sonar imaging system is under development for use by Navy divers for mine-field reconnaissance and mine-hunting systems. Divers require a small, low-power, lightweight acoustic imaging system with high resolution for examining and disposing of underwater ordinance. Sparse array technology is investigated to develop a sonar requiring the minimal number of signal processing channels while maintaining the beamwidth and sidelobe structure required for high-resolution imaging. Beamwidth and sidelobe structure are examined as a function of transducer element count, channel bandwith, and element location when optimized using various cost functions. Locally optimum solutions are computed for uniformly weighted sparse arrays. Cost functions evaluated for the optimization process compute average sidelobe energy, peak sidelobe energy and a weighted average sidelobe energy. Images will be formed using multiple CCD/CMOS time-delay beamforming circuits to compute the large number of beams required to provide a real-time high-resolution image.

Experimental and theoretical capacity analysis of space-division multiple access (SDMA) with adaptive antennas

The paper focuses on TDMA air interface techniques combined with spatial multiplexing through the space division multiple access (SDMA) method. First, results from field trials performed for a variety of operational scenarios provide experimental proof of the SDMA concept for two users. Then, the capacity of the space and time division multiple access scheme is studied with the help of simulations, but from the more general and practical point of view of the produced/achieved radiation-pattern characteristics. Up to now, the analyses for the capacity of SDMA systems have ignored the critical effects that parameters, such as the scattering of the environment, the mutual coupling among the array elements or signal mismatches, have on the produced radiation pattern. The straightforward method employed in the analysis presented here takes into account these effects and hence produces a more realistic capacity assessment. The presented results for different radiation pattern beamwidths and average sidelobe levels, different frequency reuse distances and additional interference cancellation techniques highlight the trade-offs between different performance criteria and the implications to the overall system design.