255 research outputs found
Optimization Methods for Designing Sequences with Low Autocorrelation Sidelobes
Unimodular sequences with low autocorrelations are desired in many
applications, especially in the area of radar and code-division multiple access
(CDMA). In this paper, we propose a new algorithm to design unimodular
sequences with low integrated sidelobe level (ISL), which is a widely used
measure of the goodness of a sequence's correlation property. The algorithm
falls into the general framework of majorization-minimization (MM) algorithms
and thus shares the monotonic property of such algorithms. In addition, the
algorithm can be implemented via fast Fourier transform (FFT) operations and
thus is computationally efficient. Furthermore, after some modifications the
algorithm can be adapted to incorporate spectral constraints, which makes the
design more flexible. Numerical experiments show that the proposed algorithms
outperform existing algorithms in terms of both the quality of designed
sequences and the computational complexity
Hybrid Constructions of Binary Sequences With Low Autocorrelation Sideobes
In this work, a classical problem of the digital sequence design, or more precisely, finding binary sequences with optimal peak sidelobe level (PSL), is revisited. By combining some of our previous works, together with some mathematical insights, few hybrid heuristic algorithms were created. During our experiments, and by using the aforementioned algorithms, we were able to find PSL-optimal binary sequences for all those lengths, which were previously found during exhaustive searches by various papers throughout the literature. Then, by using a general-purpose computer, we further demonstrate the effectiveness of the proposed algorithms by revealing binary sequences with lengths between 106 and 300, the majority of which possess record-breaking PSL values. Then, by using some well-known algebraic constructions, we outline few strategies for finding highly competitive binary sequences, which could be efficiently optimized, in terms of PSL, by the proposed algorithms
Volumetric diffusers : pseudorandom cylinder arrays on a periodic lattice
Most conventional diffusers take the form of a surface based treatment, and as a result can only operate in hemispherical space. Placing a diffuser in the volume of a room might provide greater efficiency by allowing scattering into the whole space. A periodic cylinder array (or sonic crystal) produces periodicity lobes and uneven scattering. Introducing defects into an array, by removing or varying the size of some of the cylinders, can enhance their diffusing abilities. This paper applies number theoretic concepts to create cylinder arrays that have more even scattering. Predictions using a Boundary Element Method are compared to measurements to verify the model, and suitable metrics are adopted to evaluate performance. Arrangements with good aperiodic autocorrelation properties tend to produce the best results. At low frequency power is controlled by object size and at high frequency diffusion is dominated by lattice spacing and structural similarity. Consequently the operational bandwidth is rather small. By using sparse arrays and varying cylinder sizes, a wider bandwidth can be achieved
Nonlinear Suppression of Range Ambiguity in Pulse Doppler Radar
Coherent pulse train processing is most commonly used in airborne pulse Doppler radar, achieving adequate transmitter/receiver isolation and excellent resolution properties while inherently inducing ambiguities in Doppler and range. First introduced by Palermo in 1962 using two conjugate LFM pulses, the primary nonlinear suppression objective involves reducing range ambiguity, given the waveform is nominally unambiguous in Doppler, by using interpulse and intrapulse coding (pulse compression) to discriminate received ambiguous pulse responses. By introducing a nonlinear operation on compressed (undesired) pulse responses within individual channels, ambiguous energy levels are reduced in channel outputs. This research expands the NLS concept using discrete coding and processing. A general theory is developed showing how NLS accomplishes ambiguity surface volume removal without requiring orthogonal coding. Useful NLS code sets are generated using combinatorial, simulated annealing optimization techniques - a general algorithm is developed to extended family size, code length, and number of phases (polyphase coding). An adaptive reserved code thresholding scheme is introduced to efficiently and effectively track the matched filter response of a target field over a wide dynamic range, such as normally experienced in airborne radar systems. An evaluation model for characterizing NLS clutter suppression performance is developed - NLS performance is characterized using measured clutter data with analysis indicating the proposed technique performs relatively well even when large clutter cells exist
High-quality dense 3D point clouds with active stereo and a miniaturizable interferometric pattern projector
We have built and characterized a compact, simple and flexible 3D camera based on interferometric fringe projection and stereo reconstruction. The camera uses multi-frame active stereo as basis for 3D reconstruction, providing full-field 3D images with 3D measurement standard deviation of 0.09 mm, 12.5 Hz 3D image capture rate and 3D image resolution of 500 × 500 pixels. Interferometric projection enables a compact, low-power projector that consumes < 1 W of electrical power. The key component in the projector, a movable micromirror, has undergone initial vibration, thermal vacuum cycling (TVAC) and radiation testing, with no observed component degradation. The system's low power, small size and component longevity makes it well suitable for space applications.publishedVersio
Development of Radar Pulse Compression Techniques Using Computational Intelligence Tools
Pulse compression techniques are used in radar systems to avail the benefits of large range detection capability of long duration pulse and high range resolution capability
of short duration pulse. In these techniques a long duration pulse is used which is either phase or frequency modulated before transmission and the received signal
is passed through a filter to accumulate the energy into a short pulse. Usually, a matched filter is used for pulse compression to achieve high signal-to-noise ratio
(SNR). However, the matched filter output i.e. autocorrelation function (ACF) of a modulated signal is associated with range sidelobes along with the mainlobe.
These sidelobes are unwanted outputs from the pulse compression filter and may mask a weaker target which is present nearer to a stronger target. Hence, these
sidelobes affect the performance of the radar detection system. In this thesis, few investigations have been made to reduce the range sidelobes using computational
intelligence techniques so as to improve the performance of radar detection system.
In phase coded signals a long pulse is divided into a number of sub pulses each of which is assigned with a phase value. The phase assignment should be such that the
ACF of the phase coded signal attain lower sidelobes. A multiobjective evolutionary approach is proposed to assign the phase values in the biphase code so as to achieve
low sidelobes. Basically, for a particular length of code mismatch filter is preferred over matched filter to get better peak to sidelobe ratio (PSR). Recurrent neural
network (RNN) and recurrent radial basis function (RRBF) structures are proposed as mismatch filters to achieve better PSR values under various noise conditions, Doppler shift and multiple target environment
Optimising Sidelobes and Grating Lobes in Frequency Modulated Pulse Compression
Pulse compression is a signal processing technique used in radar systems to achieve long range
target detection capability, which is a characteristic of long duration pulse, without
compromising the high range resolution capability, which is characteristic of a short duration
pulse. For this, the received signal at the receiver is compressed by a matched filter to produce a
compressed version of the signal for better resolution. As the range resolution is inversely
proportional to the bandwidth, high range resolution is ensured by using a transmitted pulse of
greater bandwidth. LFM pulse is better used than a constant frequency pulse because of its larger
bandwidth. The bandwidth of a signal can further be increased by taking a train of pulses with
the center frequency of consecutive pulses stepped by some frequency step ∆f. A train of pulses
with each pulse of duration T, separated by time Tr gives rise to grating lobes in its
autocorrelation function (ACF), when T∆f>1. ACF of a single LFM pulse has also sidelobes of
its own. Grating lobes and sidelobes may act individually or together to mask smaller targets in
close vicinity of a larger target, hence are needed to be reduced.
In the first part of the work, two optimization algorithms called Clonal Particle Swarm
Optimization and Differential Evolution has been used to find out specific windows that shape an
LFM pulse to reduce the ACF sidelobes to their optimal minima. Temporal windows has been
found out using three coefficient window expressions and four coefficient window expressions.
Resulting windows have been found to reduce sidelobes to an extent which was not possible by
the classical windows. Grating lobes in a train of pulses can be lowered by the use of LFM
pulses instead of fixed frequency pulses. Nullification of the ACF grating lobes is possible when
T, ∆f, and B satisfy a special relationship that puts the ACF nulls due to a single LFM pulse
exactly at the positions of grating lobes. The scheme is valid if and only if Tr/T>2, which
restricts the extent of increase in bandwidth by limiting the number of frequency steps for a
signal of particular time duration. In the second part of the work presented in this thesis, a
scheme has been proposed that allows to accommodate more bandwidth by taking Tr/T=1. It
allows more number of pulses within the same signal time, and hence more number of frequency
stepping to result a larger total bandwidth
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