2,756 research outputs found
Sparse Array DFT Beamformers for Wideband Sources
Sparse arrays are popular for performance optimization while keeping the
hardware and computational costs down. In this paper, we consider sparse arrays
design method for wideband source operating in a wideband jamming environment.
Maximizing the signal-to-interference plus noise ratio (MaxSINR) is adopted as
an optimization objective for wideband beamforming. Sparse array design problem
is formulated in the DFT domain to process the source as parallel narrowband
sources. The problem is formulated as quadratically constraint quadratic
program (QCQP) alongside the weighted mixed -norm squared
penalization of the beamformer weight vector. The semidefinite relaxation (SDR)
of QCQP promotes sparse solutions by iteratively re-weighting beamformer based
on previous iteration. It is shown that the DFT approach reduces the
computational cost considerably as compared to the delay line approach, while
efficiently utilizing the degrees of freedom to harness the maximum output SINR
offered by the given array aperture
Quadratically Constrained Beamforming Robust Against Direction-of-Arrival Mismatch
It is well known that the performance of the minimum variance distortionless response (MVDR) beamformer is very sensitive to steering vector mismatch. Such mismatches can occur as a result of direction-of-arrival (DOA) errors, local scattering, near-far spatial signature mismatch, waveform distortion, source spreading, imperfectly calibrated arrays and distorted antenna shape. In this paper, an adaptive beamformer that is robust against the DOA mismatch is proposed. This method imposes two quadratic constraints such that the magnitude responses of two steering vectors exceed unity. Then, a diagonal loading method is used to force the magnitude responses at the arrival angles between these two steering vectors to exceed unity. Therefore, this method can always force the gains at a desired range of angles to exceed a constant level while suppressing the interferences and noise. A closed-form solution to the proposed minimization problem is introduced, and the diagonal loading factor can be computed systematically by a proposed algorithm. Numerical examples show that this method has excellent signal-to-interference-plus-noise ratio performance and a complexity comparable to the standard MVDR beamformer
Sparsity-Cognizant Total Least-Squares for Perturbed Compressive Sampling
Solving linear regression problems based on the total least-squares (TLS)
criterion has well-documented merits in various applications, where
perturbations appear both in the data vector as well as in the regression
matrix. However, existing TLS approaches do not account for sparsity possibly
present in the unknown vector of regression coefficients. On the other hand,
sparsity is the key attribute exploited by modern compressive sampling and
variable selection approaches to linear regression, which include noise in the
data, but do not account for perturbations in the regression matrix. The
present paper fills this gap by formulating and solving TLS optimization
problems under sparsity constraints. Near-optimum and reduced-complexity
suboptimum sparse (S-) TLS algorithms are developed to address the perturbed
compressive sampling (and the related dictionary learning) challenge, when
there is a mismatch between the true and adopted bases over which the unknown
vector is sparse. The novel S-TLS schemes also allow for perturbations in the
regression matrix of the least-absolute selection and shrinkage selection
operator (Lasso), and endow TLS approaches with ability to cope with sparse,
under-determined "errors-in-variables" models. Interesting generalizations can
further exploit prior knowledge on the perturbations to obtain novel weighted
and structured S-TLS solvers. Analysis and simulations demonstrate the
practical impact of S-TLS in calibrating the mismatch effects of contemporary
grid-based approaches to cognitive radio sensing, and robust
direction-of-arrival estimation using antenna arrays.Comment: 30 pages, 10 figures, submitted to IEEE Transactions on Signal
Processin
On the detection of spectral ripples from the Recombination Epoch
Photons emitted during the epochs of Hydrogen () and Helium recombination ( for HeII
HeI, for HeIII
HeII) are predicted to appear as broad, weak spectral distortions of the Cosmic
Microwave Background. We present a feasibility study for a ground-based
experimental detection of these recombination lines, which would provide an
observational constraint on the thermal ionization history of the Universe,
uniquely probing astrophysical cosmology beyond the last scattering surface. We
find that an octave band in the 2--6 GHz window is optimal for such an
experiment, both maximizing signal-to-noise ratio and including sufficient line
spectral structure. At these frequencies the predicted signal appears as an
additive quasi-sinusoidal component with amplitude about nK that is
embedded in a sky spectrum some nine orders of magnitude brighter. We discuss
an algorithm to detect these tiny spectral fluctuations in the sky spectrum by
foreground modeling. We introduce a \textit{Maximally Smooth} function capable
of describing the foreground spectrum and distinguishing the signal of
interest. With Bayesian statistical tests and mock data we estimate that a
detection of the predicted distortions is possible with 90\% confidence by
observing for 255 days with an array of 128 radiometers using cryogenically
cooled state-of-the-art receivers. We conclude that detection is in principle
feasible in realistic observing times; we propose APSERa---Array of Precision
Spectrometers for the Epoch of Recombination---a dedicated radio telescope to
detect these recombination lines.Comment: 33 pages, 16 figures, submitted to ApJ, comments welcom
Time domain synthesis of pulsed arrays
Pulsed arrays are becoming popular in new ultrawideband applications to enhance the robustness of transmitted and received signals in complex environments and to identify the angle of arrival of multiple echoes. A global synthesis technique is here proposed to shape the array field in accordance to given angle-time constraints. The synthesis problem is cast as the inverse Radon transform of a desired array mask, applying the alternate projections method to include constraints over the input signals' waveform and to improve the synthesis robustness. The unknown array currents are generated as linear combinations of Hermite-Rodriguez functions in order to achieve a simple and realizable beamforming network. The effectiveness of the method is demonstrated by many examples
Understanding synthesis imaging dynamic range
We develop a general framework for quantifying the many different
contributions to the noise budget of an image made with an array of dishes or
aperture array stations. Each noise contribution to the visibility data is
associated with a relevant correlation timescale and frequency bandwidth so
that the net impact on a complete observation can be assessed. All quantities
are parameterised as function of observing frequency and the visibility
baseline length. We apply the resulting noise budget analysis to a wide range
of existing and planned telescope systems that will operate between about 100
MHz and 5 GHz to ascertain the magnitude of the calibration challenges that
they must overcome to achieve thermal noise limited performance. We conclude
that calibration challenges are increased in several respects by small
dimensions of the dishes or aperture array stations. It will be more
challenging to achieve thermal noise limited performance using 15 m class
dishes rather than the 25 m dishes of current arrays. Some of the performance
risks are mitigated by the deployment of phased array feeds and more with the
choice of an (alt,az,pol) mount, although a larger dish diameter offers the
best prospects for risk mitigation. Many improvements to imaging performance
can be anticipated at the expense of greater complexity in calibration
algorithms. However, a fundamental limitation is ultimately imposed by an
insufficient number of data constraints relative to calibration variables. The
upcoming aperture array systems will be operating in a regime that has never
previously been addressed, where a wide range of effects are expected to exceed
the thermal noise by two to three orders of magnitude. Achieving routine
thermal noise limited imaging performance with these systems presents an
extreme challenge. The magnitude of that challenge is inversely related to the
aperture array station diameter.Comment: 27 pages, 24 figures, accepted in A&A, final versio
MVDR broadband beamforming using polynomial matrix techniques
This paper presents initial progress on formulating minimum variance distortionless response (MVDR) broadband beamforming using a generalised sidelobe canceller (GSC) in the context of polynomial matrix techniques. The quiescent vector is defined as a broadband steering vector, and we propose a blocking matrix design obtained by paraunitary matrix completion. The polynomial approach decouples the spatial and temporal orders of the filters in the blocking matrix, and decouples the adaptive filter order from the construction of the blocking matrix. For off-broadside constraints the polynomial approach is simple, and more accurate and considerably less costly than a standard time domain broadband GSC
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