Time-Frequency Analysis.

Time--frequency feature representation using energy concentration: An overview of recent advances

Starting with a real-valued N-point discrete-time signal, frequency-domain algorithms are provided for computing (1) the complex-valued standard N-point discrete-time "analytic" signal of the same sample rate; (2) the complex-valued decimated N/2-point discrete-time "analytic" signal of half the original sample rate; and (3) the complex-valued interpolated NM-point discrete-time "analytic" signal of M times the original sample rate. Special adjustment of the transform end points are shown to be necessary in order to generate proper discrete-time "analytic" signals.

Computing the discrete-time "analytic" signal via FFT

Massive MIMO is a reality—What is next?: Five promising research directions for antenna arrays

The radio-frequency (RF) electromagnetic spectrum, extending from below 1 MHz to above 100 GHz, represents a precious resource. It is used for a wide range of purposes, including communications, radio and television broadcasting, radionavigation, and sensing. Radar represents a fundamentally important use of the electromagnetic (EM) spectrum, in applications which include air traffic control, geophysical monitoring of Earth resources from space, automotive safety, severe weather tracking, and surveillance for defense and security. Nearly all services have a need for greater bandwidth, which means that there will be ever-greater competition for this finite resource. The paper explains the nature of the spectrum congestion problem from a radar perspective, and describes a number of possible approaches to its solution both from technical and regulatory points of view. These include improved transmitter spectral purity, passive radar, and intelligent, cognitive approaches that dynamically optimize spectrum use.

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Radar Spectrum Engineering and Management: Technical and Regulatory Issues

The multiple-input multiple-output (MIMO) radar delay-Doppler ambiguity function is introduced as the sum of all auto- and cross-ambiguity functions of the set of K spatially diverse waveforms. The integral of this function over the range-Doppler frequency region occupied by spread clutter relates to the clutter's power at the output of the matched MIMO receiver, and it serves as a measure of orthogonality of the waveforms in this region. We derive the upper bound on the attainable "clear area" for the ambiguity function, and we show that, with respect to the conventional single-waveform ambiguity function, the maximum clear area is K times smaller for the MIMO system.

Bounds on the Volume and Height Distributions for the MIMO Radar Ambiguity Function

In this correspondence we discuss methods to produce the discrete analytic signal from a discrete real-valued signal. Such an analytic signal is complex and contains only positive frequencies. Its projection onto the real axis is the same as the original signal. Our use stems from instantaneous-frequency estimation and time-frequency signal analysis problems. For these problems the negative frequency component of real signals causes unwanted interference. The task of designing a filter to produce an approximation to the ideal analytic signal is not as simple as its formulation might suggest. Our result is that the direct methods of zeroing the negative frequencies, or using Hilbert transform filters, have undesirable defects. We present an alternative which is similar to the "quadrature" filters used in modem designs. >

Analytic signal generation-tips and traps

In multiple-input multiple-output (MIMO) radar, the efficiency of target detection in clutter that is distributed over a certain range bin-Doppler frequency region depends on the sidelobe level of the waveformspsila auto- and cross-ambiguity functions within this region. We introduce the MIMO ambiguity function as the sum of all auto- and cross-ambiguity functions of the K MIMO radar waveforms, with the integral of this function over some range-Doppler region serving as a measure of the orthogonality (separability) of the waveforms within this region of interest. We derive an upper bound on the attainable ldquoclear areardquo for the MIMO ambiguity function, and show that, with respect to the conventional single-waveform ambiguity function, the maximum clear area is K times smaller for the MIMO system.

MIMO radar performance in clutter: Limitations imposed by bounds on the volume and height distributions for the MIMO radar ambiguity function

A coarray-based near-field, wideband synthetic aperture beamformer using stepped-frequency signal synthesis and post-data acquisition processing is presented. While coarray techniques offer significant reduction in the number of array elements for a given angular resolution, the hybrid subarray-stepped frequency realization of wideband systems simplifies implementations and offers flexibility in beamforming. Proof of concept is provided using real data collected in an anechoic chamber for several pulse shapes and array weightings.

/pdf/design-and-implementation-of-near-field-wideband-synthetic-1dgqs242cf.pdf

Design and implementation of near-field, wideband synthetic aperture beamformers

The application of multi-input multi-output (MIMO) radar concepts to HF over-the-horizon radar is considered to improve radar timeline management flexibility and to permit adaptivity on transmit. MIMO radar concepts in the literature are inconsistent and in this paper the taxonomy of MIMO radar is clarified and distinctions between different MIMO radar types drawn. The term "spatially waveform diverse radar" is introduced, and performance equivalences between element-space and beamspace orthogonality discussed. Finally, we outline significant constraints on the design of waveform sets for the case of transmit arrays where (at least for some part of the operational regime) the array inter-element spacing is less than one half wavelength.

Gordon J. Frazer

Papers

Bounds on the Volume and Height Distributions for the MIMO Radar Ambiguity Function

Analytic signal generation-tips and traps

MIMO radar performance in clutter: Limitations imposed by bounds on the volume and height distributions for the MIMO radar ambiguity function

Design and implementation of near-field, wideband synthetic aperture beamformers

Spatially Waveform Diverse Radar: Perspectives for High Frequency OTHR