Patrick M. McCormick

Practical aspects of optimal mismatch filtering and adaptive pulse compression for FM waveforms

The sensitivity impact of range straddling in the form of mismatch loss is well known. What is less appreciated, however, is the effect upon dynamic range, particularly for receive filtering that seeks to minimize range sidelobes. For FM-based waveforms, which are readily implementable in a high-power radar system, it is shown that least-squares (LS) mismatched filtering (MMF) realizes a penalty in sidelobe suppression when range straddling occurs. This degradation can be partially compensated through modification of the LS MMF implementation. Alternatively, adaptive pulse compression (APC), appropriately modified for application to FM waveforms, demonstrates robustness to both straddling and eclipsing effects. Simulated and experimentally measured results are provided to demonstrate the efficacy of these filtering approaches.

Joint polarization/waveform design and adaptive receive processing

Leveraging the design freedom provided by the recently developed polyphase-coded FM (PCFM) waveform structure and the enhanced sensitivity on receive through sidelobe suppression provided by adaptive pulse compression (APC) and its variants, the impact on full polarimetric scattering estimation is examined. By incorporating a Σ-Δ hybrid combiner different polarization modulation schemes are considered. To address the limitation on achievable cross-correlation between the waveforms associated with orthogonal polarization channels, a polarimetric adaptive pulse compression (PAPC) method is derived that is used to isolate adaptively the different polarization components. Results from an open-air experiment are provided to demonstrate the efficacy of PAPC with this new emission structure. A decoupled emission structure is also tested to use as a benchmark for comparison.

Spatially-modulated radar waveforms inspired by fixational eye movement

We consider a class of MIMO radar emissions in which a coherent spatial beam is formed while the direction is modulated during the pulse width. This type of spatial modulation has a direct analog to the rapid, small movements of the human eye during fixation (staring) to enhance contrast and sensitivity to fine detail. To replicate this passive sensing capability of the eye for the active sensing modality of radar we leverage and expand the continuous phase modulation (CPM) framework for code-to-waveform implementation and thereby realize a physical delay-angle coupled emission. Through analysis of a defined angle-delay ambiguity function for specific waveform/spatial modulation examples it is shown that enhanced discrimination is enabled at the cost of some SNR loss, which may be an acceptable tradeoff for some applications.

A novel approach for embedding communication symbols into physical radar waveforms

Due to constantly increasing demand from commercial communications, defense applications are losing spectrum while still striving to maintain legacy capabilities, not to mention the need for enhanced performance. Consequently, ongoing research is focused on developing multi-function methods to share spectrum between radar and military communication. One approach is to incorporate information-bearing communication symbols into the emitted radar waveforms. However, varying the radar waveform during a coherent processing interval (CPI) causes range sidelobe modulation (RSM) that results in increased residual clutter in the range-Doppler response, thus leading to reduced target visibility. Here a novel approach is proposed to embed information into radar emissions while preserving constant envelope waveforms with good spectral containment. Information sequences are implemented using continuous phase modulation (CPM) and phase-attached to a polyphase-coded frequency-modulated (PCFM) radar waveform, the implementation of which is also derived from CPM. The resulting communication-embedded radar waveforms therefore maintain high power and spectral efficiency. More importantly, the adjustable parameterization of the proposed approach enables direct control of the degree of RSM by trading off bit error rate (BER) and/or data throughput.

Simultaneous radar and communications emissions from a common aperture, Part I: Theory

Multi-function RF systems address the growing need to provide greater functionality with fewer hardware and spectral resources. In this vein, a two-stage iterative optimization approach denoted as far-field radiated emission design (FFRED) is developed that is used here to design a set of physical multi-function waveforms that realize far-field radar and communication signals simultaneously from a common antenna array and with the same spectral support. Particular attention is paid to the efficiency of power radiated into the radar and communication spatial directions, peak-to-average-power ratio (PAPR), and the bit error rate (BER) for the communication mode. Experimental demonstration of this joint emission scheme is presented in the companion paper

Tandem-hopped OFDM communications in spectral gaps of FM noise radar

It was recently demonstrated that hopped spectral gaps can be incorporated into a physically realizable form of FM noise radar emission. Here it is shown using experimental loopback measurements how OFDM communications can be embedded into these spectral gaps and, by virtue of proper spectral shaping, realize a composite emission with low autocorrelation sidelobes. The impact of tandem hopping of the radar spectral gap and embedded communication signal is evaluated. An example of a spectrally shaped OFDM emission for use in a notional commensal radar setting is also presented.

Fast-time 2-D spatial modulation of physical radar emissions

It was recently shown that polyphase-coded frequency modulation (PCFM) waveforms can be expanded to incorporate a spatial modulation coding across a linear antenna array that enables fast-time beamsteering during a transmitted pulsewidth. Here this joint waveform / spatial modulation framework is generalized to two spatial dimensions via a planar array so that complete fast-time spatial steering freedom is available. The resulting emission represents a physically realizable manifestation of MIMO radar that provides enhanced spatial resolution and target discrimination capability using only matched filtering and non-adaptive beamforming. Spatial modulation can also be viewed as a time-varying form of phase-only transmit beam-shaping where the significant increase in degrees-of-freedom, relative to static beam-shaping, translates into many more possible physically achievable design solutions.

Adaptive Receive Processing of Spatially Modulated Physical Radar Emissions

Inspired by the fixational movements of the human eye, fast-time spatial modulation was recently demonstrated as a particular physically realizable form of a multiple-input multiple-output (MIMO) radar emission. The attendant coupling of the delay and angle dimensions has been shown to provide a modest improvement in spatial separation, even when using non-adaptive pulse compression and beamforming. Here this continuous emission paradigm is appropriately discretized and a joint delay-angle adaptive filtering strategy is developed that exploits the physical waveform-diverse emission structure to realize significant enhancement in target separability.

Simultaneous radar and communication emissions from a common aperture, Part II: Experimentation

In the companion paper the Far-Field Radiated Emission Design (FFRED) formulation was theoretically derived for the application of simultaneous generation of multi-function radar and communication emissions in the same spectrum from the same antenna array via a physically realizable MIMO transmit arrangement. Here the practical consequences of such a transmission scheme are considered by way of experimental measurements. The multi-function waveforms were implemented on an Air Force Research Laboratory (AFRL) software-defined radar testbed comprised of four independent transmit channels. Experimental results from a compact range include beampattern measurements, radar waveform validation, and bit error rate analysis of the communication beam.

Wideband MIMO Frequency-Modulated Emission Design With Space-Frequency Nulling

A design approach is presented that jointly optimizes the beampattern and spectral content of a wideband multiple-input multiple-output (MIMO) radar emission within the context of physically realizable frequency-modulated (FM) waveforms emitted from a uniform linear array. Such waveforms minimize the distortion induced by the power amplifier by virtue of being constant amplitude and inherently well-contained spectrally. The design approach is a specific form of alternating projections that shapes the emission spectrum as a function of spatial angle while intrinsically addressing the problem of reactive power that arises for the wideband MIMO emission. This scheme also permits incorporation of joint space-frequency nulling to facilitate spectrum cohabitation with other nearby RF users. The design process is performed in a discretized manner that is oversampled relative to waveform 3-dB bandwidth to capture a sufficient portion of the spectral roll-off to realize the physical waveform, which is subsequently implemented via the polyphase-coded FM structure.