Justin G. Metcalf

Performance Characteristics and Metrics for Intra-Pulse Radar-Embedded Communication

Low probability of intercept (LPI) communication generally relies on the presence of noise to obfuscate a covert signal through the use of spectral spreading or hopping. In contrast, this paper addresses the use of ambient interference from other man-made emissions as a means to mask the presence of covert communication. Specifically, the high power, wide bandwidth, and repeating structure of pulsed radar systems provide an advantageous framework within which to embed a communication signal. The operating paradigm considered here is that of an RF tag/transponder that is illuminated by the radar and intends to covertly communicate with the radar or some other desired receiver while being masked by the ambient radar backscatter to avoid detection by an intercept receiver. Communication takes place on an intra-pulse (or individual pulse) basis to maximize the data rate. The impact of multipath, and its exploitation using time reversal to achieve spatio-temporal focusing, is considered. The processing gain for the destination receiver and intercept receiver are derived analytically and subsequently used to optimize the parameterization of communication symbol design.

Analysis of symbol-design strategies for intrapulse radar-embedded communications

The design of communication symbols that may be embedded on an intrapulse basis into the backscatter generated by a high-power, pulsed radar is considered. This framework requires the asynchronous detection of transmitted symbols in a high-interference environment that degrades the capabilities of conventional intercept receivers. The impact of symbol design and filter structure upon the successful detection of covert symbols by the intended receiver and a hypothetical partially clairvoyant intercept receiver is examined.

The impact of mutual coupling on MIMO radar emissions

The effects of mutual coupling between antenna elements are considered with regard to the impact upon co-located MIMO radar emissions. Because this sensing scheme intentionally couples the spatial and fast-time (waveform) domains, it is shown that MIMO radar is sensitive to any electromagnetic mutual coupling effects that are not adequately characterized in the transmit array manifold. This sensitivity leads to mismatch that will degrade the radars sensitivity on receive.

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.

A machine learning approach to cognitive radar detection

We consider the requirements of cognitive radar detection in the presence of non-Gaussian clutter. A pair of machine learning approaches based on non-linear transformations of order statistics are examined with the goal of adaptively determining the optimal detection threshold within the low sample support regime. The impact of these algorithms on false alarm rate is also considered. It is demonstrated that the adaptive threshold estimate is effective even when the distribution in question is unknown to the machine learning algorithm.

Using time reversal of multipath for intra-pulse radar-embedded communications

This paper considers the problem of embedding a covert communication waveform into the backscatter from an illuminating radar by an RF tag/transponder within a multipath environment. We propose to exploit the multipath via time reversal, which is ideally suited for backscatter communications. Multipath time reversal induces a spatio-temporal focusing of the embedded communication waveform at the desired receiver while distorting it at all other locations thus further reducing the probability of intercept. Specifically, the paper addresses the case where the radar waveform is known to the tag/transponder thus allowing for estimation of the multipath via coherent processing (i.e. pulse compression). Within the radar-embedded communications context the use of the estimated multipath is compared with the case when no multipath knowledge is employed.

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.

Filter design to address range sidelobe modulation in transmit-encoded radar-embedded communications

In a companion paper a continuous phase modulation (CPM) based approach has been introduced to embed information sequences into physical radar emissions, yielding spectrally-efficient constant-envelope waveforms. In addition, the CPM-based approach enables direct control of the degree of range sidelobe modulation (RSM), which occurs due to the changing waveform structure during the coherent processing interval (CPI), by trading off bit error rate (BER) and/or data throughput. When not properly addressed, RSM translates to residual clutter in the range-Doppler response, and hence degraded target visibility. Here receive filter design to mitigate RSM is addressed. The objective for such filters is to produce pulse compression responses that are similar despite the pulse-to-pulse change in waveforms. Three different filter designs are proposed and compared by simulation, where it is found that coherence can be enhanced (and thus RSM reduced) at the expense of higher range sidelobes.