Ric A. Romero

Theory and Application of SNR and Mutual Information Matched Illumination Waveforms

A comprehensive theory of matched illumination waveforms for both deterministic and stochastic extended targets is presented. Design of matched waveforms based on maximization of both signal-to-noise ratio (SNR) and mutual information (MI) is considered. In addition the problem of matched waveform design in signal-dependent interference is extensively addressed. New results include SNR-based waveform design for stochastic targets, SNR-based design for a known target in signal-dependent interference, and MI-based design in signal-dependent interference. Finally we relate MI and SNR in the context of waveform design for stochastic targets.

Cognitive Radar Network: Cooperative Adaptive Beamsteering for Integrated Search-and-Track Application

Cognitive radar (CR) is a paradigm shift from a traditional radar system in that previous knowledge and current measurements obtained from the radar channel are used to form a probabilistic understanding of its environment. Moreover, CR incorporates this probabilistic knowledge into its task priorities to form illumination and probing strategies, thereby rendering it a closed-loop system. Depending on the hardwares capabilities and limitations, there are various degrees of freedom that a CR may utilize. Here we concentrate on spatial illumination as a resource, where adaptive beamsteering is used for search-and-track functions. We propose a multiplatform cognitive radar network (CRN) for integrated search-and-track application. Specifically, two radars cooperate in forming a dynamic spatial illumination strategy, where beamsteering is matched to the channel uncertainty to perform the search function. Once a target is detected and a track is initiated, track information is integrated into the beamsteering strategy as part of CRs task prioritization.

Information-theoretic matched waveform in signal dependent interference

The design of transmit waveforms is critical to the performance of a radar system. Traditionally, pulsed and wideband chirp transmit waveforms are used. Our focus here is on the design of waveforms matched to extended targets. In the literature, optimum transmit waveform design in additive receiver noise has been investigated both from SNR maximization and information-theoretic approaches. In this paper, we extend the information-based approach to the signal-dependent interference problem. In particular, we investigate the use of information theory to generate the optimum waveform matched to a Gaussian distributed target ensemble with known spectral variance in the presence of signal-dependent clutter. Several waveform design examples are shown. We also apply this waveform technique to a cognitive radar application.

Friendly Spectrally Shaped Radar Waveform With Legacy Communication Systems for Shared Access and Spectrum Management

In this paper, we investigate a friendly spectrally shaped radar waveform design that can be used in a spectral band being utilized by one or more communication systems. We specifically consider legacy communication systems as opposed to cooperative communication systems to address the ever present problem of legacy technologies. This radar waveform is able to share the spectrum with the existing communication systems such that its detection performance is not compromised while trying to help the legacy systems maintain their own symbol error rates (SERs). We show with various scenarios that the spectrally shaped radar waveform outperforms the traditional wideband pulse waveform in terms of detection performance with the communication signals acting as interference to the radar. Moreover, SERs of the legacy systems employing quaternary phase-shift keying modulation in the presence of the shaped radar waveform (acting as interference) outperform SERs of systems under traditional radar pulse interference. These SERs are very close to theoretical noise-only SERs.

Adaptive beamsteering for search-and-track application with Cognitive Radar Network

In this paper, we introduce the concept of a Cognitive Radar Network (CRN). The goal of the radar platforms in a CRN is to cooperate in illuminating the radar channel in an efficient manner in an effort to search for moving targets. Moreover, when a detection is declared, the CRN should incorporate the tracking requirement into the illumination strategy. That is, the beamsteering strategy must exploit the radar channel uncertainty, which is a function of probabilistic representation of the channel. The radar channel uncertainty is dynamic and as such the CRNs beamsteering strategy should be dynamic. Here, we demonstrate a CRN by utilizing two static radar platforms that form a dynamic integrated search-and-track beamsteering strategy matched to the radar channel uncertainty.

Improved waveform design for target recognition with multiple transmissions

This paper presents a matched waveform technique for target class identification, i.e., a multiple hypotheses testing (MHT) framework. This technique is shown to improve classification performance of SNR-based matched waveforms derived from a probability-weighted spectral variance (PWSV) approach. The technique, which allows for real-time adaptive waveform transmission, is also shown to be computationally efficient.

Radar waveform design in active communications channel

In this paper, we investigate spectrally adaptive radar transmit waveform design and its effects on an active communication system. We specifically look at waveform design for point targets. The transmit waveform is optimized by accounting for the modulation spectrum of the communication system while trying to efficiently use the remaining spectrum. With the use of spectrally-matched radar waveform, we show that the SER detection performance of the communication system is minimally affected compared to the SER performance with a classical non-adaptive pulsed radar waveform. Moreover, we show the detection performance of the adaptive waveform is less impacted by the active communication compared to that of the pulsed radar waveform design. In other words the radar is able to coexist with a friendly communication system and thus share the spectrum with a friendly communication system.

Channel probability ensemble update for multiplatform radar systems

Cognitive radar (CR) is a recently proposed concept that depicts the radar channel in a probabilistic manner. In a multiplatform or networked radar system, some parameters or dimensions of interest are visible (i.e., resolvable) to one radar and not to others depending on the geometry of the scenario. For a radar with new measurements, Bayesian methods to update the cell ensemble probabilities in the non-visible parameters are needed. Here, we show how the overall probabilistic understanding of the channel can be updated despite the fact that some cells are non-visible or “ambiguous”. Unfortunately, the number of calculations needed to accomplish a full update is exponentially related to the number of cells. As such, we also introduce a technique that reduces the calculations immensely. Finally, we apply both update techniques to a two-platform radar system trying to form a two-dimensional probability ensemble of the channel.

Estimation and cancellation of high powered radar interference for communication signal collection

In this paper, we investigate the demodulation of a communication signal that is interfered by a high-powered radar signal. We choose quaternary phase-shift keying (QPSK) modulation for illustration. A least squares estimator (LSE) is used to estimate the amplitude and phase of the interfering radar signal to be used for interference cancellation. Then a maximum likelihood detection (MLD) receiver is used. We show that the QPSK SER improvement in the radar interference environment depends on collection time. For large collection times, the SER approaches the theoretical SER for QPSK. Unfortunately, as the collection time decreases the variance of the radar estimate increases, which degrades the SER. Further, we show increases in radar estimate variance as the QPSK SNR increases, which affects the SER. We propose a form of interference cancellation based on estimating the radar signal and then subtracting it from the received signal prior to MLD of the communication signal.

Ambiguity function and detection probability considerations for matched waveform design

In this paper we investigate the range resolution of transmit waveform designs matched to extended targets. We specifically look at eigenwaveform design which is also known as SNR-based illumination waveform design. To that end, we evaluate some ambiguity functions of radar systems employing eigenwaveforms. We consider some example targets and plot the corresponding ambiguity functions. Unlike traditional waveforms whose responses totally dictate the shape of the ambiguity function, both matched illumination waveform and extended target response contribute to the shape of the ambiguity function. In other words, range and Doppler resolutions are not just functions of the transmit waveform but of the target response itself which makes for interesting ambiguity functions. Moreover, we also evaluate the detection probability of eigenwaveforms matched to extended targets and show the performance improvement over wideband pulsed waveform designs.