Matthew D. Sharp

Microwave and millimeter-wave ranging for coherent distributed RF systems

Microwave and millimeter-wave ranging systems, waveforms, and experimental results are described for coherent distributed RF systems applications. Measured results show that coherent distributed systems operating at carrier frequencies with coherence at λ/10 are possible well into the millimeter-wave regime by using widely separated two-tone ranging waveforms. The two-tone waveform in the context of continuous-wave ranging is introduced. A method for overcoming the range-ambiguous output of the matched filter processing is described. Microwave and millimeter-wave ranging measurements are shown and compared to the Cramer-Rao lower bound for range accuracy.

Benefits of Digital Phased Array Radars

In this paper, an overview is given of the radar benefits of digital arrays in comparison with conventional phased arrays. Considered are passive and active phased arrays as well as subarray- and element-level digital arrays; their key differences are highlighted. A discussion of several radar attributes and performance measures follows to show the advantages and promise of element-level digital arrays as the newest generation architecture for radar and other electronic systems. Radar attributes considered are antenna patterns and beam control (including adaptive interference cancellation), dynamic range, in-band linearity, system phase noise, and angle measurement accuracy.

Wideband photonic compressive sampling

We discuss the potential benefits of combining the high speeds, broad modulation bandwidths and precision timing of photonic sampling techniques with recent advances in the theory and algorithms of compressive sampling.

On the Estimation of Angle Rate in Radar

The direct measurement of the angle rate of moving objects using a radar with a spatially diverse electric field pattern, a measurement analogous to the measurement of the range rate of a moving objects, represents, along with the measurements of range, range rate, and angle, a fourth basic radar measurement. Recently introduced and experimentally demonstrated, the theoretical accuracy of the direct measurement of angle rate is derived in this paper, and it is compared to the measurements of range, range rate, and angle in the context of the optimal signal forms for the best measurement accuracy. Signal forms achieving optimal accuracy for each measurement are discussed; example implementations of high-accuracy measurements are compared to the optimal forms; and the limitations of simultaneous measurements of pairs of measurements are derived. Combining the angle rate measurement with the three other standard radar measurements may provide future radar systems the capability to simultaneously and instantaneously measure the position and 3-D trajectory of moving objects without compromising the accuracy of any individual measurement.

IQ imbalance decorrelation in digital array radars

Digital array radars (DAR) with element-level digital transmission and reception present new opportunities for more advanced radar functionality and performance. In order to reduce RF front-end complexity while also reducing the need for high sample rates, the analog IQ demodulation receiver, or homodyne, is proposed so that only a single local oscillator (LO) is needed per element. An important homodyne receiver architecture impairment is IQ imbalance and its mitigation is the focus of this article. We introduce a new approach to IQ imbalance compensation, which leverages the digital array architecture to decorrelate these errors and significantly reduce their impact at the digital beam-formed output. As a result, we show that the image rejection ratio, defined as the ratio of the desired-signal power level to that of the image after digital beamforming is significantly higher than at any individual channel output. We derive a single compensation factor from the imbalance statistics estimated across all array elements. The compensation technique is then applied to measured data from a 32-element X-band homodyne DAR test bed.

Waveforms and signal processing for high-accuracy microwave metrology

In this paper, the waveforms and signal processing for a high-accuracy X-Band microwave ranging system that enables a non-cooperative, open-loop coherent distributed radio frequency transmission system are presented. This sub-centimeter-accurate ranging system leverages a Two-Tone Continuous Wave (TTCW) signal as opposed to the conventional Linear-Frequency Modulated (LFM) waveform for ranging. Measurements for the TTCW are shown to achieve better accuracies than a LFM waveform given the same bandwidth and signal to noise ratio.

Accuracy of Angle Rate Measurements Using a Distributed Radar With a Correlation Receiver

Direct measurements of the angle rate of moving objects using a distributed radar have recently been demonstrated in theory and practice. In this letter, the theoretical accuracy of angle rate measurements using a simple correlation receiver is derived and compared to the theoretical accuracy of a more general but less intuitive root-multiplier receiver described in previous work. In particular, the correlation receiver bound on angle rate accuracy is shown to be dependent on the radiation intensity of the electric field rather than the field itself, as is the case for the root-multiplier receiver. The result is that the correlation receiver does not as closely represent the optimal form to achieve the theoretical bound on measurement accuracy. However, despite this, it is shown that there are clear operational regions where the correlation receiver nonetheless outperforms the more general root-multiplier receiver.

A Bandpass Sampling Receiver for Wide-Bandwidth, Spectrally-Sparse Waveforms for High-Accuracy Range Measurements

Measuring range to millimeter accuracy using microwave wireless systems is challenging due to the need for wide-bandwidth waveforms. This work presents a Sparse Ranging Receiver (SpaRR) that utilizes spectrally-sparse waveforms and bandpass sampling to achieve high range accuracy measurements without the need for expensive wideband digitizers. The accuracy of a ranging system varies inversely with the RMS bandwidth of the waveform, and the optimal ranging waveform is one that concentrates its frequency content at the edges of its bandwidth. The SpaRR utilizes widely separate two-tone waveforms from L to X band to achieve high range accuracy with a low-speed digitizer. The theory, architecture, and signal processing associated with the SpaRR are presented, and measurements are shown demonstrating accuracies on the order of 100 μm for tone separations on the order of 2 GHz.

Angle resolution using interferometrie spatial frequency modulation

The theoretical basis for a new remote sensing approach to the measurement of objects in angle is introduced. Using a time-varying spatial modulation, achieved with a scanned-beam two-element interferometer, the resolution of a large aperture can be achieved with a two-element sparse array, without spatial ambiguities. The presented approach can be implemented with simple continuous-wave signals, and may thus be a simpler method of measuring objects in angle than direction-of-arrival systems. The theoretical concept is derived, parameters affecting the angular resolution are discussed, and simulations are compared to the resolution of filled-apertures.

Bandpass signal design for passive time delay estimation

This paper analyzes the performance of passive time delay estimation with bandpass signals and generalizes the results of Weiss and Weinstein by considering a more general bandpass signal model with a parameter that allows for increasing the mean-squared bandwidth of the bandpass signal with respect to conventional flat bandpass signals. Analysis of the modified Ziv-Zakai lower bound shows (i) performance is typically improved at moderate to high signal to noise ratios due to the increased mean-squared bandwidth of the split bandpass signal but (ii) performance is typically worse at moderate to low signal to noise ratios due to increased ambiguities.