Jason E. Hodkin

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.

Open-Loop Coherent Distributed Arrays

The ability to cohere the wireless operations between separate, moving microwave systems enables significant increases in capability for remote sensing, radar, communications, and other microwave wireless applications. Open-loop coherent distributed arrays, which are distributed systems of nodes that coherently coordinate without external signal inputs from the destination, are introduced in this paper. A model of the signal received from a coherent distributed array is derived, and a statistical analysis of the variation in signal power in the presence of coordination errors is presented. Requirements on the tolerable errors for internode range measurement, beamsteering angle measurement, and internode clock phase synchronization are given as a function of the probability of achieving a given signal power. Approaches for achieving the necessary internode range and angle measurements are presented, and the experimental results of a novel one-way clock transfer approach to phase synchronization are shown. These examples demonstrate the feasibility of implementing coherent distributed arrays on moving platforms operating at frequencies extending into the microwave region.

Demonstration of a Coherent RF Repeater for Distributed Communications

In this letter, we demonstrate a coherent radio frequency repeater. The repeater adds the appropriate phase shift to amplify or null a signal generated by another platform and directed to a target point. It leverages a high-accuracy microwave ranging system to estimate the range between platforms and maintain phase coherence when the platforms are moved without additional feedback from the receiver. A continuous beamforming gain operation is achieved without phase or frequency locking the repeater to the transmitter. We include measurements of the stability of the ranging system and discuss potential sources of the variations that limit system performance. Results from outdoor experiments demonstrating successful amplification and nulling of an existing communications link with the repeater are provided.

A Distributed RF Transmitter Using One-Way Wireless Clock Transfer

A coherent distributed two-element radio-frequency (RF) transmitter implemented with wireless clock transfer is presented in this letter. This work represents the first demonstration, to the authors’ knowledge, of a distributed RF transmitter using one-way wireless clock transfer, where the slave node need not provide any information to the master node. Two 1-GHz transmitters were implemented and coherent gain above 90% of ideal signal summation was achieved with a probability of 0.97. Experimental results show near-ideal 6-dB gain from the two-transmitter system at a distance of 85 m.

Platform placement for sidelobe mitigation in mobile sparse arrays

The preferential placement of the platforms in a coherent RF network to suppress sidelobe energy close to the array mainbeam is analyzed in this work. A linear array consisting of nine platforms within a 9000λ array size is optimized with a genetic algorithm. A large set of platform layouts that achieve sidelobe suppression of nearly 3 dB exists. The layouts are tolerant to position error on the order of 10λ and suppression improves with increased waveform bandwidth.

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.

A Coherent RF repeater for distributed communications

The quality of a communications link is fundamentally limited by its transmission power. In the case of a mobile or small platform, this limitation can be particularly crippling because of energy storage constraints. In addition, a single platform, especially one equipped with an omni-directional antenna, can be limited in its ability to control where its signal is broadcast spatially. The former challenge motivates the use of a repeater to boost or null the signal power at a target; the latter prompts the concept of having that repeater act as a cooperative platform in order to provide spatial diversity as well as additional power on target. Some cooperative distributed communication methods have been demonstrated (D. Brown, P. Bidigare, and U. Madhow, ICASSP 2012), but these rely on feedback from a receiver. In a noncooperative setting, repeaters can be used to sample a transmitted signal and noncoherently retransmit a modified version of it to degrade the performance of a receiver (D. Torrieri, IEEE J. Sel. Areas Commun., vol. 7, no. 4, pp. 569–575, May 1989).

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.

One-way wireless clock transfer for coherent distributed arrays

One-way wireless clock transfer for cohering the emissions of separated radio frequency transmitters is demonstrated in this paper. The clock signal is modulated onto a carrier frequency and transferred from the master node to the slave node over a wireless link. Leveraging recent developments in high-accuracy microwave ranging developed by the authors, the one-way clock transfer method can be used to implement a simple, low-overhead distributed clock alignment method for cohering the emissions of transmitters in coherent distributed arrays.

Microwave wireless coordination technologies for coherent distributed maritime radar

Developments in communications and networking have enabled separate microwave wireless systems to coordinate at increasingly detailed levels, creating a path towards distributed wireless systems. Coherent distributed arrays, where the individual wireless systems synchronize at the level of the RF carrier phase, achieve transmit power gains on the order of the number of platforms squared, and receive gains proportional to the number of platforms. For radar applications, an array with N elements yields an overall system gain of N3 in the far field of the array, providing a significant improvement in radar sensitivity. This paper analyzes improved detection capabilities for surface maritime radars operating coherently, and discusses technologies for achieving coherent gain. Recently developed microwave wireless technologies for inter-node coordination in coherent distributed arrays are presented, and future challenges for coherent arrays of maritime radars are discussed.