John Jakabosky

Polyphase-coded FM (PCFM) radar waveforms, part II: optimization

This paper addresses polyphase code optimization with respect to the nonlinear frequency modulation waveform generated by the continuous phase modulation implementation. A greedy search leveraging the complementary metrics of peak sidelobe level, integrated sidelobe level, and spectral content yield extremely low range sidelobes relative to waveform time-bandwidth product. Transmitter distortion is also incorporated into the optimization via modeling and actual hardware. Thus the physical radar emission can be designed to address spectrum management and enable the physical realization of advanced waveform-diverse schemes.

Polyphase-coded FM (PCFM) radar waveforms, part I: implementation

Polyphase radar codes promise enhanced performance and flexibility due to greater design freedom. While the search for better codes continues, the implementation issues of transmitter bandlimiting and nonlinear distortion have precluded their widespread use in high-power systems. This paper introduces a modified continuous phase modulation implementation that converts an arbitrary polyphase code into a nonlinear frequency-modulated waveform that is constant envelope and spectrally well contained. Experimental results assess the receive sampling and pulse compression effects.

Transmitter-in-the-loop optimization of physical radar emissions

Ongoing work is exploring the optimization of physical radar emissions based on the continuous phase modulation (CPM) implementation of polyphase codes. Here a modification to the code search strategy known as Marginal Fishers Information (MFI) is presented that enables this greedy approach to further improve upon the performance of the resulting CPM-implemented continuous waveform in terms of range sidelobes. The optimization process is also expanded to include the effects of the transmitter (from both modeled and physical hardware perspectives) to facilitate the optimization of physical emissions that are specifically tuned to the transmitter. This approach is particularly useful for high-power transmitters in which the actual physical emission is a spectrally modified and non-linearly distorted version of the intended radar waveform.

Non-linear FM waveform design using Marginal Fisher's Information within the CPM framework

The optimization of radar codes for specific use within the continuous phase modulation (CPM) implementation is presented as a means to achieve both power and spectral efficiency for radar emissions. An optimization strategy known as Marginal Fishers Information (MFI), recently shown to be an efficient and effective strategy for the selection of high-dimensional codes, is here applied within the CPM framework to design continuous waveforms that are physically realizable.

Waveform design and receive processing for nonrecurrent nonlinear FMCW radar

A spectral shaping optimization scheme is used to design the autocorrelation response of individual segments of a nonrecurrent nonlinear FMCW waveform denoted as Pseudo-Random Optimized FMCW (or PRO-FMCW). Because each waveform segment is unique, the range sidelobes do not combine coherently during Doppler processing thereby providing further sidelobe suppression. The PRO-FMCW waveform can be viewed as a specific instantiation of FM noise radar where the constant amplitude permits maximum power efficiency. A segmented approach to processing the received data is used to reduce processing time and complexity. Measured results from hardware implementation are provided to demonstrate the efficacy of the proposed approach.

Ultra-low sidelobe waveform design via spectral shaping and LINC transmit architecture

A new spectral-shaping approach is used to design jointly the amplitude window and phase of a tapered NLFM waveform. This approach can produce physical radar emissions with ultra-low sidelobes that support sufficient spectral roll-off to maintain forthcoming spectral compliance requirements. A small SNR loss is traded for a substantial reduction in range sidelobes. Further, it is demonstrated that this tapered waveform scheme can be readily combined with the linear amplification using nonlinear components (LINC) architecture to realize the small yet necessary amplitude variation while operating the power amplifiers in saturation, which may be used to realize enhanced power efficiency. Experimental measurements demonstrate range sidelobes below -80 dB with less than 0.3 dB in SNR loss.

Gapped spectrum shaping for tandem-hopped radar/communications & cognitive sensing

A non-repeating FMCW waveform was recently developed and experimentally demonstrated to provide a feasible instantiation of FM noise radar. This emission scheme was subsequently examined in terms of the impact of both stationary and hopped spectral gaps with the prospect of enabling in-band interference avoidance for cognitive sensing and possibly tandem hopped radar/communications. Here this gap-hopped spectrum framework is further explored with regard to the relation between the shaping of spectral gaps and the associated time sidelobe response. Experimental loopback measurements are shown that provide a sense of how this form of emission would operate on a real system.

Optimization of “over-coded” radar waveforms

Polyphase-Coded FM (PCFM) radar waveforms generated using the power and spectrally efficient continuous phase modulation (CPM) framework can be further enhanced through the use of finer time control by subdividing each phase transition into sub-transitions and by allowing a greater phase excursion per transition interval, herein referred to as over-phasing. These two strategies are denoted collectively as “over-coding”. It is shown that various combinations of sub-transitions and over-phasing can greatly improve waveform design capabilities by expanding the available degrees-of-freedom. It is also demonstrated that the commensurate increase in computational complexity for optimization under the over-coding paradigm can largely be offset through GPGPU processing.

Optimizing polyphase-coded FM waveforms within a LINC transmit architecture

Linear amplification using nonlinear components (LINC) is a design approach used to mitigate the nonlinear distortion introduced by the transmitter components, particularly the high-power amplifier. This work investigates using the LINC approach in combination with a new polyphase-coded FM (PCFM) waveform optimization strategy to strike a balance between waveform effectiveness, power efficiency, and spectral containment. A hardware-in-the-loop optimization of LINC-PCFM waveforms is employed to enable joint consideration of the transmitter components and the excitation waveforms.

Transmitter-in-the-loop optimization of distorted OFDM radar emissions

The inherent amplitude modulation of orthogonal frequency division multiplexing (OFDM) has limited its use in radar due to transmitter-induced distortion. Specifically, there is a necessary trade-off between reduced SNR caused by power back-off to accommodate these AM effects and the waveform distortion that occurs when driving the amplifier closer to saturation. Here a method is developed to design OFDM-based radar emissions with low range sidelobes in the presence of transmitter-induced distortion. This work builds upon previous results for hardware-in-the-loop optimization of continuous phase modulation (CPM) waveforms to demonstrate how saturated OFDM emissions may potentially provide a feasible alternative waveform design scheme.