Noise correlation radar (NCR) is a form of noise radar technology that focuses on using random, or noise-like, signals for radio frequency (RF) detection and imaging. NCR has several applications, including the detection of moving targets and wireless security. The technique relies on digitally correlating the received and time-shifted replicas of transmitted signals.
Correlation of random signals was first proposed in the 1950s, but was not possible until hardware limitations were overcome with the advent of the high-speed analog-to-digital converter (ADC). Since then, ultra-wideband, pulsed, and most recently, pulsed-compressed noise waveforms have been utilized in NCR.1–4 Each of these waveforms has unique characteristics, as well as different processing requirements for effective correlation.
Our research involves the design of advanced radar waveforms that are suitable for the detection of moving targets. Specifically, we experiment with NCR and NCR-like devices and research two aspects of random signals: lower susceptibility to cochannel interference, and low probability intercept.5–7
Figure 1. Results from laboratory experiments showing the frequency response for the advanced pulse-compressed noise waveform. Above: in-phase (I) and quadrature (Q) components. Below: magnitude and phase.
Using bench-top equipment, we have successfully created and demonstrated the advanced pulse-compressed noise (APCN) radar waveform (see Figure 1). We facilitate pulse-compression by linearly increasing the wave frequency over the duration of a transmitted pulse so that it is greater than the inverse of the bandwidth, which is traditionally used. We observed the envelope of the complex signal and confirmed its random nature. We have confirmed that matched filter reception ensures minimal receiver distortion and maximizes the system's sensitivity to Doppler-induced frequencies. For reception windows shorter than the transmitted pulse duration, we estimate that stretch-processing reduces the number of data samples necessary to reconstruct the received signal. This aspect of stretch-processing is only possible using APCN waveforms and therefore represents a considerable advantage over other waveforms used in NCR.
Figure 2. The noise correlation radar baseline unit.
We are currently incorporating a real-time capability into our experimental radar (see Figure 2). The radar motherboard hosts the waveform generator/digital signal processing functionality, as well as the direct digital synthesizer, microcontroller unit (MCU), complex programmable logic device (CPLD), and field programmable gate array (FPGA).
Figure 3. The radio frequency design of the noise correlation radar used in our experiments. DDS: Direct digital synthesizer. I: In-phase component. Q: Quadrature component. WG: Waveform generator.
We are in the process of upgrading the ADCs to improve sampling of the intermediate frequency (IF) signal that will provide better waveform fidelity for the baseline program. Other hardware design changes include utilizing an advanced CPLD, a new MCU, an advanced FPGA, and expanding the external memory interface bus. The receiver architecture is incorporated in a heterodyne fashion, with decoupling of the transmitted signal not driving any of the receive-side circuitry (see Figure 3). In addition, a quadrature demodulator parses the radar returns and creates in-phase (I) and quadrature (Q) components that are used to determine if a target is moving in an inbound or outbound direction with respect to the radar. After the waveform I&Q components are parsed, they are passed along to the ADC.
The non-uniform, transmitted power envelopes of NCR has adverse effects on detection performance. Noise radar systems generally encounter target fluctuation behavior similar to that of conventional systems. The fluctuations are caused by the many scattering mechanisms that make up most real targets. In noise radar systems they are dictated by target composition and geometry, as well as by the non-uniform power envelope of their random transmitted signals.8 While we have yet to obtain experimental data that supports an additional dependency on non-uniform power envelope, our simulations suggest that a performance trade-off can be expected. With our current experimental efforts, we hope to demonstrate how NCR can effectively address the spectrum interference problems. If successful, our work will help the future development of radar systems and standards.
US Army Communications-Electronics Research, Development and Engineering Center (CERDEC) Intelligence and Information Warfare Directorate (I2WD)
Mark Govoni is a research scientist presently serving as a subject matter expert at CERDEC. He uses his expertise in wavelength-specific propagation effects, non-conventional radar waveform design, and digital signal processing schema to help develop state-of-the-art RF-based technology systems.
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4. M. A. Govoni, Linear-FM of Stochastic Radar Waveform, PhD thesis, Stevens Institute of Technology, 2011.
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8361, p. 836115, 2012. doi:10.1117/12.923096