Adaptive RF Interference Canceller in High Dynamic Range Doppler Radar for Landmine

Detection Yu Zhang1, Dan Orfeo1, Joe Keranen2, Dryver Huston1, Tian Xia1

School of Engineering, University of Vermont, Burlington, VT 05405, USA1 White River Technologies, Inc., Lebanon, NH 03766, USA 2

Email: yzhang19@uvm.edu, dan.orfeo@gmail.com, keranen@whiterivertech.com, dhuston@uvm.edu, txia@uvm.edu

Abstract— Doppler radar detects a target’s movement by characterizing the frequency variations of the reflection RF signal. In this paper, Doppler radar is utilized for detecting landmines through vibration sensing. By checking the Doppler frequency of reflection signal resulting from the landmine’s response to the vibration stimulus, the buried landmine can be detected. In this operation, large RF interferences in the system may diminish the dynamic range of Doppler radar and degrade its sensing performance. An adaptive RF interference canceller is designed to enhance the reflection signal from the landmine and increase the dynamic range of Doppler radar by more than 20 dB. For performance validation, experiments on detecting a landmine stimulated by a vibration source are conducted.

Keywords—Doppler radar; radio-frequency measurement; dynamic range; cancellation system; landmine detection.

I. INTRODUCTION

Antipersonnel mines are a significant international threat to civilians despite recent intense efforts to clear them from post-conflict regions [1-2]. Detecting landmines with accuracy and safety is of great humanitarian value. For metallic landmine detection, electromagnetic induction and magnetometer approaches have been proven effective. However, for nonmetallic mines, different sensing technologies are needed.

In [3-4], ground penetrating radar (GPR) is adopted to image buried objects for landmine detection, which involves sophisticated data processing. Recently, a new acoustic-to-seismic coupling technique has been demonstrated for detecting nonmetallic mines [5-6]. In the operation, acoustic signal sources, like subwoofers, are employed to produce strong sound toward the ground. The resulting seismic wave propagates and interacts with the mine and soil where nonlinear responses occur at their interface. The nonlinear mechanism [5] can be modeled as a one-dimensional bi-modulator spring structure where the soil and the mine have different spring constant values. Under the seismic wave excitations, oscillation occurs at the surface of soil and mine. During the compression phase, the surface stays together and then separates under the tensile phase. For mines of relatively high compliances, nonlinear response occurs. By utilizing microphones or accelerometers to measure the frequency

responses, the buried landmines can be detected. In [5] and [7], it is also pointed out that for a noncompliant burying object, like a root, a piece of metal, or a brick, the nonlinear mechanism does not occur or is not significant.

Compared with GPR sensing, the acoustic-to-seismic coupling approach involves less sensing data collections and low complexity signal processing, which facilitate a fast sensing speed and a low false rate. However, using accelerometers or microphones as detectors has limiting effects on sensing performance and effectiveness. For instance, when using the accelerometers, multiple accelerometers need to be deployed in the scanning area to measure mechanical vibrations directly. The deployment cost and safety are of concerns. While using the microphone to pick up the response signals eliminates such concerns, the sensing accuracy might be degraded as the response signal is very weak and can be smeared by interference noise sound in detection environment.

To overcome those limitations, in this paper, a Doppler radar is designed to remotely detect the frequency response from the landmine. Doppler radar [8] is a specialized radar that uses the Doppler effect to measure the velocity of moving objects at a distance. It radiates an electromagnetic (EM) wave toward a target under test whose movement causes the Doppler frequency shift or frequency modulation of the incident EM signal. Through characterizing the frequency variation, the target's velocity relative to the radar can be measured. Doppler radar has found its applications in wide scopes, including vehicle speed measurement [9] [9], noncontact periodic displacement measurement for physiological monitoring [10], human activities detection [11], etc. In our application, Doppler radar measures the frequency variations due to a landmine’s vibrations in response to the seismic stimulus.

For landmine detection, the major challenge is that the landmine reflection signal is extremely weak due to the small magnitude displacement of landmine vibration. In addition, there exist RF interference signals that can impair Doppler radar performance, i.e. the direct coupling between the transmitter antenna and the receiving antenna, the intrinsic distortion factors in the system, and strong clutter in the

sensing environment [12-16]. To achieve meaningful measurements, it is necessary to leverage the Doppler radar dynamic range by alleviating the interference and enhancing the small signal under measurement.

In [17-21], for RF interference alleviation, feedforward canceller circuits are designed utilizing vector modulators to generate a baseband signal with in-phase (I) and quadrature phase (Q) components. By manipulating the relative phase offset and amplitude parameters of I/Q components of the baseband signal [22], the frequency up-conversion is then performed to shift the signal to the desired frequency band. The synthesized signal and the interference signal are then added for cancellation. Such cancellation signal synthesis has a relatively complicated circuit structure. Moreover, for the local oscillator utilized in frequency modulation, as its phase is random with no controllability, the final signal synthesized may not precisely match the signal under cancellation. Hence the cancellation effectiveness is degraded.

This paper proposed an adaptive software-controlled RF interference canceller to improve Doppler radar dynamic range and sensitivity for landmine detection application. The software-controlled RF interference cancellation is highly configurable and has advantages over vector modulation cancellation system: (1) the ability to quickly alter and analyze the effects of key parameters, including frequency, waveform shape, amplitude and phase parameters; (2) adaptable RF cancellation in accordance with the actual interference and clutter, and the specific target detection requirements. The proposed canceller implementation aims to eliminate or reduce the interference, including transceiver circuit intrinsic distortion, antenna direct coupling, and the clutter received by the Rx antenna. This acts to enhance the small signal under detection.

The paper is organized as below: Section 2 describes the architecture and the operating principle of the RF interference canceller. Section 3 elaborates the canceller development and the calibration procedure for the Doppler radar landmine detection application. Section 4 presents the landmine detection experimental scenarios and the corresponding measurement results. Concluding remarks are summarized in Section 5.

Fig.1. RF interference cancellation system.

II. CANCELLATION SYSTEM ARCHITECTURE

A. Principal of the Cancellation System

Fig.1Fig. sketches the architecture of our proposed RF interference cancellation circuit. In the canceller configuration, the sensing signal is generated and split to form two identical copies. One is fed to the transmit (Tx) antenna for radiation. The signal received by the receiving (Rx) antenna includes the direct coupling signal from the Tx antenna, the clutter signal, the intrinsic distortion in the transceiver chain, and the landmine reflection signal under measurement. The second output signal of the splitter is sent to the phase shifter and amplitude tuner to synthesize the cancellation signal to match the amplitude of the undesired interference signal in the receiver with an opposite phase. The received signal and the cancellation signal are summed up through a combiner, so that the direct coupling, clutter and other intrinsic distortions are cancelled out while the small reflection signal from the landmine remains.

For Doppler radar landmine sensing, the transmitter emits a radio frequency signal. A mechanical vibrator is deployed for stimulating the landmine burying site. The resulting seismic wave propagates and causes the landmine to vibrate, which in turn modulates the frequency of the incident RF signal. Fig. 2 illustrates the sensing signal spectrums. The emitted RF signal is a continuous sinewave with a carrier frequency . The vibrating landmine modulates the sinewave and produces the Doppler frequency ± , where is the vibrating frequency of the landmine under detection. The received signal also contains the direct coupling signal and the background signal that have the same frequency as the sensing signal but different phases. The spectrum of the received signal is plotted in Fig. 2Fig.(b). For the buried non-metallic landmine, its scattering signal is typically 100 dB below the peak power of the emitted RF sinewave [23][23], and is extremely difficult to detect directly. For the cancellation signal, its amplitude is tuned to match the carrier frequency component in the received signal, while the phase is offset by 180-degrees. Both the cancellation signal and the received signal are fed to the combiner for interference cancellation. As depicted in Fig. 2(c), the power ratio between the small Doppler components and the carrier frequency interference increases, which makes Doppler frequency components become more pronounced.

Fig.2. Frequency spectrum of a). RF sinewave; b). Received signal upon Doppler frequency modulation; c). The signal after carrier frequency cancellation

Fig.3. Programmable digital synthesizer to generate the cancellation signal

B. Generating the Cancellation Signal

Fig. 3 sketches the programmable digital control unit for the cancellation signal synthesis. In this unit, a software program is developed to generate a digital data sequence of the cancellation signal. In the feedback calibration stage (details can be found in the following subsection), the program adjusts the phase and amplitude of the digital signal to match those of the RF interferences. The digital data are stored in the memory and read out to the digital-to-analog converter (DAC) and low-pass filter (LPF) for producing the analog cancellation signal. As the digital data are generated by the software program, it provides a great flexibility to generate signals of arbitrary waveforms. In our testing, a single tone sine wave is generated which can be expressed as sin 2 (1)

where is the frequency of the sinewave, and represents the initial phase. The corresponding discrete digital sample is sin 2 ∆ sin 2 / (2)

where is the index of the data point, and ∆ is the time interval between two data points which equals the inverse of the sampling frequency . Assuming the initial phase of the sensing signal is 0, the digital data sequence for the initialized cancellation signal can be expressed as sin 2 / (3)

In the calibration, the transmitter antenna and the receiving antenna are used to capture the RF interference signal. Assuming the time delay between the RF interferences and the cancellation signal is ∆ initially, the corresponding phase offset is ∆ 2 ∆ 2 ∆ (4)

where is period of the sinewave. The magnitude of the digital data sequence is scaled by a coefficient to match the magnitude of the RF interferences. The final digital data sequence can be expressed as α sin ∆ sin 2 ∆ (5)

Once the digital sequences are produced, they can be fed through the DAC and LPF for analog signal synthesis.

The phase of the cancellation signal is fine-tuned by iteratively adjusting the value of ∆ in Eq. (5). A threshold is used to evaluate the effectiveness of the cancellation. Specifically, if the amplitude of the interference signal is reduced by 25 dB, the iterative adaptive signal generation process is ended.

Fig.4. Diagram of RF interference canceller for Doppler radar measurement.

III. DEVELOPMENT OF CANCELLER FOR DOPPLER RADAR

LANDMINE DETECTION

For fast prototyping development, the programmable digital control unit is implemented with an arbitrary waveform generator (AWG). The AWG is connected to a computer via a LAN cable. The digital data sequence with controllable phase and amplitude parameters is generated by a MATLAB script, and then downloaded to the AWG for analog signal synthesis. As shown in Fig. 4, the Ch1 channel of the AWG produces the cancellation signal and the Ch2 channel outputs the sensing signal for vibrating landmine detection. In the canceller configuration, the Ch2 output signal is amplified by a power amplifier and emitted by the Tx antenna. The Rx antenna receives the direct coupling signal from the Tx antenna, the clutter signal reflected from the ground surface and the scattering signal from the landmine. The received signal in Rx antenna and the AWG’s Ch1 cancellation signal are summed up through a combiner, so that the RF interference is cancelled-out while the landmine reflection signal remains. The output signal of the combiner is then amplified by a low noise amplifier (LNA) before being sent to the data acquisition unit.

A. Canceller Calibration

To characterize the RF interference signal, calibration is first performed. In the calibration, the test site is stationary and no vibration is stimulated. The sensing signal data is generated in a MATLAB program according to Eq. (3) and then downloaded into the AWG Ch2 for signal generation and radiation, while Ch1 is turned off. As the target is stationary, the signal received by the Rx antenna is essentially the static interference that needs to be cancelled. The interference signal is digitized and stored in the computer for cancellation signal generation. Following Eq. (5), the cancellation signal is fine-tuned to match the magnitude of the RF interference, except with opposite phase.

B. Doppler Radar Measurement

During Doppler radar measurement, the cancellation signal is generated from AWG Ch1, and the test signal is

output from Ch2. When the landmine is vibrating, the received signal contains both the RF interference and the Doppler signal resulting from the landmine’s vibration. The Ch1 cancellation signal and Rx antenna receiving signal are summed at the combiner, where the static interference is alleviated.

C. Prototyping Implementation of Canceller for Doppler Radar Measurement

Fig. 5 illustrates the setup of a Doppler radar prototype. An Agilent N8241A AWG (15-bit 1.25 GS/s sampling rate) is employed for synthesizing the sensing signal and the cancellation signal. The maximum frequency that can be produced from this AWG is 500 MHz. To achieve a higher Doppler sensing resolution, two frequency doublers (Mini-Circuits ZX90-2-13-S+) are used on two channels to multiplier the carrier frequency from 500 MHz to 1 GHz, which halves the signal wavelength. To compensate for the insertion loss of the doublers, an amplifier (Mini-Circuits ZHL-4240W) is added in each channel. The test signal from Ch2 connects to a power amplifier before transmission. Two customized horn antennas [24] are used for signal transmission and receiving. An Agilent E4440A spectrum analyzer is utilized as the data acquisition unit. Table 1 lists all key devices and equipment utilized.

Fig.5. Prototype RF interference canceller for Doppler radar measurement.

Tab. 1 Device and equipment list

Device Name Model AWG Agilent N8241A, 1.25 GS/s-15 Bit

Amplifier Mini-Circuits ZFL-750+X2 Frequency

Multiplier Mini-Circuits ZX90-2-13-S+

Power Amplifier Mini-Circuits ZHL-4240WSplitter Mini-Circuits ZFRSC-42-S+

Spectrum Analyzer Agilent E4440A, 3 Hz – 26.5 GHzAntennas UWB Horn Antenna [24]

(a) (b)Fig.6. Doppler radar measurements on buried vibrating landmine: (a) Landmine buried in soil; (b) Doppler radar measurement setup.

IV. EXPERIMENTAL RESULTS

To evaluate the system performance, experiments for detecting the buried landmine are conducted. As shown in Fig. 6, a nonmetallic landmine is buried in soil. A concrete vibrator is inserted into a hole in soil to agitate at 240 Hz. The seismic wave propagates and stimulates the landmine vibration at the same frequency. The burying depth of the landmine measured from the ground surface is 15 cm. The concrete vibrator is 3 feet away from the landmine (not visible in Fig. 6).

During the measurement, a 1 GHz sinewave is emitted as the sensing signal by the AWG and the frequency doubler. A spectrum analyzer is used as the data acquisition and characterization unit. Two types of measurement tests are performed. In the first test, the landmine is buried in soil and stimulated to vibrate at 240 Hz by the concrete vibrator. In the second test, the concrete vibrator still simulates the ground at 240 Hz while the landmine is removed.

The experimental results when landmine is buried in soil are plotted in Fig. .7. In this setup, since the vibration source is 240 Hz and the test signal is 1 GHz, the data acquisition unit receives signals of two frequencies: one is the carrier frequency 1 GHz, the other one is the Doppler frequency 1 GHz ± 240 Hz. Fig. 7(a) shows the test signal spectrum when the RF interference canceller is turned on. Note, in the spectrum plot, the central frequency is 1 GHz. The Doppler signal frequency is measured as 240 Hz and its power is -128.7 dBm. The power of the carrier signal is -87.17 dBm. The ratio between the Doppler signal and the carrier signal is -41.53 dB. For comparison, Fig. 7(b) plots the test result without interference cancellation. As shown, the power of the Doppler frequency component is -121.5 dBm, while the power of the carrier frequency is -56.49 dBm. Correspondingly, the ratio between the Doppler signal and the carrier signal is -65.01 dB. The measurement results indicate that, with the RF interference canceller, 23.48 dB [(-41.53) - (-65.010)] improvement on signal-to-interference ratio is achieved. Note, upon using the RF interference canceller, the Doppler signal’s power decreases from -121.5 dB to -128.7 dB (by 7.2 dB). This is mainly due to the signal loss introduced by the splitter in the signal chain.

(a)

(b) Fig.7. Doppler measurement results when landmine is buried in soil: (a) Doppler measurement with canceller; (b) Doppler measurement without canceller.

The experimental results when the landmine is removed from the soil are plotted in Fig. 8. The motivation of this testing is to compare the measurement difference with or without landmine buried in soil. Fig. 8(a) shows the signal spectrum with cancellation circuit Fig. 8(b) plots the Doppler measurement without cancellation circuit while the vibrating stimulus maintains 240 Hz operation. Comparing Fig. 7(a) and Fig. 8(a), the Doppler measurements of two testing scenarios show different results. When landmine is buried, the 240 Doppler frequency appears as a peak in Fig. 7(a). Moreover, the higher order harmonics also exist although small. However, when landmine is removed as shown in Fig. 8Fig. (a) or Fig. 8(b), only the carrier frequency, RF interference (the peak at 93.33 Hz) and noises exist in the spectrum measurement. The difference between the two measurements validates that the Doppler radar can detect the landmine by checking the landmine’s response to vibration stimulus.

V. DISCUSSIONS AND CONCLUSIONS

The effectiveness of Doppler radar for landmine detection by checking the landmine’s response to vibration stimulus is evaluated and validated. An adaptive RF interference cancellation circuit is designed to improve the dynamic range of Doppler radar for the landmine detection application. Experimental results indicate the RF interference canceller can improve the ratio between the Doppler signal and the interference signal by more than 20 dB.

(a)

(b) Fig. 8. Doppler measurement results when landmine is removed from the soil: (a) Doppler measurement with canceller; (b) Doppler measurement without canceller.

The achievable performance relies on the phase and amplitude tuning accuracy and resolution of the cancellation signal synthesizer. In future study, dynamic interference signal characterization and real-time feedback tuning will be explored to alleviate the changing interference. Furthermore, the effects of antenna polarization will also be studied.

ACKNOWLEDGEMENT

This research is support in part by U.S. Army contract/grant ARL A16A-T004 STTR Phase II, W911NF-13-1-0301, and ONR grant N00014-16-1-2981. The views, opinions, findings, and conclusions reflected in this paper are the responsibility of the authors only and do not represent the official policy or position of the funding agencies or other entity.

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