A Doppler aliasing free micro-motion parameter estimation method in the terahertz band
© The Author(s). 2017
Received: 27 September 2016
Accepted: 21 March 2017
Published: 31 March 2017
Micro-Doppler, induced by micro-motion of targets, is an important characteristic for target recognition once extracted via parameter estimation. However, micro-Doppler is usually too significant to result in aliasing in the terahertz band. According to this problem, a Doppler aliasing free micro-motion parameter estimation method based on the modulo Generalized Hough transform is proposed in this paper. Its basic idea is to search and match the parameters of aliasing micro-Doppler curves in the time-frequency distribution image. The high estimation precision and excellent noise immunity of this method are verified through simulations based on CST data and experiments based on a 0.33 THz radar system.
Terahertz (THz) waves usually refer to electromagnetic waves with frequencies between 0.1 and 10 THz. The terahertz band lies between the millimeter wave and infrared, which is a transitional band from electronics to photonics. Its position in the spectrum confers special properties and applications on terahertz waves that differ from other bands [1–3]. With breakthroughs in terahertz sources, signal detectors and other devices, the terahertz radar technology has developed rapidly, and many terahertz radar systems have been established, mainly for the study of high resolution imaging [4–7]. Some terahertz devices and radar systems are getting mature, and examples are the 0.85 THz vacuum-based power amplifier designed by Northrop Grumman Corporation  and the ultra-high-resolution radar Miranda 300 designed by the FGAN Research Institute for High Frequency Physics and Radar Techniques . However, researches on micro-motion targets, as a common type of object in the real world, are still very limited, although terahertz waves, due to their short wavelengths, are more sensitive to Doppler than microwaves and hence more suited for micro-Doppler imaging. Also, the serious atmospheric attenuation of terahertz waves has little influence on their applications in exoatmosphere or near space. As a result, the research on parameter estimation of micro-motion targets with terahertz radar is significant. Despite the advantages mentioned above, micro-motion parameter estimation in the terahertz band faces a significant hurdle, i.e., the Doppler aliasing induced by the inadequate pulse repetition frequency (PRF). Terahertz radar usually utilizes frequency modulated continuous wave (FMCW) signals due to low peak power requirement. For example, both the 220 GHz COBRA radar of FGAN  and the 580 GHz imaging radar of Jet Propulsion Laboratory (JPL)  utilize FMCW. In order to ensure the linearity of the FMCW transmitted signal, the equivalent PRF can’t be too large, and therefore the limited PRF sets an upper limit to the maximal observable non-aliasing Doppler values. Specifically, the Doppler frequency can be unambiguously observed only when it lies in the interval between –PRF/2 and PRF/2, and it will be aliasing or folded up when Doppler values are outside this interval. In addition, micro-Doppler in the terahertz band tends towards being aliasing as the carrier frequency is much higher than that in the microwave band.
For micro-motion in the terahertz band, Robert W. McMillan, et al. introduced an experimental coherent pulsed radar operating at 225 GHz early in 1991, and obtained the Doppler spectrum of a stationary truck with its engine running . Li J et al. performed a theoretical analysis of typical micro-motion forms to establish the micro-Doppler signature model, and then compared the micro-Doppler effect in the terahertz band and that in X band by using the joint time-frequency analysis method . X. Zhengwu et al. analyzed the characteristics of micro-motion and investigated methods for motion parameter estimation and micro-Doppler signature extraction by the Radon transform . However, the parameter estimation method in micro-Doppler aliasing situation was not considered, just because the micro-Doppler values they assumed are relatively smaller than PRF/2. In practice, micro-Doppler aliasing in the terahertz radar system is inevitable, and thus these algorithms are inapplicable for our concerns.
Herein, we propose an algorithm for micro-motion parameter estimation based on the modulo Generalized Hough transform (GHT). Its main idea is derived from the GHT in image processing field, which searches sinusoidal curves or other types of curves hidden in images. However, the GHT cannot be used in the aliasing situation directly. In order to obtain micro-motion parameters in the aliasing situation, we firstly obtain the time-frequency distributions of micro-motion targets, and then match the micro-Doppler curves of scattering centers in the time-frequency image with aliasing reference curves obtained by modulo. Finally, we map the parameters of curves to parameter spaces and estimate them by extracting positions of peaks in parameter spaces.
The paper is organized as follows. Section 2 analyzes the micro-Doppler characteristics of a precession warhead model in the terahertz band. In section 3, an aliasing free parameter estimation method based on modulo GHT is proposed and the detailed procedures are given. In section 4, the simulation and experimental results are shown and the performance of the method is analyzed. The conclusions are presented in section 5.
2 Micro-Doppler characteristics of a precession warhead model
2.1 Theoretical model of echo signals
A reference frame O-XYZ that takes the mess center of the model as the origin is established. Considering the model precessing around O-Z axis, the spin angular velocity is Ω, the precession angular velocity is ω, the precession angle is θ, the azimuth and pitch angles of the light of sight (LOS) are α and β, the initial distance between the radar and the model is R 0. According to the theoretical calculation and the experimental measurement, every scattering center corresponds to a discontinuity in the Stratton-Chu integral, that is the discontinuities of the curvature or surface from the view point of geometry. For the model shown in Fig. 1, the scattering centers include the cone-top P1, the discontinuities in the intersection line of the plane Γ (plane Γ is composed of the LOS and the symmetrical axis of the model Oz) and the model P2, P3, P4, and P5.
where A ω is the maximal micro-Doppler frequency, and φ ω is its initial phase. The micro-Doppler of scattering centers on the model are sinusoidally modulated with a period 2π/ω. Therefore, the extraction of precession parameters is equivalent to the estimation of periods, amplitudes and initial phases of sinusoidal curves in time-frequency images. In general, amplitudes and initial phases usually reflect the relative positions of scattering centers, which can be used for space reconstruction and imaging of micro-motion targets.
2.2 Characteristics of Doppler aliasing
In micro-Doppler aliasing situations, the observed Doppler values are no longer the correct values, but are projections on the interval from –PRF/2 to PRF/2. Estimation of the real Doppler value for SAR or ISAR becomes necessary as we cannot obtain perfect SAR/ISAR images without the real Doppler value.
2.3 Aliasing effect on parameter estimation methods
Traditional parameter estimation methods such as the Fourier transform and the Inverse Radon transform (IRT) are not applicable for the aliasing situation because aliasing changes the signal properties and destroys the completeness of time-frequency curves.
3 Aliasing free parameter estimation based on modulo GHT
3.1 Algorithm principle
The algorithm in this paper depends on the time-frequency distributions, and we chose short-time Fourier transform (STFT) as a basis as it has no cross term interference. There is a phase error because of the discrepancy in the start time between STFT and the signal and can be compensated during parameter estimation. We can estimate the micro-motion parameters after obtaining time-frequency distribution images of echo signals. It is well known that GHT is an effective way to detect curves in digital image processing when the curve expressions are known [14–16]. The basic idea of GHT is to map the curves in measurement space to peaks in the parameter space. Curves which share the parameters correspond to the same peak in the parameter space. We can then extract the parameters by identifying the positions of peaks in parameter spaces. Returning to the problem in this paper, the primary objective is to map the aliasing sinusoidal curves in time-frequency distribution images to peaks in parameter spaces. GHT cannot be directly applied to the Doppler aliasing situation as no analytical expressions exist, necessitating an improvement in this case.
3.2 Algorithm steps
Obtain the time-frequency distribution image TF of a micro-motion target by an appropriate time-frequency analysis method (such as STFT).
Estimate the period of micro-motion by the autocorrelation method in order to reduce the parameter space.
Establish a parameter space K = (A k , φ k ) and identify an appropriate search scope and step length based on both effectiveness and estimation precision. In this paper, we set the upper limit of the micro-Doppler frequency F reasonably according to the experimental conditions. The integration time is T = 1s, the micro-Doppler step length is Δf = 1Hz, and the phase step length Δφ = 1∘.
Identify the coordinates of reference pixels in the time-frequency distribution image TF, especially in the micro-Doppler aliasing situations. The reference pixel coordinates can be expressed as (t, mod(A k sin(ωt + φ k ) + PRF/2 , PRF)), where mod(·) is the modulo operation and that is different from GHT.
Search and match the parameters, and average the values of reference pixels in each group in order to reduce detrimental effects of noise.
Extract positions of peaks in the parameter space. Their abscissa values represent the initial phases and vertical ordinate values of the maximum micro-Doppler values.
4 Data processing results and analysis
4.1 Simulation results
For each group of results, (a) is the time-frequency image of scattering centers, and (b) is the parameter space obtained through the modulo GHT method. The peaks in parameter spaces correspond to time-frequency curves of scattering centers. The results taken together show that the algorithm based on modulo GHT does well both in situations of aliasing and non-aliasing, and the noise immunity is very good from the results of No. 2 and No. 3. We can estimate the parameters even though the shapes of time-frequency distributions are no longer observed due to low SNR, i.e., we can still find the right peaks in the parameter space when the SNR is −3 dB, even though the time-frequency distribution image is polluted severely by noise. Furthermore, the algorithm has a high reliability in multi-scattering centers situations from the comparison of No. 5 with others. Finally, the relative frequency and phase errors of each experiment are within the range of 1% from Fig. 11, so the estimation precision of parameters is very high.
4.2 Experimental results
Experiments based on a 0.33 THz radar system are carried out. The terahertz radar system adopted in this paper is based on linear frequency modulated (LFM) pulse principle and has a 322 GHz of central frequency with a synthetic bandwidth of 10 GHz, thereby realizing a 1.5 cm theoretical range resolution. The terahertz signal is transmitted by the cone-shaped horn antenna with an azimuth beam angle 11° after 36 times frequency multiplication of an X-band sweeping generator in the transmitting chain. The pulse repetition frequency (PRF) of the transmitting signal is 1000, and the transmitting power is greater than 3 mW.
4.3 Performance analysis
4.3.1 Computation load analysis
The computation load is directly proportional to the upper limit frequency and the integration time and inversely proportional to the frequency and phase precision from Eq. (3). So, we need to make a tradeoff between the computation load and parameter estimation precision or adopt the variable step-length searching strategy mentioned above.
4.3.2 SNR analysis
Researches on parameter estimation of micro-motion in the terahertz band are of great value to exploit the advantages of terahertz band and promoting the applications of terahertz radars. In this paper we have proposed a modulo GHT algorithm, and applied it to the estimation of micro-motion parameters in micro-Doppler aliasing situation. Its essence is to search the matching parameters of micro-Doppler curves in time-frequency distribution images, and it is especially suited for the micro-Doppler aliasing situation in the terahertz band. The simulation and experimental results demonstrate the high estimation precision and excellent noise immunity, especially when the SNR is above −12.5 dB.
The authors would like to thank the editors and reviewers for their insightful comments.
The problem of Doppler aliasing resolution with terahertz radar was arisen from the discussions between BD and HW. YQ built the 0.33 THz radar system and wrote Section 1. The Doppler aliasing free micro-motion parameter estimation method was derived and implemented by QY. HW wrote the Abstract and BD wrote the Conclusions. Section 2, Section 3 and Section 4 were written by QY. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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