Linear Frequency Modulated Continuous Wave Signal Research Articles

FPGA implementation of closed‐loop compensation for LFMCW signal non‐linear distortions

Linear-frequency-modulated continuous-wave (LFMCW) radars with deramping technique have several advantages such as compact size, low cost, and easy implementation, compared to traditional pulse radars; however, the non-linear distortions caused by analogue modules, particularly power amplifiers, may deteriorate the radar detection performance. To solve this problem, a digital closed-loop compensation scheme for the LFMCW signal is proposed, which can compensate the LFMCW signal in real time, when the non-linear distortions change with the working environment. The proposed compensation scheme improves the detection performance, noticeably. Considering the speed limitations of field programmable gate arrays (FPGAs), the proposed scheme is implemented in an FPGA at an intermediate-frequency level, using a polyphase processing structure.

Multitarget AOA Estimation Using Wideband LFMCW Signal and Two Receiver Antennas

Linear frequency modulated continuous wave (LFMCW) signals have been widely used in target detection and localization, thanks to their good detection sensitivity and high range resolution. However, estimation of the angle of arrival (AOA) of the LFMCW signal reflected from each target in a multitarget environment is still quite limited at present. In this paper, we propose a novel AOA estimation framework in which one transmitter antenna and two receiver antennas are employed to transmit a wideband LFMCW signal and receive the signals reflected from different targets, respectively. The signals received at two receiver antennas are processed in three steps. First, each of them is demodulated by mixing with the transmitted signal to transform the wideband nonstationary signal into superposition of a series of single-tone signals. Second, signals received at each receiver antenna from different targets are separated using bandpass filtering. Third, the corresponding AOA of each target is estimated using the phase difference between two receiver antennas. Different from the traditional subspace-based methods, the number of targets is not limited by the number of receiver antennas. The theoretical analysis and simulation results show that the proposed algorithm can achieve accurate AOA estimation for multiple targets even at a low signal to noise ratio.

Photonics-based broadband radar for high-resolution and real-time inverse synthetic aperture imaging.

A photonics-based radar with generation and de-chirp processing of broadband linear frequency modulated continuous-wave (LFMCW) signal in optical domain is proposed for high-resolution and real-time inverse synthetic aperture radar (ISAR) imaging. In the proposed system, a broadband LFMCW signal is generated by a photonic frequency quadrupler based on a single integrated electro-optical modulator, and the echoes reflected from the targets are de-chirped to a low frequency signal by a microwave photonic frequency mixer. The proposed radar can operate at a high frequency with a large bandwidth, and thus achieve an ultra-high range resolution for ISAR imaging. Thanks to the wideband photonic de-chirp technique, the radar receiver could apply low-speed analog-to-digital conversion and mature digital signal processing, which makes real-time ISAR imaging possible. A K-band photonics-based radar with an instantaneous bandwidth of 8 GHz (18-26 GHz) is established and its performance for ISAR imaging is experimentally investigated. Results show that a recorded two-dimensional imaging resolution of ~2 cm × ~2 cm is achieved with a sampling rate of 100 MSa/s in the receiver. Besides, fast ISAR imaging with 100 frames per second is verified. The proposed radar is an effective solution to overcome the limitations on operation bandwidth and processing speed of current radar imaging technologies, which may enable applications where high-resolution and real-time radar imaging is required.

An Approximate Cramer–Rao Lower Bound for Multiple LFMCW Signals

In this paper we focus on deriving an approximate Cramer–Rao lower bound (CRLB) for the parameters of a multicomponent linear frequency modulated continuous wave (LFMCW) signal corrupted by complex additive white Gaussian noise. The approximation is necessary due to the discontinuities inherent in the mathematical model of the instantaneous phase of each LFMCW signal model. By comparing our approximate bound to a simulation of the maximum likelihood estimator (MLE) of the LFMCW parameters, we confirm our analysis. In general, the CRLB is a useful tool for feasibility studies or in evaluating the degree of suboptimality that non-MLE methods exhibit. For passive detection and estimation of LFMCW signals, the Generalized Likelihood Ratio Test and the associated MLE are difficult to implement in practice, primarily due to their large computational requirements. So, lower bounds on performance, such as those provided by the CRLB, are necessary to evaluate suboptimal methods that are more suited for practical implementations.

基于周期FRFT的LFMCW信号检测与参数估计

Linear frequency-modulated continuous wave (LFMCW) signals have low probability interception (LPI) characteristics, and contain multicomponent LFM signals. The detection and estimation of LFMCW signals is a difficult problem for radar intelligence and reconnaissance systems. In addressing this problem, first the fractional Fourier transform (FRFT) of the LFMCW signal is analyzed. By periodic modulating of the FRFT kernel function, the periodic fractional Fourier transform (PFRFT) is defined, and the relationship between the PFRFT and FRFT is derived. By analyzing the PFRFT of the LFMCW, a novel method based on the PFRFT to detect and estimate an unknown LFMCW is presented. The detection and estimation performance are also discussed. Finally, simulation verifies its effectiveness and rationality. Compared with the FRFT, PFRFT increases the LFMCW SNR with periodic accumulation, and fits a periodic modulated signal to the LFMCW.

330 GHz FMCW Image Sensor for Homeland Security Applications

A single-pixel imaging remote sensor operating at 330 GHz is described. It is based on a frequency modulated continuous wave (FMCW) and aimed at detection of concealed objects for ranges up to 40 m. The system consists of 2 horn-lens antennas integrated with a homodyne transceiver. The synthesized linear FMCW signal at X-band is multiplied by a factor of 32 to generate the transmitted signal. An intermediate frequency (IF) signal obtained in the output port of the 2-nd harmonic mixer is employed for image processing. Distance measurements were made by performing data acquisition unit based on LabView interface and resulting in a range resolution about 1 cm. Examples of 2D and 3D images reconstructed with this sensor are presented.