Guofu Zhu

Adaptive Through-Wall Indication of Human Target with Different Motions

Through-wall indication of human targets is highly desired in many applications. Generally, human targets behind wall are noncooperative, and rare prior knowledge about the circumstance behind wall could be available. Thus, it requires the ability to indicate human targets with different motions from clutters. To investigate this problem, we first examine the conventional time-domain indication methods, and find that their performances are controlled by the historical pulse number adopted to estimate background, which corresponds to the tap-length from the angle of filter. Then, based on an intermittent mode of human target echoes, we define the optimum tap-length as the shortest tap-length that makes the filter output signal-to-clutter-and-noise ratio reach maximum and develop an adaptive indication method with a gradient tap-length control scheme to search the optimum tap-length. Finally, through-wall experiments with an impulse through-wall radar demonstrate that the proposed method can obtain a good adaptive indication performance on human target with different motions.

Preliminary results of ultra-wideband through-the-wall life-detecting radar

It is often a difficult task of knowing the location or movement of people behind barriers. To address this problem, we develop a through-the-wall life-detecting radar (TWLDR) prototype. It is a kind of ultra-wideband (UWB) radar using impulse waveform, which can penetrate non-metallic walls to detect static and moving targets behind walls. We first introduce configurations of the radar system, then describe signal processing techniques, and present experimental results in different situations at last. Results indicate that the UWB TWLDR can penetrate non-metallic walls to detect people with different locomotion states such as standing, walking along cross-range direction, walking along range direction, and the signal processing algorithm could be applied to other complex environments.

Shadow Effect Mitigation in Indication of Moving Human Behind Wall via MIMO TWIR

In through-wall indication of a moving human target in enclosed structures, a shadow effect because of the human target blocking parts from illumination on the back wall will emerge, referred to as a “ghost” in indication results. The shadow ghost moves as the human target does, which makes causal change detection (CD) invalid to separate them. To mitigate the shadow ghost, we analyze its differences from the moving human target. Based on the difference that the illumination is only blocked in partial channels of the multiple-input-multiple-output (MIMO) array while target echoes exist in most channels and the fact that shadow ghosts overlap more between successive indication results than the imaged targets as a result of their larger size, we proposed a mitigation method including a coherence factor and noncausal CD processing. Through-wall experiments via a MIMO through-wall imaging radar validate the proposed method.

Automatic human target detection of ultra-wideband through-wall radar

This paper presents a time domain, moving target detection (MTD) processing formulation for detecting human motion behind walls. The proposed MTD processing formulation consists of exponential averaging background subtraction, ordered statistics constant false alarm rate detection (OS-CFAR) and binary accumulation. We demonstrate the effectiveness of the MTD processing formulation using data collected by an impulse, ultra-wideband through-wall radar. We also analyze the false alarm and detection rate of human with static standing and radial movement.

Sparse Sequential Single-Input–Multiple-Output Array Design for Ultra-Wideband Radar

This letter investigates the array design methods of the sequential single-input-multiple-output (SSIMO) array. Antennas of SSIMO array are both transmitters and receivers, and when one antenna transmits, others receive. Compared with the widely used multiple-input-multiple-output (MIMO) array, SSIMO array has the similar azimuth resolution but lower grating lobe level (GLL). As a general rule, periodic and irredundant (PIR) ultra-wideband one-way array provides a means of improving the azimuth resolution and lowering the GLL. Based on this principle, we first propose the design method for the SSIMO array with four antennas. However, if the number of antennas exceeds four, the effective array of the SSIMO array cannot be PIR. But we have successfully proposed a novel design method to deal with this problem. The simulation results and design example verify the correctness and effectiveness of the proposed method.

An improved OFDM chirp waveform scheme for GMTI in clutter environment

Orthogonal frequency division multiplexing (OFDM) chirp waveform is a new kind of waveform scheme that combine the OFDM principle with the linear frequency modulation (LFM) waveform. In this paper, the OFDM chirp waveform generation model is improved to the condition with arbitrary numbers of orthogonal basic signals, and the detection performance under the energy detector (ED) of this waveform in clutter environment is also analyzed. The simulation results proved that the OFDM chirp waveform performs better than the traditional OFDM and LFM waveforms in clutter environment.

Grating lobe mitigation based on extended coherence factor in sparse MIMO UWB array

Many multiple-input-multiple-output (MIMO) ultra-wideband (UWB) array systems are restricted to a limited array with limited antenna elements and thus suffer from the azimuth grating lobes. To combat the azimuth grating lobes, in this paper, firstly a simple analysis about the grating lobes in the sparse MIMO UWB array is performed. Then, an extended coherence factor (ECF) method taking advantage of the frequency point flexibility of the stepped frequency continuous wave (SFCW) signals is proposed. Finally, numerical simulations are carried out and validate the proposed method.

Moving human target CFAR detection along slow-time profile in ultrawide band through-wall radar

Constant false alarm rate (CFAR) methods are generally applied for the target detection after clutter mitigation in the through-wall sensing applications using ultra-wide band through-wall radar. Current studies of CFAR only focus on detection of moving human targets without considering the effect of residual clutters. However, in the cases with the presence of strong stationary clutters, little residual clutters after clutter mitigation are strong enough to cause false alarms in the following target detection, compared with the weak human target echoes. Unfortunately, conventional CFAR methods fail to remove the effect of residual clutters and suffer from false alarms. To combat this problem and obtain a reliable detection performance, a moving target CFAR detection method along slow-time profile is proposed in this paper. Experiments with an impulse through wall radar show that the proposed method not only has good detection performance for moving human targets, but also successfully gets rid of the problems brought by the strong residual stationary clutters.

RFI suppression method based on amplitude equalization in ultra-wide band through-wall radar

Radio Frequency Interference (RFI) usually has serious effect on through-wall radar (TWR) using ultra-wide band (UWB) signal to achieve good penetrating capacity. Conventional parametric and nonparametric RFI suppression techniques generally work on the fast-time domain and suffer from computation cost not allowed for real-time processing. The waveform average method and median filter method are performed on slow-time domain, but the former decrease the pulse repetition frequency (PRF) and the latter performance is very sensitive to the length of the window. An amplitude equalization algorithm with computation efficiency and parameter independency is proposed according to the signature of RFI in the range-time image. The performance of the method is analyzed. Experimental results are presented to illustrate and validate the approach.

Grating Lobe Suppression in Sparse Array-Based Ultrawideband Through-Wall Imaging Radar

The use of a sparse through-wall radar array can significantly reduce the complexity and the costs of a radar system. However, due to insufficient spatial sampling, a sparse array often suffers from strong grating lobes that seriously degrade image quality. To suppress grating lobes, we first divided the whole array aperture into two subapertures to examine the method of aperture difference. However, this method suppressed only the grating lobes in the edge region rather than in the middle region. Thus, cross-correlation of the two subapertures was proposed, which would mainly reduce grating lobes in the middle region. In this letter, a novel method of cross-correlation weighting of aperture difference is proposed, which shows great feasibility in suppressing grating lobes in both regions. Results from experiments are in agreement with the theory and simulation outcomes and show a dramatic improvement in image quality.