Detection of Multiple Stationary Humans Using UWB MIMO Radar

Fulai Liang, Fugui Qi, Qiang An, Hao Lv, Fuming Chen, Zhao Li, Jianqi Wang
2016 Sensors  
Remarkable progress has been achieved in the detection of single stationary human. However, restricted by the mutual interference of multiple humans (e.g., strong sidelobes of the torsos and the shadow effect), detection and localization of the multiple stationary humans remains a huge challenge. In this paper, ultra-wideband (UWB) multiple-input and multiple-output (MIMO) radar is exploited to improve the detection performance of multiple stationary humans for its multiple sight angles and
more » ... -resolution two-dimensional imaging capacity. A signal model of the vital sign considering both bi-static angles and attitude angle of the human body is firstly developed, and then a novel detection method is proposed to detect and localize multiple stationary humans. In this method, preprocessing is firstly implemented to improve the signal-to-noise ratio (SNR) of the vital signs, and then a vital-sign-enhanced imaging algorithm is presented to suppress the environmental clutters and mutual affection of multiple humans. Finally, an automatic detection algorithm including constant false alarm rate (CFAR), morphological filtering and clustering is implemented to improve the detection performance of weak human targets affected by heavy clutters and shadow effect. The simulation and experimental results show that the proposed method can get a high-quality image of multiple humans and we can use it to discriminate and localize multiple adjacent human targets behind brick walls. Sensors 2016, 16, 1922 2 of 17 of multiple human bodies. The mutual interference mainly includes the strong sidelobes of multiple humans and the shadow effect. Due to individual difference, reflection intensity of different humans has significant difference. Sidelobes of human target with strong reflection will submerge the signal of the adjacent human targets with weak reflection. Humans are no longer point targets in high-resolution imaging radar applications. For example, the frequency range of a common high resolution radar system is usually 0.5-3 GHz or even larger. If the frequency band is 2 GHz, the range resolution is 7.5 cm. However, the size of a common human is 1.8 m × 0.5 m × 0.35 m [8] . In this case, the human will occupy several resolution units. Considering the relationship between human size and resolution, human must be considered as an extended target, which will obscure the EM propagation. Thus, the EM waves, which should irradiate a certain human and reflected back to receiving element, will be partly blocked by the nearby humans. Therefore, the signal reflected from the distant humans will be weakened. We call the above phenomenon the shadow effect [9]. If the above interactions are misconducted, the leakage alarm rate will increase when there exist multiple humans in the detection region. The secondary damage may occur in the actual post-disaster rescue scenes. Much research effort has been made to solve the multiple stationary humans detection problem. Continuous wave (CW) radars are by far the most popular platform of stationary human detection, as they require relatively small hardware expense. Zhou et al. [10] developed a generalized likelihood ratio test (GLRT) for CW radar to distinguish between the presences of 2, 1 or no subjects. However, the drawback of CW radar is that they do not allow the localization of the humans and will lead to increased difficulty in interference suppression and multiple stationary humans discrimination. Further performance improvements can be achieved by the ultra-wideband (UWB) radar. UWB waveform provides high range resolution ability, and thus has the potential to determine the distance of humans with high accuracy [11, 12] . This allows accurate localization of the breathing subject and tracking of the small movements of the diaphragm during breathing [13] . In [14] , UWB impulse radar was used to monitor the breathing rates of two subjects through a cement wall. Wang et al. [15] proposed a logarithmic method (LM) utilizing the phase variation of the reflected pulses caused by the periodic thorax displacements to monitor multiple subjects at low power consumption. The stepped frequency continuous wave (SFCW) is a new form of UWB radar waveform, which transmits a series of discrete frequencies in a stepwise manner, covering the radar bandwidth in the time domain to realize the UWB. The SFCW radar technology is superior to the time domain impulse radar for its high reliability and relative easy implementation. Liu et al. [16] used SFCW radar to detect vital signs from a human subject under laboratory conditions. Cardiologic signals can be achieved when the human subject was in the line of sight. However, these UWB systems use a single input and single output (SISO) channel, in which only range profile image of humans from single sight angle can be attained. For the applications of non-line-of-sight (NLOS), the detection performance of SISO degrades [16] . Meanwhile, three-dimensional space information of vital signs is projected onto the range dimension, which made it difficult to mitigate the mutual interference between multiple human bodies due to the aliasing problem. These challenges constrain the usage of SISO UWB radar in multiple stationary humans detection. The radar system with multiple receiving channels, which can be termed as single-input multiple-output (SIMO), is used to achieve further improvement [17, 18] . SIMO radar systems fuse the information from multiple channels to improve the detection performance. Akiyama et al. [19] used a system with one transmitting antenna and four receiving antennas to improve the signal-to-noise ratio (SNR) with correlation processing. Liu et al. [18] demonstrated that the SIMO radar systems have the ability to resolve multiple sources and obtain the angle-of-arrival (AOA) of multiple human targets. Multiple-input and multiple-output (MIMO) radar is a special type of multiple channels radar which emerged in recent years. The MIMO array with M transmitting elements and N receiving elements can obtain a virtual aperture with M × N virtual transceivers, which greatly reduces the weight and cost of the radar system. MIMO radar echo data can be decomposed as the data from
doi:10.3390/s16111922 pmid:27854356 pmcid:PMC5134581 fatcat:btdon3bom5dvpcz7br4l42g5cy