# Selecting Target Range with Accurate Vital Sign Using Spatial Phase Coherency of FMCW Radar
## 1 Introduction  
Respiration and heartbeat can provide the basic physiological state of human beings, and can be used as healthcare indicators or in clinical diagnosis [1; 2; 3; 4; 5; 6]. Traditionally, stretch sensors [7], pneumotachographs [8; 6], photoplethysmographs [9; 10], and electrocardiographs [11; 12] have been used to measure respiration and heartbeat. However, traditional monitoring sensors that are attached to the body can cause discomfort and restrict behavior. Radar can detect micro-displacement in the human body surface without contact [13; 14; 15], and this ability has been exploited for contactless monitoring of vital signs (VSs). Radar can monitor VSs continuously, and without contact, so the monitoring system has been applied in various applications such as medical care surveillance for severe burns or infectious diseases [16; 17], driver assistance [18; 19], sleep monitoring for sudden infant death syndrome, apnea detection [20; 21; 22], and homecare for the elderly [23; 24; 25]. In particular, frequency-modulated continuous wave (FMCW) radar, which has ranging capability, can spatially separate VSs from clutters [26; 27]. Additionally, FMCW radar uses a millimeter-wave band, and therefore has the advantages of small sizes and low power consumption. Therefore, FMCW radar has been widely used for contactless monitoring of VSs [28; 29; 30; 31; 32; 33; 34; 35; 36].  
Methods to extract VSs from the FMCW radar signal are generally divided into spectral analysis and range-bin selection [37; 38; 39; 40; 41]. The discrete Fourier transform (DFT) is mostly used for spectral analysis, which is a range analysis of the FMCW signal. The results of DFT at each range (or spectral) bin, namely magnitude and phase, contain information about displacement of the target. Information pertaining to respiration and heartbeat exists in specific range-bins, such as range-bins near the abdomen or thorax for respiration [39; 42], and range-bins near the neck for heartbeat [43], so the range-bin that includes the VSs must be selected precisely. In conventional methods, the magnitude or phase in a single range-binis used to search for the range-bin that includes the VSs [37; 38; 39], or signals in multiple range-bins are integrated [40]. The method in [37] uses the coherency between magnitude and phase as a criterion for range-bin selection, and therefore enables more accurate selection of range-bins that contain information about VSs, compared to the methods in [38; 39; 40], which exploit the extent of magnitude, phase fluctuation, or integrated phase. However, the magnitude does not give an accurate measurement of VSs, so range-bins that contain inaccurate information on VSs may be selected. When a range-bin that contains inaccurate VS information is selected, the accuracy of the measurement of VSs degrades. Therefore, range-bin that contains accurate VS information must be selected exactly.  
This paper introduces a range-bin selection criterion that exploits coherency of the phase in multiple range-bins to select a range-bin that contain an accurate VS signal. Considering the radar signal model, phase is preferred for measuring the VS displacement to magnitude. In addition, respiration and heartbeat occur in spatially-various parts of the body, so the displacement with vital periods observed simultaneously in various range-bins is expected to be respiration or heartbeat. Thus, spatial phase information is used for range-bin selection instead of using magnitude, as in [37; 38]. A spatial phase coherency (SPC) index was devised to quantify the coherency between displacements in spatially different range-bins. Then, a pair of range-bins that had the highest SPC was selected for vital extraction. Experimental results demonstrate that the proposed method selects range-bins that contain more accurate respiration and heartbeat than the bins selected by using conventional methods.