## 5 Experimental Data
### Free Space Detection  
First, the radar shines directly on a metal wall. The front view and the side view of the experimental scenario are shown in Figure 10. The radar transmits the FMCW, and the frequency range is 1.2 GHz to 3 GHz. Transmitting and receiving antennas use Vivaldi antennas. They are parallel to each other and placed towards the metal wall. In this case, the metal wall is obtained by fixing a large metal plate to the wall of the room. The distance between the metal wall and the center of transmitting and receiving antennas is 0.9 m.  
Ideally, the beat signal consists of two parts: the directly coupled wave from the transceiver antenna and the reflected wave from the metal wall. The former is greater than the latter. Because of the phase-locked loop spuriousness in the system, the beat signal contains two pairs of clutter in addition to the two parts. A pair of clutter is generated along with the directly coupled wave, located on both sides of its main lobe. The other pair of clutter is generated along with the reflected wave from the metal wall, located on both sides of its main lobe. The beat signal is processed according to the steps in Section 3. Only time-varying clutter is left. After accumulating along the slow time, the clutter is highlighted, as shown in Figure 11a. The red dashed axis in Figure 11a is the position of the main lobe of the directly coupled wave from the transceiver antenna, and the green dashed axis is the position of the main lobe of the wave reflected from the metal wall. They are eliminated in the treatment of highlighting the clutter. The locations of the two axes are denoted as \(R_{red}=0\) m, \(R_{green}=0.9\) m. Only two spikes are observed in Figure 11a,  
Figure 10: The experimental scenario in free space. (**a**) The front view of the experimental scenario; (**b**) The side view of the experimental scenario. which are clutter waves located to the right of the main lobe. The clutter symmetrical to it does not appear. The positions corresponding to the two peaks are denoted as \(R_{c1}=3.6\) m, \(R_{c2}=4.5\) m, respectively. Accessing long delay lines before the transmitting antenna and after the receiving antenna which has an equivalent range of \(\Delta R=4.6\) m, four peaks are observed, as shown in Figure 11b. The distances corresponding to these four peaks are \(R_{d1}=1\) m, \(R_{d2}=1.9\) m, \(R_{d3}=8.2\) m and \(R_{d4}=9.1\) m, from left to right. The positions of the two axes become \(R_{red}^{\prime}=\Delta R\) and \(R_{green}^{\prime}=R_{green}+\Delta R\). The two pairs of clutter waves are symmetric about each of the two axes, i.e., \(R_{red}^{\prime}=\frac{R_{d1}+R_{d3}}{2}\), \(R_{green}^{\prime}=\frac{R_{d2}+R_{d4}}{2}\). And the distances of the left and right sides clutter to the corresponding symmetry axes are both equal to \(R_{x}=3.6\) m.  
Using the proposed method, the result obtained by calculating the correlation coefficients of this range-slow time matrix by rows is shown in Figure 11c. Four bright spots appear, arranged in a rectangular shape. The primary diagonal is the correlation coefficient between pairs of clutter before and after. The secondary diagonal is the correlation coefficient between different pairs of clutters. The result shows that the strongest correlation is between pairs of clutter before and after, and there is also some correlation between different pairs of clutter. Figure 11d shows the result after clutter removal using the proposed method. There are no more visible protrusions, indicating that the clutter has been removed. Figure 11: Identification and removal of the clutter in free space. (**a**) The distribution of unprocessed clutter along the distance; (**b**) Symmetrical clutter after connecting long delay lines; (**c**) The correlation coefficient matrix; (**d**) The result after removing clutter. Additionally, the radar is moved backward in steps of 0.3 m away from the metal wall.