## 3 Integrating the Antennas for RF Sensing and Communication and Experiments  
The elemental antennas which were designed and explained in the previous section are integrated into one module to be the core part of the ISAC equipment shown below. The communication block is located at the top of the surface-sensor block because the antenna for communication, which works at the higher frequency, is the smaller of the two antennas. Figure 5 displays the scenarios and experimental configurations used to examine the functions and properties of the small and different surface conditions, like in Figure 5a. The communication and sensing links on the line-of-sight (LoS) are initially observed as in Figure 5b, which is realized by Figure 5c. Secondly, the direct path is changed to the sum of the incident and reflected paths by assuming an object. This mimics the scattering of the wireless signal caused by reflections from cars or buildings in an urban site. As mentioned earlier, the pointing error of the radiated wave seriously influences the mmW system, and it is assisted by the information acquired by the sensor which recognizes a barrier in the communication link. This is one of the reasons why ISAC is introduced to ensure good quality in 6G mobile services. The EM links of the ISAC module are tested with reference to reflection by the typical conducting plane as in Figure 5d,e, and with a reflecting metasurface, as in Figure 5f,g as a non-conventional object to be sensed. Its  
Figure 4: The antenna for RF sensing. (**a**) Front side of the array. (**b**) Backside of the array. (**c**) S\({}_{11}\) of the antenna (**d**). Far-field pattern of the two-dimensional array antenna expressed in from red (strongest) through green (middle) to blue (weakest). (**e**) Measured beam-pattern. characteristics are mentioned in Appendix A. The two reflecting surfaces with the area of 11.5 cm \(\times\) 11.5 cm are compared in Figure 4(b). The distance between the two ports of the LoS tests is denoted as \(Dist_{\text{-LoS}}\) in Figure 4(b), and there is nothing in the path between the two sides, as in Figure 4(c). The straight path becomes \(Dist_{\text{-RoundT}}\), broken into \(Dist_{\text{-Inc.}}\) as the distance of the incident segment and \(Dist_{\text{-Roff}}\) as the distance of the reflected segment, as shown in Figure 4(d). The frequencies dealt with here belong to a very high frequency group, in other words, 28 GHz and 24 GHz, and most of surfaces of the structures may seem like conducting facets to the impinging mmW signals. The left picture in Figure 4(b) is a copper plane. These days, reconfigurable intelligent surfaces (RISs) draw attention from researchers studying 6G mobile service as a way of reducing the blockage error, which is rooted in the reflective array as the metasurface [14]. As such, it also hinders the signal. Figure 5: _Cont._  
The experiments are conducted largely from four standpoints that are the sensing signal link vs. the comm. signal link, LoS vs. the round-trip link by reflection, the reflection of the typical conducting plane vs. the reflection by RS01, and a long distance vs. a longer distance. In detail and one by one, the first viewpoint means that the sensing signal link and comm. signal link are mainly driven by 24 GHz and 28 GHz signals, respectively. The second viewpoint is that the S\({}_{21}\) values of the LoS and round trips by two different kinds of system are compared. The third point is that the strengths of the reflected signals by the copper plane and the metasurface are compared. The fourth viewpoint is that considering that the wavelengths at 24 GHz and 28 GHz are 12.5 mm and 10.7 mm, respectively, 1 m and 1.5 m are given as the distances of the direct and round-trip links, respectively. The input power of \(-3\) dBm, which is a very low level of RF power, and the angles of incidence from TX and reflection to RX, which are set at 45\({}^{\circ}\), are applied to all the tests here.