## 3 Integrating the Antennas for RF Sensing and Communication and Experiments
Figure 6a-c shows the S\({}_{21}\) of 24 GHz input signal cases with the distance \(Dist_{LoS}\) and \(Dist_{RoundT}=1\) m going with LoS, reflection by the metal plane vs. LoS, and the reflection by RS01 with the metasurface vs. reflection by the metal plane vs. LoS, in that order. Figure 6a,b presents what happens when a very highly directive beam for the direct path and the round trip of reflection by a large metal plane travels the same distance, i.e., their S\({}_{21}\)-curves are identical. This proves the electromagnetic theories. Compared with them, the S\({}_{21}\) of the reflected signal due to RS01 behaves differently, which is inferred to be because non-conventional scattering occurs by the changed boundary condition of the reflecting surface [14; 15; 16; 17; 18; 19; 20]. Common to the three tests, the RX receives the strongest signal at 24 GHz, which was transmitted through the sensing channel. The next three cases focus on the communication link, which uses the 28 GHz signal. Figure 6d-f shows the S\({}_{21}\) of the distance \(Dist_{Round}\) and \(Dist_{RoundT}=1\) m going with LoS, the reflection by the metal plane vs. LoS, and the reflection by RS01 of the metasurface vs. the reflection by the metal plane vs. LoS, respectively. Figure 6d,e shows a deviation of the curve of the round trip from that of the direct path, which results from a fraction of the fan beam from the 28 GHz 1D array going over the edge of the reflecting surface. \(Dist_{LoS}=1\) m is already a very long distance equivalent to many wavelengths. Thinking of the low input power of the TX in the VNA, \(Dist_{LoS}\) feels extremely far for mmW bands. Moreover, the distance is extended to \(Dist_{LoS}\) and \(Dist_{RoundT}=1.5\) m to confirm the advantage of the high directivity of the designed antennas. Figure 6g-i shows the S\({}_{21}\) of 24 GHz input signal cases with the longer distance given to LoS, the reflection by the metal plane vs. LoS, and the reflection by RS01 of the Ka band reflect array vs. reflection by the metal plane vs. LoS, respectively. Figure 6g,h shows that even though they are very close, the error between them still occurs, since the new distance makes the finite sized plane look waning, and the returned signal becomes weaker than in Figure 6b. Despite there being a decrease in the signal strength, it is of a small scale because  
Figure 5: The module integrating the communication and surface-sensing antennas put into the experiments. (**a**) Prototype of the module. (**b**) Scheme of LoS link tests. (**c**) Real LoS link tests. (**d**) Scheme of the typical reflection link tests (**e**). Scheme of the typical reflection link tests. (**f**) Scheme of RS01(metasurface) reflection link tests. (**g**) Scheme of RS01(metasurface) reflection link tests. (**h**) Typical metal reflector and a 28 GHz-beam-tilting metasurface. of the advantage of a very highly directive beam. Like in Figure 6c, the reflected signal from RS01, as in Figure 6i, has a different behavior from the other cases, which comes from the changed boundary condition surface. The last three cases relate to the communication link, which uses the 28 GHz signal and a distance of 1.5 \(\times\) 10\({}^{3}\) mm. Figure 6j-l shows the S\({}_{21}\) curves of the LoS, the reflection by the metal plane vs. LoS, the reflection by RS01, and the the metasurface vs. the reflection by the metal plane, respectively. Figure 6j,k shows rather a large deviation of the curve of the round trip from that of the direct path, which occurs because a portion of the fan beam from the 28 GHz 1 \(\times\) 4-array misses the reflecting surface, which is located at a farther position and looks shrunken as a result of the increased path. Interestingly, the returned 28 GHz signal is made stronger, reaching \(-\)50 dB instead \(-\)58 dB, when the metal plane is replaced by the metasurface, as shown in Figure 6l.