## 2 Issue of Ultra-Narrow Spot in mmWave and Terahertz  
The mmWave and terahertz bands exhibit spatially limited communication zones in specific directions through the use of directional high-gain antennas (Figure 2a). When these bands are installed on mobile platforms such as UAVs, the resultant zones also move. Thus, they exhibit spot-like characteristics, being both temporally and spatially confined, and thereby enable communication solely within distinct timeframes, periods, and positions. This perspective will be discussed below, incorporating analytical methods. In general, elevating the frequency of a given wireless system leads to reduced received power. Theoretically, the decrease in received power at higher frequencies stems from two factors: a decrease in the transmitter's output power and free-space propagation loss. [PERSON] et al. have conducted a survey revealing that output power is empirically inversely proportional to the cube of the carrier frequency \(f_{c}\)[18],  
\[P_{t}\propto 1/f_{c}^{3} \tag{1}\]  
where \(P_{t}\) is the output power from the transmitter. This effect arises due to the complexities of RF implementations involving semiconductors, known as the Johnson limit [19], making the fabrication of high-power transmitters progressively more challenging as the carrier frequency rises. Considering free-space path loss, we integrate the aforementioned power reduction effect noted in Equation (1) into the well-known Friis transmission equation [20],  
\[P_{r}=G\bigg{(}\frac{\lambda}{4\pi d}\bigg{)}^{2}P_{t}=G\Big{(}\frac{c}{4\pi d }\bigg{)}^{2}\frac{P_{t}}{f_{c}^{2}}\propto\frac{1}{f_{c}^{5}} \tag{2}\]  
where \(P_{r}\) is the power available at the receiver, \(G\) is the total antenna gain, and \(d\) is the distance between antennas. This equation indicates that the received power in a wireless communication system at a given frequency is roughly inversely proportional to the fifth power of that frequency. For instance, elevating the frequency of a 30 GHz mmWave system to 300 GHz terahertz, a tenfold increase, would result in a \(-\)50 dB reduction in received power. Note that even stronger attenuation caused by atmospheric gases may occur, potentially exceeding the effect predicted by the theory presented in Equation (2), particularly at frequencies resonating with gas molecules such as oxygen (O\({}_{2}\)), nitrogen dioxide (N\({}_{2}\)), and water vapor (H\({}_{2}\)O) [11]. To counteract this power attenuation and maintain consistent received power, we can employ high-gain antennas which satisfy the following relationship, cancelling out the frequency effect. \[G\propto f_{c}^{5} \tag{3}\]  
Assuming utilization of the generated communication spot through intersection (cross-sectional path within the zone), the cross-sectional spot size, \(s\), as defined in Figure 2a, can be geometrically calculated based on the half power beam width (HPBW) [21],  
\[HPBW=\sqrt{16\ln 2\frac{(180/\pi)^{2}}{10^{6/20}}}, \tag{4}\]  
\[s=2d\tan\bigg{(}\frac{HPBW}{2}\frac{\pi}{180}\bigg{)}. \tag{5}\]  
Figure 2b shows the theoretical correlation between frequency and spot size as derived from Equations (3)-(5). As previously mentioned, the received power diminishes at higher frequencies, resulting in a reduced communication range. Consequently, the need arises to  
augment antenna gain to compensate for this impact, which in turn leads to the narrowing of the spot size. Assuming a distance of 5 m, while the 28 GHz band used in 5G exhibits a spot size of about 3 m, the 60 GHz band targeted in this paper features an approximate one-meter spot size. Even higher frequencies have even more restricted spot sizes, just 16 cm for the 300 GHz band. Despite the constraints imposed by these extremely high frequencies, the available communication spot offers the advantage of enabling data transmission via wider bandwidths at elevated carrier frequencies, facilitating exceedingly high-speed transmissions within the delimited zone.