# Key Technologies for THz Wireless Link by Silicon CMOS Integrated Circuits
## 3 Features and Applications of 300 GHz Band Communication
### The Future of Terahertz Communication Spreading in Space  
Inter-server communication for data centers, backhaul and fronthaul of networks, and high-speed communication in electronic devices are assumed applications of terahertz communication but these are forms of fixed wireless communication that have been realized by the conventional optical fiber network. Merely replacing fixed communication, however, does not achieve the maximum potential of terahertz communication. On the other hand, since terahertz air attenuation is large, it may be thought that it is unsuitable for long distance communication. However, as mentioned in Section 2, there is an atmosphere window in the 300 GHz band. In addition, if the antenna size is maintained, even if the wavelength is short, the directivity increases but the propagation loss decreases. The received power increases as the frequency increases if the effective area of the antenna is constant, so ultra-high-speed terahertz communication may be utilized in the universe without atmospheric attenuation if a high-power terahertz transmitter and a high-gain antenna are available. In the 300 GHz band, the gain of a parabola antenna with a 1.2-m diameter will be 70 dBi from Equation 5 if the aperture efficiency is 70%. The diameter \(\phi\) of the beam produced by the antenna with the gain \(G\) at the distance \(d\) is given by:  
\[\phi\simeq\frac{4d}{\sqrt{G}} \tag{7}\]  
regardless of the frequency. With a 70 dBi antenna, the diameter of the beam 1000 km ahead will be 1.3 km. Terahertz communication in which the beam diameter is wider than the laser beam helps alleviate the requirement of position control of the antenna. Furthermore, in the terahertz communication in the universe without atmospheric attenuation, a frequency band exceeding 300 GHz can be utilized. Although not in the 300 GHz band, as shown in Figure 16, in the frequency band from 121 GHz to 154 GHz, the distance at which the atmospheric attenuation becomes 10 dB exceeds 8 km. If this wide frequency bandwidth that is not currently allocated to communication is available, stable high-speed communication to the universe is possible in the desert without rain attenuation located at an altitude of 5000 m at which the air density is about half that of the sea level. Figure 17: In IEEE\({}^{3}\) 802.15.3d, channels in multiple bands are allocated in the frequency band from 252.72 GHz to 321.84 GHz. (**a,b**) are examples of channels allocated in IEEE 802.15.3d. (**c**) Assigning the channel of the 34.56 GHz band facilitates connection with the wired communication interface CEI-28 G (Common Electrical Interface 28 Gigabit-per-second). In the future, can we use terahertz communication to solve social problems? For example, to simulate various phenomena in the real world accurately on a large scale, it is indispensable to improve the performance of supercomputers. What are the challenges that must be solved when performance improvement of supercomputers is expected in the future? The computing performance of the supercomputer has improved exponentially, by 1000 times in about 10 years [29]. Currently, development is progressing toward the realization of an exascale computer that performs floating-point calculations \(10^{18}\) times per second in 2020. If the computation speed continues to improve after 2020, a zettascale computer will be created in 2030 that will perform floating point calculations at a tremendous rate of \(10^{21}\) times per second. One problem here is power consumption. It is foreseen that the power consumption of an exascale computer will be 20 MW [29]. On the other hand, in a zettascale computer with 1000 times higher computing capacity, the power consumption will be 20 GW if the hardware scale is simply multiplied by 1000. Even if low-power technology is developed, the power consumption of the zettascale computer will be around 1 GW. The power consumption of 1 GW is roughly equivalent to the output of one nuclear power generator.