# Key Technologies for THz Wireless Link by Silicon CMOS Integrated Circuits
## 3 Features and Applications of 300 GHz Band Communication
### Characteristics of Terahertz Band Communication  
You may imagine that the terahertz band has a large atmospheric attenuation and is limited to near-field communication. How large is the effect of atmospheric attenuation? Figure 15 shows atmospheric attenuation from 30 GHz to 3 THz [22; 23]. If the atmospheric attenuation is less than 10 dB, the effect is relatively small. On the other hand, for example, if the atmospheric attenuation at 1 km exceeds 10 dB, kilometer-class communication may become difficult. Such frequencies are 60 GHz, 183 GHz, 325 GHz and above 351 GHz. Apart from these frequency bands, the influence of atmospheric attenuation is relatively small for medium-range communications of at least about 1 km. In Figure 16, to estimate the communication distance, the distance at which the atmospheric attenuation is 10 dB is shown. There is a wide radio-wave window with a communication distance of 2 km from 192 GHz to 298 GHz [24]. This is one reason why the 300 GHz band is attracting attention as a communication application.  
On the other hand, it is necessary to consider free space propagation loss besides atmospheric attenuation. Using received power \(P_{r}\), transmit power \(P_{t}\), transmitter antenna gain \(G_{t}\), receiver antenna gain \(G_{r}\), wavelength \(\lambda\), and communication distance \(d\), propagation loss is given by:  
\[\frac{P_{r}}{P_{t}}=G_{t}G_{r}\bigg{(}\frac{\lambda}{4\pi d}\bigg{)}^{2}. \tag{3}\]  
Equation (3) is known as the Friis transmission equation. According to (3), as the wavelength \(\lambda\) decreases, the received power \(P_{r}\) decreases and the propagation loss increases. On the other hand, if the effective areas of the transmitting and receiving antennas are \(A_{t}\) and \(A_{r}\) respectively, the antenna gain is given by:  
\[G_{t}=\frac{4\pi}{\lambda^{2}}A_{t},\text{and} \tag{4}\]  
\[G_{r}=\frac{4\pi}{\lambda^{2}}A_{r}. \tag{5}\]  
The antenna gain is proportional to the effective area and increases in inverse proportion to the square of the wavelength \(\lambda\). Substituting (4) and (5) into (3) gives:  
\[\frac{P_{r}}{P_{t}}=\frac{A_{t}A_{r}}{\left(\lambda d\right)^{2}}. \tag{6}\]  
Figure 14: Performance comparison of terahertz transceivers. The communication distance is normalized by the wavelength, the received signal-to-noise ratio, and the antenna gain, where \(d\) is communication distance, \(\lambda\) is wavelength, SNR is received signal-to-noise ratio, \(G_{t}\) is transmitter antenna gain, and \(G_{r}\) receiver antenna gain.  
Equation (6) is nothing other than the equation shown in [PERSON] original paper [25]. Equation (6) shows that if the effective area of the antenna is constant, the received power increases in inverse proportion to the square of the wavelength, and the propagation loss decreases. In terahertz communication, it is important not to reduce the antenna area to realize medium to long distance transmission. Since the antenna gain increases as the frequency increases with the antenna area constant, terahertz communication is more suitable for one-to-one communication than broadcasting.