# Key Technologies for THz Wireless Link by Silicon CMOS Integrated Circuits
## 1 Introduction  
The data rate of communication has been increasing exponentially year-by-year, as shown in Figure 1[1]. In particular, the improvement of the data rate of wireless communication has been remarkably rapid compared with wired communication. If the data rate continues to improve with this rate of advancement, rates of 100 gigabits per second--which cannot currently be realized except via optical communication--could be realized by wireless communication in 2020. In the field of ultra-high-speed wireless communication, wireless transceivers with data rates exceeding 100 gigabits per second have recently been released as if to support this. Extrapolating the graph as if this data rate growth will continue in the future allows the possibility that the wireless data rate will catch up with the wired data rate in 2030 [2]. Of course, there are many remaining technical challenges that will prevent wireless data rates catching up with the wired data rate, and equivalence cannot simply be realized. However, the era of low data rates in wireless communication will soon end.  
The key to realizing wireless communication approaching wired data rates is the terahertz band, which has an intermediate property between radio waves and light with an electromagnetic wave around 1 THz. Terahertz wireless communication is one of the important applications covered by the terahertz photonics field [3]. Signals in this frequency band are generated and detected by photonics devices and electronic devices but, because of their significant technical difficulties, they are not yet widely used. However, communication using the terahertz band has attracted attention in recent years. This is because, unlike microwave and millimeter waves, the overwhelmingly wide frequency band near 100 GHz can be used. Such a wide frequency band is typically used in optical communication, in which high data rates have been realized, but has also become available for wireless communication.  
The 300 GHz band corresponding to the low frequency band in the terahertz band is considered promising in wireless communication. Since this frequency band is included in the 220-330 GHz band of the WR-3 (3.4-inch rectangular waveguide, one of the specifications of a rectangular waveguide)waveguide standard, communication in this frequency band has been actively studied in recent years. In this paper, we will refer to communication using this WR-3 band as 300 GHz band communication.  
Why is the 300 GHz band promising for communication? The most significant reason is the frequency allocation rule. Since radio waves are finite resources and the usage of each frequency is determined, the frequency band that can be used for communication is limited. Although a wide frequency band exists in the ultra-high frequency band higher than the millimeter wave, the frequency is allocated to various applications related to space observation, and it is difficult to use the continuous broadband for communication. A band of 252 GHz to 275 GHz has already been allocated to mobile and fixed wireless communications [4]. Further, a discussion on utilizing the frequency band exceeding 275 GHz, which is currently unallocated, for communication is underway within the International Telecommunication Union Radiocommunication Sector (ITU-R) [5]. Since the terahertz band has strong directivity and interference is not stringent, sharing of communication and observation is being considered. Therefore, the possibility of using the 300 GHz band for communication is increasing. As a result, it is expected that a continuous wide frequency band exceeding 252 GHz and extending over 275 GHz will be available soon for wireless communication.  
In this paper, we discuss the silicon integrated circuit technology that realizes 300 GHz band communication, and new applications of terahertz communication, including using the 300 GHz band.