1. Introduction
On January 30, 2014, the Ministry of Internal Affairs and Communications (MIC) of Japan officially
revised the radio regulations to allocate the band from 116 GHz to 134 GHz for the 120-GHz band
wireless link for broadcasting services. This is the first industrial allocations of over-100-GHz carrier
frequencies, a beginning frequency edge of terahertz waves defined as electromagnetic waves
from 100 GHz (0.1 THz) to 10 THz. It has passed 14 years since a born of the 120-GHz band
wireless link in 2000 [1]. At that time, 120-GHz band signals were generated and modulated by
means of photonics technologies, since electronic components such as oscillators and modulators
with sufficient bandwidth were not available. Its unprecedented data rate of 10 Gb/s attracted the
broadcasters who had a need to transmit multiple channels of high-definition (HD) TV data over the
distance of 1 km [2]. In 2004, the MIC approved the photonics-based transmitter as an experimental
radio station which was introduced to examine the usefulness and practicality of the link in the
outdoor. The photonics-based 120-GHz wireless technology also activated a development of
electronic devices and integrated circuits such as amplifier MMICs to strengthen the wireless
technology, and finally all electronic MMIC-based systems were successfully developed and
deployed in real-world events such as the 2008 Beijing Olympic Games [3].
Now, the demand for much higher speed wireless technology is ever increasing in accordance
with a rapid advancement of mobile networks and rich contents handled by the networks. The
prospective data rate for wireless communications in the marketplace will be 100 Gb/s within ten
years. Against this background, researchers have recently been seeking the use of radio waves