High-speed data transfer and high-performance imaging can be realized by using radio-waves in high-frequency bands, such as millimeter-waves and THz-waves, where wide frequency bands are available. However, the cell size would be smaller than a few hundred meters, due to large free space propagation loss and large atmospheric attenuation. Thus, many base stations, which are connected by networks, are required to offer nationwide or global network services by such high-frequency radio-bands. The networks would be constructed by various transmission media including optical fibers and fixed wireless links, where many media converters are required. This paper reviews various technologies for seamless bridges between radio and optical links. For the time being, congestion of radio spectrum in THz bands is not significant. However, if we look at the history of radio-wave technologies, spectral congestion has been high even in newly developed high frequency bands. In active radio services in millimeter-wave or THz-wave bands, interference mitigation with passive services such as radio astronomy and Earth observation satellites is an important issue, as of now. This paper describes research trends of THz-wave technologies from the point of view of a figure of merit defined by a product of the carrier frequency and spectral efficiency, to discuss the significance of spectral efficiency enhancement in the high-frequency region. Analysis of power consumption of short-distance radio systems is also shown to discuss expected performance of THz-wave links.
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