Radio links using terahertz (THz) waves can provide high-speed wireless transmission whose bitrate is higher than 100Gb/s. Congestion of radio spectrum in THz bands (0.1-10 THz) is not so significant for the time being. However, multi- level modulation formats would be required in in THz bands as well as in conventional millimeter- wave or microwave bands, to increase spectral efficiency. We provide overviews on spectral efficiency and transmission capacity of high-speed radio links using THz bands. Reduction power consumption in radio equipment is also very important to reduce operation cost of networks. We describe a survey result on power consumption of short-distance wireless systems. The result implies that THz high-speed radio links would be useful to reduce power consumption per bit in transmission. However, transmission distances in THz systems would be shorter than a few kilometers due to attenuation in the air. Thus, we should rely on seamless networks consisting of THz radio- links and optical fibers, where devices developed for optical fiber transmission can be used to THz wave generation and detection.