The THz band is a new and vast frequency resource expected to be used for future ultrahigh-speed wireless communications. The research group has developed a transmitter that achieves a communication speed of 105 gigabits per second using the frequency range from 290 GHz to 315 GHz. This range of frequencies are currently unallocated but fall within the frequency range from 275 GHz to 450 GHz, whose usage is to be discussed at the World Radiocommunication Conference (WRC) 2019 under the International Telecommunication Union Radiocommunication Section (ITU-R). Last year, the group demonstrated that the speed of a wireless link in the 300-GHz band could be greatly enhanced by using quadrature amplitude modulation (QAM). This year, they showed six times higher per-channel data rate, exceeding 100 gigabits per second for the first time as an integrated-circuit-based transmitter. At this data rate, the whole content on a DVD (digital versatile disk) can be transferred in a fraction of a second.
"This year, we developed a transmitter with 10 times higher transmission power than the previous version's. This made the per-channel data rate above 100 Gbit/s at 300 GHz possible," said Prof. Minoru Fujishima, Graduate School of Advanced Sciences of Matter, Hiroshima University. "We usually talk about wireless data rates in megabits per second or gigabits per second. But we are now approaching terabits per second using a plain simple single communication channel. Fiber optics realized ultrahigh-speed wired links, and wireless links have been left far behind. Terahertz could offer ultrahigh-speed links to satellites as well, which can only be wireless. That could, in turn, significantly boost in-flight network connection speeds, for example. Other possible applications include fast download from contents servers to mobile devices and ultrafast wireless links between base stations," said Prof. Fujishima. "Another, completely new possibility offered by terahertz wireless is high-data-rate minimum-latency communications. Optical fibers are made of glass and the speed of light slows down in fibers. That makes fiber optics inadequate for applications requiring real-time responses. Today, you must make a choice between 'high data rate' (fiber optics) and 'minimum latency' (microwave links). You can't have them both. But with terahertz wireless, we could have light-speed minimum-latency links supporting fiber-optic data rates," he added.
“Now THz wireless technology is armed with very wide bandwidths and QAM-capability. The use of QAM was a key to achieving 100 gigabits per second at 300 GHz,” said Prof. Minoru Fujishima, Graduate School of Advanced Sciences of Matter, Hiroshima University.
“Today, we usually talk about wireless data-rates in megabits per second or gigabits per second. But I foresee we’ll soon be talking about terabits per second. That’s what THz wireless technology offers. Such extreme speeds are currently confined in optical fibers. I want to bring fiber-optic speeds out into the air, and we have taken an important step toward that goal,” he added.
The research group plans to further develop 300-GHz ultrahigh-speed wireless circuits.
“We plan to develop receiver circuits for the 300-GHz band as well as modulation and demodulation circuits that are suitable for ultrahigh-speed communications,” said Prof. Fujishima.
IEEE Journal of Solid-State Circuits - A 300 GHz CMOS Transmitter with 32-QAM 17.5 Gb/s/ch Capability over Six Channels
A 300 GHz transmitter (TX) fabricated using a 40 nm CMOS process is presented. It achieves 17.5 Gb/s/ch 32-quadrature amplitude modulation (QAM) transmission over six 5 GHz-wide channels covering the frequency range from 275 to 305 GHz. With the unity-power-gain frequency fmax of the NMOS transistor being below 300 GHz, the TX adopts a power amplifier-less QAM-capable architecture employing a highly linear subharmonic mixer called a cubic mixer. It is based on and as compact as a tripler and enables the massive power combining necessary above fmax without undue layout complication. The frequency-dependent characteristics of the cubic mixer are studied, and it is shown that even higher data rates of up to 30 Gb/s are possible at certain frequencies, where the channel signal-to-noise ratio is high. The design and the operation of the power-splitting and power-combining circuits are also described in detail. The measurements reported herein were all made "wired" via a WR3.4 waveguide.
SOURCES Hiroshima University, IEEE Journal of Solid-State Circuits