Terahertz wireless could lead to fiber-optics speed in-flight and mobile metropolitan internet

February 14, 2017

Terahertz wireless links to spaceborne satellites could one day make gigabit-per-second connection speeds available to anyone, anytime, anywhere on the face of the earth, on the ground or in flight (credit: Fujishima et al./Hiroshima University)

Hiroshima University researchers and associates have developed a terahertz* (THz) transmitter capable of transmitting digital data over a single channel at a speed of 105 gigabits per second (Gbps), and demonstrated the technology at the International Solid-State Circuits Conference (ISSCC) 2017 conference last week.

For perspective, that’s more than 100 times faster than the fastest (1 Gbps) internet connection in the U.S. or more than 3,000 times faster than the 31 Mbps available to the average U.S. household in 2014, according to an FCC report. It’s also ten times or more faster than the fastest rate expected to be offered by fifth-generation mobile networks (5G) for metropolitan areas around 2020.

Major uses: faster in-flight and metropolitan internet, high-frequency trading

Applications of this forthcoming THz technology include higher-speed in-flight network connection speeds via satellite, fast download of videos and other large files for mobile devices, and ultrafast wireless links between base stations, according to Hiroshima University professor Minoru Fujishima.

An important business application is faster high-frequency trading, which requires minimal latency (delay). Recently, the time it takes to execute these trades has gone from milliseconds (thousandths of a second) to microseconds (millionths of a second), as KurzweilAI has explained. However, long-distance fiber optics connections (currently used for long-distance trading) have significant latency because light (as radio waves) travels 50%  faster in a vacuum than through glass fiber, while microwaves traveling in air have a less than a 1% speed reduction.

The National Institute of Information and Communications Technology and Panasonic Corporation are also partners in this research.

* Terahertz, a frequency range that is 1,000 times higher than gigahertz, or 1012 Hz, actually starts at 100 GHz or .1 THz. The researchers transmitted in this unregulated THz band — a vast new frequency resource expected to be used for future ultrahigh-speed wireless communications — using the frequency range from 290 GHz to 315 GHz. The full range of frequencies in the THz band (275 GHz to 450 GHz) are currently unallocated, but are expected to be discussed at the World Radiocommunication Conference 2019.

Abstract of A 105Gb/s 300GHz CMOS Transmitter

“High speed” in communications often means “high data-rate” and fiber-optic technologies have long been ahead of wireless technologies in that regard. However, an often overlooked definite advantage of wireless links over fiber-optic links is that waves travel at the speed of light c, which is about 50% faster than in optical fibers as shown in Fig. 17.9.1 (top left). This “minimum latency” is crucial for applications requiring real-time responses over a long distance, including high-frequency trading [1]. Further opportunities and new applications might be created if the absolute minimum latency and fiber-optic data-rates are put together. (Sub-)THz frequencies have an extremely broad atmospheric transmission window with manageable losses as shown in Fig. 17.9.1 (top right) and will be ideal for building light-speed links supporting fiber-optic data-rates. This paper presents a 105Gb/s 300GHz transmitter (TX) fabricated using a 40nm CMOS process.

Abstract of A 300GHz 40nm CMOS transmitter with 32-QAM 17.5Gb/s/ch capability over 6 channels

The vast unallocated frequency band lying above 275GHz offers enormous potential for ultrahigh-speed wireless communication. An overall bandwidth that could be allocated for multi-channel communication can easily be several times the 60GHz unlicensed bandwidth of 9GHz. We present a 300GHz transmitter (TX) in 40nm CMOS, capable of 32-quadrature amplitude modulation (QAM) 17.5Gb/s/ch signal transmission. It can cover the frequency range from 275 to 305GHz with 6 channels as shown at the top of Fig. 20.1.1. Figure 20.1.1 also lists possible THz TX architectures, based on recently reported above-200GHz TXs. The choice of architecture depends very much on the transistor unity-power-gain frequency fmax. If the fmax is sufficiently higher than the carrier frequency, the ordinary power amplifier (PA)-last architecture (Fig. 20.1.1, top row of the table) is possible and preferable [1-3], although the presence of a PA is, of course, not a requirement [4,5]. If, on the other hand, the fmax is comparable to or lower than the carrier frequency as in our case, a PA-less architecture must be adopted. A typical such architecture is the frequency multiplier-last architecture (Fig. 20.1.1, middle row of the table). For example, a 260GHz quadrupler-last on-off keying (OOK) TX [6] and a 434GHz tripler-last amplitude-shift keying (ASK) TX [7] were reported. A drawback of this architecture is the inefficient bandwidth utilization due to signal bandwidth spreading. Another drawback is that the use of multibit digital modulation is very difficult, if not impossible. An exception to this is the combination of quadrature phase-shift keying (QPSK) and frequency tripling. When a QPSK-modulated intermediate frequency (IF) signal undergoes frequency tripling, the resulting signal constellation remains that of QPSK with some symbol permutation. Such a tripler-last 240GHz QPSK TX was reported [8]. However, a 16-QAM constellation, for example, would suffer severe distortion by frequency tripling. If the 300GHz band is to be seriously considered for a platform for ultrahigh-speed wireless communication, QAM-capability will be a requisite.