Research into terahertz technology is now receiving increasing attention around the world, and devices exploiting this waveband are set to become increasingly important in a very diverse range of applications. Here, an overview of the status of the technology, its uses and its future prospects are presented.

Cutting-edge terahertz technology

This Review covers the state-of-the-art technologies on photonics-based terahertz communications, which are compared with competing technologies based on electronics and free-space optical communications. Future prospects and challenges are also discussed. Almost 15 years have passed since the initial demonstrations of terahertz (THz) wireless communications were made using both pulsed and continuous waves. THz technologies are attracting great interest and are expected to meet the ever-increasing demand for high-capacity wireless communications. Here, we review the latest trends in THz communications research, focusing on how photonics technologies have played a key role in the development of first-age THz communication systems. We also provide a comparison with other competitive technologies, such as THz transceivers enabled by electronic devices as well as free-space lightwave communications.

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Advances in terahertz communications accelerated by photonics

Recent changes in how people consume multimedia services are causing an explosive increase in mobile traffic. With more and more people using wireless networks, the demand for the ultra-fast wireless communications systems is increasing. To date, this demand has been accommodated with advanced modulation schemes and signal-processing technologies at microwave frequencies. However, without increasing the carrier frequencies for more spectral resources, it may be quite difficult to keep up with the needs of users. Although there are several alternative bands, recent advances in terahertz-wave (THz-wave) technologies have attracted attention due to the huge bandwidth of THz waves and its potential for use in wireless communications. The frequency band of 275 ~ 3000 GHz , which has not been allocated for specific uses yet, is especially of interest for future wireless systems with data rates of 10 Gb/s or higher. Although THz communications is still in a very early stage of development, there have been lots of reports that show its potential. In this review, we will examine the current progress of THz-wave technologies related to communications applications and discuss some issues that need to be considered for the future of THz communications.

Present and Future of Terahertz Communications

A wireless communication system with a maximum data rate of 100 Gbit s−1 over 20 m is demonstrated using a carrier frequency of 237.5 GHz. The photonic schemes used to generate the signal carrier and local oscillator are described, as is the fast photodetector used as a mixer for data extraction.

Wireless sub-THz communication system with high data rate

According to Edholm’s law, the demand for point-to-point bandwidth in wireless short-range communications has doubled every 18 months over the last 25 years It can be predicted that data rates of around 5–10 Gb/s will be required in ten years In order to achieve 10 Gb/s data rates, the carrier frequencies need to be increased beyond 100 GHz Over the past ten years, several groups have considered the prospects of using sub-terahertz (THz) and THz waves (100–2000 GHz) as a means to transmit data wirelessly Some of the reported advantages of THz communications links are inherently higher bandwidth compared to millimeter wave links, less susceptibility to scintillation effects than infrared wireless links, and the ability to use THz links for secure communications Our goal of this paper is to provide a comprehensive review of wireless sub-THz and THz communications

Review of terahertz and subterahertz wireless communications

This paper describes the development of a InP-HEMT MMIC chipset for 120-GHz wireless applications. The transmitter chip includes a frequency doubler for carriers, an ASK modulator, an RF band-pass filter, and a power amplifier. The receiver chip includes a low-noise amplifier and an ASK demodulator. A back-to-back test of the chipset has shown it to be fully functional at 10-Gbit/s data rate with BER=e-12 at -45.7-dBm input power of the receiver chip. To our knowledge, this is the first report of the development of highly integrated MMIC chipset operating at 120 GHz for wireless data communication.

120-GHz Tx/Rx chipset for 10-Gbit/s wireless applications using 0.1 /spl mu/m-gate InP HEMTs

We measured the effects of rain attenuation on 120-GHz band wireless link during the heavy rainy period (from July to September, 2008) and annual period (from March to December, 2008). The heavy rainy period data are used as a basis for the wireless link design, and the annual data are used to calculate the reliability of the wireless link. The 120-GHz band wireless link with V polarization was operated on a 400-m-long path from March to December 2008 in Atsugi, Japan. The rain rate distribution for Atsugi is in good agreement with the conditional M distributions. As for rain attenuation, the annual data coincide with the ITU and Laws and Parsons models. Cumulative distributions of attenuation and bit-error-rate deterioration due to rain were obtained.

Effect of Rain Attenuation for a 10-Gb/s 120-GHz-Band Millimeter-Wave Wireless Link

We present a high-directivity photonic emitter with a high-gain antenna and waveguide-output photodiode module (WG-PM) for extending the transmission distance of a wireless link that uses a 120-GHz millimeter wave. The module employs a uni-traveling-carrier photodiode, broad-band high electron-mobility transistor (HEMT) amplifier, and planar-circuit-to-waveguide transition substrate. The maximum output power of the WG-PM is 8 dBm at a frequency of 120 GHz, and it has a 3-dB bandwidth of over 16 GHz. The wireless link with the high-directivity photonic emitter achieved 10-Gbit/s wireless data transmission, and using a high-gain Gaussian optic lens antenna and an HEMT amplifier reduced the input optical power necessary for error-free transmission. The transmission characteristics of the link showed that its transmission distance can be extended to over 100 m.

High-directivity photonic emitter using photodiode module integrated with HEMT amplifier for 10-Gbit/s wireless link

This paper describes the development of wavegude modules for 120-GHz wireless applications. The MMIC in the modules were fabricated using 0.1-mum-gate InP-HEMTs and coplanar waveguides. The transmitter (Tx) module contains a Tx MMIC and a multiplier MMIC for carrier generation. The Tx MMIC contains a frequency doubler, ASK modulator, and an amplifier. Output power of the Tx module is 0 dBm. The receiver (Rx) module contains a Rx MMIC with a low-noise amplifier and ASK demodulator. For high-power operation of transmitter, we developed a power amplifier (PA) waveguide module that attaches to the Tx module. This PA module contains one-chip PA MMIC, and has 13-dBm output power with associated gain of 10.5 dB at 125 GHz. A back-to-back test of the Tx/Rx modules with and without the PA module have shown it to be fully functional at 10-Gbit/s data rate with BER = 1e-10 at -34.5-dBm and -36.1-dBm input powers for the receiver, respectively

120-GHz Tx/Rx Waveguide Modules for 10-Gbit/s Wireless Link System

We have developed a 120-GHz-band wireless link that uses MMICs for the generation, modulation, and amplification of 125-GHz signals. We succeeded in the error-free (BER=10-12) transmission of a 10-Gbit/s signal over a distance of 800 m, and the maximum transmission distance was estimated to be about 2 km.

Toshihiko Kosugi

Papers

120-GHz Tx/Rx chipset for 10-Gbit/s wireless applications using 0.1 /spl mu/m-gate InP HEMTs

Effect of Rain Attenuation for a 10-Gb/s 120-GHz-Band Millimeter-Wave Wireless Link

High-directivity photonic emitter using photodiode module integrated with HEMT amplifier for 10-Gbit/s wireless link

120-GHz Tx/Rx Waveguide Modules for 10-Gbit/s Wireless Link System

10-Gbit/s MMIC wireless link exceeding 800 meters