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

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

The increasing demand of unoccupied and unregulated bandwidth for wireless communication systems will inevitably lead to the extension of operation frequencies toward the lower THz frequency range. Higher carrier frequencies will allow for fast transmission of huge amounts of data as needed for new emerging applications. Despite the tremendous hurdles that have to be overcome with regard to sources and detectors, circuit and antenna technology and system architecture to realize ultrafast data transmission in a scenario with extensive transmission loss, a new area of research is beginning to form. In this article we give an overview of emerging technologies and system research that might lead to ubiquitous THz communication systems in the future.

A Review on Terahertz Communications Research

A 120-GHz-band wireless link that uses millimeter-wave (MMW) photonic techniques was developed. The output power and noise characteristics of 120-GHz-band MMWs generated by converting a 125-GHz optical subcarrier signal were evaluated. It was then shown that the noise characteristics of the 125-GHz signal generated with these photonic technologies is sufficient for 10-Gb/s data transmission. We constructed a compact 120-GHz-band wireless link system, and evaluated its data transmission characteristics. This system achieved error-free transmission of OC-192 and 10-GbE signals over a distance of more than 200 m with a received power of below -30 dBm.

120-GHz-band millimeter-wave photonic wireless link for 10-Gb/s data transmission

We have developed a 120-GHz-band wireless link whose maximum transmission data rate is 11.1 Gbit/s. The wireless link uses millimeter-wave monolithic integrated circuits (MMICs) for the generation of a 120-GHz-band millimeter-wave wireless signal. The MMICs were fabricated using 0.1-mum-gate InP-HEMTs and coplanar waveguides. The wireless link can handle four kinds of data rate for OC-192 and 10-Gbit Ethernet standards with and without forward error correction (FEC). We succeeded in the error-free transmission of a 10-Gbit/s signal over a distance of 800 m. The introduction of FEC into the 120-GHz-band wireless link decreased the minimum received power for error-free transmission, and improved the reliability of the link.

10-Gbit/s Wireless Link Using InP HEMT MMICs for Generating 120-GHz-Band Millimeter-Wave Signal

The transmission characteristics of a 120-GHz-band millimeter-wave wireless link are described. The wireless link uses photonic technologies for generation, modulation, and transmission of millimeter-wave signals. This configuration enables set up of the photonic millimeter-wave generator and transmitter core separately; therefore, the wireless link can be used as a kind of radio-over-fiber system. The effects of transmitting 120-GHz-band optical subcarrier signals through single-mode fibers were theoretically and experimentally investigated. It was confirmed that the time shift of the code edges, because of chromatic dispersion, limits the transmission distance. A data stream at 10-Gbit/s was successfully transmitted over the 120-GHz-band millimeter-wave wireless link, with a bit error rate (BER) below 10-12 over a distance of 250 m. The results also demonstrated the stability of the wireless link, which satisfied the 10-Gb Ethernet standard under clear weather conditions.

Transmission Characteristics of 120-GHz-Band Wireless Link Using Radio-on-Fiber Technologies

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 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.

R. Yamaguchi

Papers

120-GHz-band millimeter-wave photonic wireless link for 10-Gb/s data transmission

10-Gbit/s Wireless Link Using InP HEMT MMICs for Generating 120-GHz-Band Millimeter-Wave Signal

Transmission Characteristics of 120-GHz-Band Wireless Link Using Radio-on-Fiber Technologies

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

10-Gbit/s MMIC wireless link exceeding 800 meters