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

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 demonstrate the generation of 90-125 GHz millimeter-wave signal with a phase noise of less than -75 dBc/Hz at 100 Hz by using optical heterodyning that employs an arrayed-waveguide grating (AWG) integrated with 3-dB combiners.

Low-phase noise photonic millimeter-wave generator using an AWG integrated with a 3-dB combiner

This paper describes a 120-GHz-band quadrature phase shift keying (QPSK) transmitter and receiver fabricated on microwave monolithic integrated circuits (MMICs). The MMICs were fabricated using 0.1-µm-gate InP HEMTs and can handle 20-Gbit/s data streams. The transmitter MMIC consists of a QPSK modulator with direct modulation, differential amplifiers for data, a power amplifier, and an output-power monitor circuit. A differentially coherent detector, voltage-controlled phase shifter, and low-noise amplifier are monolithically integrated on a receiver MMIC. The QPSK transmitter and receiver MMICs were mounted in WR-8 waveguide modules to evaluate the data transmission performance. The bit error rate (BER) for 22.2-Gbit/s pseudorandom binary sequence (PRBS) 27−1 data is smaller than 10−10 at a receiver power of −35 dBm.

http://ieeexplore.ieee.org/iel7/6476772/6483692/06483799.pdf

120-GHz-band 20-Gbit/s transmitter and receiver MMICs using quadrature phase shift keying

This paper describes a 10-Gbit/s fully integrated wireless link for last-one-mile access in optical fiber networks. The wireless equipment employs quadrature phase-shift keying (QPSK) for modulation. The transmitter (TX) head of the link includes a 120-GHz-band QPSK TX module and an encoder board for generating an in-phase and quadrature signal from a 10-Gbit/s optical signal. The receiver (RX) head consists of an RX module using QPSK differentially coherent detection and a decoder board. To obtain sufficient output power for a link distance of over 1 km, we designed a power amplifier (PA) monolithic microwave integrated circuit (MMIC) using a 0.1- μm-gate indium-phosphide HEMT. The MMIC uses a novel RF pad for high isolation between input and output ports. The measured gain of the amplifier is over 15 dB from 110 to 140 GHz. The saturation output power at 128 GHz is 15 dBm. The TX, RX, and PA modules provide sufficient data-transmission performance: a bit error rate of less than 10-10 and an error vector magnitude of 10%. The integrated TX head exhibits the 99% power bandwidth of 8.4 GHz and the spectral efficiency is 1.32 bit/s/Hz. We demonstrated real-time 10-Gbit/s building-to-building connection using a new QPSK wireless link over the distance of 170 m.

120-GHz-Band Fully Integrated Wireless Link Using QSPK for Realtime 10-Gbit/s Transmission

This paper presents a 120-GHz-band 10-Gbit/s quadrature phase-shift-keying (QPSK) modulator and demodulator. To reduce system complexity, the modulator employs direct modulation, and the demodulator uses differentially coherent detection. We fabricated the modulator monolithic microwave integrated circuit (MMIC) and demodulator MMIC with 0.1-μm-gate InP HEMTs. The test element of the modulator had a static error-vector magnitude of 10%. We mounted the modulator and demodulator MMICs in separate modules. The size of the modules was 20 mm × 8 mm × 25 mm. The main lobe in the spectrum of 10-Gbit/s QPSK signals ranged from 123 to 133 GHz. The bit error rate for 10-Gbit/s 27 - 1 pseudorandom binary sequence data was smaller than 10-10 at a received power of -38.5 dBm.

Hiroyuki Takahashi

Papers

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

Low-phase noise photonic millimeter-wave generator using an AWG integrated with a 3-dB combiner

120-GHz-band 20-Gbit/s transmitter and receiver MMICs using quadrature phase shift keying

120-GHz-Band Fully Integrated Wireless Link Using QSPK for Realtime 10-Gbit/s Transmission

10-Gbit/s Quadrature Phase-Shift-Keying Modulator and Demodulator for 120-GHz-Band Wireless Links