Terahertz receiver for 6G wireless communications

Future mobile network: Small radio cells (orange) are connected by wireless high-speed terahertz links (green). Credit: IPQ, KIT / Nature Photonics

by Monika Landgraf

https://phys.org/news/2020-09-terahertz-6g-wireless.html

Future wireless networks of the 6th generation (6G) will consist of a multitude of small radio cells that need to be connected by broadband communication links. In this context, wireless transmission at THz frequencies represents a particularly attractive and flexible solution. Researchers at Karlsruhe Institute of Technology (KIT) have now developed a novel concept for low-cost terahertz receivers that consist of a single diode in combination with a dedicated signal processing technique. In a proof-of-concept experiment, the team demonstrated transmission at a data rate of 115 Gbit/s and a carrier frequency of 0.3 THz over a distance of 110 meters. The results are reported in Nature Photonics.

5G will be followed by 6G: The sixth generation of mobile communications promises even higher data rates, shorter latency, and strongly increased densities of terminal devices, while exploiting Artificial Intelligence (AI) to control devices or autonomous vehicles in the Internet-of-Things era. "To simultaneously serve as many users as possible and to transmit data at utmost speed, future wireless networks will consist of a large number of small radio cells," explains Professor Christian Koos, who works on 6G technologies at KIT together with his colleague Professor Sebastian Randel. In these radio cells, distances are short such that high data rates can be transmitted with minimum energy consumption and low electromagnetic immission. The associated base stations will be compact and can easily be mounted to building facades or street lights.

To form a powerful and flexible network, these base stations need to be connected by high-speed wireless links that offer data rates of tens or even hundreds of gigabits per second (Gbit/s). This may be accomplished by terahertz carrier waves, which occupy the frequency range between microwaves and infrared light waves. However, terahertz receivers are still rather complex and expensive and often represent the bandwidht bottleneck of the entire link. In cooperation with Virginia Diodes (VDI) in Charlottesville, U.S., researchers of KIT's Institute of Photonics and Quantum Electronics (IPQ), Institute of Microstructure Technology (IMT), and Institute for Beam Physics and Technology (IBPT) have now demonstrated a particularly simple inexpensive receiver for terahertz signals. The concept is presented in Nature Photonics.

Highest Data Rate Demonstrated So Far for Wireless THz Communications over More Than 100 Meters

"At its core, the receiver consists a single diode, which rectifies the terahertz signal," says Dr. Tobias Harter, who carried out the demonstration together with his colleague Christoph Füllner in the framework of his doctoral thesis. The diode is a so-called Schottky barrier diode, that offers large bandwidth and that is used as an envelope detector to recover the amplitude of the terahertz signal. Correct decoding of the data, however, additionally requires the time-dependent phase of the terahertz wave that is usually lost during rectification.

To overcome this problem, researchers use digital signal processing techniques in combination with a special class of data signals, for which the phase can be reconstructed from the amplitude via the so-called Kramers-Kronig relations. The Kramers-Kronig relation describe a mathematical relationship between the real part and the imaginary part of an analytic signal. Using their receiver concept, the scientists achieved a transmission rate of 115 Gbit/s at a carrier frequency of 0.3 THz over a distance of 110 m.

"This is the highest data rate so far demonstrated for wireless terahertz transmission over more than 100 m," Füllner says. The terahertz receiver developed by KIT stands out due to its technical simplicity and lends itself to cost-efficient mass production.

Fujitsu Makes a Terahertz Receiver Small Enough for a Smartphone

http://spectrum.ieee.org/tech-talk/telecom/wireless/fujitsu-makes-a-terahertz-receiver-small-enough-for-a-smartphone
By John Boyd

It’s a good time to be alive for pixel peepers. TV makers are pushing 4K-resolution sets to replace our present 1080p screens; Apple’s iMacs sport a 5K resolution; and NHK, Japan’s national broadcaster, is testing 8K broadcasting equipment, targeting 2020 and the Tokyo Olympics for its introduction.

To help wireless devices cope with the higher speeds demanded by such applications, Fujitsu has developed a 300-GHz prototype receiver compact enough to fit into a cellphone. Though limited to about 1 meter in range, the company says the device can download 4K and 8K video almost instantly.

Today’s cellphones operate in frequency ranges between 0.8 to 2.5-GHz, and are capable of download speeds of around 230 megabits per second, while the top speed for 802.11n Wi-Fi operating in the same frequency range can reach speeds as high as 600 Mb/s. Fujitsu touts its new receiver as operating in theterahertz band—frequencies of over 300 GHz—where terminals can communicate at speeds hundreds of times faster than today’s mobile handsets.

Devices to enable such high speeds have been developed, but because terahertz-band waves quickly attenuate, receiver-amplifier chips need to be sensitive enough to deal with a weak signal. Present designs rely on a separate antenna, which in turn requires a waveguide component to transport the incoming signal from the antenna to the chip. This makes the overall combination far too bulky for cellphone use, says Fujitsu.

The goal, then, is to create a receiver-amplifier module with a built-in antenna  to increase miniaturization. This has been achieved for devices employed in millimeter wave-band equipment operating at 60-GHz to 80-GHz frequencies, for instance, and used in applications such as collision-avoidance radar. These modules connect the antenna to the receiver-amplifier  chip through an internal printed-circuit substrate making a waveguide unnecessary.

Illustration: Fujitsu

“Typical printed-circuit-substrate materials used in these higher frequency ranges are ceramics, quartz ,and Teflon,” says Yasuhiro Nakasha, a research manager at Fujitsu’s Devices & Materials Lab. “But when these are used in terahertz-band communications, there is significant signal attenuation and loss of receiving sensitivity.”

To get round this, Fujitsu has micro-fabricated a printed-circuit substrate using a polyimide (a heat-resistant synthetic polymer) material.  Signals from the antenna are transmitted to the receiver-amplifier chip through a connecting circuit on the substrate.

In order to ensure stable signal transmission with low loss, the top and bottom faces of the printed circuit substrate are grounded and connected using through-hole metalized vias. This and the connecting circuit together form a grounded coplanar-waveguide structure: a transmission pathway designed to enhance high frequency signal propagation. To reduce signal interference from the printed circuit substrate, the vias need to be spaced apart less than one-tenth of the signal’s wavelength—in this case less than a few tens of micrometers.

Though the polyimide material experiences a signal loss ten-percent greater than quartz, Fujitsu says the material’s processing accuracy is more than four times higher than the latter. This makes it possible to space the vias closer together, thereby halving the overall signal loss compared to using a quartz substrate.

To facilitate a strong connection between the antenna connecting-circuit on the printed-circuit substrate and the receiver-amplifier chip, Fujitsu adapted a millimeter-mounting technology to handle terahertz transmission. This method let the receiver-amplifier circuitry directly face the printed circuit substrate.

The outcome is a module with an overall volume of just 0.75 cubic centimeters—not including output terminals—small enough to be incorporated into a mobile phone. Download speeds obtained so far in the lab reached 20 Gb/s.

Fujitsu will begin field-testing by the end of March 2016, and aims to launch the technology in 2020. The application the engineers envision include instant downloading of large volumes of data from servers and terminals, electronic versions of printed guides and brochures used at events, and downloading video and music from kiosks.

Nakasha isn’t looking beyond 2020 at the moment, but he believes the technology has the potential to one day achieve speeds of 100 Gb/s.

Part of the research used was obtained from an R&D project on expanding radio spectrum resources commissioned by Japan’s Ministry of Internal Affairs and Communications.