Device Doubles Capacity for Terahertz Signals

Researchers in Australia and Japan have developed a signal mixer that enhances the capacity of terahertz communications. The mixer, called a polarization multiplexer, merges two polarized signals of the same frequency into a single beam and operates at frequencies being explored for future 6G networks. The all-silicon integrated device uses novel features that deal with some of the challenges of communicating in the terahertz (THz) spectrum, such as limited power and transmission range.

The researchers report transmitting aggregated data rates of up to 155 and 190 gigabits per second (with the help of two common error correction methods). The reported transmission speed represents an enormous increase over the average 1 Gbps bandwidths for 5G networks in operation today.

Because it enables two data streams to be transmitted simultaneously, the new multiplexer effectively doubles any given signal’s transmission capacity. The device is also exceptionally flexible for a range of frequencies in the 6G spectrum, says Withawat Withayachumnankul, professor of electrical and electronic engineering at the University of Adelaide in Australia. To quantify the multiplexer’s flexibility, Withayachumnankul cites a standard metric for communications devices called fractional bandwidth. (Fractional bandwidth is found by dividing a device’s frequency range by its central operating frequency. A higher number suggests a more flexible and versatile device.)

For this multiplexer, Withayachumnankul says, “The fractional bandwidth of nearly 40 percent is a record for any integrated multiplexer found in any frequency range.”

He adds that while there are, for instance, optical polarization multiplexers in the marketplace today, their fractional bandwidths are much narrower, and therefore aren’t useful at this 6G device’s terahertz frequency range. “Adopting these [optical multiplexer] designs directly into the terahertz domain is not feasible,” says Withayachumnankul. “Furthermore, efficiency is critical to preserving the limited power available from terahertz resources.”

So, to boost efficiency, they cut signal loss and interference by carefully selecting the right materials for the device—including using a very pure form of silicon called float-zone silicon. Other innovations included optimizing the tapering on the device’s couplers—structures that combine or split signals based on their polarization states to ensure efficient transmission between waveguides.

Masayuki Fujita, an associate professor of engineering science at Osaka University, says one of the chief challenges of terahertz waves is their high attenuation rates traveling through the atmosphere. Terahertz signals, in other words, can’t be effectively broadcast in every direction like radio or microwave signals. They need to be directed in a beam toward a receiving station or device. “Therefore,” says Fujita, “beam steering of terahertz waves is essential for 6G wireless application.”

Which is why the researchers previously developed a beamforming device that can be integrated with the new multiplexer. Weijie Gao, a postdoctoral researcher in Osaka University, says that combining this device with the new multiplexer “enables beam steering and sensing. Consequently, we expect it will help pave the way for high-speed 6G wireless networks.”

So What’s the Status of High-Speed 6G Wireless?

So will the 6G networks be ready anytime soon for these kinds of devices? A number of telecom executives told CNBC last year, for instance, that next-generation 6G mobile communications technology was likely to launch in 2030.

Yet, according to a July survey on advancements in terahertz technology published on the arXiv preprint server, many technological advancements are still needed for 6G networks to be ready to be deployed on anything like the executives’ end-of-decade timetable. The survey noted, for instance, improvements required in making better high-frequency components, more advanced sub-micron transistors for digital 6G signal processing, more efficient circuit designs to improve system performance, better antennas that are less expensive to make, and smarter software to handle the complex nature of terahertz signals. The survey also cited the need for “thorough investigations to understand the potential long-term biological effects of [terahertz] radiation.”

Then there are international standards that have to be defined to ensure interoperability around the globe, and specifications established for industry-specific applications such as augmented reality, remote surgery, and satellite communications.

“Will we solve all the challenges facing terahertz communications before the expected introduction of 6G?” asks Josep Jornet, professor and interim chair of the Department of Electrical and Computer Engineering at Northeastern University in Boston. “Probably not. Industry buzz suggests [even] sub-terahertz systems will not make the cut by the first release of 6G. And we should be fine with that. For we need more time to build systems that can really leverage the physics of terahertz systems.”

Nevertheless, he adds that an early version of 6G could still be launched around 2030, though it will likely depend on mid-band frequencies—those between 7 and 24 GHz—rather than terahertz frequencies.

The researchers tested the current multiplexer in the sub-terahertz J-band (220 to 330 GHz) and reported their results in Laser & Photonic Reviews in August.