Multiplexing, the ability to send multiple signals over a single channel, is a fundamental feature of any voice or data communication system. An international research team has demonstrated for the first time a method of multiplexing data carried over terahertz waves, high-frequency radiation that could enable the next generation of very high-speed wireless networks.
In the review Nature Communicationthe researchers report the transmission of two video signals in real time through a terahertz multiplexer at an aggregate data rate of 50 gigabits per second, approximately 100 times the optimal data rate of today’s fastest cellular network.
“We have shown that we can transmit separate streams of data over terahertz waves at very high speeds and with very low error rates,” said Daniel Mittleman, professor at Brown’s School of Engineering and corresponding author of the paper. ‘article. “This is the first time anyone has characterized a terahertz multiplexing system using real data, and our results show that our approach could be viable in future terahertz wireless networks.”
Today’s voice and data networks use microwaves to carry wireless signals. But the demand for data transmission is rapidly exceeding what wireless networks can handle. Terahertz waves have higher frequencies than microwaves and therefore a much greater capacity to carry data. However, scientists have only just begun to experiment with terahertz frequencies, and many of the basic components needed for terahertz communication do not yet exist.
A multiplexing and demultiplexing system (also known as mux/demux) is one of these basic components. It’s the technology that allows one cable to carry multiple TV channels or hundreds of users to access a wireless Wi-Fi network.
The mux/demux approach developed by Mittleman and his colleagues uses two metal plates placed parallel to each other to form a waveguide. One of the plates is pierced with a slit. When the terahertz waves pass through the waveguide, some of the radiation escapes from the slit. The angle at which the radiation beams escape depends on the frequency of the wave.
“We can put multiple waves at multiple different frequencies – each carrying a stream of data – into the waveguide, and they won’t interfere with each other because they’re different frequencies; that’s multiplexing”, Mittleman said. “Each of these frequencies escapes the slot at a different angle, separating the data streams; this is demultiplexing.”
Due to the nature of terahertz waves, signals in terahertz communication networks will propagate as directional beams, not omnidirectional broadcasts as in existing wireless systems. This directional relationship between propagation angle and frequency is the key to enabling multiplexing/demultiplexing in terahertz systems. A user at a particular location (and therefore at a particular angle of the multiplexing system) will communicate on a particular frequency.
In 2015, Mittleman’s lab first published a paper describing their waveguide concept. For this initial work, the team used a broadband terahertz light source to confirm that different frequencies were indeed emerging from the device at different angles.
While an effective proof of concept, Mittleman said, this latest work has taken the critical step of testing the device with real data.
In collaboration with Guillaume Ducournau at the Institute of Electronics, Microelectronics and Nanotechnology, CNRS/University of Lille, France, the researchers encoded two high-definition television broadcasts on terahertz waves of two different frequencies: 264.7 GHz and 322.5GHz. They then transmitted the two frequencies together into the multiplexer system, with a television receiver tuned to detect the signals as they exited the device. When the researchers aligned their receiver to the angle from which the 264.7 GHz waves were emitted, they saw the first channel. When they aligned on 322.5 GHz, they saw the second.
Other experiments showed transmissions to be error-free up to 10 gigabits per second, which is much faster than today’s standard Wi-Fi speeds. Error rates increased somewhat when the speed was increased to 50 gigabits per second (25 gigabits per channel), but were still well within the range that can be corrected using forward error correction , which is commonly used in today’s communication networks.
In addition to demonstrating that the device worked, Mittleman says the research revealed startling details about data transmission over terahertz waves. When a terahertz wave is modulated to encode data – that is, turned on and off to make zeros and ones – the main wave is accompanied by sideband frequencies which must also be detected by a receiver in order to transmit all the data. Research has shown that the angle of the detector relative to the sidebands is important in reducing the error rate.
“If the angle is a little off, we might be picking up full signal strength, but we’re getting one sideband a little better than the other, which increases the error rate.” Mittleman explained. “So it’s important to get the right angle.”
Fundamental details like this will be critical, Mittleman said, when it comes time to begin designing the architecture for full terahertz data systems. “This is something we didn’t expect, and it shows how important it is to characterize these systems using data rather than just an unmodulated radiation source.”
Researchers plan to continue developing this and other terahertz components. Mittleman recently received a license from the FCC to perform outdoor testing at terahertz frequencies on the Brown University campus.
“We believe we have the highest frequency license currently issued by the FCC, and we hope this is a sign that the agency is starting to think seriously about terahertz communication,” Mittleman said. “Companies are going to be reluctant to develop terahertz technologies until regulators make a serious effort to allocate frequency bands for specific uses, so this is a step in the right direction.”
This work was supported by the US National Science Foundation, the US Army Research Office, the WM Keck Foundation and the National Research Agency under the COM’TONIQ and TERALINKS research grants and under the CPER “Photonics for Society” developed in the Hauts-de-France region.
