First reported terahertz multiplexer data transmission

Multiplexing, the ability to send multiple signals through 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 ultra-high bandwidth wireless networks.

In the review Nature Communication, the researchers report the transmission of two real-time video signals through a terahertz multiplexer at an overall data rate of 50 gigabits per second, about 100 times the optimal data rate of today’s fastest cellular network.

“We have shown that we can transmit separate data streams 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 ‘article. “This is the first time that someone has characterized a terahertz multiplexing system using real data, and our results show that our approach may be viable in future terahertz wireless networks.”

Today’s voice and data networks use microwaves to carry signals wirelessly. But the demand for data transmission is rapidly exceeding what microwave networks can handle. Terahertz waves have higher frequencies than microwaves and therefore a much greater capacity to carry data. However, scientists have only just started experimenting with terahertz frequencies, and many of the basic components needed for terahertz communication do not yet exist.

A multiplexing and demultiplexing system (also called mux / demux) is one of these basic components. It is 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 has a slit cut out. When the terahertz waves pass through the waveguide, part 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 – in 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 from 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 as omnidirectional broadcasts as in existing wireless systems. This directional relationship between the angle of propagation and frequency is the key to enabling mux / demux in terahertz systems. A user at a particular location (and therefore at a particular angle in the multiplexing system) will communicate on a particular frequency.

In 2015, Mittleman’s lab first published an article describing his waveguide concept. For this initial work, the team used a broadband terahertz light source to confirm that different frequencies did indeed emerge from the device at different angles.

While this is an effective proof of concept, Mittleman said, this latest work took 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 coded two high-definition television broadcasts on terahertz waves of two different frequencies: 264.7 GHz and 322.5 GHz. They then broadcast the two frequencies together through the multiplexing system, with a television receiver tuned to detect the signals as they came out of the device. When the researchers aligned their receiver to the angle of emission of the waves at 264.7 GHz, they saw the first channel. When they lined up at 322.5 GHz, they saw the second.

Other experiments have shown that transmissions are error-free at up to 10 gigabits per second, which is much faster than current 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 showing that the device worked, Mittleman says research has revealed surprising details about transmitting data over terahertz waves. When a terahertz wave is modulated to encode data, i.e. 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 to transmit all data. Research has shown that the angle of the detector to the sidebands is important in keeping the error rate low.

“If the angle is a bit off, we can detect the full strength of the signal, but we are receiving one sideband a little better than the other, which increases the error rate.” Mittleman explained. “So it’s important to have the right angle.”

Fundamental details like this will be critical, Mittleman said, when the time comes to start designing the architecture for full terahertz data systems. “This is something we weren’t expecting, and it shows how important it is to characterize these systems using data rather than just a source of unmodulated radiation.”

Researchers plan to continue to develop this component as well as 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 seriously consider terahertz communication,” Mittleman said. “Companies will 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.”