Towards 1 Tbps throughput using sub-terahertz bands

In order to enable the near-instantaneous communication promised by 6G, ultrahigh data speeds will be needed on wireless channels. In a study recently published in IEICE Electronics Express, researchers from Osaka University and IMRA AMERICA have found a way to increase these data speeds by reducing the noise in the system through lasers.

To pack in large amounts of data and keep responses fast, the sub-terahertz (sub-THz) band from 100 to 300 GHz will be used by 6G transmitters and receivers. A sophisticated approach called ‘multi-level signal modulation’ is used to further increase the data transmission rate of these wireless links. However, when operating at the top end of these extremely high frequencies, multi-level signal modulation becomes highly sensitive to noise. To work well, it relies on precise reference signals, and when these signals begin to shift forward and backward in time (a phenomenon called ‘phase noise’), the performance of multi-level signal modulation drops.

“This problem has limited 300 GHz communications so far,” said Keisuke Maekawa, lead author of the new study. “However, we found that at high frequencies, a signal generator based on a photonic device had much less phase noise than a conventional electrical signal generator.”

Specifically, the team used a stimulated Brillouin scattering laser, which employs interactions between sound and light waves to generate a precise signal. They then set up a 300 GHz-band wireless communication system that employs the laser-based signal generator in both the transmitter and receiver. The system also used online digital signal processing (DSP) to demodulate the signals in the receiver and increase the data rate.

“Our team achieved a single-channel transmission rate of 240 Gbps,” said Professor Tadao Nagatsuma, PI of the project. “This is the highest transmission rate obtained so far in the world using online DSP.”

The results of the study are a significant step towards 300 GHz-band wireless communication, with the researchers anticipating that, with multiplexing techniques (where more than one channel can be used) and more sensitive receivers, the data rate can be increased to 1 terabit per second (Tbps) — ushering in a new era of near-instantaneous global communication. And such an era could be closer than we think, as Nagatsuma has separately been working with French researchers to break the 1 Tbps barrier using a different frequency band.

In order to reach 1 Tbps speeds, several tens or hundreds of gigahertz of bandwidth are needed, which makes higher frequencies like sub-THz appealing. And while there is extensive research into creating early sub-THz communications systems, components for these frequencies are still under development and very scarce.

In a paper presented at the 2023 Asia-Pacific Microwave Conference, Nagatsuma and the University of Lille’s Professor Guillaume Ducournau revealed that they had built a system using a combination of terahertz photodiodes and an electronics-based receiver covering a range of 500–724 GHz. In this frequency band, they used channel aggregation with 14 carriers and a range of 16 to 64 quadrature amplitude modulation (QAM) to achieve a total data throughput of 1.04 Tbps.

“I am … so happy to reach a single-lane 1 Tbps data rate,” Nagatsuma said. “That is a long-time dream of terahertz communication researchers.”

To measure the performance of their state-of-the-art system, the researchers used a 4-channel Keysight Infiniium UXR‑series oscilloscope coupled with vector signal analysis (VSA) software. According to Ducournau, “The combination of wideband terahertz photodiode, receiver and the unique performance of Keysight’s UXR really enabled [us] to succeed in these experiments.”

Ducournau said he was “excited to see that photonics enabled the first aggregated greater than 1 Tbps sub-THz system, as photonics technologies have accelerated to boost terahertz communication research”. Photonics is just one new technology that is being investigated to enable communications at sub-THz bands; whether or not it will be widely adopted in the future remains to be seen.

Image credit: iStock.com/ko_orn