Tokushima University Demonstrates First-Ever 100 Gbps-Class Wireless Communication Beyond 420 GHz: Ultra-High-Speed Mobile Backhaul Technology for Photonic 6G

■ Points
- Conventional electronic technologies face difficulties generating high-frequency signals above 350 GHz, necessitating new approaches to realize ultra-high-speed wireless communication for 6G.
- Through terahertz communication using an optical fiber-connected micro-optical comb, researchers demonstrated a single-channel wireless transmission of 112 Gbps in the 560 GHz band.
- This achievement is expected to establish the technological foundation for ultra-high-speed mobile backhaul communication and optical-wireless converged networks in 6G.

■ Overview
Mobile communications have continuously advanced toward higher speeds and larger capacities by increasing the wireless carrier frequencies. In next-generation mobile communications (6G), expected to launch in the 2030s, the utilization of terahertz waves above 300 GHz is anticipated. However, in the region exceeding 350 GHz, it has been considered difficult to achieve stable and high-speed wireless communication due to the limitations of signal generation via conventional electronics and the increase in phase noise.

To overcome these challenges, a research group led by Lecturer Yu Tokizane, Associate Professor Hiroki Kishikawa, Professor Naoya Kuse, and Professor Takeshi Yasui from the Institute of Post-LED Photonics (pLED) and the Photonics Health Frontier Research Center at Tokushima University, Graduate Student Takumi Kikuhara from the Graduate School of Sciences and Technology for Innovation at Tokushima University, Visiting Professor Tadao Nagatsuma from pLED at Tokushima University, along with Professor Shintaro Hisatake from the Faculty of Engineering at Gifu University, developed a micro-optical comb-driven terahertz communication system. This system combines terahertz wave generation using an optical fiber-connected micro-optical comb with multi-level modulation technology (Figure 1). By leveraging the highly stable frequency characteristics of the micro-optical comb, the research generated low-phase-noise terahertz carriers and demonstrated a single-channel 112 Gbps wireless transmission in the 560 GHz band. This achieved speeds surpassing the conventional tens of Gbps class.

This outcome marks the first demonstration of the feasibility of 100 Gbps-class wireless communication in the unmapped frequency domain above 420 GHz, acting as a crucial technological base for realizing ultra-high-speed backhaul and optical-wireless converged networks for 6G.

■ Research Background
Mobile communication has attained greater speed and capacity through the adoption of higher frequency bands. While 5G utilizes the millimeter-wave band, next-generation 6G networks aim to utilize terahertz waves of 300 GHz and above to meet surging communication demands. However, generating signals over 350 GHz using conventional electronics poses significant issues, including declining output power and increased phase noise. Thus, new technologies capable of producing stable, high-quality signals have been urgently required.

Addressing this, the research group focused on optical technology as an alternative. They advanced studies on terahertz signal generation utilizing a micro-optical comb—a type of optical frequency comb—for wireless communication applications, termed 'Photonic 6G'. Despite the micro-optical comb's high frequency stability, simultaneously achieving stable high-frequency signal generation and high-speed data transmission through high-order modulation at frequencies beyond 350 GHz remained elusive until now.

The present research aimed to resolve these technical bottlenecks and successfully demonstrate 100 Gbps-class communication beyond 350 GHz.

■ Research Content and Results
First, to realize a stable and compact terahertz signal source, the team developed a micro-optical comb device utilizing an optical fiber-connected micro-optical resonator. By adopting a structure where an optical fiber is directly bonded to a silicon nitride micro-optical resonator using an optical adhesive, they eliminated the need for precise optical adjustments and significantly downsized the equipment (Figure 2). This configuration also vastly improved the temporal stability of the pump light coupling efficiency, enabling the use of high-power pump light and establishing long-term, highly stable, and low-noise signal generation.

Next, the team constructed a terahertz wireless communication system using this device. They generated highly stable, dual-wavelength optical carriers via optical injection locking and applied multi-level modulation (QPSK and 16QAM) in the optical domain. Through photomixing, a 560 GHz multi-level modulated terahertz wave was generated and wirelessly transmitted. On the receiving end, the signal was demodulated via heterodyne detection using a subharmonic mixer. Consequently, they achieved wireless transmission speeds of 84 Gbps with QPSK modulation and 112 Gbps with 16QAM modulation (Figure 3). This is the first practical demonstration of 100 Gbps-class communication in the unexplored frequency band above 420 GHz.

■ Future Developments
This study proves that 100 Gbps-class wireless communication is feasible in terahertz bands exceeding 350 GHz. Future efforts will focus on further reducing the phase noise of the micro-optical comb to improve signal quality, enabling higher-order modulation for even faster, higher-capacity communications. To extend the practical communication range, selecting frequency bands less affected by atmospheric absorption, increasing terahertz wave output, and introducing high-gain antennas will be vital steps toward applying this technology to next-generation infrastructure.