"Beyond 5G" Demonstration of the first transparent fiber-millimeter-wave-fiber system by NICT, Sumitomo Osaka Cement Co. Ltd., and Waseda University

The National Institute of Information and Communications Technology (NICT); Sumitomo Osaka Cement Co., Ltd.; and Waseda University announced on July 15 that they have developed a new transparent fiber-millimeter-wave-fiber system, and succeeded in transmitting a high-speed millimeter-wave radio signal with a capacity of over 70 Gb/s in the 100-GHz band directly to an optical fiber. The system was realized using two newly developed technologies: a broadband modulator for direct conversion of a millimeter-wave signal to an optical signal, and a photonic down-conversion technology that can transmit a remote optical carrier to the antenna site. The utilization of a low-loss broadband optical modulator for the direct conversion of a millimeter-wave signal to an optical signal significantly simplified the millimeter-wave radio receiver because electronic devices such as signal generators used in existing systems will no longer be required. In addition, by adopting direct photonic down-conversion technology for simultaneous detection and down-conversion of the signal to the microwave band, the fiber-radio receiver and the subsequent digital signal processing could be considerably simplified, rendering the proposed system a promising solution for high-capacity, low-latency, and low-power consumption fiber-wireless transmission in 5G and beyond networks.

The 5G (5th generation mobile communication system) service is being deployed, and high-speed communications of 10 Gbps or more is expected using millimeter-wave radio signals. Although the communication speed is improved using millimeter waves, high-speed electronic devices are required. Hence, the power consumption of wireless transceivers can be increased. Furthermore, because millimeter-wave signal has a shorter transmission distance than those in the microwave band in the conventional 4G networks (4th generation mobile communication networks), the number of wireless antennas will significantly increase. Thus, low power consumption and low cost of wireless antennas become critical issues.

NICT has been engaged in the research and development of radio-fiber technology and optical devices. They have succeeded in development of of simple wireless antenna transmitter for direct conversion of an optical signal to a radio signal. However, simplification of the receiving unit that converts a radio signal to an optical signal for further transmission has been an issue.

In this research, the three organizations jointly developed two key technologies for the millimeter-wave wireless receiver and succeeded in receiving a high-speed millimeter-wave signal and transmitting it to an optical fiber directly without a frequency conversion. The first technology is a high-speed optical modulator using a ferroelectric electro-optical crystal (lithium niobate) for direct conversion of a millimeter-wave signal to an optical signal. By reducing the substrate thickness to below 100 μm, which is less than one-fifth of the conventional structure, they succeeded in achieving a high-speed operation of up to 110 GHz. The second technology is a photonic down-conversion method to simultaneously detect and down-convert the signal to the microwave band.

In this technology, two optical signals with a frequency separation that is approximately equal to the frequency of the millimeter-wave signal is generated from a single light source. One of the signals is modulated by the millimeter-wave signal, and an optical double-sideband carrier-suppressed signal is generated. One of the modulated sidebands is selected using optical filtering. Subsequently, the modulated and unmodulated signals are combined and input to a low-speed photodetector to be converted to an electrical signal in the microwave band. Using this technology, the receiver complexity and the frequency offset and phase noise of the received signal can be significantly reduced, as compared to the coherent detection used in the previous systems.

Using the developed broadband modulator and photonic down-conversion technology, the group successfully demonstrated a high-speed transmission of more than 70 Gbit/s over a wired and wireless converged system consisting of two optical fiber links and a 20 m radio link at 101 GHz.

The paper containing the results of this demonstration was published at the 2021 International Conference on Optical Fiber Communication (OFC 2021, June 6 (Sun.) to June 11 (Fri.)), one of the largest international conferences in the field of optical fiber communications. It was highly evaluated and was presented in the Post Deadline session, which is known to release the latest important research achievements, on June 11 (Fri) 2021 local time.