- A 100-GHz band fiber-millimeter-wave–fiber transparent system was constructed based on direct millimeter-wave-to-optical conversion using a low-loss optical modulator with direct photonic down-conversion.
- 70-Gbit/s high-capacity transmission over the transparent fiber–millimeter-wave–fiber system at 101 GHz was demonstrated using 64-QAM OFDM.
- This demonstration opens the door for transparent fiber-millimeter-wave systems in the field of high-capacity, low-latency, and low-power consumption communications in the 5G and beyond era.
The National Institute of Information and Communications Technology (NICT, President: TOKUDA Hideyuki, Ph.D.), Sumitomo Osaka Cement Co., Ltd. (President: MOROHASHI Hirotsune), and Waseda University (President: TANAKA Aiji) jointly developed the first transparent fiber–millimeter-wave–fiber system in the 100-GHz band using a low-loss broadband optical modulator with direct photonic down-conversion. The developed broadband modulator and photonic down-conversion technology were utilized to successfully demonstrate 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 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 it included only a radio front end and an optical modulator. 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, thus 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 results of this demonstration were published as a post-deadline paper presentation at the 2021 International Conference on Optical Fiber Communications (OFC 2021).
Fiber–wireless systems in high-frequency bands are a promising technology for inter-building connections, disaster recovery, and mobile transport networks, especially in 5G and beyond era. To date, most systems rely on the use of electronics-based receivers for radio-to-optical conversion, which generally feature less bandwidth and complicated antenna sites. Achieving fully transparent radio–optical conversion using photonic solutions is promising for increasing the transmission capacity and simplifying the antenna sites. However, the frequency of radio links in the previous systems that utilized the photonic conversion method was limited to below 90 GHz owing to the limited bandwidth of optical modulators. Recently, a plasmonic modulator was employed to realize a transparent bridge system in high-frequency bands. Generally, plasmonic modulators exhibit high insertion loss, which requires the use of optical amplifiers. However, this increases the optical noise, system cost, and the antenna site complexity.
On the other hand, most of the previous systems utilized coherent detection by using free-running lasers for signal detection at the fiber-radio receiver, which significantly increased the system complexity, frequency offset, and phase noise of the detected signal, thus requiring complicated digital signal processing algorithms for signal recovery. Therefore, employing a direct photonic down-conversion technology to simultaneously detect and down-convert the signal to the microwave band using a coherent two-tone optical signal generation is promising for simplifying the system and reducing the cost and power consumption.
In this work, we demonstrated the first transparent fiber–millimeter-wave–fiber system in the 100-GHz band (see Fig. 1) using two key element technologies: (i) a low-loss broadband optical modulator, and (ii) direct photonic down-conversion. For direct conversion of a millimeter-wave signal to an optical signal, we fabricated and employed a broadband modulator for operation up to 110 GHz. This was achieved by performing Ti diffusion on the x-cut lithium niobate (LiNbO3 thickness:≤100µm) in the low dielectric constant layer. In addition, we employed a photonic down-conversion method based on a coherent two-tone optical signal generation technology to simultaneously detect and down-convert the signal to the microwave band. This significantly simplified the system and reduced the frequency offset and phase noise, as compared to systems utilizing coherent detection. Using the technologies developed in this study, we successfully transmitted 64-quadrature amplitude modulation (QAM) orthogonal frequency division multiplexing (OFDM) signal with a line rate of 71.4 Gbit/s over a system consisting of two fiber links and a 20 m radio link at 101 GHz.
The system consists of the following key element technologies:
- A broadband optical modulator with a low half-wave voltage and low loss in the high-frequency band for direct conversion of millimeter-wave signals to optical signals.
- Direct photonic detection and down-conversion of signals to the microwave band utilizing coherent two-tone optical signal generation based on optical modulation technology.
- High-spectral efficiency 64-QAM OFDM signal transmission.
The optical carrier for data modulation at the antenna site was remotely generated and distributed from the fiber-radio receiver, which significantly simplified the antenna site and eased its operation and management. In addition, owing to the use of direct photonic down-conversion and detection technology at the fiber-radio receiver, the frequency offset and phase noise of the detected signal could be largely suppressed. This considerably reduced the receiver complexity and the subsequent digital signal processing. The proposed system is promising for high-speed, low-latency, and low-power-consumption communication links in 5G and beyond networks.
In the future, we will further study the millimeter-wave-to-optical conversion device and fiber wireless technology that were developed in this study to further increase the radio frequency and transmission capacity. In addition, we will promote international standardization activities and social implementation activities related to fiber wireless communication systems.
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.