We propose different fronthaul systems for facilitating future mobile networks based on the seamless convergence of fiber-optic and wireless systems in the millimeter-wave (mmWave) bands. First, a flexible and high-performance wireless fronthaul system is proposed through an encapsulation of radio signals onto a converged fiber-mmWave system. A simultaneous transmission of three radio signals over the system is successfully demonstrated. Second, a high-performance optical self-heterodyne system is proposed and demonstrated for the generation and transmission of radio access signals in high-frequency bands. Third, a high-spectral-efficiency optical fronthaul system for the simultaneous transmission of multiple radio signals in different frequency bands is proposed using a subcarrier-multiplexing intermediate-frequency-over-fiber system. Satisfactory performance is experimentally confirmed for the transmission of three different radio signals in the microwave and low- and high-mmWave bands. The proposed systems can overcome the challenges and bottlenecks of the current mobile fronthaul systems and can be useful in different usage scenarios of 5G and beyond networks.

To develop highly reliable fixed wireless links, such as mobile backhaul and fronthaul systems, it is essential to evaluate the link budget under extreme weather conditions. In this paper, we presented a link budget evaluation for fixed wireless links using radio signals at four different carrier frequencies: 75.375, 85.375, 275, and 400 GHz. As examples of extreme weather conditions, we focused on storms which can be defined as the combination of high temperature, violent rain, and strong winds. A theoretical model was proposed to evaluate the attenuation by the wind effect, which was due to mechanical vibration. Using the proposed model, we evaluated the link attenuation with different weather parameters, such as clear weather condition, different rain rates, and strong wind. In addition, the maximum channel capacities were estimated based on the model. According to the calculation results, it can be confirmed that the attenuation caused by heavy rain is the dominant factor to the whole system. Besides, the attenuation due to antenna gain degradation induced by the wind effect is considerable for the wireless link at high frequencies (275 and 400 GHz). This manuscript includes a comprehensive discussion regarding various weather factors including gas, rain, and wind, which impact the fixed wireless links. The obtained results would be useful for designing a high reliability fixed wireless link to operate under different weather conditions.

We designed and fabricated a 100-GHz range integrated photoreceiver with zero-bias operational uni-traveling-carrier photodiode and pseudomorphic high electron mobility transistor amplifier, which performed the roles of photonic power generation and 100-GHz RF signal generation. Using the integrated photoreceiver, high data rate radio- and power-over-fiber transmission through multicore fiber communication was demonstrated. Subsequently, a bit error rate of 1×10 ^-3 could be achieved with 12 Gb/s, orthogonal frequency division multiplexing, 16 quadrature amplitude modulation, and an intermediate frequency of 92 GHz. The crosstalk issue in radio- and power-over-fiber transmission through multicore fiber was discussed.

We demonstrate the first fiber–millimeter-wave–fiber system in the 100-GHz band using a low-loss optical modulator and direct photonic down-conversion technology. The utilization of a low-loss optical modulator for the direct conversion of a millimeter-wave signal to an optical signal significantly simplifies the millimeter-wave radio receiver because it includes 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 millimeter-wave signal to the microwave band, the fiber–radio receiver and subsequent digital signal processing can be considerably simplified. Using the proposed technologies, we successfully transmitted 32-/64-quadrature amplitude modulation orthogonal frequency division multiplexing and single-carrier signal with a line rate of 71.4 Gb/s and a net data rate of 53.7 Gb/s over a wired and wireless converged system consisting of two optical fiber links and a 20 m radio link at 101 GHz. The proposed system is promising for high-capacity, low-latency, and low-power consumption fiber-wireless transmission in beyond 5G networks.

We discuss and propose millimeter-wave linear cell systems for signal distribution on railway radiocommunication systems between train and trackside, and a foreign object debris detection system for airport runways using radio-over-fiber network technology. Linear cell configurations based on wavelength-division multiplexing (WDM) and power-splitter-based distribution networks are discussed and demonstrated experimentally. A single-sideband modulation for data transmission in an intermediate frequency over a fiber system is successfully transmitted with an ultrafast wavelength-tunable laser diode for WDM routing to track the train car within 3 μs. Radar signal distribution by an optical double-sideband suppressed-carrier modulation mitigates the dispersion effect of the fiber cable to transmit a 32-GHz signal into the remote radar heads.

A linear-cell-based radio-over-fiber (LC-RoF) system is proposed and demonstrated for efficient mobile communication in high-speed trains without hard handover processes. The configuration of the LC-RoF network and possible router architectures are discussed for the field trial test of the proposed system and for advanced systems in the future. Via the LC-RoF network, 90-GHz, millimeter-wave radio access between the ground and a train car is operated in a centralized manner. The resulting throughput of 1.5 Gbit/s is achieved in the field trial test, which is comprised of 250-Mbaud four-subcarrier differential quadrature phase-shift keying with a forward-error correction code implemented in field-programmable gate arrays to the Shinkansen train traveling at 240 km/h without any interruptions in the connections at the change of radio access units.