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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 intermediatefrequency-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.

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We developed a high-speed and handover-free communication network for high-speed trains (HSTs) using an ultrafast and switchable wavelength-division multiplexing fiber-wireless backhaul system in the W band. We successfully transmitted approximately 20-Gb/s and 10-Gb/s signals over the switched fiberwireless links in the downlink and uplink directions, respectively. An ultrafast radio-cell switching of less than 10 µs was experimentally demonstrated in both downlink and uplink directions. Moreover, the possibility of connecting a central station to many remote radio cells was evaluated, confirming that an uninterrupted communication network up to several tens of kilometers can be achieved for HSTs. The proposed system can overcome the current challenges in mobile networks and can provide a potential solution for the provision of advanced services to users on HSTs in future 5G and beyond networks.

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We propose a millimeter-wave (MMW) coherent radio-over-fiber (RoF) transmission system for application to an access network with a direct broadband last-mile wireless connection to an optical fiber network. The coherent RoF system comprises an optical twotone RoF signal generator with an advanced modulation format for high-throughput transmission and employs a digital-signal-processing-aided coherent detection technique for MMW radio, which is a technique similar to that of optical digital coherent detection. As proof of concept, a 20-GBd quadrature phase-shift keying RoF transmission over 20 km of optical fiber before a radio transmission over 20 m of air is demonstrated. The results of the radio transmissions using Cassegrain-type antenna pairs with a gain of 50 dBi are consistent with theoretical estimations; this suggests the possibility of developing a midrange transmission system using a high-power MMW amplifier.