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This paper reports an end-to-end demonstration of a fiber-wireless mobile fronthaul (MFH) network based on hybrid intermediate-frequency-over-fiber (IFoF) and radio-over-fiber (RoF) links using a 28-GHz millimeter-wave (MMW) band. We successfully transmitted 24 64-QAM OFDM signals with a total bandwidth of 9.12 GHz over 20-km broadband IFoF, 1-km narrowband IFoF, 500-m RoF, and 10-m free-space links with an aggregate capacity of up to 34.2 Gbps, which complies with the required peak data capacity of IMT-2020. A digital demultiplexer was exploited to flexibly perform channel selection and frequency conversion. The obtained results show the feasibility of implementing fiber-wireless systems with the hybrid IFoF/RoF links for future mobile fronthaul networks.

We experimentally demonstrate a multi-beam steering system using an optical phase control for a multi-user-MIMO (MU-MIMO) scenario in 5G mobile system. We successfully transmitted three 380-MHz wireless OFDM signals at 28 GHz simultaneously with 30° beam separation over a free space of 5m.

RF signal phase shift method is successfully demonstrated in radio-over-fiber system. Radio wave signal is transmitted with single side band modulation. Optical phases of the optical carrier and the sideband are controlled in planar lightwave circuit, thus attaining RF signal phase shift.

This paper describes the background, motivation, approach and experimental results of our research and development of mobile fronthaul based on analog radio-over-fiber and related optical transmission technique. Future evolution scenario toward Beyond-5G era is also discussed.

We demonstrate an IFoF transmission using a commercial EML TOSA for the first time. We successfully transmitted 40×400-MHz 256QAM OFDM signals by using the TOSA with a record net bit rate of 128 Gb/s.

A 2-D millimeter-wave beamforming system is demonstrated by using a developed compact 4×4 antenna module integrated with photodiodes and RF chains. We successfully transmitted a 400-MHz 64-QAM signal over a free space of 5 m.

Recently, GaN-based light-emitting diodes (LEDs) find their applications in, other than general lighting, visible light communication (VLC) for high-speed wireless data networks. The pursuit for more bandwidth from the LED can be realized by engineering the LED material structure and device design, or by modulation techniques. In this paper, we proposed GaN-based LEDs with the photonic crystal (PhC) structure for VLC applications. Contradictory to general thought that higher small frequency response translates to better large-signal performance, our results reveal that a PhC LED may not perform well at low data rate due to its relatively poorer signal-to-noise ratio (SNR). However, at higher data rates, the performance of PhC LEDs surpasses conventional LED structure. We showed data transmission capability of PhC LEDs with a bit rate up to 400 Mb/s using on–off keying modulation. To demonstrate the chip capability for large-signal transmission, without using preemphasis or postemphasis, VLC with 16-quadrature amplitude modulation orthogonal frequency-division multiplexing transmission capacity up to 2 Gbit/s is achieved with an error vector magnitude of 17.07%, SNR of 15.35 dB, and bit error rate of $3.3 \times {10}^{-3}$ . Our LEDs embedded with PhCs pass the forward error correction criterion. The results reveal the advantages of LEDs with PhC for achieving higher data rate transmission.

In this paper, the optical frequency response of gallium nitride-based blue micro-LEDs with different quantum well numbers in the active region is presented. Single quantum well (SQW) blue micro-LEDs have about 1.6 times higher modulation bandwidth ( $f_{-3\,\text {dB}}$ ) by comparison with triple quantum well blue micro-LEDs. The larger bandwidth enhancement of the SQW micro-LEDs can be attributed to more carriers stored in a quantum well leading to higher recombination rate and lower recombination lifetime. The high $f_{-3\,\text {dB}}$ up to 752 MHz is demonstrated in the SQW micro-LED with a light-emitting area of $10\,\,\mu \text {m} \times 10\,\,\mu \text{m}$ . It suggests that micro-LEDs designed with fewer quantum well numbers may have the potential for the application of visible light communication (VLC) technology.

Up to 18-Gbps direct encoding of blue laser diode (BLD) is demonstrated for free-space data transmission. By reshaping the orthogonal frequency multiplexed (16-QAM OFDM) stream with sidelobe filtering, the raw data rate expedites from 17.2 to 18.4 Gbps. Employing an ultrafast p-i-n photodiode with smaller active area diameter and lower noise equivalent power significantly enlarges the data rate by 1.6 Gbps or upgrades the signal-to-noise ratio (SNR) by 0.2 dB. Replacing the 80-mW BLD with the 120-mW one essentially increases the received SNR by 0.4 dB under enhanced modulation throughput. Reinforcing the beam collimation and collection by increasing the numerical aperture with a plano-convex hyper-hemispherical lens further improves the SNR by 0.6 dB. After optimization, the 16-QAM OFDM data with and without sidelobe filtering are respectively delivered at raw data rates of 16.4 and 18 Gbps with spectral-density usage efficiency as high as 4 bit/s/Hz over 16 m in free space, wherein the BLD carried QAM-OFDM data stream remains its capacity after reformation with sidelobe filtering as the superior inter-carrier-interference immunity reinforces.

An 850-nm multi-mode vertical cavity surface emitting laser (VCSEL) bare chip with high-indium-density InGaAs/AlGaAs quantum-well pairs is demonstrated for directly encoded QAM-OFDM transmission in multi-mode fiber (MMF). By directly encoding the 850-nm VCSEL bare chip with a pre-leveled 14-GHz 16-QAM OFDM data, >50-Gb/s transmission over 100-m-long OM4 MMF can be realized without using data recovery circuit. Increasing the bias current of the VCSEL beyond 7.5Ith improves the signal-to-noise ratio (SNR) and the bit error ratio (BER) of received QAM-OFDM data to 15.5 dB and $2.9\times 10^{-3}$ , respectively. The 100-m-long OM4 MMF transmission degrades the SNR with its covered bandwidth reducing to 13 GHz. The OFDM subcarrier pre-leveling technique with a slope of 0.2 dB/GHz ensures the 16-QAM-OFDM data transmission with an error vector magnitude of 17.1% and a BER of $3.4\times 10^{-3}$ .

Wideband signal generation using frequency/phase modulation (FM/PM) is the key fundamental function for various applications such as radar and analog communication systems. It is well known that analog FM/PM communication systems can dramatically improve signal quality by spectral expansion. In classical communication theories, the Armstrong indirect method is one of the most popular methods for bandwidth expansion of FM signals. In the Armstrong method, a narrowband signal is converted to a broadband signal with the help of a nonlinear frequency multiplier. In this paper, we propose a photonic Armstrong method enabled by direct detection. By utilizing the nonlinearity caused by direct detection, we can increase bandwidth so that it is double that of the original signal in the photonic Armstrong method. Also, it completely eliminates signal-to-signal beat interference (SSBI) at the same time. We conducted an experiment to verify the concept and confirmed these advantages experimentally.

A 850-nm single-mode VCSEL with 3-dB bandwidth of 22.1 GHz enables 16-QAM OFDM transmission at nearly 100 Gbit/s over 100-m-Iong OM4 MMF link is demonstrated over 100-m-long OM4 MMF with BER of 3.5×10−3.

Recently, GaN based light-emitting diodes (LEDs) have become a promising candidate for visible light communication (VLC). In this work, we demonstrated 2-Gbit/s data rate transmission using GaN-based LEDs with photonic crystal (PhC) structure.

Through the reduction of mirror-resistance of vertical-cavity surface-emitting lasers, the optical bandwidth of 22.5 GHz is successfully achieved by implementation of zinc-diffusion process. It shows that lower mirror-resistance provides better transmission quality under similar optical bandwidth. With 20 ohm resistance reduction, the bandwidth limitation by electrical parasitics can be effectively improved from 6.4 to 14.71 GHz confirmed by the small-signal extraction results.

Directly 64-QAM OFDM encoding the colorless laser diode with power-to-frequency pre-leveling is demonstrated for 60-Gbit/s BtB and 55.8-Gbit/s 25-km fiber transmissions with its selectable channel wavelength to construct uniform transmitter for DWDM-PON channels.

A few-mode (FM) vertical cavity surface emitting laser (VCSEL) chip with heavily zinc-diffused contact layer and oxide-confined cross-section is demonstrated for carrying pre-leveled 16-quadrature amplitude modulation orthogonal frequency division multiplexing (QAM-OFDM) data in OM4 multi-mode fiber (MMF) over 100 m for intra-data-center applications. The FM VCSEL chip, which has an oxide-confined emission aperture of 5 μm, demonstrates high external quantum efficiency, provides an optical power of 2.2 mW at 38 times threshold condition, and exhibits 3 dB direct-modulation bandwidth beyond 22 GHz at a cost of slight heat accumulation. At a DC bias point of 5 mA (22.6Ith) the FM VCSEL chip, with sufficiently normalized modulation output, supports Baud and data rates of 25 and 100 Gb/s, respectively, with forward error correction (FEC) certifying receiving quality after back-to-back transmission. After passing through 100 m OM4 MMF with a receiving power penalty of 4 dB, the FM VCSEL chip demonstrates FEC-certified transmission of the pre-leveled 16-QAM OFDM data at 92 Gb/s.

We demonstrate transmitting 28-GHz band radio signals from cascaded IFoF based C-RAN mobile fronthaul system to inside a vehicle by re-radiation of the cost-effective analog radio over multi-mode fiber employing a directly modulated VCSEL.