In this study, we demonstrate the intermediate-frequency-over-fiber (IFoF) based mobile fronthaul technology implemented in a 28 GHz millimeter-wave (mmWave) based 5G prototype supporting Giga-bit mobile broadband services. In order to confirm the technical feasibility of the proposed IFoF based mobile fronthaul, the clock signal for frequency synchronization and mobile payload data are simultaneously transmitted in the form of IF carriers. Phase noise (PN) and error vector magnitude (EVM) measurements were performed on separate and simultaneous clock transmission paths, respectively. The results confirm an acceptable impact on performance, and the simultaneous clock transmission method reduces system costs and complexity, and improves the flexibility and efficiency of wavelength operation. We also successfully demonstrate the operation of real-time Giga-bit mobile services such as 4K video streaming over IFoF based mobile fronthaul for 5G prototypes with a 28 GHz mmWave. The achieved peak data-rate per user is up to 1.5 Gb/s, which complies with the requirements of the 5G vision of IMT-2020.

We demonstrate 4 × 4 multiple-input and multiple-output (MIMO) supporting mmWave-based indoor 5G mobile network based on a distributed antenna system (DAS) that has over-the-air interface to receive the broadband wireless signal from, e.g., remote radio head (RRH) of mobile fronthaul. To realize the cost/bandwidth-efficient and low-latency-inducing indoor 5G DAS network, we exploit the radio-over-fiber (RoF) system that is based on the intermediate frequency-over-fiber (IFoF) technology. To be more specific, we make simultaneous use of wavelength- and frequency-division multiplexing in the IFoF link so as to transport 4 × 4 MIMO 5G signal of which the bandwidth is effectively 3.2 GHz (= 4 × 800 MHz, where each antenna deals with 800 MHz 5G signal). The 4 × 4 MIMO-supporting IFoF link has error vector magnitude (EVM) performance of <; 4.5% at all channels of downlink and uplink for up to 2 km. The whole RoF system, exploiting the IFoF link, mmWave frequency conversion module, and control /management plane, proves itself to meet the 3GPP-defined EVM requirement (i.e., 8%) at all channels. Based on the RoF system, we demonstrate the indoor 5G DAS network in conjunction with Korea Telecom 5G mobile network, i.e., 5G baseband unit and RRH, investigating the downlink and uplink throughputs. We achieve ~4 Gb/s total throughput for end user, where the additional latency induced by the DAS network is only a few hundreds of nanoseconds other than the single mode fiber transmission delay.

In this study, we report the successful demonstration of an intermediate-frequency-over-fiber (IFoF)–based radio access network (RAN) for 28 GHz millimeter-wave (mmWave)-based 5G mobile communication. In order to increase the network coverage of the mmWave-based 5G networks, we propose a distributed antenna system (DAS) that uses the IFoF technology. An IFoF-based DAS with 2 × 2 multiple-input multiple-output (MIMO) configuration was deployed in the PyeongChang area to provide 5G trial demonstration during the Winter Olympics. 5G trial services such as high-speed data transfer and autonomous vehicle driving were offered to the public through the IFoF-based DAS. A downlink throughput of ~1 Gb/s and uplink throughput of ~200 Mb/s were achieved in the DAS-deployed area. We also present an IFoF-based 5G mobile fronthaul that can overcome the bandwidth bottleneck in RANs. We performed real-time transmission of mmWave-based 5G wireless access networks using the IFoF-based mobile fronthaul. The real-time downlink throughput achieved per 5G terminal was approximately 9 Gb/s, when using a 4 × 4 MIMO configuration. An outdoor demonstration was performed to verify the technical feasibility of the 5G fronthaul based on IFoF technology. When moving the 5G terminal between remote radio heads at a speed less than 60 km/h, 5G mobile broadband services could be provided with real-time throughput more than 5 Gb/s. Thus, we confirmed that the IFoF technology was capable of supporting RANs for mmWave-based 5G networks and providing real-time multi-Gb mobile services.

Homodyne detection provides the simplest digital signal processing (DSP) solution to optical coherent detection and minimizes the receiver bandwidth requirements. These features make it promising for high spectrally efficient formats such as optical orthogonal frequency domain multiplexing (OFDM), which has a flat optical spectrum and which is thus inherently sensitive to high-frequency distortions, e.g., due to limited detector bandwidth. The key to homodyne detection is recovery of the carrier from the received signal all optically (as opposed to frequency offset compensation via DSP. Herein, we use optical injection locking (OIL) in conjunction with carrier tone-assisted OFDM to achieve this. In contrast to previous reports, we show that OIL carrier recovery with subsequent homodyne detection can operate without the need for any optical prefiltering. First, we evaluate the performance as a function of the carrier tone guardband bandwidth. Further, we improve the robustness of this technique using a slow-phase lock loop that compensates for drift in the laser's temperature/current control electronics. Using this improved setup, we compare our all-optical-carrier-recovered homodyne and the “traditional” DSP-assisted intradyne detection for the case of OFDM-16QAM signals. Finally, we compare the computing complexity necessary for both approaches and estimate the intradyne performance limitations due to the carrier-local oscillator frequency offset.

We propose a theoretical model of reflective modulators for a seeded dense wavelength-division multiplexing (DWDM) system based on amplified spontaneous emission light. A travelling-wave amplifier model is modified to take into account the finite front-facet reflectivity. The net gain and phase shift are updated at every round trip of the cavity to have complex gain for the injected field. With the proposed model, we calculate side-mode suppression ratio, gain, relative intensity noise suppression as a function of injection power, and wavelength detuning for antireflection-coated Fabry-Perot laser diodes. We also investigate the performance of the seeded DWDM system by using the model. The simulation results match well with the experimental results.

We demonstrate the use of an optical injection phase locked loop (OIPLL) as a regenerative amplifier for optical frequency transfer applications. The optical injection locking provides high gain within a narrow bandwidth ($<100\mathrm{MHz}$) and is capable of preserving the fractional frequency stability of the incoming carrier to better than ${10}^{-18}$ at 1000 s. The OIPLL was tested in the field as a mid-span amplifier for the transfer of an ultrastable optical carrier, stabilized to an optical frequency standard, over a 292 km long installed dark fiber link. The transferred frequency at the remote end reached a fractional frequency instability of less than $1\times {10}^{-19}$ at averaging time of 3200 s.