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.

We propose an indoor network with a sub-terahertz- band wireless link for 6G applications. In our proposed indoor network, an optical hub unit (OHU) that controls the entire system is optically linked to THz remote nodes (RNs) over optical distribution fibers. The THz RNs communicate with the user equipment through a sub-THz wireless link. The function of the THz RNs is to provide an interface between the optical link and the sub-THz wireless link. For downlink transmission, a photonics-based sub-THz-band signal generation method is adopted to take advantage of the broadband characteristics of the optical components. An electronics-based sub-THz mixer is also used for uplink transmission because of its cost-effectiveness and low energy consumption. A digital signal processor (DSP) is designed to recover the original transmitted baseband signal. The DSP provides frequency offset compensation over a wide frequency range and reduces the probability of cyclic slip. The performance of the proposed system was investigated experimentally with commercially available optical/electrical components. We demonstrate 100 Gb/s 2.5-m wireless transmission with a 16-quadrature amplitude modulation (16-QAM) signal for configuring the downlink. The optical transmission distance was set to 10 km, and the power penalty measured by optical transmission was negligible. We also investigated the scalability and tunability of the photonics-based sub-THz transmitter to confirm the upgradability of our proposed indoor network to consider future capacity expansion. To establish an uplink, a 25 Gb/s 1.5-m wireless transmission with a quadrature phase shift keying (QPSK) signal was employed. A directly modulated laser was used for cost-effective optical transmission. Unlike downstream transmission, a measured bit error rate (BER) penalty caused by the optical transmission was observed. This is due to the interplay between the frequency chirp of the directly modulated laser and the chromatic dispersion in the fiber. Despite this penalty, BERs less than the soft-decision forward error correction (FEC) threshold (2 × 10⁻²) with 20% overhead were achieved. We discuss several remaining technical challenges in real-field deployment. These include THz Tx power improvement, photonic integration, reducing form-factor, polarization insensitivity, and automatic beam steering. Our recent efforts to address these issues are also introduced and examined.

Abundant available spectral resources in terahertz (THz) bands enables opportunities to develop next-generation mobile communications. However, devices with large operating bandwidths are required to exploit the benefits offered by the THz band, which significantly increases the cost and complexity of implementation. In addition, THz frequencies experience a relatively large propagation loss, which inevitably reduces network coverage; thus, numerous antenna sites are required to provide reliable mobile services. Therefore, implementing antenna-sites for a THz-band 6G mobile network may be limited by various design restrictions, such as the high power, complexity, and cost requirements. A photonic-based approach for a THz communication system can offer an efficient solution for simplified antenna-site architecture. THz-wave generation using optical heterodyne can reduce the implementation burden on the antenna-site through the use of complex electrical signal up-conversion. When combined with radio-over-fiber (RoF) technology, the benefits of analog-waveform transmission can be achieved, such as bandwidth-efficient transmission in a mobile transport link. This study presents a comprehensive investigation of design guidelines for implementing an RoF-based photonic THz communication system. We investigate a major performance-limiting factor by analyzing noise contribution in a fiber-wireless link through theoretical and experimental verifications considering the quality of an RoF-based analog photonic link. Furthermore, we demonstrate the THz-band fiber-wireless transmission of an aggregated mobile signal using multiple intermediate frequency (IF) carriers to confirm the technical feasibility of the RoF-based photonic THz system for a 6G mobile network. The experimental results show that a mobile signal of 32 IF carriers with a total occupied bandwidth of 12.8 GHz was successfully transmitted over a 5-m free-space wireless link at a frequency of 0.3 THz, which satisfies the 16-quadrature amplitude modulation requirements. We also discuss the design issues related to the optical-link budget considering potential architectures of the RoF-based photonic THz communication system.

In this study, we demonstrate fifth-generation (5G) trial services using intermediate frequency-over-fiber (IFoF) based distributed antenna system (DAS) implemented in a 28-GHz millimeter-wave (mmWave) during Pyeongchang Winter Olympics. To extend mmWave-based 5G wireless coverage, IFoF-based DAS with 2 × 2 multiple-input multiple-output (MIMO) configuration is presented. Error vector magnitude (EVM) measurements of the 5G signal on the IF and mmWave bands are performed for both downlink and uplink transmissions over 5-km single mode fiber. The experimental results confirm that the minimum requirement for 5G signals, defined by the technical specification of the third generation partnership project, are satisfied by EVMs less than 9%. In addition, the developed IFoF-based DAS that supports 5G mobile service using 28-GHz mmWave was deployed in the city of Pyeongchang, during Winter Olympics. An ultra-high speed 5G data service was offered to the general public for 5G trial demonstration. The reference signals received power and data throughput of the 5G handset are measured while moving in a DAS section and 5G connected car section. We confirm that the downlink throughput of ~1 Gb/s and uplink throughput of ~200 Mb/s are achieved for the 5G trial service area in which an IFoF based DAS is deployed. We also successfully demonstrate the 5G connected car service, which is one of the 5G key services, through deployed IFoF-based DAS. At the intersection of baseband unit (BBU) and DAS deployed sections, a seamless handover is performed to smoothly support 5G-based autonomous driving service.

The telecommunication society is paving the way toward ultra-high frequency regions, including the millimeter wave (mmWave) and sub-terahertz (sub-THz) bands. Such high-frequency electromagnetic waves induce a variety of physical constraints when they are used in wireless communications. Inevitably, the fiber-optic network is deeply embedded in the mobile network to resolve such challenges. In particular, the radio-over-fiber (RoF)-based distributed antenna system (DAS) can enhance the accessibility of next-generation mobile networks. The inherent benefits of RoF technology enhance the DAS network in terms of practicality and transmission performance by enabling it to support the 5G mmWave and 6G THz services simultaneously in a single optical transport link. Furthermore, the RoF allows the indoor network to be built based on the cascade architecture; thus, a service zone can be easily added on request. This study presents an RoF-based multi-service DAS network and experimentally investigates the feasibility of the proposed system.

We propose and demonstrate a cascaded distributed antenna system (DAS) for efficiently extending the coverage of the mmWave-based 5G indoor network. For this, we exploit the radio-over-fiber (RoF) system based on the intermediate-frequency-over-fiber (IFoF) transmission technique that is enabled to add/drop the specific wavelength to the designated remote antenna unit (RAU) with using optical splitters and coarse wavelength division multiplexing (CWDM) filters. Moreover, the IFoF transceivers (TRx) perform the subcarrier multiplexing (SCM) in order to transmit $2\times 8$ frequency allocation (FA) 5G signals per a single optical carrier, where each FA has 100 MHz bandwidth, leading each RAU to support $2\times 2$ MIMO operation. Consequently, the cascaded structure allows for the adaptive and flexible configuration of the order of MIMO in accordance of the required data throughput at the specific indoor area. We introduce the cascaded IFoF link structure that can support up-to 13.5 dB optical power budget with following error-vector-magnitude (EVM) performance characterizations. And then we experimentally demonstrate the RoF-based cascaded DAS network, showing that more than 1 Gb/s total throughput can be achieved per a single antenna. Furthermore, we examine the use of avalanche photodiode (APD) to further increase the optical power budget (i.e., the coverage) based on experiment as well as simulation.