Joonyoung Kim

The recent advances in mobile network technologies are based on pioneering higher radio frequency ranges such as millimeter wave and (sub-)terahertz bands. Most likely, the simultaneous use of multiple bands of such high frequency as well as mid/low frequency ranges should be considered to implement heterogeneous mobile environments where diverse conventional and emerging applications can coexist. This study aims to propose, demonstrate, and evaluate a radio-over-fiber (RoF)-based optical transport system that meets the aforementioned future indoor network’s requirements. In particular, we suggest a novel distributed antenna system architecture that can support next-generation mobile services such as millimeter wave 5G and terahertz 6G as well as commercial 5G systems employing low/mid bands. Following the experimental validation of the downlink and uplink transmission of radio signals, we investigate the cost reductions obtained by implementing the proposed system in comparison to conventional topologies such as point-to-point and wavelength division multiplexing-based point-to-multipoint connections.

This paper presents a real-time remote water level monitoring system based on dense wavelength division multiplexing (DWDM)-passive optical fiber sensor (OFS) network for the application of the Internet of Things (IoT). This network employs a broadband light source based on amplified spontaneous emission (ASE) as a seed light. This ASE light is spectrum-sliced by an athermal type arrayed waveguide grating (200 GHz × 16 channel), then distributed towards multiple sensing units (SU). Here, 16 SUs are installed vertically at the specified height in the water pool according to the design specification (i.e., spatial resolution). Then, each SU reflects an optical spectrum having a different reflection coefficient depending on the surrounding medium (e.g., air or water). By measuring these reflected optical spectra with an optical spectrum analyzer, the water level can be easily recognized in real time. However, as the sensing distance increases, system performance is severely degraded due to the Rayleigh Back-Scattering of the ASE light. As a result, the remote sensing capability is limited at a short distance (i.e., <10 km). To overcome this limitation, we propose a simple signal processing technique based on feature extraction of received optical spectra, which includes embedding a peak detection algorithm with a signal validation check. For the specific, the proposed signal processing performs the peak power detection, signal quality monitoring, and determination/display of the actual water level through three function modules, i.e., data save/load module, signal processing module, and Human–Machine Interface display module. In particular, the signal quality of the remote sensing network can be easily monitored through several factors, such as the number of spectral peaks, the wavelength spacing between neighboring peaks and the pattern of detected peak power. Moreover, by using this validation check algorithm, it is also possible to diagnose various error types (such as peak detection error, loss of data and so on) according to the pattern of measured optical spectra. As a result, the IoT sensor network can recognize 17 different level statuses for the water level measurement from a distance of about 25 km away without active devices such as optical amplifiers (i.e., passive remote sensing).

With the advances in mobile network technologies, the fronthaul is being evolved to centralized and cloud networks, leading to the emergence of various optical multiplexing architectures. Among others, the Ethernet-based packet switching network is considered one of the most promising concepts due to the inherent merits of Ethernet such as backward compatibility, high reliability, and cost reduction. This work aims at evaluating the Ethernet-based mobile fronthaul network by comparing it with other configuration options. In particular, we calculate and compare the total cost of ownership (TCO) in order to emphasize the cost efficiency of the 5G Ethernet fronthaul network. For this, we analyze the packet aggregation capabilities of the Ethernet fronthaul through queueing latency modeling. Hence, we show that the statistical multiplexing enabled by packet switching reduces the cost related to optical transceiver modules and subsequently fiber optic cabling, which eventually allows for the TCO to be less than 70% of the active transparent system.

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.

Water management is a critical mission required to protect the water resources that is essential in diverse industrial applications. Amongst a variety of parameters such as level (or depth), temperature, conductivity, turbidity, and pH, the water level is the most fundamental one that needs to be monitored on a real-time basis for securing the water management system. This paper presents an overview of water level monitoring technologies based on optical fiber sensor (OFS) networks. Firstly, we introduce and compare the passive distributed and quasi-distributed (discrete) sensor networks with the recent achievements summarized. The performance (i.e., sensing range and resolution) of the OFS networks can be enhanced through diverse multiplexing techniques based on wavelength, time, coherence, space, etc. Especially, the dense wavelength division multiplexing (DWDM)-based sensor network provides remote sensing (where its reach can be extended to >40 km) with high scalability in terms of the channel number that determines the spatial resolution. We review the operation principle and characteristics of the DWDM-based OFS network with full theoretical and experimental analysis being provided. Furthermore, the key system functions and considerations (such as the link protection from physical damages, self-referencing, management of sensing units, and so on) are discussed that could be a guideline on the design process of the passive OFS network.

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.

We present an erratum for our recent paper [ Opt. Express 28, 23397 (2020)10.1364/OE.398460 ] to include funding information in the funding section.

This paper presents a passive optical fiber sensor network based on the dense wavelength division multiplexing (DWDM) to remotely monitor the water level of the spent fuel pool in nuclear power plants. In states of emergency, such as a tsunami, safety information must be secured for rapid response, in spite of all power losses in the plant. We consider the proposed passive sensor network to be one of the best solutions that is able to provide the remote (more than tens of kilometers) monitoring station with the highly reliable on-site information. The principle of water level measurement is based on the change of Fresnel reflection power coefficient at sensing units, which are installed according to the water levels in a row. The sensing units that play the role of reflector and modulator at the same time are connected to an arrayed waveguide grating (AWG) for DWDM. By measuring the spectrum of the optical signal transferred from the sensing units, the water level can be determined in real-time. However, in the remote sensing, the system performance can be seriously degraded due to the Rayleigh Back-Scattering (RBS) of the seeded amplified spontaneous emission (ASE) light that is induced at the fiber-optic link. As such, we investigate the effect of RBS on the remote (more than tens of kilometers) sensing performance of the proposed network. Following the theoretical analysis, we propose a simple network configuration to overcome the RBS issue by utilizing two different transmission paths: one for downstream of the ASE seed light, and the other for upstream of the optical signals coming from the sensing units. Based on the proposed configuration, the maximum sensing distance can be increased up to 42.5 km without the support of any optical amplifier.