Ill-Keun Rhee

Bio: Ill-Keun Rhee is an academic researcher from Hannam University. The author has contributed to research in topics: Direction of arrival & Smoothing. The author has an hindex of 2, co-authored 6 publications receiving 210 citations.

Clock Synchronization in Wireless Sensor Networks: An Overview

Abstract: The development of tiny, low-cost, low-power and multifunctional sensor nodes equipped with sensing, data processing, and communicating components, have been made possible by the recent advances in micro-electro-mechanical systems (MEMS) technology. Wireless sensor networks (WSNs) assume a collection of such tiny sensing devices connected wirelessly and which are used to observe and monitor a variety of phenomena in the real physical world. Many applications based on these WSNs assume local clocks at each sensor node that need to be synchronized to a common notion of time. This paper reviews the existing clock synchronization protocols for WSNs and the methods of estimating clock offset and clock skew in the most representative clock synchronization protocols for WSNs.

Uplink Interference Adjustment for Mobile Satellite Service in Multi-beam Environments

Abstract: In this paper, uplink interference, which can occur when frequencies are reused, and new measures are mainly discussed. First, many different types of scenarios on uplink interference in Mobile Satellite Service in both normal and reverse mode are designed. Then the simulator which analyzes uplink interference of a user link, among interferences that can occur when reuse frequencies to make efficient use of the limited frequency resources in multi-beam satellite environments, is designed. The simulator provides the amount of interference in MSS and the way to reuse the frequency efficiently. The results of interference analysis are shown in I/N. With the given data, resolutions are proposed to minimize the interference by modifying satellite cells, diameter of the terrestrial cells, reuse of frequency and protective area.

Performance Comparison of Time-Frequency Analysis Methods for Radar Signature Analysis

Abstract: In this paper, four different time-frequency analysis techniques are applied to the scattering analysis of radar targets. Short-time fourier transform (STFT), continuous wavelet transform (CWT), adaptive wavelet transform (AWT) and evolutionary AWT (EAWT) are applied to scattering analysis. The two scattering phenomenon which are important for radar target signature analysis are scattering centers and resonance frequencies. In the simulation using wire target (Caravelle-like) data, the comparisons of four different time-frequency analysis techniques are presented. The results show that the EAWT is the best technique to analyze the scattering centers and resonance frequency simultaneously.

Signal Parameter Extraction via Component Cancellation Using Evolutionary Programming

Abstract: In this paper, a novel direction of arrival (DOA) estimation scheme is proposed. The proposed algorithm, called the signal parameter extraction via component cancellation (SPECC), is an evolutionary optimization-based method and extracts the amplitudes of signal sources and DOA impinging on a sensor array in a step-by-step procedure, different from other algorithms such as MUSIC, root-MUSIC, ESPRIT, etc., which extract those parameters at the same time. Our algorithm has robustness to noise and high resolution in DOA estimation. In the simulation, comparisons with root-MUSIC, which has been known as one of the best algorithms, are presented to illustrate the superiority of the proposed SPECC algorithm.

Improved DOA estimation of correlated signals in correlated antenna noises environment

Abstract: This paper presents a high resolution direction-of-arrival (DOA) algorithm which can effectively estimate highly correlated multiple signals in the presence of correlated antenna array noises at the receiver. The proposed algorithm in this paper is obtained by combining the forward/backward spatial smoothing algorithm that is an improved spatial smoothing scheme with the second-order technique which is known as an effective DOA algorithm to resolve very closely spaced multiple signals in the presence of correlated antenna noises. Analysis has been made in several aspects to compare the performance between the spatial smoothing, the spatially smoothed second-order, and the proposed algorithms in DOA estimation. Computer simulations are statistically performed to show the usefulness of the analysis.

Array signal processing

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Re-evaluation of RF electromagnetic communication in underwater sensor networks

Abstract: Most underwater wireless networks use acoustic waves as the transmission medium nowadays, but the chances of getting much more out of acoustic modems are quite remote. Optical links are impractical for many underwater applications. Given modern operational requirements and digital communications technology, the time is now ripe for re-evaluating the role of electromagnetic signals in underwater environments. The research presented in this article is motivated by the limitations of current and established wireless underwater techniques, as well as the potential that electromagnetic waves can offer to underwater applications. A case study is presented that uses electromagnetic technology in a small-scale underwater wireless sensor network. The results demonstrate the likely effectiveness of the designated network.

Design of a Water Environment Monitoring System Based on Wireless Sensor Networks

Abstract: A water environmental monitoring system based on a wireless sensor network is proposed. It consists of three parts: data monitoring nodes, data base station and remote monitoring center. This system is suitable for the complex and large-scale water environment monitoring, such as for reservoirs, lakes, rivers, swamps, and shallow or deep groundwaters. This paper is devoted to the explanation and illustration for our new water environment monitoring system design. The system had successfully accomplished the online auto-monitoring of the water temperature and pH value environment of an artificial lake. The system's measurement capacity ranges from 0 to 80 °C for water temperature, with an accuracy of ±0.5 °C; from 0 to 14 on pH value, with an accuracy of ±0.05 pH units. Sensors applicable to different water quality scenarios should be installed at the nodes to meet the monitoring demands for a variety of water environments and to obtain different parameters. The monitoring system thus promises broad applicability prospects.

Collaborative Sensor Network Localization: Algorithms and Practical Issues

Abstract: Emerging communication network applications including fifth-generation (5G) cellular and the Internet-of-Things (IoT) will almost certainly require location information at as many network nodes as possible. Given the energy requirements and lack of indoor coverage of Global Positioning System (GPS), collaborative localization appears to be a powerful tool for such networks. In this paper, we survey the state of the art in collaborative localization with an eye toward 5G cellular and IoT applications. In particular, we discuss theoretical limits, algorithms, and practical challenges associated with collaborative localization based on range-based as well as range-angle-based techniques.

Robust Time-Based Localization for Asynchronous Networks

Abstract: Time-based localization approaches attract a lot of interest due to their high accuracy and potentially low cost for wireless sensor networks (WSNs). However, time-based localization is tightly coupled with clock synchronization. Thus, the reliability of timestamps in time-based localization becomes an important yet challenging task to deal with. In this paper, we propose robust time-based localization strategies to locate a target node with the help of anchors (nodes with known positions) in asynchronous networks. Two kinds of asynchronous networks are considered: one only with clock offsets, labeled quasi-synchronous networks, whereas the other with not only clock offsets but also clock skews, labeled fully asynchronous networks. A novel ranging protocol is developed for both networks, namely asymmetric trip ranging (ATR), to reduce the communication load and explore the broadcast property of WSNs. Regardless of the reliability of the timestamp report from the target node, closed-form least-squares (LS) estimators are derived to accurately estimate the target node position. As a result, we counter the uncertainties caused by the target node by ignoring the timestamps from this node. Furthermore, in order to simplify the estimator in fully asynchronous networks, localization and synchronization are decoupled. A simple yet efficient method is proposed to first Calibrate the Clock Skews of the anchors, and then Estimate the Node Position (CCS-ENP). Finally, Cramer-Rao bounds (CRBs) and simulation results corroborate the efficiency of our localization schemes.