Sung-Ju Lee

Seoul National University, Sŏul, Seoul, South Korea

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Publications (69)23.22 Total impact

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    ABSTRACT: Spectrum management and device coordination for dynamic spectrum access (DSA) networks have received significant research attention. However, current wireless devices have yet to fully embrace DSA networks due to the difficulties in realizing spectrum-agile communications. We address the practical hurdles and present solutions toward implementing DSA devices, answering an important question “what is a simple practical extension to current wireless devices that makes them spectrum-agile?” To this end, we propose RODIN, a general per-frame spectrum-shaping protocol that has the following features to support DSA in commercial off-the-shelf (COTS) wireless devices: direct manipulation of passband signals from COTS devices, fast FPGA-based spectrum shaping, and a novel preamble design for spectrum agreement. RODIN uses an FPGA-based spectrum shaper together with a preamble I-FOP to achieve per-frame spectrum shaping with a delay of under 10 μs.
    IEEE Transactions on Mobile Computing 01/2014; 13(1):20-34. · 2.40 Impact Factor
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    ABSTRACT: The growing demand for real-time streaming video on portable devices has increased the importance of multimedia multicast in mobile wireless networks. A defining characteristic of such multicast networks is its heterogeneity in both the channel states and the MIMO capabilities of its clients. However, current wireless multicast schemes adapt poorly to such heterogeneity. We introduce Procrustes, a multimedia multicast scheme that is built upon a novel PHY-layer rateless code. Unlike bit-level rateless codes (such as Raptor [14] codes), Procrustes clients automatically adjust the PSNR of the received multicast video stream to match both the instantaneous channel state and the number of active receive antennas. We demonstrate the performance of Procrustes in a simulated environment.
    INFOCOM, 2013 Proceedings IEEE; 01/2013
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    ABSTRACT: Light field is a large set of spatially correlated images of the same static scene captured using a 2D array of closely spaced cameras. Interactive light field streaming is the application where a client continuously requests successive light field images along a view trajectory of his choosing, and in response the server transmits appropriate data for the client to correctly reconstruct desired images. The technical challenge is how to encode captured light field images into a reasonably sized frame structure a priori (without knowing eventual clients' view trajectories), so that at stream time, expected server transmission rate can be minimized, while satisfying client's view-switch requests. In this paper, using I-frames, redundant P-frames and distributed source coding (DSC) frames as building blocks, we design coding structures to optimally trade off storage size of the frame structure with expected server transmission rate. The key novelty is to facilitate the use of “landmarks” in the structure-popular reference frames cached in the decoder buffer-so that the probability of having at least one useful predictor frame available in the buffer for disparity compensation is greatly increased. We first derive recursive equations to find the optimal caching strategy for a given coding structure. We then formulate the structure design problem as a Lagrangian minimization, and propose fast heuristics to find near-optimal solutions. Experimental results show that the expected server streaming rate can be reduced by up to 93.6% compared to an I-frame-only structure, at twice the storage required.
    IEEE International Conference on Acoustics, Speech and Signal Processing (ICASSP2012), Kyoto, Japan; 03/2012
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    ABSTRACT: Spectrum management and device coordination for Dynamic Spectrum Access (DSA) networks have received significant research attention. However, current wireless devices have yet to fully embrace DSA networks due to the difficulties in realizing spectrum-agile communications. We address the practical hurdles and present solutions towards implementing DSA devices, answering an important question "what is a simple practical extension to current wireless devices that makes them spectrum-agile?" To this end, we propose RODIN, a general per-frame spectrum-shaping protocol that has the following features to support DSA in commercial off-the-shelf (COTS) wireless devices: (a) direct manipulation of passband signals from COTS devices, (b) fast FPGA-based spectrum shaping, and (c) a novel preamble design for spectrum agreement. RODIN uses an FPGA-based spectrum shaper together with a preamble I-FOP to achieve per-frame spectrum shaping with a delay of under 10 μ s.
    01/2012;
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    ABSTRACT: One of the key features of high speed WLAN such as 802.11n is the use of MIMO (Multiple Input Multiple Output) antenna technology. The MIMO channel is described with fine granularity by Channel State Information (CSI) that can be utilized in many ways to improve network performance. Many complex parameters of a MIMO system require numerous samples to obtain CSI for all possible channel configurations. As a result, measuring the complete CSI space requires excessive sampling overhead and thus degrades network performance. We propose CSI-SF (CSI Sampling & Fusion), a method for estimating CSI for every MIMO configuration by sampling a small number of frames transmitted with different settings and extrapolating data for the remaining settings. For instance, we predict CSI of multi-stream settings using CSI obtained only from single stream packets. We evaluate the effectiveness of CSI-SF in various scenarios using our 802.11n testbed and show that CSI-SF provides an accurate, complete knowledge of the MIMO channel with reduced overhead from traditional sampling. We also show that CSI-SF can be applied to network algorithms such as rate adaptation, antenna selection and association control to significantly improve their performance and efficiency.
    Proceedings - IEEE INFOCOM 01/2012;
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    ABSTRACT: We present the design and experimental evaluation of Simultaneous TRansmission with Orthogonally Blinded Eavesdroppers (STROBE). STROBE is a cross-layer approach that exploits the multi-stream capabilities of existing technologies such as 802.11n and the upcoming 802.11ac standard where multi-antenna APs can construct simultaneous data streams using Zero-Forcing Beamforming (ZFBF). Instead of using this technique for simultaneous data stream generation, STROBE utilizes ZFBF by allowing an AP to use one stream to communicate with an intended user and the remaining streams to orthogonally “blind” (actively interfere with) any potential eavesdropper thereby preventing eavesdroppers from decoding nearby transmissions. Through extensive experimental evaluation, we show that STROBE consistently outperforms Omnidirectional, Single-User Beamforming (SUBF), and directional antenna based transmission methods by keeping the transmitted signal at the intended receiver and shielded from eavesdroppers. In an indoor Wireless LAN environment, STROBE consistently serves an intended user with an SINR 15 dB greater than an eavesdropper.
    Proceedings - IEEE INFOCOM 01/2012;
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    ABSTRACT: Light field is a large set of spatially correlated images of the same static scene captured using a 2D array of closely spaced cameras. Interactive light field streaming is the application where a client continuously requests successive light field images along a view trajectory of her choosing, and in response the server transmits appropriate data for the client to correctly reconstruct desired images. The technical challenge is how to encode captured light field images into a reasonably sized frame structure a priori (without knowing eventual clients' view trajectories), so that during streaming session, expected server transmission rate can be minimized, while satisfying client's view requests. In this paper, we design efficient frame structures, using I-frames, redundant P-frames and distributed source coding (DSC) frames as building blocks, to optimally trade off storage size of the frame structure with expected server transmission rate. The key novelty is to optimize structures in such a way that decoded images in caches of neighboring cooperative peers, connected together via a secondary network such as ad hoc WLAN for content sharing, can be reused to further decrease the server-to-client transmission rate. We formulate the structure design problem as a Lagrangian minimization, and propose fast heuristics to find near-optimal solutions. Experimental results show that the expected server streaming rate can be reduced by up to 83% compared to an I-frame-only structure, at less than twice the storage required.
    Multimedia and Expo (ICME), 2011 IEEE International Conference on; 08/2011
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    ABSTRACT: We provide a comparative analysis of various routing strategies that affect the end-to-end performance in wireless mesh networks. We first improve well-known link quality metrics and routing algorithms to enhance performance in wireless mesh environments. We then investigate the route optimality, i.e., whether the best end-to-end route with respect to a given link quality metric is established, and its impact on the network performance. Network topologies, number of concurrent flows, and interference types are varied in our evaluation and we find that a non-optimal route is often established because of the routing protocol’s misbehavior, inaccurate link metric design, interflow interference, and their interplay. Through extensive simulation analysis, we present insights on how to design wireless link metrics and routing algorithms to enhance the network capacity and provide reliable connectivity.
    Ad Hoc Networks. 01/2011; 9:1343-1358.
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    ABSTRACT: We explore the design of a high capacity multi-radio wireless network using commercial 802.11n hardware. We first use extensive real-life experiments to evaluate the performance of closely located 802.11n radios. We discover that even when tuned to orthogonal channels, co-located 802.11n radios interfere with each other and achieve significantly less throughput than expected. Our analysis reveals that the throughput degradation is caused by three link-layer effects: (i) triggering of carrier sensing, (ii) out of band collisions and (iii) unintended frequency adaptation. Using physical layer statistics, we observe that these effects are caused by fundamental limitations of co-located radios in achieving signal isolation. We then consider the use of beamforming antennas, shielding and antenna separation distance to achieve better signal isolation and to mitigate these problems. Our work profiles the gains of different physical isolation approaches and provides insights to network designers to realize high-performance wireless networks without requiring synchronization or protocol modifications.
    Proceedings of the 8th Annual IEEE Communications Society Conference on Sensor, Mesh and Ad Hoc Communications and Networks, SECON 2011, June 27-30, 2011, Salt Lake City, UT, USA; 01/2011
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    ABSTRACT: We present an experimental performance evaluation study of WiFi links in an open-space outdoor environment. We consider a large scale wireless sensor network scenario of seismic data collection from sensors that are buried in ground and as et of access points (APs) form the hierarchical aggregation layer and the backbone of the network. We conduct two different link characterization studies. First, we evaluate the links between the sensor nodes and a wireless AP using IEEE 802.11a/b/g. We construct the path loss model and investigate the reachability distance of this link for different protocols and different sensor node antenna heights. We then characterize the long distance wireless backhaul links between the APs. We use 802.11n and high gain directional antenna for high throughput and long distance. We evaluate how different PHY and MAC layer enhancements of 802.11n impacts its performance in an open outdoor environment. We observed up to 148 Mb/s throughput at 800 meter line-of-sight links without sophisticated tuning of antenna orientation. We believe our findings can be a benchmark for WiFi based outdoor network deployment, especially for high throughput long distance links.
    Proceedings of the 8th Annual IEEE Communications Society Conference on Sensor, Mesh and Ad Hoc Communications and Networks, SECON 2011, June 27-30, 2011, Salt Lake City, UT, USA; 01/2011
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    ABSTRACT: In Wireless Local Area Networks (WLANs), channel management is important in achieving reliable data communications and satisfying QoS requirements. The key aspects of wireless channel management are monitoring the channel quality and adapting quickly to the network conditions by switching to a better channel. We propose a wireless channel monitoring system with co-located monitoring surrogates. Our system works on multi-radio Access Points (APs) where a co-located surrogate radio monitors the condition of various channels while the master radio serves the clients for data communication. Although we have designed our system for generic WLANs, we believe it will be most useful for IEEE 802.11n networks where there are a large number of channels and dynamic frequency selection is required. Our system enables intelligent, fast channel adaptation, reduces service disruption time, and consequently helps realize the performance potential of 802.11n. We present our multi-radio co-located wireless channel monitoring surrogate system and evaluate its effectiveness on our IEEE 802.11n network testbed. We also perform case studies to demonstrate the benefit our system brings compared against the existing schemes.
    Communications (ICC), 2010 IEEE International Conference on; 06/2010
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    ABSTRACT: Recently, there has been a tremendous growth in the number of installed distributed computing platforms such as those for content distribution networks, cloud computing infrastructures, and distributed data centers. Such distributed platforms need a scalable end-to-end (e2e) network monitoring component to provide Quality of Service (QoS) guarantees to the services and improve the overall performance. An important challenge for a network monitoring infrastructure is the periodicity of the measurements as this aspect trades off the monitoring overheads with staleness of the results. In the Network Genome project, we explore the relationships between different e2e network metrics with the aim of leveraging such relationships for reducing monitoring costs while maintaining measurement accuracy. We perform our analysis using long range network measurements from PlanetLab, where we have been collecting e2e network data (route, number of hops, capacity bandwidth and available bandwidth) as part of the S3 system since January 2006. In this paper, we focus on the correlation between the Capacity and Available Bandwidth metrics between host pairs in the PlanetLab testbed. Our analysis shows that the ranking of hosts with respect to their Capacity to/from a set of nodes is a good indicator of the ranking of hosts with respect to their Available Bandwidth to/from the same set of nodes.
    Proceedings of the Global Communications Conference, 2010. GLOBECOM 2010, 6-10 December 2010, Miami, Florida, USA; 01/2010
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    ABSTRACT: The exploding popularity of 802.11 Wireless Local Area Networks (WLAN) has drawn intense research interest in the optimization of WLAN performance through channel assignment to access points (AP), AP-client association control, and transmission scheduling-we refer to any combination of the three approaches as WLAN management. No matter what degrees of freedom are enabled in WLAN management for performance optimization in a particular WLAN setting, a fundamental question is the corresponding maximum achievable system throughput. We show that for a particular network setting, the derivation of the system throughput (where system throughput is aggregate throughput of all clients or max-min throughput), for any combination of channel assignment, association control and transmission scheduling, is NP-hard and hard to approximate in polynomial time.
    IEEE Communications Letters 01/2010; 14:906-908. · 1.16 Impact Factor
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    ABSTRACT: In wireless networks, modeling of the physical layer behavior is an important yet difficult task. Modeling and estimating wireless interference is receiving great attention, and is crucial in a wireless network performance study. The physical layer capture, preamble detection, and carrier sense threshold are three key components that play important roles in successful frame reception in the presence of interference. Using our IEEE 802.11a wireless network testbed, we carry out a measurement study that reveals the detailed operation of each component and in particular we show the terms and conditions (interference timing, signal power difference, bitrate) under which a frame survives interference according to the preamble detection and capture logic. Based on the measurement study, we show that the operations of the three components in real IEEE 802.11a systems differ from those of popular simulators and present our modifications of the IEEE 802.11a PHY models to the NS-2 and QualNet network simulators. The modifications can be summarized as follows. (i) The current simulators’ frame reception is based only on the received signal strength. However, real 802.11 systems can start frame reception only when the Signal-to-Interference Ratio (SIR) is high enough to detect the preamble. (ii) Different chipset vendors implement the frame reception and capture algorithms differently, resulting in different operations for the same event. We provide different simulation models for several popular chipset vendors and show the performance differences between the models. (iii) Based on the 802.11a standard setting and our testbed observation, we revise the simulator to set the carrier sense threshold higher than the receiver sensitivity rather than equal to the receiver sensitivity. We implement our modifications to the QualNet simulator and evaluate the impact of PHY model implementations on the wireless network performance; these result in an up to six times increase of net throughput.
    Computer Networks 01/2010; · 1.23 Impact Factor
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    ABSTRACT: Peak Signal-to-Noise Ratio (PSNR) is the simplest and the most widely used video quality evaluation methodology. However, traditional PSNR calculations do not take the packet loss into account. This shortcoming, which is amplified in wireless networks, contributes to the inaccuracy in evaluating video streaming quality in wireless communications. Such inaccuracy in PSNR calculations adversely affects the development of video communications in wireless networks. This paper proposes a novel video quality evaluation methodology. As it not only considers the PSNR of a video, but also with modifications to handle the packet loss issue, we name this evaluation method MPSNR. MPSNR rectifies the inaccuracies in traditional PSNR computation, and helps us to approximate subjective video quality, Mean Opinion Score (MOS), more accurately. Using PSNR values calculated from MPSNR and simple network measurements, we apply linear regression techniques to derive two specific objective video quality metrics, PSNR-based Objective MOS (POMOS) and Rates-based Objective MOS (ROMOS). Through extensive experiments and human subjective tests, we show that the two metrics demonstrate high correlation with MOS. POMOS takes the averaged PSNR value of a video calculated from MPSNR as the only input. Despite its simplicity, it has a Pearson correlation of 0.8664 with the MOS. By adding a few other simple network measurements, such as the proportion of distorted frames in a video, ROMOS achieves an even higher Pearson correlation (0.9350) with the MOS. Compared with the PSNR metric from the traditional PSNR calculations, our metrics evaluate video streaming quality in wireless networks with a much higher accuracy while retaining the simplicity of PSNR calculation.
    INFOCOM 2010. 29th IEEE International Conference on Computer Communications, Joint Conference of the IEEE Computer and Communications Societies, 15-19 March 2010, San Diego, CA, USA; 01/2010
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    ABSTRACT: We develop a new wireless link quality metric, ECOT (Estimated Channel Occupancy Time) that enables a high throughput route setup in wireless mesh networks. The key feature of ECOT is being applicable to diverse mesh network environments where IEEE 802.11 MAC (Medium Access Control) variants are used. We take into account the detailed operational features of various 802.11 MAC protocols, such as 802.11 DCF (Distributed Coordination Function), 802.11e EDCA (Enhanced Distributed Channel Access) with BACK (Block Acknowledgment), and 802.11n A-MPDU (Aggregate MAC Protocol Data Unit), and derive an integrated link metric that enables finding maximum throughput end-to-end routes. Through simulations in randomized topological environments, we evaluate the performance of the proposed link metric and routing strategy to demonstrate that our proposed schemes can achieve up to 354.4% throughput gain over existing ones.
    Personal, Indoor and Mobile Radio Communications, 2009 IEEE 20th International Symposium on; 10/2009
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    ABSTRACT: We provide a comparative analysis of various routing strategies that affect the end-to-end performance in wireless mesh networks. We first improve well-known link quality metrics and routing algorithms to better operate in wireless mesh environments. We then investigate the route optimality and its impact on the network performance by comparing the achieved end-to-end performance with the optimal offline routing. Various network topologies, number of concurrent flows, and interference types are considered in our evaluation and we reveal that a nonoptimal route is easily established because of routing protocol's misbehavior, interflow interference, and their interplay, thus affecting the end-to-end performance.
    Personal, Indoor and Mobile Radio Communications, 2009 IEEE 20th International Symposium on; 10/2009
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    ABSTRACT: Wireless mesh networks aim to provide high-speed Internet service without costly network infrastructure deployment and maintenance. The main obstacle in achieving high-capacity wireless mesh networks is interference between the mesh links. In this article, we analyze the carrier sensing and interference relations between two wireless links and measure the impact of these relations on link capacity on an indoor 802.11a mesh network testbed. We show that asymmetric carrier sensing and/or interference relations commonly exist in wireless mesh networks, and we study their impact on the link capacity and fair-channel access. In addition, we investigate the effect of traffic rate on link capacity in the presence of interference.
    IEEE Communications Magazine 08/2009; · 3.66 Impact Factor
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    ABSTRACT: In IEEE 802.11-based wireless mesh networks, routing is crucial in achieving high throughput in face of both interflow and intra-flow interference. Prior work focuses on finding the maximum available bandwidth path when a new flow enters the network. However, few has considered the effect of the new flow on the throughput of the existing flows. We propose a routing framework that uses the topology map of a mesh network with the carrier sense and interference relations and estimates the available bandwidth of a candidate path. We propose two algorithms for finding a route for a new flow: (1) FIRM searches for the maximum bandwidth path for the new flow, and (2) FIRM<sup>+</sup> not only considers the available bandwidth of a path for the new flow, but also the amount of throughput degradation of existing flows. We implement and evaluate FIRM and FIRM<sup>+</sup> with the IRU routing algorithm on a 15 node indoor IEEE 802.11a testbed. Various experiments reveal that FIRM<sup>+</sup> achieves the highest total throughput of all flows.
    Sensor, Mesh and Ad Hoc Communications and Networks Workshops, 2009. SECON Workshops '09. 6th Annual IEEE Communications Society Conference on; 07/2009
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    ABSTRACT: We present a transmission rate adaptation algorithm called AGILE (ACK-Guided Immediate Link rate Estimation) for IEEE 802.11 networks. The key idea of AGILE is that the transmitter adjusts the transmission rate by means of measuring the SNR (Signal-to-Noise Ra- tio) during any frame reception including the ACK (Acknowledgment) frame, and estimating the corresponding maximum achievable through- put using a profile, which is materialized by extensive off-line measure- ment. AGILE is equipped with an advanced RTS (Request-To-Send)/CTS (Clear-To-Send) activation algorithm, eRTS filter that intelligently switches on/off RTS frame transmission to enhance the achievable throughput depending upon the existence of multiple contending (or even hidden) stations. The effectiveness of AGILE is evaluated in our MadWifi-based testbed implementation and we compare its performance with different rate adaptation schemes in various scenarios.
    Future Generation Information Technology, First International Conference, FGIT 2009, Jeju Island, Korea, December 10-12, 2009. Proceedings; 01/2009

Publication Stats

831 Citations
23.22 Total Impact Points

Institutions

  • 2007–2011
    • Seoul National University
      • • Department of Electrical and Computer Engineering
      • • School of Computer Science and Engineering
      Sŏul, Seoul, South Korea
  • 2008–2010
    • University of California, Davis
      • Department of Computer Science
      Davis, California, United States
  • 2001–2009
    • Hewlett-Packard
      Palo Alto, California, United States
  • 2006
    • University of Michigan
      • Department of Electrical Engineering and Computer Science (EECS)
      Ann Arbor, Michigan, United States
  • 2004
    • The University of Arizona
      • Department of Electrical and Computer Engineering
      Tucson, AZ, United States