Hanwook Jung

A scheduling algorithm combined with zero-forcing beamforming for a multiuser MIMO wireless system

In this paper, we introduce a scheduling algorithm over a MIMO broadcast channel. Given a set of users, the sched- uler selects more than one user and transmits independent data to them simultaneously by using zero-forcing beamforming. Taking computational complexity into account, a greedy method finding the best and most orthogonal channel vectors is proposed. Addi- tionally, considering fairness among asymmetric users, we also propose an asymptotically fair scheduling algorithm.

Enhancement of a WLAN-based internet service in Korea

A wireless LAN (WLAN)-based Internet service, called NESPOT, of Korea Telecom (KT), the biggest telecommunication and Internet service company in Korea, has been operational since early 2002. As the numbers of subscribers and deployed access points (APs) increase, KT has been endeavoring to improve its service quality as well as the network management. In this paper, we introduce a joint effort between Seoul National University (SNU) and KT to achieve it. We have been addressing two major issues as part of the joint project thus far: (1) a unified WLAN management tool; and (2) real-time traffic support enhancement. We present our on-going efforts as well as some preliminary results. Some issues, which need further attention for the future NESPOT service enhancement, are also introduced.

P-DCF: enhanced backoff scheme for the IEEE 802.11 DCF

The IEEE 802.11 standard is the most dominant MAC protocol in wireless LANs. This paper introduces a novel backoff scheme to improve the performance of the IEEE 802.11 DCF. The original IEEE 802.11 DCF with the binary exponential backoff mechanism suffers from frequent packet collisions under high traffic load. Our proposed backoff scheme termed P-DCF enables mobile nodes to choose their next backoff times in the collision-free backoff range by continuously listening to the medium. P-DCF ensures high throughput and low packet latency by reducing the packet collision probability. We show through simulation that the P-DCF greatly outperforms the original IEEE 802.11 DCF in the saturated network condition.

Integrated hybrid MAC and topology control scheme for M2M area networks

This paper presents the design and performance evaluation of an integrated hybrid MAC and topology control scheme (abbreviated IMTC) for machine-to-machine (M2M) area networks that require comparatively the high data rate communications. In order to maximize the network performance, IMTC is designed with a cross-layer concept of MAC and network layers, which provides the multi-channel TDMA scheduling based on the information of network topology optimized by transmission power control. Simulation results show that by using the proposed scheme, the network throughput and the energy efficiency can be significantly improved.

Reachability and Goodput Enhancement via Fragmentation in Public IEEE 802.11b WLAN

In wireless packet networks, such as IEEE 802.11 wireless LAN, users can suffer from bad channel quality at cell boundaries, and wireless stations may not overcome the bad communication condition, in spite of using the lowest transmission rate via link adaptation. Unless handoff is considered due to the deployment of APs, the case of initial access to a WLAN network, and so on in such an area, fragmentation can be a unique and compatible solution for coping with the channel limitation. We have evaluated a public WLAN service network, called NESPOT, which is operated by Korea Telecom, especially the case of initial access. We evaluate delay and error performances of NESPOT initial access procedure via both mathematical analysis and simulations. From the results, we conclude that WLAN user can experience enhanced goodput, enlarged reachability, and shortened delay via fragmentation. We also propose a practical method, TCP maximum segment size spoofing to get the same effect as fragmentation. Finally, we propose a policy, called cut-off delay, for WLAN users not to wait too long time to get an IP address without severe performance degradation

Participation strategy for cooperative diversity with multiple partners

Cooperative diversity represents an effective way of combating multipath fading through inter-terminal cooperation in wireless networks. In this letter, we propose a new participation strategy that increases the chance of cooperation and present the closed-form expression for outage probability. Numerical results demonstrate that new participation strategy improves the outage performance. key words: user cooperation, diversity, outage probability, wireless communications

UM3 data exchange model and encoding rules

M2M (Machine to machine) service has been designed to provide the customer with remote monitoring and control for various sensors and controllers. M2M platform is composed of a bunch of the software and devices equipped with network connectivity providing data manipulation and exchange among devices and applications. It is also required to be equipped with data management and analysis functions independent of the specific services. In this paper, it is proposed a highly scalable and flexible data exchange protocol, named UM3, which is independent to industry and products. The proposed UM3 data exchange protocols with data model and its encoding rules have strong interoperability to the various M2M related industry standards. UM3 data model based on ITU-T ASN.1 specification language has the capability of representing a time varying unstructured data format. The binary encoding rule of UM3 uses T-NL-N-L-V PDU format. It uses character string instead of numerical tag values as its attribute identifier. UM3 encoding rule provides XML and JSON encoding schemes as its text-based encoding methods. The merits of binary and text based encoding rules are more self-descriptive compared to numerical values based BER, XER and BACnet, etc. By the proposed in this paper self-descriptive capability and compatibility to the other industry protocols, integration of various M2M devices with different industry M2M platforms shall be more easer and expandable by modular way resulting general purpose M2M services. Finally, the pros and cons by character string based data exchange protocol and the future directions of M2M platform for different type of sensors and industry would be discussed and suggested.

Management and diagnosis architecture for a large-scale public WLAN

In Korea, a large-scale IEEE 802.11b-based public wireless LAN (WLAN) service, called NESPOT, has been in operation by Korea Telecom (KT) across the country during the last four years. Along with a fast growth of the service, however, the service quality problems have been encountered. In order to manage and overcome such problems properly, we have developed a diagnostic tool, which is composed of a database for wireless connection-related dissatisfaction, a log collector and analyzer, and a mechanism to report breakdown of access points (APs). To our best knowledge, this is the first attempt to address an integrated management and develop a diagnosis architecture for large-scale commercial WLANs

Analysis of campus users' pattern in NESPOT wireless LANs

Korea Telecommunication and Seoul National University initiated a joint project as an effort of collaboration for building U-Campus early this year. As a part, our project aims to develop a measurement infra system and measure wireless traffic in SNU NESPOT Zone. Currently, SNU NESPOT Zone is successfully built, so the registered users are able to use the Internet anywhere in Seoul National University. In this paper, we first introduce how to implement a NESPOT Zone monitoring and managing system (NMMS) in campus-wide wireless networks. This infra consists of two main parts: a data collection and a real-time monitoring system. Then, we really measured wireless traffic in SNU NESPOT Zone and analyzed measured data on the basis of both user IDs and MAC addresses. From measured results, we would derive a resident type and a migratory type as user mobility patterns