Daewon Jung

Asynchronous Medium Access Protocol for Multi-User MIMO Based Uplink WLANs

Multi-user multiple-input multiple-output (MIMO) technology makes it possible for wireless nodes to successfully receive multiple packets from simultaneous transmitters in wireless networks. As it can provide more transmission opportunities without causing collisions, the network throughput performance can be dramatically improved. In this paper, we propose an asynchronous medium access control (MAC) protocol, which enables senders to independently start their transmissions if the access point (AP) can receive more simultaneous packets up to its multi-packet reception capability. This asynchronous protocol makes the multi-user MIMO channel more efficiently used, especially in wireless networks where transmission durations are dynamically varying due to different packet sizes and transmission rates. Through our performance analysis and extensive simulations, we show that the proposed asynchronous MAC protocol achieves significantly higher uplink throughput performance in multi-user MIMO wireless networks.

Opportunistic MAC Protocol for Coordinating Simultaneous Transmissions in Multi-User MIMO Based WLANs

Multi-user MIMO technology makes it possible for wireless nodes to successfully receive multiple packets from simultaneous transmitters in wireless networks. As it can provide more transmission opportunities without causing collisions, the network throughput performance can be dramatically improved. In this letter, we propose a medium access control (MAC) protocol, which allows more nodes to opportunistically transmit packets even though they do not exchange any control packets for transmission coordination if the AP can concurrently receive more packets due to the multi-packet reception capability. Through extensive simulations, we show that the proposed MAC protocol achieves significantly higher throughput performance in multi-user MIMO wireless networks.

Adaptive contention control for improving end-to-end throughput performance of multihop wireless networks

In multihop wireless networks, packets of a flow originating from a source node are relayed by intermediate nodes (relay nodes) and travel towards their destination along a multihop wireless path. Since the traffic forwarding capability of each node varies according to its level of contention, ideally, a node should not transmit more packets to its relay node than the corresponding relay node can forward. Instead, each node should yield its channel access opportunity to its neighbor nodes so that all the nodes can evenly share the channel and have similar forwarding capabilities. In this manner, nodes can utilize the wireless channel effectively, and further increase the end-to-end throughput of a multihop path. We propose a fully distributed contention window adaptation (CWA) mechanism, which adjusts the channel access probability depending on the difference between the incoming and outgoing traffic at each node, in order to equate the traffic forwarding capabilities among all the nodes in the path. We implement the proposed adaptive contention algorithm on Madwifi Linux kernel driver for Wi-Fi interface with Atheros chipset and carry out an empirical study in our division building. The experiment results demonstrate how the proposed mechanism can improve end-to-end throughput performance in the multihop wireless networks.

Power-saving strategy for balancing energy and delay performance in WLANs

In wireless local area networks (WLANs), power conservation for mobile devices is considered as one of the most important issues because it effectively prolongs the battery life of mobile devices. The IEEE 802.11 standard specifies a power-saving mode that allows mobile nodes to adaptively operate in sleep and wake modes to reduce the overall energy consumption. In the IEEE 802.11 power-saving mode, the access point (AP) can adjust the number of nodes in wake mode at every beacon interval. In this paper, we first investigate how the number of nodes in wake mode affects both energy consumption and delay performance in WLANs. We then propose a balanced power-saving strategy, which determines an appropriate number of nodes in wake mode based on a trade-off between energy consumption and packet delay. Through a performance analysis and extensive simulations, we show that our proposed scheme effectively reduces overall energy consumption while retaining low packet delay.

[New Introduction] Search Method for Similar Images from Representative Video Frame Images

With rapid advances in computer and communications technology, the internet and video information is being used in a variety of areas?and the amount of this information is increasing exponentially. However, existing information analysis systems mainly depend on text, so it is not easy to differentiate video information. In order to solve this problem, we propose an algorithm that efficiently analyzes large amounts of video information and searches for similar images. We extract representative frames from a video via histogram analysis of the video information through low computational complexity of frame intervals. Then, we search for images with high similarity using information about colors and shapes. For obtaining color feature information in images and shape feature information, we use both hue?saturation?intensity and moment invariants.

TDoA localization for wireless networks with imperfect clock synchronization

With the advent of an ubiquitous computing environment, the need to capture location information and incorporate it into applications has become urgent. In particular, time difference-of-arrival (TDoA) localization is considered to be a cost effective and accurate localization technique. However, the local clocks of the access points (APs) are required to be fully synchronized with each other. If clock offsets exist, TDoA localization can yield a poor localization performance. In this paper, we propose a TDoA-based localization technique that simultaneously estimates the client location and clock offsets of the APs. The simulation and experimental results show that the proposed localization algorithm alleviates the impact of imperfect clock synchronization among APs, thereby improving the localization accuracy.

Power control for multiple access communication systems with multi-packet reception capability

This paper addresses the power control problem in wireless multiple access communication systems with a multi-packet reception (MPR) channel. Driven by advances in signal processing and multiuser detection (MUD) technologies such as UWB and MIMO, it has become possible for a wireless node to simultaneously receive several packets from other transmitters. In this case, the receiver node can determine the set of transmitters from which it wants to receive data and instruct them to adjust their transmit power levels in order to maximize the overall network capacity by exploiting the MPR capability. We first show that the efficiency of the MPR channel utilization can be maximized by equalizing the transmission durations of simultaneously transmitting nodes, and then propose a power control that increases/decreases the transmit power of the nodes to equalize the transmission durations. Under the proposed power control, a node with a long transmission duration is allowed to use a higher transmit power if the next higher transmission rate is achievable with the increased transmit power. Through extensive simulations, we show that the proposed power control significantly improves the overall network capacity in multiple access.

Ubiquitous Sensor Networks and Image Processing Based Environmental Monitoring System for Fire Safety

In recent years, since the tall buildings are getting increased and getting dense in the city area, those buildings can be more and more exposed to risks such as fire and natural disasters. In case if the high-rise building is on fire, it is not easy to put out the fire even with the modernization of the firefighting equipment. These risks can directly lead to human casualties so that it is necessary to quickly have an initial evacuation or initial suppression risks. In this paper, we propose the integrated method for reducing the fire risks through sensors such as temperature and humidity, ZigBee technology, camera control and image processing technologies, and real-time data transmission technology. Based on our proposed method, it is expected that our integrated system directly leads to the expansion of risk safety management system as well as the development of USN technologies.

Probabilistic packet dropping for load control in multihop wireless networks

In multihop wireless networks, packets of a traffic flow originating from a source node are relayed by intermediate nodes (relay nodes) and travel toward their destination node along a multihop wireless path. If a source node transmits excessive packets at a high rate, its relay node may not have enough channel access opportunities to forward them, resulting in a significant throughput decrease on an end-to-end perspective of multihop path. In this paper, we propose a probabilistic packet dropping scheme that regulates the traffic load offered by source nodes and reduces their interference with neighboring relay nodes on the multihop paths. Through ns-2 simulations, we show that the proposed scheme can significantly improve the end-to-end throughput performance without exploiting topological information under chain and cross topologies.

Beacon selection for localisation in IEEE 802.11 wireless infrastructure

Given the widespread deployment of IEEE 802.11-based access points APs, received signal strength RSS-based localisation algorithms, which estimate the location of clients by measuring RSS at the installed APs, have drawn considerable attention. However, the accuracy of RSS-based localisation depends heavily on the RF and geometry characteristics between the client and the APs. In order to improve the localisation accuracy, the selection of an appropriate AP set without outliers is an important and challenging issue. In this paper, we first propose to use Cramer-Rao Bound, obtained from the average Fisher Information Matrix, as a criterion for selecting an appropriate AP set. Then, based on the proposed selection criterion, we develop a batch beacon selection algorithm that searches all the possible AP sets. Furthermore, to implement real-time mobile client localisation by alleviating computational complexity, we devise an online beacon selection algorithm.