With the spread of broadband and wireless Internet access, there is a growing need for a nomadic network environment that enables the use of network services anywhere, via various access media. In a nomadic network environment, however, the connectivity is decreased because users move among different access networks, and the bandwidth is narrow and fluctuating, especially for radio propagation in wireless networks. To solve these problems, we propose a multilink system with three key functions: IPinIP tunneling, dynamic distribution of packets, and reordering of distributed packets. In particular, our distribution function includes a novel algorithm based on available bandwidth estimation. A prototype of our system was evaluated through experiments using real wireless environments and its efficiency is discussed.

Probability distribution of the node degree has long been regarded as one of the important metrics to characterize the wireless ad hoc networks. However, there is no exact formulation for this probability distribution when shadow fading effects are considered. In this letter, by deriving the conditional probability of the node degree and the joint distribution of the distances between nodes and a randomly selected reference node (RSRN), we show that the probability distribution of the node degree for the wireless ad hoc networks in the shadow fading environments is binomial.

Test compression / decompression using variable-length coding is an efficient method for reducing the test application cost, i.e., test application time and the size of the storage of an LSI tester. However, some coding techniques impose slow test application, and consequently a large test application time is required despite the high compression. In this paper, we clarify the fact that test application time depends on the compression ratio and the length of codewords and then propose a new Huffman-based coding method for achieving small test application time in a given test environment. The proposed coding method adjusts both of the compression ratio and the minimum length of the codewords to the test environment. Experimental results show that the proposed method can achieve small test application time while keeping high compression ratio.

In ad-hoc networks, mobile nodes are limited by a range of radio coverage and have an irregular source of power due to their battery. In ad-hoc networks, there are a lot of situations that all mobile nodes need to agree on their key not at the same time but in part and then merge themselves subsequently. This is because ad-hoc networks have specific features such as mobility and allow various conditions during configuration. In this thesis, we propose MCP (Merging Clusters Protocol), a simple key agreement scheme that can effectively deal with merging different adjacent clusters in mobile ad-hoc networks. When nodes of each cluster have already agreed on their own group keys and intend to merge themselves for further secure communications, MCP can be used in an efficient and secure way. In addition, it can be utilized for efficient group key agreement in a large ad-hoc network. We analyze the security and efficiency of MCP and discuss the experimental results according to practical implementation scenarios.

An integrated multi-level simulation environment is developed for a highly collision-resistant RFID system. An analog/mixed-signal (AMS) simulator for a circuit-level description of analog front-end power/signal transmission through electro-magnetic coupling is concurrently connected to a tailored software simulator for system-level description of digital back-end processing of TH-CDMA based anti-collision communication. The feasibility of the RFID system in which more than 1,000 transponders can be identified by a single reader in 400 msec is successfuly explored, under a practical presence of field disturbances such as background noises in communication channels as well as variations of electro-magnetic coupling strengths for power transmission.

Transmission schemes that gain content from multiple servers concurrently have been highlighted due to their ability to provide bandwidth aggregation, stability on dynamic server departure, and load balancing. Previous approaches employ parallel downloading in the transport layer to minimize the receiver buffer size and maximize bandwidth utilization. However, they only focus on the receiver operations and induce considerable overhead at the senders in contradiction to the main goal of a multi-provider environment, offloading popular servers through replication. In the present work, the authors propose MTCP, a novel transport layer protocol that focuses on reduction of the sender overhead through the elimination of unnecessary disk I/Os and efficient buffer cache utilization. MTCP also balances trade-off objectives to minimize buffering at receivers and maximize the request locality at senders.

For execution of computation-intensive applications, one of the most important paradigms is to divide the application into a large number of small independent tasks and execute them on heterogeneous parallel computing environments (abbreviated by HPCEs). In this paper, we aim to execute independent tasks efficiently on HPCEs. We consider the problem to find a schedule that maximizes the throughput of task execution for a huge number of independent tasks. First, for HPCEs where the network forms a directed acyclic graph, we show that we can find, in polynomial time, a schedule that attains the optimal throughput. Secondly, for arbitrary HPCEs, we propose an (+ε)-approximation algorithm for any constant ε(ε>0). In addition, we also show that the framework of our approximation algorithm can be applied to other collective communications such as the gather operation.

This paper presents an admission control technique for multi-carrier systems with an FRF(frequency reuse factor) of 1. The FRF of 1 is very attrative for more improved channel throughput but the forward link capacity is rapidly decreased at the cell boundary region due to the increase in the ICI(InterCell Interference). By measuring a region-based channel capacity and deriving a closed form of blocking probability, a QoS(Quality of Service) maintenance technique and mobility model can be acquired. In the simulation, the proposed scheme demonstrates a blocking probability reduction of up to 40% compared to the cell-based link capacity scheme.

We investigate the performance of a code division multiple access (CDMA) system employing local polynomial approximation (LPA) smart antenna under moving user scenario. A closed form for average signal to interference plus noise power ratio (SINR) is derived, where the angular velocity of a target user is invariant during an observation interval. This SINR is independent of user velocity, and consequently it induces the independence of bit error rate (BER) with respect to the user velocity, while the use of conventional smart antenna shows significant degradation in the system performance by moving user.

This paper presents a numerical analysis of reverse link capacity by obtaining a closed form of ICI (InterCell Interference) over OFDM (Orthogonal Frequency Division Multiplexing)-based broadband wireless networks. In the analysis, shadowing factors are taken into account for determining the home BS (Base Station) of each MS (Mobile Station) over multicell environments. Under the consideration, a more accurate analysis of link capacity can be performed compared to Gilhousen's approximation. In the numerical results, it turns out that the actual interference is lower than Gilhousen's approximation with a decrease of around 20% in the interference.

In this study we have employed an effective technique for dosimetric analyses of base station antennas in an underground environment. The technique combines a ray-tracing method and the finite-difference time-domain (FDTD) method to calculate the specific absorption rate (SAR) in the human body. The ray-tracing method was applied to evaluate the incident fields in relation to the exposed subject in a three-dimensional space, while the FDTD method was used to calculate the detailed SAR distributions in the human body. A scenario under an underground passage with the installation of a top-loaded monopole antenna was analyzed to investigate the relationship between the actual antenna exposure and a plane-wave exposure. The results show that the plane-wave exposure overestimated the whole-body average SAR in most cases, although this was not always true for peak SAR. The finding implies not only the usefulness of the present uniform-exposure-based reference level for the whole-body average SAR evaluation but also the necessity of modeling actual underground environment for high-precision local peak SAR evaluation.

Multiple-input multiple-output (MIMO) systems using eigenbeam space division multiplexing (E-SDM) perform well and have increased capacities compared with those using conventional space division multiplexing (SDM). However, channel state information (CSI) is required at a transmitter, and the performance of E-SDM systems depends much on the accuracy of the CSI at a transmitter and a receiver. In time-varying fading environments, the channel change between the transmit weight determination time and the actual data transmission time causes the system performance to degrade. To compensate for the channel error, a linear extrapolation method has been proposed for a time division duplexing system. Unfortunately, the system performance still deteriorates as the maximum Doppler frequency increases. Here, two new techniques of channel extrapolation are proposed. One is second order extrapolation, and the other is exponential extrapolation. Also, we propose maximum Doppler frequency estimation methods for exponential extrapolation. Simulation results for 4tx 4rx MIMO systems showed that using the proposed techniques, E-SDM system performs better in a higher Doppler frequency region.

In wide-band code division multiple access (WCDMA), orthogonal variable spreading factors (OVSF) codes are assigned to different users to preserve the orthogonality between users' physical channels. In this letter, we present the dual OVSF code, which can transmit the variable data rates by suing two different modulated signals without loss of the orthogonality. The bit error rate (BER) performance under a multi-user environment suffering the additive white Gaussian noise (AWGN) channel and correlations of those codes are evaluated. The results demonstrate that the proposed dual OVSF scheme could provide flexible rates and lower correlation values with a slight increase in complexity.

We present a novel precomputed radiance transfer method for efficient relighting under all-frequency environment illumination. Environment illumination is represented as a set of environment lights. Each environment light comprises a direction and an intensity. In a preprocessing step, the environment lights are clustered into several clusters, taking into account only the light directions. By experiment, we confirmed that the environment lights can be clustered into a much smaller number of clusters than their original number. Given any environment illumination, sampled as an environment map, an efficient relighting is then achieved by computing the radiance using the precomputed clusters. The proposed method enables relighting under very high-resolution environment illumination. In addition, unlike previous approaches, the proposed method can efficiently perform relighting when some regions of the given environment illumination change.

3D virtual environment provides a limited amount of information, mainly focusing on visual information. This is the main cause of users losing the sense of direction in the environment. Many researches for developing a navigation tools that address this problem have been carried out. In this study, a navigation tool is designed by applying topic map, one of the technologies for semantic web construction, to a 3D virtual environment. Topic map constructs a semantic link map by defining the connection relation between topics. According to an experiment done to evaluate the proposed navigation tool, the tool was more helpful in finding detailed object than highly represented objects. Also, it could be seen that providing the surrounding knowledge is effective for object selection by users when that target for searching is not defined.

Assistive technology (AT) is becoming increasingly important for improving the mobility and language learning capabilities of persons with disabilities, thus enabling them to function independently and to improve their social opportunities. The Morse code has been shown to be a valuable tool in assistive technology, augmentative and alternative communication, and rehabilitation for people with neuromuscular diseases such as amyotrophic lateral sclerosis, multiple sclerosis, and muscular dystrophy. In this paper, we designed and implemented a wireless environmental control aid system using the Morse code as an adapted access communication tool, which includes three types of switch: single-switch, double-switch, and six-switch types. People with disabilities can easily control all types of electronic appliance without restrictions owing to spatial arrangements using a signal transmission based on radio frequency (RF). Experimental results revealed that three participants with disabilities were able to gain access to electronic facilities after six weeks of practice with the new system.

Recently, much research has focused on providing high data rate transmissions, by applying Orthogonal Frequency Division Multiplexing (OFDM) technology to mobile/wireless environments. To use this technology effectively, it is essential to enlarge total cell capacity, guarantee the performance of users in cell edge areas, and provide the users with seamless service. In this paper, a technique employing distributed antennas in positions where low C/I (carrier to interference ratio) levels are anticipated in multi-cell environments, is presented. The specific locations and transmission power are calculated based on antenna gains and interference from adjacent cells. The extent of the performance enhancement in terms of overall cell throughput, throughput per ring, and packet error rate per user, is analyzed. The proposed distributed antennas are found to be efficient for servicing real time traffic, while also enhancing the performance of the users in cell edge areas, and overall cell performance.

Modification of ITU-R P.1411 model to enhance the prediction accuracy in urban environments having variable heights of buildings is proposed in this paper by introducing two kinds of novel correction factors. One is considering the relationship of the highest building height and the transmitter (Tx) antenna height, and the other is considering the effect of receiver (Rx) position on crossroads. After introducing two correction factors, the prediction accuracy is shown to be improved.

Refactoring is one of the promising techniques for improving program design by means of program transformation with preserving behavior, and is widely applied in practice. However, it is difficult for engineers to identify how and where to refactor programs, because proper knowledge and skills of a high order are required of them. In this paper, we propose the technique to instruct how and where to refactor a program by using a sequence of its modifications. We consider that the histories of program modifications reflect developers' intentions, and focusing on them allows us to provide suitable refactoring guides. Our technique can be automated by storing the correspondence of modification patterns to suitable refactoring operations. By implementing an automated supporting tool, we show its feasibility. The tool is implemented as a plug-in for Eclipse IDE. It selects refactoring operations by matching between a sequence of program modifications and modification patterns.

This paper proposes a new compensation method of acoustic scores in the Viterbi search for robust speech recognition. This method introduces noise models to represent a wide variety of noises and realizes robust decoding together with conventional techniques of subtraction and adaptation. This method uses likelihoods of noise models in two ways. One is to calculate a confidence factor for each input frame by comparing likelihoods of speech models and noise models. Then the weight of the acoustic score for a noisy frame is reduced according to the value of the confidence factor for compensation. The other is to use the likelihood of noise model as an alternative that of a silence model when given noisy input. Since a lower confidence factor compresses acoustic scores, the decoder rather relies on language scores and keeps more hypotheses within a fixed search depth for a noisy frame. An experiment using commentary transcriptions of a broadcast sports program (MLB: Major League Baseball) showed that the proposed method obtained a 6.7% relative word error reduction. The method also reduced the relative error rate of key words by 17.9%, and this is expected lead to an improvement metadata extraction accuracy.