We study the advantages and limitations of InP/GaAsSb/InP DHBTs for high-speed digital circuit applications. We show that the high-current performance limitation in these devices is electrostatic in nature. Comparison of the location of collector current blocking in various collector designs suggests a smoother, more gradual onset of blocking effects in type-II collectors. A comparison of collector current blocking effects between InP/GaAsSb--based and various designs of InP/GaInAs--based DHBTs provides support for our analysis.

We have developed Thai speech synthesis by rule using cepstral parameters. In order to synthesize the pitch contour of Thai tones, we have applied an extension of Fujisaki's model. A mid tone is unique for Thai when compared to Chinese. For the extension of Fujisaki's model to Thai tones, we assumed that the mid tone is neutral and we adopted its phrase component as the phrase components for all tones. According to our study on the pitch contour of five Thai tones using this model, the result shows that the command pattern for the local F0 components needs both positive and negative commands. Listening tests showed that the intelligibility of the Thai tones measured in terms of error rate were 0.0%, 0.7% and 2.7% for analysis/synthesis, Fujisaki's model and the polynomial model, respectively. Therefore, it is shown that the extension of Fujisaki's model is effective for Thai.

The Bluetooth system provides point-to-point or point-to-multipoint communications with no peer-to-peer communication. For peer-to-peer communication, a slave should be able to communicate with other slaves as well as a master. To do this, the master should indirectly forward the packets. However, the end-to-end delay of forwarding packets may be increased because the packets should be forwarded at network layer. Forwarding packets for peer-to-peer communications may also bring about some needless waste of wireless resource. In this paper, we propose the addressing policy for efficient peer-to-peer communications in Bluetooth. We also address the dynamic master selection policy in which a master is dynamically selected in order to minimize the average channel occupancy. To dynamically change master in a piconet, the master-slave switching should be executed as soon as possible. Therefore, we propose the fast master-slave switching mechanism in order to minimize the switching delay. It is observed from the simulations that our proposed policies perform better than the naive Bluetooth specification.

We propose multibits/sequence-period optical code division multiple access (MS-OCDMA) systems with double optical hardlimiters (DHL) in the presence of APD noise, thermal noise, and channel interference. We apply Reed-Solomon (RS) codes to MS-OCDMA to further improve the error rate performance. We show that the MS-OCDMA receiver with DHL improves the bit error probability of MS-OCDMA systems when the received laser power is large. We also show that the performance of RS coded MS-OCDMA system is better than that of on-off keying OCDMA (OOK-OCDMA) system at the same bit rate and at the same chip rate, respectively.

In this paper, we describe the operation of circuits capable of more than 40-Gbit/s that we have developed using InP HEMT technology. For example, we succeeded in obtaining 43-Gbit/s operation for a full-rate 4:1Multiplier (MUX), 50-Gbit/s operation for a Demultiplexer (DEMUX), 50-Gbit/s operation for a D-type flip-flop (D-FF), and a preamplifier with a bandwidth of 40 GHz. In addition, the achievement of 90-Gbit/s operation for a 2:1MUX and a distributed amplifier with over 110-GHz bandwidth indicates that InP HEMT technology is promising for system operations of over 100 Gbit/s. To achieve these results, we also developed several design techniques to improve frequency response above 80 GHz including a symmetric and separated layout of differential elements in the basic SCFL gate and inverted microstrip.

In this letter, we show the effectiveness of a double-loop algorithm based on the concave-convex procedure (CCCP) in decoding linear codes. For this purpose, we numerically compare the error performance of CCCP-based decoding algorithm with that of a conventional iterative decoding algorithm based on belief propagation (BP). We also investigate computational complexity and its relation to the error performance.

This paper describes a method for the fast evaluation of the Sommerfeld integrals for modeling a vertical dipole antenna array in a borehole. When we analyze the antenna inside a medium modeled by multiple cylindrical layers with the Method of Moment (MoM), we need a Green's function including the scattered field from the cylindrical boundaries. We focus on the calculation of Green's functions under the condition that both the detector and the source are situated in the innermost layer, since the Green's functions are used to form the impedance matrix of the antenna. Considering bounds on the location of singularities on a complex wave number plane, a fast convergent integration path where pole tracking is unnecessary is considered for numerical integration. Furthermore, as an approximation of the Sommerfeld integral, we describe an asymptotic expansion of the integrals along the branch cuts. The pole contribution of TM01 and HE11 modes are considered in the asymptotic expansion. To obtain numerical results, we use a fast convergent integration path that always proves to be accurate and efficient. The asymptotic expansion works well under specific conditions. The Sommerfeld integral values calculated with the fast evaluation method is used to model the array antenna in a borehole with the MoM. We compare the MoM data with experimental data, and we show the validity of the fast evaluation method.

In this letter, we propose (1) a new sub-optimum minimum distance search (sub-MDS), whose search complexity is reduced considerably compared with optimum MDSs and (2) a termination criterion, called near optimality condition, to reduce the average number of decoding iterations with little degradation of error performance for the proposed decoding using sub-MDS iteratively. Consequently, the decoding algorithm can be applied to longer codes with feasible complexity. Simulation results for several Reed-Muller (RM) codes of lengths 256 and 512 are given.

We propose an image generation method for an immersive multi-screen environment that contains a motion ride. To allow a player to look around freely in a virtual world, a method to generate an arbitrary direction image is required, and this technology has already been established. In our environment, displayed images must also be updated according to the movement of the motion ride in order to keep a consistency between the player's viewpoint and the virtual world. In this paper, we indicate that this updating process can be performed by the similar method to generate looking-around images and the same data format can be applicable. Then we discuss the format in terms of the data size and the amount of calculations need to consider the performance in our display environment, and we propose new image formats which improve on the widely-used formats such as the perspective, or the fish-eye format.

Given the importance of the traffic on modern communication networks, advanced error correction methods are needed to overcome the changes expected in channel quality. Conventional countermeasures that use high dimensionality parity codes often fail to provide sufficient error correction capability. We propose a parity code with high dimensionality that is iteratively decoded. It provides better error correcting capability than conventional decoding methods. The proposal uses the steepest descent method to increase code bit reliability and the coherency between parities and code bits gradually. Furthermore, the quantization of the decoding algorithm is discussed. It is found that decoding with quantization can keep the error correcting capability high.

There has been a growing interest in multihop wireless ad hoc networks in recent years. Previous studies have shown that, in a wireless multihop network using the slotted ALOHA as the medium access control mechanism, the maximum throughput can be achieved if the number of neighbors is six to eight. We show that, when using the IEEE 802.11 DCF protocol in a wireless ad hoc network, the maximum end-to-end goodput is achieved when all nodes are within transmission range of each other. The main reason is that the channel spatial reuse factor gained from the multihop network does not match the increase in additional transmission hops that a packet needs to travel in a multihop network. For a multihop network, its MAC frame delivery capacity is approximately fixed at a value dependent on its spatial reuse factor. If the offered load increases, less capacity will be spent on delivering packets that eventually reach their destinations and hence resulting in lower end-to-end goodput.

This paper focuses on flow control in high-speed networks. Each node in a network handles its local traffic flow on the basis of only the information it is aware of, but it is preferable that the decision-making of each node leads to high performance of the whole network. To this end, we investigate the relationship between the flow control mechanism of each node and network performance. We consider the situation in which the capacity of a link in the network is changed but individual nodes are not aware of this. Then we investigate the stability and adaptability of the network performance, and discuss an appropriate flow control model on the basis of simulation results.

In this paper, a new Hierarchical Sum of Double Difference metric, HSDD, is introduced. It is shown, as opposed to conventional Sum of Absolute Difference (SAD) metric, how this zerotree coding aware metric can jointly constrain the motion vector searching for both temporal and spatial (quad-tree) directions under multiresolution motion estimation framework. The reward from the temporal-spatial co-optimization concept of HSDD is that fewer bits are spent later for describing the isolated zeros. The embedded wavelet video coder using HSDD metric was tested with a set of video sequences and the compression performance seems to be promising.

We propose a frequency stabilization system for laser diodes (LDs), in which the electrical feedback loop response can be determined using an on-line genetic algorithm (GA) so as to attain lower LD frequency noise power within the specific Fourier frequency range of interest. At the initial stage of the stabilization, the feedback loop response has been controlled through GA, manipulating the proportional gain, integration time, and derivative time of conventional analog PID controller. Individuals having 12-bit chromosomes encoded by combinations of PID parameters have converged evolutionarily toward an optimal solution providing a suitable feedback loop response. A fitness function has been calculated for each individual in real time based on the power spectral density (PSD) of the frequency noise. The performance of this system has been tested by stabilizing a 50 mW visible LD. Long-term (τ > 0.01 s) frequency stability and its repeatability have been improved.

A source code for N symbols can be represented by a codeword length sequence (1,2,,N). A binary code is compact if it satisfies = 1. Also we may assume that 1 1 2 N. In this paper, we show that these two constraints can be replaced by max i for 1 i N - 1. Using the latter characterization, we construct a tree T N representing all binary compact codes. In fact every leaf of T N represents a compact code and vice versa. This technique can also be used to generate all compact codes for a given (1,2,,k), k < N.

A rectangular-to-radial waveguide transformer through a crossed slot is proposed as a feeder of a radial line slot antenna (RLSA) for use in a system of solar power satellite (SPS). The transformer is analyzed and designed by using the MoM with numerical eigenmode basis functions. The measured ripple of the amplitude is 3.0 dB in the φ-direction and a 7.0% frequency bandwidth for rotating mode with the ripple below 6 dB is obtained. This bandwidth is wider than that of conventional ring slot or cavity resonator with a coaxial feeder. The antenna measurements at 5.8 GHz show reasonable rotational symmetry both in the near-field distribution and the far field radiation patterns. The reflection is -27.7 dB at the design frequency and below -15 dB in the 7.0% bandwidth. The gain of the antenna with the diameter of 300 mm is 22.7 dBi and the efficiency is 56%.

Multipath is a big problem for TCP traffic in traffic engineering. To solve it, hash functions such as CRC-16 are usually applied over source and destination address segments in packet headers. Through simulations and performance comparison of several multipath algorithms, it is found out that high network utilization achieved by using hash functions is at the expense of low fairness among coexisting TCP flows. It is also illustrated that packet size has significant influence on performance.

The general public is expected to demand in not too distant future instituting more stringent certification procedures for computing parts of traditional and new-generation safety-critical application systems. Such quality-of-service (QoS) certification processes will not and can not rely solely on the testing approach. Design-time guaranteeing of timely service capabilities of various subsystems is an inevitable part of such processes. Although some promising developments in this area have been occurring in recent years, the technological challenges yet to be overcome are enormous. This paper is a summary of the author's perspective on the remaining challenges and promising directions for tackling them.

In light of the increasing number of computer applications that rely heavily on multimedia data, the database community has focused on the management and retrieval of multidimensional data. Nearest Neighbor queries (NN queries) have been widely used to perform content-based retrieval (e.g., similarity search) in multimedia applications. Incremental NN (INN) query is a kind of NN queries and can also be used when the number of the NN objects to be retrieved is not known in advance. This paper points out the weaknesses of the existing INN search algorithms and proposes a new one, called Batch-Incremental Nearest Neighbor search algorithm (denoted B-INN search algorithm), which can be used to process the INN query efficiently. The B-INN search algorithm is different from the existing INN search algorithms in that it does not employ the priority queue that is used in the existing INN search algorithms and is very CPU and memory intensive for large databases in high-dimensional spaces. And it incrementally reports b(b > 1) objects simultaneously (Batch-Incremental), whereas the existing INN search algorithms report the neighbors one by one. In order to implement the B-INN search, a new search (called k-d-NN search) with a new pruning strategy is proposed. Performance tests indicate that the B-INN search algorithm clearly outperforms the existing INN search algorithms in high-dimensional spaces.

Many cooperated web cache systems and protocols have been proposed. These systems, however, require expensive resources, such as external bandwidth and CPU power or storage of a proxy, while inducing hefty administrative costs to achieve adequate client population growth. Moreover, a scalability problem in the cache server management still exists. This paper suggests peer-to-peer client-clustering. The client-cluster provides a proxy cache with backup storage which is comprised of the residual resources of the clients. We use DHT based peer-to-peer lookup protocol to manage the client-cluster. With the natural characteristics of this protocol, the client-cluster is self-organizing, fault-tolerant, well-balanced and scalable. Additionally, we propose the Backward ICP which is used to communicate between the proxy cache and the client-cluster, to reduce the overhead of the object replication and to use the resources more efficiently. We examine the performance of the client-cluster via a trace driven simulation and demonstrate effective enhancement of the proxy cache performance.