The software rejuvenation is one of the most effective preventive maintenance technique for operational software systems with high assurance requirement. In this paper, we propose the workload-based software rejuvenation scheme for a server type of software system, and develop stochastic models to determine the optimal software rejuvenation schedules for some dependability measures. In numerical examples, we evaluate quantitatively the performance of workload-based software rejuvenation scheme and compare it with the time-based rejuvenation scheme.

A 15-50 GHz-band GaAs MMIC variable attenuator has been developed for microwave and millimeter-wave wireless communications systems. The attenuator employs a balanced distributed configuration using shunt connected HEMT's in order to realize a broadband operation, a low insertion loss and a good impedance matching with simple control bias scheme. The MMIC was fabricated with a pseudomorphic HEMT device technology. It has exhibited an insertion loss as low as 1.6 dB with an attenuation control range as wide as 21 dB at 26 GHz. It has also shown a good linearity of an input power of more than 12 dBm at 1-dB compression point and that of 26.3 dBm at a 3rd-order intercept point.

We describe a new system for high-speed wireless access systems between base stations and mobile terminals. In the proposed system, the base station has an array antenna and tracks mobile terminals by using a new tracking algorithm. A radio-on-fiber technique is used to simplify and miniaturize the components of the base station. Estimating the direction-of-arrival of the signals from a mobile terminal is important in implementing the proposed system. We propose a new tracking algorithm that uses directions-of-arrival, angular velocities of mobile terminals, and scatter modeling in multipath communications channels to improve the tracking performance. We also developed experimental equipment to demonstrate the feasibility of the proposed millimeter-wave broadband wireless access system and the efficiency of the tracking algorithm using an array antenna system. In this paper, we describe our system and present a new approach for tracking mobile terminals, which is the key feature of the system. We also discuss our simulation and experimental results.

This paper presents improved versions of three-stage positive-output and two-stage negative-output Dickson charge-pump circuits which are intended to replace switching regulators in video-product CCD driver applications (where 12 V and -6.5 V are needed), and are designed and fabricated in a custom CMOS process. From a power supply Vdd of 4.0 to 5.5 V, the positive charge pump generates a positive output voltage of greater than 3.9Vdd, while the negative charge pump generates a negative voltage of greater than -1.9Vdd, both with efficiencies of greater than 94% at 2 mA output currents.

Rectangular/circular-to-radial waveguide tra-nsformers through a ring slot have been proposed for the feeder of radial line slot antennas (RLSAs) in millimeter wave application. Rotating electric modes are excited by a set of ring slot and perturbation dog bone slot. Basic operation is observed in 12 GHz band. Concentric array radial line slot antennas fed by these transformers are fabricated and the antenna gain of 26.9 dBi with the efficiency more than 60% is measured. The applicability for millimeter wave is verified for 38 GHz band RLSA fed by the rectangular waveguide. The measured gain of the antenna is 22.5 dBi with the efficiency of 53% with the diameter of 46mm and 26.4 dBi with 61% with the diameter of 66mm.

This paper proposes a multisegment dielectric resonator (MSDR) placed on a slotline for millimeter-wave filter applications. The MSDR structure, including a rectangular dielectric lump and a thin low-dielectric insert, is quite useful for adjusting the coupling between the slotline mode and the resonant mode, leading to improve the filter performances. In addition, by tuning dimensions of the MSDR, a sharp and clear notch response can be designed in the transmission parameter. We have demonstrated the filter characteristics both theoretically and experimentally, and showed the practical procedure for the design of MSDR filters.

A power combining technique using a Fabry-Perot resonator with many more active devices is investigated. The Fabry-Perot power combiner consists of two mirrors with a circular groove mounted with the active devices. The power combiner has an axially symmetrical structure and operates at an axially symmetrical TEM01n mode so that uniform device-field coupling required for perfect power combining can be realized. By numerical calculation using the boundary element method, it was shown that high combining efficiency can be obtained when the active devices are mounted in the circular groove of larger radius on either of the two mirrors. In experiments at X-band, power combining efficiency over 90% was obtained for the case of twelve devices on either of the mirrors and the output powers of the twenty or twenty-four devices on both the mirrors were almost perfectly combined.

The millimeter wave filter using two whispering-gallery mode dielectric disk resonators is presented in this paper. The coupling coefficients of dual disk resonators and the external Q values of the single resonator excited by a dielectric waveguide are investigated theoretically and experimentally. A 2-stage bandpass filter which is designed at the center frequency of 69.85 GHz with a bandwidth of 500 MHz shows a low-loss property of 1.8 dB insertion loss.

This paper describes the numerical analysis for terahertz electromagnetic-wave oscillation/detection properties of plasma-wave field-effect transistors (PW-FET's) and their applications to future smart photonic network systems. The PW-FET is a new type of the electron device that utilizes the plasma resonance effect of highly dense two-dimensional conduction electrons in the FET channel. By numerically solving the hydrodynamic equations for PW-FET's, the plasma resonance characteristics under terahertz electromagnetic-wave absorption are analyzed for three types of FET's; Si MOSFET's, GaAs MESFET's, and InP-based HEMT's. The results indicate that the InP-based sub-100-nm gate-length HEMT's exhibit the most promising oscillation/detection characteristics in the terahertz range with very wide frequency tunability. By introducing the PW-FET's as injection-locked terahertz-frequency-tunable oscillators and terahertz mixers, a new idea of coherent heterodyne detection utilizing terahertz IF (intermediate-frequency) bands is proposed for the future smart photonic network systems that enable real-time adaptive wavelength routing for add-drop multiplexing. The plasma resonance of PW-FET's by means of different frequency generation based on direct photomixing is also proposed as an alternative approach to injection-locked terahertz oscillation. To realize it, virtual carrier excitations by the polariton having photon energy lower than the bandgap of the channel is a possible mechanism.

The current-voltage characteristics of InP-based HBTs with InAlAs-InP composite emitters have been measured as a function of the thickness of the InP layer in the emitter. As the thickness varies, characteristics such as the gain and the ideality factor vary qualitatively as expected from the changes in position of the InAlAs barrier in the emitter. Quantitatively, however, the variations indicate that the interfaces vary systematically with InP thickness, becoming more abrupt for emitters with thicker InP layers.

We propose a three-dimensional structure on a planar substrate employing micromachining technology. A low-loss suspended structure incorporating a BCB membrane employing deep trench etching technology has been newly proposed. A micromachined suspended line structure using BCB membrane film enables us to realize a low loss planar resonator, which achieved an unloaded quality factor (Q-factor) of more than 280 at 60 GHz. We design low-loss filters and antennas built into silicon in a 60 GHz band. A low-loss filter realizes an insertion loss of 1.0 dB at 60 GHz and a patch antenna obtains a 3% bandwidth. In addition, we demonstrate a 60 GHz receiver front-end IC incorporating the planar filter and the antenna, and obtain good results. These techniques enable us to integrate various functions into a compact package even in millimeter-wave bands.

A periodic approach introduced previously is applied to the TM wave scattering from a finite periodic surface. A mathematical relation is proposed to estimate the scattering amplitude from the diffraction amplitude for the periodic surface, where the periodic surface is defined as a superposition of surface profiles generated by displacing the finite periodic surface by every integer multiple of the period . From numerical examples, it is concluded that the scattering cross section for the finite periodic surface can be well estimated from the diffraction amplitude for a sufficiently large .

In this paper, we propose an approach to solve the power control issue in a DS-CDMA cellular system using genetic algorithms (GAs). The transmitter power control developed in this paper has been proven to be efficient to control co-channel interference, to increase bandwidth utilization and to balance the comprehensive services that are sharing among all the mobiles with attaining a common signal-to-interference ratio(SIR). Most of the previous studies have assumed that the transmitter power level is controlled in a constant domain under the assumption of uniform distribution of users in the coverage area or in a continuous domain. In this paper, the optimal centralized power control (CPC) vector is characterized and its optimal solution for CPC is presented using GAs in a large-scale DS-CDMA cellular system under the realistic context that means random allocation of active users in the entire coverage area. Emphasis is put on the balance of services and convergence rate by using GAs.

This paper introduces current-mode biquad using multiple current output operational transconductance amplifiers (OTAs) and grounded capacitors. The circuit configuration is obtained from a second-order integrator loop structure with loss-less and lossy integrators. The proposed circuit can realize low-pass, band-pass, high-pass, band-stop and all-pass transfer functions by suitably choosing the input and output terminals. And the circuit characteristics can be electronically tuned through adjusting the transconductance gains of OTAs. It is also made clear that the proposed circuit has very low sensitivities with respect to the circuit active and passive elements. An example is given together with simulated results by PSpice.

Focusing on a distributed control service-control-node (SCP) that houses a database (DB) distributed across multiple modules, this paper proposes an autonomous distributed SCP architecture using multicasting access to the distributed DB, and highlights its application areas. We assume as a basic condition that neither the network nor the other modules in the system are aware of the DB configuration. Based on this condition, we propose two basic methods: a unicast approach in which the DB management module that is selected at random by the network routes the DB access request to the module where the target data resides (Method A), and a multicast method in which DB access requests are broadcast to all modules (Method B). A quantitative evaluation is made of the number of required modules and required communications performance between modules which is determined by the capacity of the main memory and processing capacity of the processors. Based on the results, we conclude that Method B better exploits the advantages of module autonomous distribution technology within the limits that the economy of inter-module communication overhead is not impaired. Furthermore, in the event a module fails in Method B, a scheme is proposed in which the defective module is cut out of the multicast group, and multicasting continues. This could be implemented most effectively using a separate route under hardware control that is independent of the on-line communications route between modules.

In 2000, Wang et al. proposed a new (t,n) threshold signature scheme with (k,l) threshold shared verification. Meanwhile, integrating the idea of message recovery, they also proposed a (t,n) threshold authenticated encryption scheme with (k,l) threshold shared verification. However, this article will show that both proposed schemes are insecure, because any malicious attacker can obtain the group secret keys from two valid threshold signatures. Thus, the attacker may solely forge or verify a threshold signature. An improvement to overcome the attacks is proposed.

There is an ever-increasing demand for better performance and scalability in workflow systems. We describe how mobile agents can be used to satisfy such a requirement. For the purpose two important design issues are pointed out in workflow execution and architecture levels. Agent delegation models and a 3-layer architecture are suggested in mobile agent based workflow systems as a solution for each consideration. Workload is statically distributed over task performers based on the proposed method. As a result the performance and the scalability are improved. The effectiveness is shown through stochastic Petri-nets simulation through comparison with client-server based- and another mobile agent-based workflow systems.

A universal channel decoder for a given family of channels is a decoder that can be designed without prior knowledge of the characteristics of the channel. Nevertheless, it still attains the same random coding error exponent as the optimal decoder tuned to the channel. This paper investigates the duality between universal channel decoders and universal source encoders. First, for the family of finite-state channels, we consider a sufficient condition for constructing universal channel decoders from universal source encoders. Next, we show the existence of a universal channel code that does not depend on the choice of the universal decoder.

We study the reliability functions or the minimum r-achievable rates of the lossless coding for the general sources in the sense of Han-Verdu, where r means the exponent of the error probability. Han has obtained formulas for the minimum r-achievable rates of the general sources. Our aim is to give alternative expressions for the minimum r-achievable rates. Our result seems to be a natural extension of the known results for the stationary memoryless sources and Markov sources.

A class E low dv/dt PWM synchronous rectifier regulating the output voltage at a fixed frequency is presented, analyzed and verified experimentally. This rectifier is derived from the class E low dv/dt rectifier by replacing the controlled switch (MOSFET with its anti-parallel diode) with the rectifier diode in class E low dv/dt rectifier, and by using the synchronized PWM signal to control the output voltage at desired value. The ZVS condition of the controlled switch can be maintained from full-loaded to open-loaded. The experimental results measured at switching frequency 1 MHz are in good agreement with the theoretical prediction.