Resonant properties of resistively shunted tunnel junctions dominate the high-frequency performance of Josephson array oscillators. To improve the operating frequency, we have developed resistively shunted Nb/AlOx/Nb tunnel junctions with a small parasitic inductance. The inductance was minimized by reducing the inductive length between the tunnel junction and the contact hole to be about 1µm. By fitting the measured I-V characteristics of the shunted tunnel junction to the simulated characteristics, we estimated the inductance to be about 105 fH. The analysis of resonant properties showed that the shunted tunnel junctions with the small parasitic inductance have a high-frequency performance up to the Nb gap frequency. Josephson array oscillators using 11 such junctions were designed and fabricated to operate at 650 GHz and 1 THz. Shapiro steps induced by Josephson oscillation were clearly observed up to 1 THz. By fitting the step heights to the simulated results, we estimated the output power of the Josephson oscillator delivered to the load resistor to be about 10 µW at 625 GHz and 50 nW at 1 THz.

Video on demand is regarded as one of the most important applications in future broadband networks. Although several trial runs in various parts of the world do not bring instant success, its inherent requirements for bandwidth and bounded delay make it a suitable candidate for studying various networks and protocols in real-time applications. Recently, the Hybrid Fiber Coaxial (HFC) networks emerged as network architecture for bringing broadband services to the home. The co-axial cable network uses a tree-like architecture and requires complex protocols for synchronizing and sharing uplink communications among many users. This paper proposes that a dual bus network may be a simpler and cost effective alternative. We implement an interactive VOD (IVOD) system as an example to demonstrate its simplicity and suitability as a community network. The system uses client/server architecture with guaranteed bandwidth and bounded delays. Simulations are conducted to test the network load and response time for different number of users. The results indicate its feasibility and the delay time can be further reduced by releasing bandwidth during the pause operation.

In the commonsense reasoning, priorities among rules are often required to be found out in order to derive the desired conclusion as a theorem of the reasoning. In this paper, first we present the bottom-up and top-down abduction procedures to compute skeptical explanations and secondly show that priorities of circumscription to infer a desired theorem can be abduced as a skeptical explanation in abduction. In our approach, the required priorities can be computed based on the procedure to compute skeptical explanations provided in this paper as well as Wakaki and Satoh's method of compiling circumscription into extended logic programs. The method, for example, enables us to automatically find the adequate priority w. r. t. the Yale Shooting Problem to express a human natural reasoning in the framework of circumscription.

We propose a novel soft-decision decoding algorithm for cyclic codes based on energy minimization principle. The well-known soft-decision decoding algorithms for block codes perform algebraic (hard-decision) decoding several times in order to generate candidate codewords using the reliability of received symbols. In contrast, the proposed method defines energy as the Euclidean distance between the received signal and a codeword and alters the values of information symbols so as to decrease the energy in order to seek the codeword of minimum energy, which is the most likely codeword. We let initial positions be the information parts of signals obtained by cyclically shifting a received signal and look for the point, which represents a codeword, of minimum energy by moving each point from several initial positions. This paper presents and investigates reducing complexity of the soft-decision decoding algorithm. We rank initial positions in order of reliability and reduce the number of initial positions in decoding. Computer simulation results show that this method reduces decoding complexity.

In this paper, to obtain higher spectral efficiency than a conventional M-ary/Spread Spectrum (SS) communication system using an orthogonal code, we propose Double M-ary/SS communication systems in which transmitting sequences are produced by multiplying two sequences obtained from different M-ary/SS systems. First, we estimate the system performance of a Double M-ary/SS in which transmitting sequences are composed of two kinds of sequences which have the same chip duration and the same sequence length in AWGN environment by theoretical analysis and computer simulations using random sequences. And we show that the bit error rate (BER) of the system can be improved, compared to that of the conventional M-ary/SS. Next, we propose two concrete examples of the Double M-ary/SS systems. We investigate the performance of these systems in AWGN environment by computer simulations, and show that the BER and spectral efficiency can be improved by the two systems compared to those of the conventional M-ary/SS. Further, we consider a modified method of the demodulation for one of the examples of Double M-ary/SS systems, and show that we can reduce quantity of calculations by the method.

CDMA unslotted ALOHA system with finite size of queueing buffers is discussed in this paper. We introduce an analytical model in which the system is divided into two Markov chains; one is in the user part, and the other is in the channel part. In the user part, we can model the queueing behavior of the user station as an M/G/1/B queue. In the channel part, we can consider the number of simultaneously transmitted packets as an M1 + M2/D///K queue. We analyze the queueing system by using this analytical model, and evaluate the effect of buffer capacity in terms of the throughput, the rejection probability and the average delay. As a result, increase in the buffer size brings about an improvement in the grade of service in terms of higher throughput and lower rejection probability.

This paper presents the performance of FH/MFSK systems, which exploit silent gaps in speech to accommodate more users, over Rayleigh fading channels. Two kinds of receivers are considered: one uses a threshold on the received signal strength to declare whether the signals were present or not, and the other is assumed to have perfect transmitter-state information obtained from using additional bandwidth. Results show that, if the codeword dropping and codeword error are assumed to be equally costly, the former can achieve slightly better performance than the latter in the decoding error probability. This finding suggests that, for the system to exploit silent gaps in speech, it is advantageous for the receiver to use a threshold to declare whether signals were present or not instead of relying on the transmitter-state information.

We shall generalize B. S. Clarke and A. R. Barron 's analysis of the Bayes method for the FSMX sources. The FSMX source considered here is specified by the set of all states and its parameter value. At first, we show the asymptotic codelengths of individual sequences of the Bayes codes for the FSMX sources. Secondly, we show the asymptotic expected codelengths. The Bayesian posterior density and the maximum likelihood estimator satisfy asymptotic normality for the finite ergodic Markov source, and this is the key of our analysis.

It is urgently required to protect copyrights of digital contents since the digital contents can be easily copied without degradation of quality. In this paper, we propose a new watermarking method which spreads an ID pattern with a random sequence and embeds it throughout the spatial domain of a color image. The random sequence is a key for extracting the ID pattern. As an ID pattern is spread throughout an image, we can extract the ID pattern from a part of the image, that is clipped image. We can also confirm authenticity by extracting the same ID pattern from several parts of an image. The proposed method is robust to disturbance by noise addition and image conversion such as brightness-contrast conversion and JPEG compression.

In this paper, we discuss the error correcting capability of the constant amplitude coding. In orthogonal multi-code CDMA systems, the transmitted signal has large amplitude fluctuation and the amplitude fluctuation causes large nonlinear distortion. In order to avoid the amplitude fluctuation, we have proposed "constant amplitude coding" in our previous study. The constant amplitude coding can achieve constant amplitude transmission by utilizing the redundancy of the coding, i. e. , the coding can have an error correcting effect. In this paper, we examine the effectiveness of the error correcting capability of the constant amplitude coding and show that the constant amplitude coding is very effective against nonlinear distortion under the consideration of the error correcting capability.

The DC component suppressing method, called Guided Scrambling (GS), has been proposed, where a source bit stream within a data block is subjected to several kinds of scrambling and a RLL (Run Length Limited) coding to make the selection set of channel bit streams, then the one having the least DC component is selected. Typically, this technique uses a convolutional operation or GF (Galois field) conversion. A review of their respective symbol error properties has revealed important findings. In the former case, the RS (Reed-Solomon) decoding capability is reduced because error propagation occurs in descrambling. In the latter case, error propagation of a data block length occurs when erroneous conversion data occurs after RS decoding. This paper introduces expressions for determining the decoded symbol error probabilities of the two schemes based on these properties. The paper also discusses the difference in code rates between the two schemes on the basis of the result of calculation using such expressions.

Privacy, voter uncoercibility, collision freedom, verifiability, and tally correctness are essential properties of modern electronic election systems. None of the single-authority election systems proposed in the literatures achieves all the above five properties. In this paper we propose a universal single-authority election system that satisfies the five properties. In particular, the privacy of each voter is protected against the authority and other voters, and no voter can coerce any other voter into changing the value of his vote in our proposed system. We also show that it is impossible for a collision-free single-authority election system to possess the voter uncoercibility and authority uncoercibility at the same time.

Fitt's law is commonly used to model target selection. But Fitts' law deals with only one kind of selection strategy. Our question is, do changes in target size, distance and direction to a target affect the differences in performance between target selection strategies? We performed the first empirical tests on a pen-based system to evaluate differences in performance between six selection strategies for selecting a target. Three target sizes, eight pen-movement-directions and three pen-movement-distances were applied to all six strategies. The results show that differences between selection strategies are affected by variations in target size but not by the other parameters (distance and direction).

A new numerical procedure called asymptotic periodic waveform evaluation (APWE) for finding the steady state solution of nonlinear circuits driven by one tone periodic input signal is presented. APWE starts by constructing a virtual system which gives the same periodic steady state waveform as the original system's but with a shorter transient duration. Thus the periodic steady state (PSS) response can be obtained by simply performing transient analysis of the newly derived system for a few periods. An efficient method for solving the nonlinear equations occurred during the transient analysis is presented. To improve the convergence rate of PSS waveform, APWE is combined with extrapolation method. Some simulation results are shown.

Mechanism of UHF radiowave propagation into multistory office buildings are explored by using ray-tracing based models, which include a three-dimensional (3-D) ray-tracing model and a direct-transmitted ray (DTR) model. Prediction accuracy of the models is ascertained by many measured data and the measurements are carried out at many specific sites with different propagation scenarios. Their measured results also demonstrate some important propagation phenomena. It is found that (1) the direct transmitted wave may be the dominant mode; (2) the path loss neither increases nor decreases monotonically as a function of increasing floor level; and (3) there is not much difference of the average path loss among the receiving positions in the same room.

This paper proposes a media synchronization mechanism for live media streams. The mechanism can also handle stored media streams by changing parameter values. The authors have implemented the mechanism on a lip-synch experimental system. Live video and voice streams input at a source workstation are transferred, and then they are synchronized and output at a destination workstation. This paper also evaluates the system performance such as mean square error of synchronization, average output rate, and average delay.

In this paper, we propose a new conference key distribution scheme and the supervision of a conference when users are in a level-based hierarchy. In a conference key distribution system, one message is transmitted to the participants from a chairman, a legitimate member can decrypt it and reveal the common session key. The proposed scheme can be implemented without using any tamper-proof hardware. For users in a level-based hierarchy, by applying the key distribution scheme, the higher priority users can derive the conference key and supervise the lower level users' communications. Further, the users in the same level who are not members of the conference or in lower levels can not expose the conference key. To break the common session key, a malicious user has to suffer from the difficulty of factorization and discrete logarithm problems.

The development of saturation diving in civil and defense applications has enabled man to work in the sea at great depths and for long periods of time. This advance has resulted, in part, as a consequence of the substitution of helium for nitrogen in breathing gas mixtures. However, utilization of HeO2 breathing mixture at high ambient pressures has caused problems in speech communication; in turn, helium speech enhancement systems have been developed to improve diver communication. These speech unscramblers attempt to process variously the grossly unintelligible speech resulting from the effect of breathing mixtures and ambient pressure, and to reconstruct such signals in order to provide adequate voice communication. It is known that the glottal excitation is quasi-periodic and the vocal tract filter is quasi-stationary. Hence, it is possible to use an auto regressive modelisation to restore speech intelligibility in hyperbaric conditions. Corrections are made on the vocal tract transfer function, either in the frequency domain, or directly on the autocorrelation function. A spectral subtraction or noise reduction may be added to improve speech quality. A new VAD enhanced helium speech unscrambler is proposed for use in adverse conditions or in speech recognition. This system, implementable on single chip DSP of current technology, is capable to work in real time.

Distributed web caching reduces retrieval latency of World Wide Web (WWW) objects such as text and graphics. Conventional distributed web caching methods, however, require many query messages among cache servers, which limits their scalability and reliability. To overcome these problems, we propose a query caching method in which each cache server caches not only WWW objects but also a query history. This method of finding cached objects can reduce the number of query messages among cache servers, making it possible to construct a large-scale distributed web cache server. We also propose an algorithm for constructing efficient query relationships among cache servers.

We discuss the concept of coding over the ring of integers modulo m. This method of coding finds its origin in the early work by Varshamov and Tenengolz. We first give a definition of the codes followed by some general properties. We derive specific code constructions and show computer-search results. We conclude with applications in 8-phase modulation and peak-shift correction in magnetic recording systems.