A maximal l-diameter tree cover of a graph G(V,E) is a spanning subgraph C(V,EC) of G such that each connected component of C is a tree, C contains no path with more than l edges, and adding any edge in EEC to C yields either a path of length l1 or a cycle. For every function f from positive integers to positive integers, the maximal f-diameter tree cover prolem (MDTC(f) problem for short) is to find a maximal f(n)-diameter tree cover of G, given an n-node graph G. In this paper, we give two parallel algorithms for the MDTC(f) problem. The first algorithm can be implemented in time O(TMSP(n,f(n))log2n) using polynomial number of processors on an EREW PRAM, where TMSP(n,f(n) is the time needed to find a maximal set of vertex disjoint paths of length f(n) in a given n-node graph using polynomial number of processors on an EREW PRAM. We then show that if suitable restrictions are imposed on the input graph and/or on the magnitude of f, then TMSP(n,f(n))O(logkn) for some constant k and thus, for such cases, we obtain an NC algorithm for the MDTC(f) problem. The second algorithm runs in time O(n log2n/{f(n)1}) using polynomial number of processors on an EREW PRAM. Thus if f(n)Ω(n/logkn) for some kO, we obtain an NC algorithm for the MDTC(f) problem.

By adding some functions to memories, highly parallel computation may be realized. We have proposed memory-based parallel computation models, which uses a new functional memory as a SIMD type parallel computation engine. In this paper, we consider models with communication between the words of the functional memory. The memory-based parallel computation model consists of a random access machine and a functional memory. On the functional memory, it is possible to access multiple words in parallel according to the partial match with their memory addresses. The cube-FRAM model, which we propose in this paper, has a hypercube network on the functional memory. We prove that PSPACE is accelerated to polynomial time on the model. We think that the operations on each word of the functional memory are, in a sense, the essential ones for SIMD type parallel computation to realize the computational power.

Circuit complexity of a Boolean function is defined to be the minimum number of gates in circuits computing the function. In general, the circuit complexity is established by deriving two types of bounds on the complexity. On one hand, an upper bound is derived by showing a circuit, of the size given by the bound, to compute a function. On the other hand, a lower bound is established by proving that a function can not be computed by any circuit of the size. There has been much success in obtaining good upper bounds, while in spite of much efforts few progress has been made toward establishing strong lower bounds. In this paper, after surveying general results concerning circuit complexity for Boolean functions, we explain recent results about lower bounds, focusing on the method of approximation.

The semantics of a language for communicating processes is investigated, and three full abstractness results for are established. The language contains atomic actions, termination, inaction, sequential composition, alternative composition, parallel composition, action restriction, and a form of guarded recursion. (The guardedness restriction on recursion is needed to establish one of the full abstractness results.) Three Plotkin-style operational semantics WL, CWL, and IWL of the language are defined. These semantics are linear in that the meaning of each program in any of these semantics is a set of action sequences the program may perform, and are weak in that the action sequences are obtained by ignoring (finte sequences of) internal moves. All the three semantics distinguish divergence (an infinite sequence of internal moves) from deadlock. The semantics IWL differs from the other two in that IWL is a so-called interal action semantics taking into account only internal moves under the assumption that the environment allows no (external) communication actions, and hence, the only possible actions for processes are internal moves, whereas the other two semantics take into account communication actions in addition to internal moves. The two semantics WL and CWL differ each other in that CWL is a so- called completed trace semantics, whereas WL is not. Then, two compositional models RF and IRF for the language are proposed, and the full a bstractness of RF (resp. of IRF) w. r. t. WL and CWL (resp. w. r. t. IWL), as expressed in the following, is established:s1s2(*) S[][S[]is a context of ⇒S[s1]S[s2]],where ,RF,WL, RF,CWL, IRF,IWL. A similar full abstractness result has been established by Bergstra, Klop, and Olderog for a language without recursion or internal moves. Moreover, Rutten investigated the semantics of a language similar to , in the framework of complete metric spaces, and showed that the failures model is fully abstract w. r. t. a strong linear semantics L, where L is strong in that it does not abstract from internal moves. The full abstractness of RF w. r. t. CWL, expressed in (*) with ,RF,CWL, is an extension of the result of Bergstra et al. to a language with recursion and internal moves. Also, the full abstractness of IRF w. r. t. IWL, expressed in (*) with ,IRF,IWL, is an extension of the Rutten's result, to the case of weak linear semantics with divergence.

Multiple context-free grammars (mcfg's) are a subclass of generalized context-free grammars introduced by Pollard in order to describe the syntax of natural languages. First, this paper shows that the universal recognition problem for mcfg's is EXP-POLY time-complete, where the universal recognition problem is the one to decide whether G generates w for a given grammar G and string w. Next, it is shown that the problem for linear context-free rewriting systems introduced by Vijay-Shanker et al., which is a proper subclass of mcfg's, is PSPACE-complete.

It is known that the problem of finding a largest common subgraph is NP-hard for general graphs even if the number of input graphs is two. It is also known that the problem can be solved in polynomial time if the input is restricted to two trees. In this paper, a randomized parallel (an RNC) algorithm for finding a largest common subtree of two trees is presented. The dynamic tree contraction technique and the RNC minimum weight perfect matching algorithm are used to obtain the RNC algorithm. Moreover, an efficient NC algorithm is presented in the case where input trees are of bounded vertex degree. It works in O(log(n1)log(n2)) time using O(n1n2) processors on a CREW PRAM, where n1 and n2 denote the numbers of vertices of input trees. It is also proved that the problem is NP-hard if the number of input trees is more than two. The three dimensional matching problem, a well known NP-complete problem, is reduced to the problem of finding a largest common subtree of three trees.

There have been several studies related to a reduction of the amount of computational resources used by Turing machines. As consequences, Linear speed-up theorem", tape compression theorem" and reversal reduction theorem" have been obtained. In this paper, we discuss a leaf reduction theorem on alternating Turing machines. Recently, the result that one can reduce the number of leaves by a constant factor without increasing the space complexity was shown for space- and leaf-bounded alternating Turing machines. We show that for time- and leaf-bounded alternating Turing machines, the number of leaves can be reduced by a constant factor without increasing time used by the machine. Therefore, our result says that a constant factor on the leaf complexity does not affect the power of time- and leaf-bounded alternating Turing machines.

The nonlinear optical properties of organics with unsaturated bonds were compared with those of inorganics including semiconductors and dielectrics. Because of the mesomeric effect, namely quantum mechanical resonance effect among configurations, aromatic molecules and polymers have larger optical nonlinear parameters defined as δ(n)X(n)/(X(1))n both for the second (n2) and third-order (n3) nonlinearities. Experimental results of ultrafast nonlinear response of conjugated polymers, especially polydiacetylenes, were described and a model is proposed to explain the relaxation processes of photoexcitations in the conjugated polymers. Applying the model constructed on the basis of the extensive experimental study, we propose model polymers to obtain ultrafast resonant optical nonlinearity.

We study the depth of planar Boolean circuits. We show that planar Boolean circuits of depth D(n) are simulated by on-line Turing machines in space O(D(n)). From this relationship, it is shown that any planar circuit for computing integer multiplication requires linear depth. It is also shown that a planar analogue to the NC-hierarchy is properly separated.

In this letter we study the wave propagation in a diode-equipped microstrip line and show that soliton generation at microwave frequencies is possible with monolithic fabrications of such nonlinear transmission lines. We propose that this phenomenon be utilized as a new method of obtaining ultrashort electrical pulses with picosecond durations. The perdicted soliton generation has been confirmed by computer simulations based on the harmonic balance method.

In this paper, we present a new image compression scheme, Projection-VQ, based on reconstruction from vector quantized projections. We can easily deal with the blocks of larger size in Projection-VQ than in conventional VQ schemes, because the dimension of vectors in projection domain is, in general, much smaller than that in the spatial domain. In Projection-VQ, the image can be reconstructed without destroying edge sharpness in the process since the projection data having the edge information are preferentially transmitted. There are several good algorithms of reconstructing an image from projections. However, we use a new modified reconstruction algorithm suitable for a variable bit rate image coding system. We allocate the bits depending on the characteristics of the block images. Our simulation results show that the performances are superior to the ordinary VQ schemes in PSNR, and that the improvement in subjective image quality is substantial.

In this paper we review the recent progress and basic technology of vertical cavity surface emitting lasers together with related parallel surface operating optical devices. First, the concept of surface emitting lasers is presented, and then currently developed device technologies will be reviewed. We will feature several technical issues, such as multi-layer structures, 2-dimensional arrays, photonic integration, etc. Lastly, future prospects for parallel lightwave systems will be discussed.

We propose Polarization-Shift-Keying (POLSK) homodyne system using phase-diversity receivers and theoretically analyze its bit-error-rate (BER) performance. Since the proposed system uses polarization modulation and homodyne detection, it can cancel the phase noise and is attractive at a high bit-rate transmission. It is found that the receiver sensitivity of the proposed POLSK homodyne system is the same as that of POLSK heterodyne system and is much better than that of DPSK phase-diversity homodyne systems at high signal-to-noise ratio (SNR). We also cosider theoretically the effect of the fluctuation of state of polarization (SOP) on the BER performance of POLSK homodyne system.

This article briefly looks at the future of telecommunication education in the universities as it evolves from present concerns and trends. Five year bachelor's programs and top-down curricular design will be common. Textbooks supplemented by advance organizers, instruction and testing according to individual learning styles and global integration of education using multi-media services and broadband technology will be some of the other features. Finally, the importance of industry-university partnership in all aspects of engineering education is emphasized.

This paper proposes a knowledge-based design method of a protocol of a communication network system based on the knowledge-based design methodology for computer communication systems. In the proposed method, two knowledge models, i.e., the communication network architecture model (CNAM) and the communication protocol architecture model (CPAM), are introduced and a protocol design task is modeled as a successive transformation process of these knowledge models. Giving CNAM which represents the users' requirements concerning a communication network system, the requirements specification of a protocol is derived from CNAM and represented as CPAM. Then, the detailed requirements specification of a protocol is also derived from CPAM and represented by the formal description technique (FDT-Expressions). The derivations of CPAM and FDT-Expressions are executed by the transformation rules which represent the mappings between knowledge models. Due to formally defined knowledge models and mappings, the proposed method provides a framework of a systematic support of knowledge-based protocol design. In this paper, the formal definitions of CNAM and CPAM are given, then the derivation process of FDT-Expressions of a protocol is also formalized based on these knowledge models. Furthermore, a design example is demonstrated by using LOTOS as one of the FDT-Expressions of a protocol.

Transmittance distribution along a horizontal line in LCDs addressed by amorphous silicon TFTs was investigated using measurements and calculations. Nonuniformity of the distribution, in which the transmittance increased with increasing distance from the left edge of the LCD, was observed in a 10 inch diagonal TFT-LCD. The cause of the nonuniformity was attributed to the decrease in voltage drop due to the gate source parasitic capacitance and the increase in gate voltage fall time due to large line resistance, based on the measurements of voltage drops in TFT test elements and calculations considering the decrease in voltage drop. The distribution could be improved by reducing the line resistance and parasitic capacitance in the actual LCD.

This paper describes the future perspective of automatic telephone interpretation using a multimedia intelligent communication network. The need for language interpretation over a telecommunication system creates a strong drive toward integrating information modalities for voice, image, data, computation and conferencing into modern systems using the capability of language interpretation. An automatic telephone interpretation system will solve the problems of language differences in international human-to-human communication. The future prospective of advanced multimedia language communication will be stated as the versatile application of an integrated intelligent network.

We propose Polarization-Shift-Keying (POLSK) homodyne system using phase-diversity receivers and theoretically analyze its bit-error-rate (BER) performance. Since the proposed system uses polarization modulation and homodyne detection, it can cancel the phase noise and is attractive at a high bit-rate transmission. It is found that the receiver sensitivity of the proposed POLSK homodyne system is the same as that of POLSK heterodyne system and is much better than that of DPSK phase-diversity homodyne systems at high signal-to-noise ratio (SNR). We also cosider theoreically the effect of the fluctuation of state of polarization (SOP) on the BER performance of POLSK homodyne system.

This paper describes a new configuration and control method for an uninterruptible power supply (UPS) with a bidirectional cycloconverter. When commercial AC power is operating normally, the load is supplied by commercial AC power and the bidirectional cycloconverter operates as a battery charger. During interruptions of commercial AC power, the bidirectional cycloconverter operates as an inverter and supplies AC power to the load. Unlike a conventional UPS, this new configuration does not require a battery charger, so it can be small, light-weight, cost-effective, and highly efficient. The output voltage characteristics and the transient voltage drop in the output when commercial AC power fails are also discussed by numerical analysis and experiments.

This paper proposes a new construction of the minimum knowledge undeniable signature scheme which solves a problem inherent in Chaum's scheme. We formulate a new proof system, the minimum knowledge interactive bi-proof system, and a pair of languages, the common witness problem, based on the random self-reducible problem. We show that any common witness problem has the minimum knowledge interactive bi-proof system. A practical construction for undeniable signature schemes is proposed based on such a proof system. These schemes provide signature confirmation and disavowal with the same protocol (or at the same time).