In high-speed TDMA mobile communications, frequency-selective fading is a serious problem because a delay time difference between multipath signals is large in comparison with symbol duration. We have proposed a spatial-domain RAKE receiver using a multibeam adaptive antenna to reduce frequency-selective fading and to realize path-diversity. The multibeam adaptive antenna resolves multipath signals in the spatial domain, and combines array outputs. In this paper, we propose the application of MUSIC algorithm to estimation of the time delays of multipath signals to make the incident signals coincide with a common reference signal. Because the MUSIC algorithm can estimate the time delays accurately, the BER performance of the proposed scheme is improved. Furthermore, we propose weighting factors which easily realize the maximal-ratio combining.

Developments in new frequency bands for wireless communications make a broadband channel for new services possible. Great effort has been made researching and developing broadband wireless communication in the 60-GHz millimeter-wave band since the early 1990s. In this paper, we design an ATM (asynchronous transfer mode)-based indoor millimeter-wave wireless local area network (WLAN) that supports multimedia transmissions and focus on the wireless access topic for implementation of wireless ATM. We propose an integrated multimedia transmission protocol, based on the MAC (medium access control) protocol, called RS-ISMA (reservation-based slotted idle signal multiple access). It supports CBR (constant bit rate), VBR (variable bit rate), ABR (available bit rate) and UBR (unspecified bit rate) transmissions and provides QoS (quality of service)-dependent adaptive retransmissions. An RS-ISMA-based prototype full-duplex indoor high-speed WLAN in the 60-GHz band was developed.

Recent development of compact and powerful portable computers and mobile phones and proliferation of the Internet will enable mobile multimedia communications. From the viewpoint of implementing multimedia services into mobile communications, it allows us to predict that traffic characteristics of mobile networks change. For planning, designing, and operating mobile multimedia networks, it is important to investigate traffic models which take the effect of multimedia services into consideration. This paper investigates population of active users in a micro-cell and proposes a traffic model for mobile multimedia networks. This model describes a population process of active users in a micro-cell in diffusion model, and its characteristics include self-similarity and activity of mobility. We also made an evaluation of network performance by using simulation, in order to show that characteristics of the proposed traffic model have impact on planning and designing networks.

A blind technique for adaptive signal suppression in multipath DS-CDMA communication channels for the downlink is considered. Its performance is degraded when mismatch problem occurs when multipath components arrive with fractional-chip delays. In order to surmount this problem, Multiple Finger Expansion Optimal Filter (MFE-OF) was recently proposed to estimate the received desired signal subspace using fractionally delayed despreading fingers. However, MFE-OF requires much computational complexity for good performance. In this paper, a modification to the MFE-OF is introduced by utilizing decision-directed steering vector to reduce the number of fingers required by MFE-OF down to that of the conventional OMF-RAKE without much performance degradation. This modified receiver is called Decision-Directed Optimal Filter (DDOF). Computer simulation validates the effectiveness of the new receiver to increase the downlink capacity of DS-CDMA systems.

We compare the numerical results for electron direct tunneling currents for single gate oxides, ON- and ONO-structures. We demonstrate that stacked dielectrics can keep the tunneling currents a few orders of magnitude lower than electrostatically equivalent single oxides. We also discuss the impact of gate material and of the modeling of electron transport in silicon.

In this paper the novel method of "weighted OFDM" is addressed. Different types of weighting factors (including Rectangular, Bartlett, Gaussian, Raised cosine, Half-sin and Shanon) are considered. The impact of weighting of OFDM on the peak-to-average power ratio (PAPR) is investigated by means of simulation and is compared for the above mentioned weighting factors. Results show that by weighting of the OFDM signal the PAPR reduces. Bit error performance of weighted multicarrier transmission over a multipath channel is also investigated. Results indicate that there is a trade off between PAPR reduction and bit error performance degradation by weighting.

In this paper, we propose a projection based design of near perfect reconstruction QMF banks. An advantage of this method is that additional design specifications are easily implemented by defining new convex sets. To apply convex projection technique, the main difficulty is how to approximate the design specifications by some closed convex sets. In this paper, introducing a notion of Magnitude Product Space where a pair of magnitude responses of analysis filters is expressed as a point, we approximate design requirements of QMF banks by multiple closed convex sets in this space. The proposed method iteratively applies a convex projection technique, Hybrid Steepest Descent Method, to find a point corresponding to the optimal analysis filters at each stage, where the closed convex sets are dynamically improved. Design examples show that the proposed design method leads to significant improvement over conventional design methods.

The Infostation concept has been proposed to provide convenient and cost effective access to high-speed mobile data services. An efficient IP packet transmission protocol is required to compensate for the high error rate inherent to fading radio channels. In this paper, a novel link layer retransmission scheme is proposed. Using the channel state and fading state estimators, the scheme adjusts the retransmission parameters dynamically in order to achieve the optimum performance under time-varying channel conditions. A theoretical analysis is presented for the case of a random error channel. Furthermore, a simulation tool is developed for evaluating the performance of the scheme in a fading channel with various parameters. The analysis and simulation results show that this new retransmission scheme can provide substantial improvement over traditional schemes. It gives a robust performance in both slow and fast fading conditions. In addition, the algorithm's sensitivity to parameter values and channel characteristics, such as Doppler frequency and fading statistics, is investigated. A unique attribute of this algorithm and performance analysis is that throughput is evaluated in IP packets rather than in physical layer packets.

When a mobile station with a call in progress moves across cell boundary in a cellular mobile communications system, the system must switch the circuit to the base station in the destination cell to enable uninterrupted communications in a process called "handoff. " However, if a circuit to the destination base station cannot be secured when a handoff is attempted, the call is forcibly terminated. Studies have therefore been performed on methods of decreasing the percentage of forcibly terminated calls by giving handoff calls priority. With the aim of simplifying system design, we propose a system for automatically setting the number of circuits reserved for handoff based on the handoff block rate. In this paper, we describe this system and evaluate static traffic characteristics taking into account reattempt calls, the occurrence of which can have a major effect on system performance. We also consider the effects of the proposed system on service quality since giving priority to handoff calls and decreasing the rate of forced terminations results in a tradeoff with the blocking rate of new call attempts. Finally, we evaluate the traffic characteristics associated with the number of control requests, an important element in estimating the processing capacity required by control equipment at the time of system design.

This paper describes the formula for dynamic power dissipation of a track/hold circuit as a function of the input frequency, the input amplitude, the sampling frequency, the track/hold duty cycle, the power supply voltage and the hold capacitance for a sinusoidal input.

This paper analyses the performance of a joint receiving structure for DS-CDMA communications systems. To reduce undesirable performance degradation due to the multiple access interferences and the near-far problem in multipath fading channel environment, this paper exploits the receiving structure for the multiuser communication composed of a beamformer-RAKE receiver and a decorrelating multiuser detector. The proposed DS-CDMA receiving structure mitigates the performance impairment invoked from the noise enhancement and reveals less computational complexity by utilizing the multipath temporal combiner prior to accessing the decorrelating detection. Also an efficient block Toeplitz inversion technique using the matrix Levinson polynomials is introduced to further diminish the computational burden encountered from applying the decorrelating multiuser detection process as in usual. Simulation results are conducted to demonstrate the superior performance of the proposed multiuser detection structure in multipath fading CDMA channel.

A circuit-level electrothermal simulator, MICS (MItsubishi Circuit Simulator), is presented with parasitic bipolar transistor action and lattice heating taken into account. Diffusion capacitance in parasitic bipolar transistors is introduced to cover turn-on behavior under short rise-time current. Device temperatures are simulated from calculated electrical characteristics and the closed-form solution of the heat transfer equation. Simulation results show that this tool is valuable in evaluating electrostatic discharge (ESD) robustness in integrated circuits (ICs).

This paper presents the implementation of a 3 V low power multi-rate of 156, 622, and 1244 Mbps clock and data recovery circuit (CDR) for optical communications tranceiver using new parallel clock recovery architecture based on dual charge-pump PLL. Designed circuit recovers eight-phase clock signals which are one-eighth frequency of the input signal. While the typical system uses the method that compares the input data with recovered clock, the proposed circuit compares a 1/2-bit delayed input data with the serial data generated by the recovered eight-phase clock signals. The advantage of the circuit is that the implementation is easy, since each sub blocks have one-eighth frequency of the input data signal. Morevover, since the circuit works at one-eighth frequency of the input data, it dissipates less power than conventional CMOS recovery circuit. Simulation results show that this recovery circuit can work with power dissipation of less than 40 mW with a single 3 V supply. All the simulations are based on HYUNDAI 0.65 µm N-Well CMOS double-poly double-metal technology.

A generalized algorithm for designing an optimum VQ source codec in systems with channel coding is presented. Based on an AWGN channel model, the algorithm derives the distribution of the channel decoder soft-output and substitutes it in the expression for the system end-to-end distortion. The VQ encoder/decoder pair is then optimized by minimizing this end-to-end distortion. For a Gauss-Markov source, the proposed algorithm outperforms the conventional SOVQ source coding scheme by 5.0 dB in the decoded source SNR. Application of this algorithm for designing optimum low-bit-rate speech codec is given. A 4.0 kbps VQ based CELP codec is designed for performance evaluations, where all the CELP parameter encoder/decoder pairs are optimized by minimizing their end-to-end distortions, respectively. As a result, the speech distortion over the noisy channel is minimized. Subjective tests show that the proposed algorithm improves the decoded speech quality by 2.5 MOS relative to a regular SOVQ CELP speech coding system. The performances of the algorithm under channel mismatch conditions are also shown and discussed.

Appropriate meshes are crucial for accurate and efficient 3D process simulation. In this paper, we present a set of tools operating on surface and interface triangulations. These tools allow the improvement of the accuracy of interfaces, the reduction of the number of triangles, and the removal of obtuse not coplanarily compensated triangles. The first tool is used within integrated topography simulation environments based on different data structures, e.g. cell-based and segment-based. The latter two are particularly important for providing appropriate input to mesh generation for 3D process simulation.

This paper proposes a method for extracting subimages from a huge reference image by learning lifting wavelet filters. Lifting wavelet filters are biorthogonal wavelet filters containing free parameters developed by Sweldens. Our method is to learn such free parameters using some training subimages so as to vanish their high frequency components in the y- and x-directions. The learnt wavelet filters have the feature of training subimages. Applying such wavelet filters to the reference image, we can detect the locations where the high frequency components are almost the same as those of the target subimage.

A three-dimensional mesh generation method in which triangulation of the domain boundary is performed first is desirable since such a method would make it easier to achieve the requirements for the mesh around the boundary. We have developed a mesh generator for a 3D device simulator based on this approach. This mesh generator recursively subdivides a box that includes the whole domain into smaller boxes (cells), a method known as the octree technique. Although our mesh generator is similar to previously reported mesh generators in the sense that it utilizes recursive subdivision of elements, its major difference is that it constructs a triangular mesh upon boundaries of the domain first and this triangular mesh is not changed in the following processes. In order to generate a mesh suitable for the control volume method, a "forbidden region" is introduced and mesh points in the domain are allocated outside of this region. Since the triangular mesh is determined prior to tessellation of the domain, this method is suitable for handling layered mesh along the boundary, which is often necessary to estimate large flows parallel to the boundary precisely. A simple method to provide a layered mesh for a planar boundary is incorporated into the mesh generator. This mesh generator is integrated within our in-house three-dimensional device simulation system. The simulator's practicality is demonstrated through analysis of the reverse narrow channel effect for MOSFETs with LOCOS isolation structures. The effect of protection of the boundary by the layered mesh is also examined by calculating Id-Vg characteristics of a MOSFET with an oblique Si surface, and it is shown that protection of the whole surface of the channel region is necessary to estimate drain current correctly.

A surface extraction algorithm with NURBS has been developed for the mesh generation from the scattered data after a cell-based simulation. The triangulation of a surface is initiated with a step of describing the geometry along the polygonal boundary with multiple points. In this work, an NURBS surface can be generated with scattered data for each polygonal surface by employing a multilevel B-spline surface approximation. The NURBS mesh in accordance with our algorithm excellently represents the surface evolution of the topography on the wafer. A dynamically allocated topography model, so-called cell advancing model, is proposed to resolve an extensive memory requirement for the numerical simulation of a complicated structure on the wafer. A concave cylindrical DRAM cell capacitor was chosen to test the capability of our model. A set of capacitance present in the cell capacitor and interconnects was calculated with three-dimensional tetrahedral meshes generated from the NURBS surface on CRAY T3E supercomputer. A total of 5,475,600 (130 156 270) cells was employed for the simulation of semiconductor regions comprising four DRAM cell capacitors with a dimension of 1.3 µm 1.56 µm 2.7 µm . The size of the required memory is about 22 Mbytes and the simulation time is 64,082 seconds. The number of nodes for the FEM calculation was 70,078 with 395,064 tetrahedrons.

Silicon self-interstitial atom diffusion and implantation induced damage were studied by using molecular dynamics methods. The diffusion coefficient of interstitial silicon was calculated using molecular dynamics method based on the Stillinger-Weber potential. A comparison was made between the calculation method based on the Einstein relationship and the method based on a hopping analysis. For interstitial silicon diffusion, atomic site exchanges to the lattice atoms occur, and thus the total displacement-based calculation underestimates the ideal value of the diffusivity of the interstitial silicon. In addition with calculating the diffusion constant, we also identified its migration pathway and barrier energy in the case of Stillinger-Weber potential. Through a study of molecular dynamics calculation for the arsenic ion implantation process, it was found that the damage self-recovering process depends on the extent of damage. That is, damage caused by a single large impact easily disappears. In contrast, the damage leaves significant defects when two large impacts in succession cause an overlapped damage region.

We present calculations of the linear-response conductance of a SiGe based single-electron transistor (SET). The conductance and the discrete charging of the quantum dot are calculated by free-energy minimization. The free-energy calculation takes the discrete level-spectrum as well as complex many-body interactions into account. The tunneling rates for tunneling through the source and lead barrier are calculated using Bardeen's transfer Hamiltonian formalism. The tunneling matrix elements are calculated for transitions between the zero-dimensional states in the quantum dot and the lowest subband in the one-dimensional constriction. We compare the results for the conductance peaks with those from calculations with a constant tunneling rate where the shape of the peaks is only due to energetic arguments.