In a DC 50 V/5 A circuit, the relationship between the number of breaking arcs and the spatial distribution of the spectral intensity of breaking arcs of long duration near the cathode in palladium contact were examined through substitution of the contact surfaces of three different shapes: flat and spherical (1 mm radius and 2 mm radius). Findings show the distribution of spectral intensity in Pd arcs to be influenced remarkably by the shape of contact surface and the number of breaking arcs. However, the temperature of Pd arcs was affected neither by the shape of contact surface nor by the number of breaking arcs. The metal-vapor quantity present differed for flat and spherical surface contacts; however, it was not affected by the radius of the curved contact surfaces or by the number of breaking arcs. Additionally, the longer the duration of the breaking arc, the more metal-vapor was presented in the beginning of the arc. Furthermore, arc tracks on contact surfaces were observed with microscopes, clarifying that the relationship between the area of the clouded white metal on the cathode and the shape of contact surface is the same as the relationship between the existent area of measured spectra and the shape of the contact surface.

This paper proposes a frequency diversity transmission scheme that obtains a frequency diversity gain and does not degrade spectrum efficiency; it utilizes multiple carrier frequencies alternately, not simultaneously. This scheme improves the bit error rate (BER) of significant information bits by sacrificing that of insignificant bits in fading channels. Simulation results show that the error floor of significant information bits is reduced to less than 1/5 while that of insignificant bits is doubled. They also show that the proposed scheme improves the received 4-bit ADPCM voice signal-to-noise ratio (SNR) by approximately 4 dB even when the frequency correlation is 0. 5.

FIR digital filters composed of parallel multiple subfilters are proposed. A binary expression of an input signal is decomposed into multiple shorter words, which drive the subfilters having different length. The output error is evaluated by mean squared and maximum spectra. A fast algorithm is also proposed to determine optimal filter lengths and coefficients of subfilters. Many examples confirm that the proposed filters generate smaller output errors than conventional filters under the condition of specified number of multiplications and additions in filter operations. Further, multiplier and adder structures (MAS) to perform the operations of the proposed filters are also presented. The number of gates used in the proposed MAS and its critical path are estimated. The effectiveness of the proposed MAS is confirmed.

The development of a far-infrared imaging system based on ultrafast THz time-domain spectroscopy has opened a new field of applications of femtosecond technology. We describe the principle of this new imaging technique and report recent progress to augment the possibilities of "T-ray" imaging. These include sub-wavelength-resolution near-field imaging and three-dimensional tomographic reconstruction of a samples refractive index profile.

Requirements engineering refers to activities of gathering and organizing customer requirements and system specifications, making explicit representations of them, and making sure that they are valid and accounted for during the course of the design lifecycle of software. One very popular software development practice is the incremental development practice. The incremental development refers to practices that allow a program, or similarly specifications, to be developed, validated, and delivered in stages. The incremental practice is characterized by its depth-first process where focuses are given to small parts of the system in sequence to fair amounts of detail. In this paper, we present a development and validation of specifications in such an incremental style using a tool called ASADAL, a comprehensive CASE tool for real-time systems. ASADAL supports incremental and hierarchical refinements of specifications using multiple representational constructs and the evolving incomplete specifications can be formally tested with respect to critical real time properties or be simulated to determine whether the specifications capture the intended system behavior. In particular, we highlight features of ASADAL's specification simulator, called ASADAL/SIM, that plays a critical role in the incremental validation and helps users gain insights into the validity of evolving specifications. Such features include the multiple and mixed level simulation, real-value simulation, presentation and analysis of simulation data, and variety of flexible simulation control schemes. We illustrate the overall process using an example of an incremental specification development of an elevator control system.

The symbol basis side information generated by Viterbi's ratio threshold test technique is proposed to improve the performance of the asynchronous slow-frequency-hopped multiple access system with BFSK signaling in the frequency non-selective fading channel. By properly setting the ratio threshold to produce erasure decisions for the received symbols, the system performances are optimized. The relationship among the hit symbols in a hop duration is exploited by this symbol basis side information to greatly reduce the packet error probability. This packet error rate improvement can be as large as two order of magnitude, compared with perfect hop basis side information systems.

This paper proposes and investigates a vehicular radar system that can measure the distance to, the relative speed of and the direction of arrival (DOA) of the reflected waves from multiple targets or vehicles using the direct-sequence spread spectrum (DS-SS) technique. In particular, we propose a DOA estimation scheme using a multi-beam antenna. In order to show that the proposed system can accurately measure the above mentioned quantities, the performance is evaluated numerically in a multipath environment. Moreover, optimal multi-beam pattern is derived to minimize error probability of DOA estimation.

In this paper, a simple frame synchronization system for M-ary Spread Spectrum (M-ary/SS) communication system is analyzed. In particular, synchronization performance, bit error rate performance, and Spread Spectrum Multiple Access (SSMA) performance are analyzed. The frame synchronization system uses the racing counters. The transmitted signal contains framing chips that are added to spreading sequences. In the receiver, the framing chips are detected from several frames. The authors have proposed the simple frame synchronization system that detects framing chips from consecutive 2 frames. In this system, as the number of framing chips increases, synchronization performance improves and bit error rate performance degrades. In this paper a frame synchronization system that improves bit error rate performance is treated and analyzed. As the rusult, when the number of reference frames is 3, the bit error rate is much improved than the conventional system.

It is well known that M-ary/spread spectrum (M-ary/SS) system is superior to direct-sequence spread spectrum system under AWGN, and can achieve high spectral efficiency. On the other hand, however, the main drawback of this system is that the synchronization acquisition is difficult. In this paper, we propose a new synchronization acquisition method of M-ary/SS system. This method acquires the code synchronization by introducing a symmetrical property in spreading sequences, and detecting this property with the differential decoding technique. As spreading sequences, a set of orthogonal sequences and a set of non-orthogonal sequences are considered. The strong features of proposed systems are that the systems can acquire the code synchronization in carrier band and can reduce the complexity of calculation greatly. Among the comparison results of the systems with newly proposed orthogonal and some specific non-orthogonal spreading sequences, it is especially noted that the latter can reduce the mean acquisition time and calculation complexity much greater than the former.

In this paper, we study and analyze the overall acquisition performance of the combined acquisition-tracking synchronization loop for direct-sequence spread-spectrum (DS-SS) signals in the presence of Doppler shift. We consider both the change of effective search rate and the impact on the detection probability due to Doppler for the acquisition loop. We also determine the acquisition behavior of the digital delay lock loop (DDLL) in the presence of code Doppler. As a result, the influence of the DDLL's acquiring capability on the complete acquisition process is investigated and some numerical results are presented to demonstrate the acquisition performances of this combined loop which are quite different from the previous reports.

This letter proposes and investigates a method of estimating the direction of arrival (DOA) of wideband signals such as spread spectrum signals, in a multipath channel. The DOA estimation method can reduce the effect of signal distortion due to bandwidth of signals by creating a spatial spectrum wihch satisfies the sampling theory in the time domain. The DOA estimate calculated from this spatial spectrum is robust against signal distortion due to multipath. Computer simulations numerically evaluate the proposed method. In comparison with conventional MUSIC algorithm, the proposed method achieves superior performance in a multipath channel.

A slotted ALOHA direct sequence spread spectrum system with random signatures is considered. The system is applicable in cases where a large number of terminals transmit to a single hub station like in cellular digital radio, personal mobile systems and wireless LANs. It is shown that significant improvements in packet throughput capacity are obtained if the adaptive receiver structures are used. Systems for the comparison are the spread spectrum slotted ALOHA system and the conventional slotted ALOHA system.

We propose a spread-spectrum power line communication system considering the cyclic features of the noise in the lines. For this purpose, we model the noise as the sum of a time-invariant stationary process and two cyclostationary proceses, i.e., cyclic continuous noise and cyclic impulsive noise. The proposed system employs two different countermeasures to each of these two classes of cyclic noise. For the cyclic continuous noise, it uses multiple-processing-gain spread spectrum technique: the smaller processing gains are assigned for the periods with lower instantaneous noise power and the larger ones for the periods of higher noise power. Considering the cyclic impulsive noise, convolutional coding with interleaving is applied. In order to analyze the performance improvement due to the employment of multiple processing gains, we introduce a simple model of the continuous noise. The overall performance is evaluated by computer simulation with the actual noise wave-form measured in power lines.

The performance of a constrained (that is, minimal order) Yule-Walker (CYW) single tone frequency estimator is studied. A closed form expression for the asymptotic error variance is derived. It is shown that CYW does not satisfactorily utilize the informaiton in data, and estimators with improved performance are proposed. Simulation results which lend support to the theoretical findings are included.

In addition to additive thermal noise, a received direct-sequence spread spectrum (DS/SS) signal may suffer from intersymbol interference (ISI) and interference caused by cochannel narrowband users or other narrowband radio frequency interference (RFI). This paper presents a workable solution for removing narrowband interference (NBI) and reducing ISI or inter-chip interference (ICI) when the communication channel can be modeled as an FIR filter and the NBI comes from multiple CW tones, an AR-modeled Gaussian process, or a BPSK signal. Unlike earlier solutions, the proposed scheme is capable of performing the functions of NBI-rejection, ISI/ICI suppression and data detection (code despreading) simultaneously. It is easy to implement and, more importantly, it yields lower bit error rate (BER) and smaller mean squared error (MSE).

A wireless communications system with a transmission rate of 10 Mbit/s using Japanese ISM band (2471-2497 MHz) is presented. This system employs a novel spread spectrum multiple access method named "CFO-SS (Carrier Frequency Offset-Spread Spectrum)" method. In the CFO-SS system, a single PN code is commonly assigned to all the multiple carriers, and the frequency offset between the carriers is determined by the information symbol rate, which is small as compared with the spread bandwidth of the signal. Bit error rate performance of the proposed CFO-SS system under multipath environments is investigated by computer simulation, and the performance of the CFO-SS method is confirmed for wireless LAN systems using the 2.4 GHz ISM band.

In this paper we discuss the binary spreading sequences whose spectral distributions are DC free and spectral distribution's shapes can be easily controlled by a certain parameter denoted by δ. The newly developed sequences, referred to as modified antisymmetric M-sequences, are modified-versions of the conventional antisymmetric (AS)M-sequences. The proposed sequences are designed to increase the varieties of spectral distribution's shapes and improve the correlation properties when compared to those of the FM coded M-sequences which have already proposed by Tsuzuki et al. Some typical line coded M-sequences, i.e. the (differential) Manchester coded M-sequences and the FM coded M-sequences, and the conventional AS M-sequences are included in the set of proposed sequences. The improvement of the average BER (bit error rate) performance for asynchronous DS/SSMA (direct sequence/spread spectrum multiple access) systems using the proposed sequences in comparison to the system using the conventional AO/LSE (auto-optimal phase with least sidelobe energy) M-sequences is also shown.

Recently, the low earth orbit satellite communications has been attracting much attention. These communications have many strong features, however, the communication performances are influenced by carrier frequency offset (CFO) and, particularly, it is hard to acquire the synchronization. A large number of publications have so far been made on the synchronization acquisition of DS/SS systems under CFO and most of them make use of the maximum likelihood decision in finding the maximum values of Fourier transform outputs. However, the implementations of Fourier transforms usually require high cost and large space. In this paper, we propose a new simple acquisition scheme using half-symbol differential decoding technique for DS/SS systems under CFO. This scheme makes use of the addition and subtraction of baseband signals and their delayed versions, (omitting Fourier transforms), together with integrations by recursive integrators, and thus resulting in much simpler implementation. In general, it is shown that the proposed scheme can acquire the code synchronization under carrier frequency offset with much smaller computational complexities and the sacrifice of longer acquisition time.

Three-dimensional (3-D) instrumentation using an image sequence is a promising instrumentation method for intelligent systems in which accurate 3-D information is required. However, real-time instrumentation is difficult since much computation time and a large memory bandwidth are required. In this paper, a 3-D instrumentation VLSI processor with a concurrent memory-access scheme is proposed. To reduce the access time, frequently used data are stored in a cache register array and are concurrently transferred to processing elements using simple interconnections to the 8-nearest neighbor registers. Based on a row and column memory access pattern, we propose a diagonally interleaved frame memory by which pixel values of a row and column are stored across memory modules. Based on the concurrent memory-access scheme, a 40 GOPS vprocessor is designed and the delay time for the instrumentation is estimated to be 42 ms for a 256256 images.

A method for recovering the LPC spectrum from a microphone array input signal corrupted by less directional ambient noise is proposed. This method is based on the subspace method, in which directional signal and non-directional noise is classified in the subspace domain using eigenvalue analysis of the spatial correlation matrix. In this paper, the coherent subspace (CSS) method, a broadband extension of the subspace method, is employed. The advantage of this method is that is requires a much smaller number of averages in the time domain for estimating subspace, suitable feature for frame processing such as speech recognition. To enhance the performance of noise reduction, elimination of noise-dominant subspace using projection is further proposed, which is effective when the SNR is low and classification of noise and signals using eigenvalue analysis is difficult.