One of unique features of CDMA slotted ALOHA (CDMA S-ALOHA) is that user must synchronize his transmission to given slot. Thus orthogonal sequence as spreading sequence would achieve ideal throughput if each of packets accomplish perfect synchronization. In the presence of any ambiguity in synchronizations, however, quasi-synchronous (QS) sequences suit well with CDMA S-ALOHA system. In this paper, we introduce new QS-sequences obtained from the orthogonal Gold sequences and discuss their performance when applying to CDMA S-ALOHA systems. As a result, withstanding to access timing error, good performance is ensured with this sequence under the environment of AWGN, MAI (multiple access interference) and frequency non-selective fading, that is, micro or pico cellular systems and indoor wireless LANs.

We consider a finite-capacity single-server queue with constant service and vacation times, which is seen in the time division multiple access (TDMA) scheme. First we derive the probability that j customers remain in the queue when a test customer arrives. Using this probability we then evaluate the probability that the test customer who arrives during the vacation or service time has to wait in the queue for longer than a given time. From these results, we obtain the waiting time distribution for the customer arriving at an arbitrary time. We also show a practical application to wireless TDMA communications systems.

Backfire quadrifilar helical antennas combined with parasitic loops are investigated in detail, focusing on clarifying the function of parasitic loops. First, the basic property is examined for the case of one parasitic loop, and it is found that the loop behaves as a director when the circumferential length of the loop is nearly 0. 9λ, and a reflector when the circumferential length of the loop is nearly 1. 2λ provided the distance between the parasitic loop and the top plane of helical antennas is approximately 0. 1λ, where λ is the wavelength. Next, the function of the parasitic loop is investigated by comparing the current distributions on the helices and the loop with those on a monofilar helix with a ground plane. It is found that the function of the parasitic loop is quite different from that of the ground plane. Then, the case of two parasitic loops is examined, and it is shown that the use of two parasitic loops is very effective and simple measures to control the radiation pattern and gain of the backfire quadrifilar helical antennas. Finally, for this type of antennas with two parasitic loops, an example of structural parameters suited to the use in satellite communications is presented.

Space Division Multiple Access (SDMA) will form an important part of the new Wideband Code Division Multiple Access (WCDMA) standard that will realize the Universal Mobile Telephone System (UMTS). This paper addresses a few issues of importance when SDMA techniques are used in a cellular CDMA system. Firstly, a brief overview of SDMA techniques are presented followed by a theoretical analysis of a SDMA/CDMA system. The analysis is focused on a single cell, multipath Rayleigh fading scenario with imperfect power control. As system performance measure Bit Error Rate (BER) is used to investigate the influence of user location, number of antennas and power control error. An important parameter in a SDMA system is the antenna array element spacing. In our analysis a Uniform Linear Array (ULA) is considered and a measure is defined to determine the optimal antenna element spacing in a CDMA cellular environment. Normally the mobile users in a cell are assumed to be uniformly distributed in cellular performance calculations. To reflect a more realistic situation, we propose a novel probability density function for the non-uniform distribution of the mobile users in the cell. It is shown that multipath and imperfect power control, even with antenna arrays, reduces the system performance substantially.

Loosely wound short-arm two-wire Archimedean spiral antennas are investigated. It is shown that good circularly polarized waves with axial ratio less than 2 dB are obtained when the outer circumference C of the spiral antenna is in the range of about 1. 3λ < C < 1. 5λ, where λ is the free-space wavelength. To improve the antenna characteristics further, spiral antennas combined with a parasitic loop are examined. It is clarified that the parasitic loop greatly contributes to the improvement of the axial ratio and power gain.

We apply evolutionary algorithms to neural network model of associative memory. In the model, some of the appropriate configurations of the synaptic weights allow the network to store a number of patterns as an associative memory. For example, the so-called Hebbian rule prescribes one such configuration. However, if the number of patterns to be stored exceeds a critical amount (over-loaded), the ability to store patterns collapses more or less. Or, synaptic weights chosen at random do not have such an ability. In this paper, we describe a genetic algorithm which successfully evolves both the random synapses and over-loaded Hebbian synapses to function as associative memory by adaptively pruning some of the synaptic connections. Although many authors have shown that the model is robust against pruning a fraction of synaptic connections, improvement of performance by pruning has not been explored, as far as we know.

A partial buffer sharing scheme is proposed as loss-priority control for a finite buffer with batch inputs. A partial batch acceptance strategy is used for a batch arriving at a finite buffer. Customer loss probabilities for high- and low-priority customers are derived under this batch acceptance strategy, using a supplementary variable method that is a standard tool for queueing analysis. A comparison of the partial buffer sharing scheme and a system without loss-priority control is made in terms of admissible offered load.

One of the objectives of ABR traffic control is a fair allocation of transmission rate to every connection. Algorithms which have been proposed can achieve the fair allocation of transmission rate in persistent traffic, even though the connections have different number of hops. In bursty traffic, however, the transmission rate during the burst period, called average burst throughput, may become degraded when the number of hops increases. In this paper, we evaluate the average burst throughput characteristics of ERICA switch algorithm and investigate the effect of the number of hops. We propose a new bandwidth allocation method which can improve the performance degradation by taking the number of hops into account. The effectiveness of our method is evaluated by computer simulation.

Array redistribution is required very often in programs on distributed memory parallel computers. It is essential to use efficient algorithms for redistribution, otherwise the performance of programs may degrade considerably. In this paper, we focus on automatic generation of communication routines for multi-dimensional redistribution. The principal advantage of this work is to gain the ability to handle redistribution between arbitrary source and destination processor sets and between arbitrary source and destination distribution schemes. We have implemented these algorithms using Parallelware communication library. Some experimental results show the efficiency and flexibility of our techniques compared to the other redistribution works.

In this paper, a new method is proposed for solving forbidden state problems in non-ordinary controlled Petri nets (NCPNs) with uncontrollable transitions. Using a precedence subnet and a boundary subnet with decision-free properties, the behavior of markings are analyzed structurally. An efficient algorithm is presented for calculating the number of total tokens in forbidden places reachable from a marking. This paper derives necessary and sufficient conditions for identifying admissible markings and boundary markings in terms of the precedence subnet and the boundary subnet.

CEFLAR is one way of realizing ATM-ABR with no cell loss. This paper shows that the transition characteristics of CEFLAR(transition time to achieve fair share), important when addressing network fairness, strongly depend on the acceleration-ratio coefficient, not the rate decrease factor or the distance between source and congestion estimation nodes. This paper also shows that the average throughput of a transmission line in transition degrades as the rate decrease factor decreases and as the distance between the source and congestion estimation nodes increases.

In a shared-memory multiprocessor, shared data are usually accessed in a critical section that is protected by a lock variable. Therefore, the order of accesses by multiple processors to the shared data corresponds to the order of acquiring the ownership of the lock variable. This paper presents a selective write-update protocol, where data modified in a critical section are stored in a write cache and, at a synchronization point, they are transferred only to the processor that will execute the critical section following the current processor. By using QOLB synchronization primitives, the next processor can be determined at the execution time. We prove that the selective write-update protocol ensures data coherency of parallel programs that comply with release consistency, and evaluate the performance of the protocol by analytical modeling and program-driven simulation. The simulation results show that our protocol can reduce the number of coherence misses in a critical section while avoiding the multicast of write-update requests on an interconnection network. In addition, we observe that synchronization latency can be decreased by reducing both the execution time of a critical section and the number of write-update requests. From the simulation results, it is shown that our protocol provides better performance than a write-invalidate protocol and a write-update protocol as the number of processors increases.

Parallel multiple context-free grammar (PMCFG) and multiple context-free grammar (MCFG) were introduced to denote the syntax of natural languages. By the known fastest algorithm, the recognition problem for multiple context-free language (MCFL) and parallel multiple context-free language (PMCFL) can be solved in O(ne) time and O(ne+1) time, respectively, where e is a constant which depends only on a given MCFG or PMCFG. In this paper, we propose the following two algorithms. (1) An algorithm which reduces the recognition problem for MCFL to the boolean matrices multiplication problem. (2) An algorithm which reduces the recognition problem for PMCFL to the recognition problem for MCFL. The time complexity of these algorithms is O(ne-3i+1 M(ni)) where e and i are constants which depend only on a given MCFG or PMCFG, and M(k) is the time needed for multiplying two k k boolean matrices. The proposed algorithms are faster than the known fastest algorithms unless e=e, i=1 for MCFG, and e=e, i=0 for PMCFG.

A convenient signaling scheme, termed orthogonal on-off BPSK (O3BPSK), along with a simple one-shot linear decorrelating detector (LDD) has been proposed by Zheng and Barton as a technique for near-far resistant detection in the asynchronous DS/CDMA systems. The temporally adjacent bits from different users in the received signals are decoupled by using the on-off signaling, and the data rate is maintained with no increase in transmission rate by adopting an orthogonal structure. The system performance of this signaling scheme in terms of bit error rate (BER) has been analyzed over an AWGN channel by Zheng and Barton. In this paper, we further study the system performance over Nakagami-m fading channel. A closed form for the BER of such a scheme is successfully derived. Numerical results show that the O3BPSK signal scheme along with the LDD receiver still offers a good near-far resistant property over Nakagami-m channel.

Differences in the behavior of dispenser cathodes and oxide cathodes in laminar-flow type and crossover type electron guns were investigated by experiments and simulations under high-current-density conditions. When an oxide cathode is operated under such conditions, the heating effect due to Joule heat in the oxide layer exceeds the cooling effect, depending on the product of the work function and the cathode current, resulting in a rise in the cathode temperature. This rise in cathode temperature aggravates deterioration of emission characteristics during the life of an oxide cathode. In the case of the dispenser cathode, however, the cathode temperature decreases under high-current-density conditions. When an oxide cathode in a crossover type electron gun is operated, equipotential surfaces are formed in the curved surface in the oxide layer. The formation of an equipotential surface leads to relaxation of the loading. It is considered that this is the reason for the longer life of an oxide cathode in a crossover type electron gun than that of an oxide cathode in a laminar-flow type electron gun.

A unified theory for the characteristics of dual modes in a circular resonator is elucidated in simple analytical expressions. First, a circular resonator is considered as a ring transmission line which allows two oppositely traveling waves. The essential quantities that characterize a transmission line, i. e. , the propagation constant and characteristic impedance are obtained theoretically and/or experimentally. Secondly, any circular resonator is described by a ring resonator model which can be treated analytically, and the resonant frequencies are obtained when perturbations are added along the periphery of a circular resonator. A two stage BPF is created by adding I/O ports to the perturbed circular resonator. Its center frequency and bandwidth is calculated based on the ring resonator model. The circuit condition for obtaining two attenuation poles at both sides of the passband is given together with the method for their control.

We have proposed a highly scattering optical transmission (HSOT) polymer for use as a high efficiency light source medium. This polymer contains specified internal microscopic heterogeneous structures for controlling light-transmission properties. An LCD backlighting system having a new light pipe made of this polymer has twice the brightness of the conventional one. A light scattering phenomenon inside the HSOT polymer was quantitatively analyzed by a ray tracing simulation based on the Mie scattering theory and the Monte Carlo method. The illumination of the backlight which is optimized by using the simulation program has enough uniformity of intensity and color because of specified multiple light scattering phenomena inside the HSOT polymer. We propose the new backlighting system having fewer components and twice efficiency of the conventional one.

Ion beam irradiation effects on a novolac positive-tone photoresist and its application to micron-size field emitters have been investigated. Irradiation of Ar and P ions was examined. The electrical resistivity of the photoresist film is found to decrease after Ar ion implantation at doses on the order of 1016 cm-2. Baking of the photoresist prior to irradiation at a high temperature is preferred to produce electrical conductivity. P ions show weaker effects than Ar ions. Raman spectroscopy shows that carbon-carbon bonds such as the graphite bond are produced due to ion bombardment. The field emission of electrons is observed from emitters made of the ion-irradiated photoresist. The emission current is shown to be fairly stable when it is compared with an emission characteristic of synthesized diamond. Fabrication of field emitter arrays using a mold technique is demonstrated. The field emitter array shows emission at a current level of about 40 µA.

Cursor RAMs have been composed of two memory planes. A cursor pattern is stored in these planes with 2-bit data depth. While the pixel port requires data from both planes at the same time, the MPU port accesses either one of the planes at a time. Since the address space is defined differently between the ports, conventional cursor RAMs could not have dealt with these different access ways at real time. This paper proposes a dual port cursor RAM with a dynamic data alignment architecture. The architecture processes the different access ways at real time, and reduces a large amount of control circuitry. Conventional cursor RAMs have been organized with a single port memory because dual port memory cells have been large. We have applied the port swap architecture which has reduced the cell size. The control block is further simplified because the controller no longer emulate a dual port memory. The cursor RAM with these architectures is fabricated with a double metal 0. 5 µm CMOS process technology. The active area is 1. 51. 6 mm2 including a couple of shift registers and a control block. It operates up to 263 MHz at the supply voltage of 3. 3 V.

As the current Internet becomes popular in information access, demands for real-time display and playback of continuous media are ever increasing. The applications include real-time audio/video clips embedded in WWW, electronic commerce, and video-on-demand. In this paper, we present a new control protocol R3CP for real-time applications that transmit stored MPEG video stream over a lossy and best-effort based network environment like the Internet. Several control mechanisms are used: a) packet framing based on the meta data; b) adaptive queue-length based rate control scheme; c) data preloading; and d) look-ahead pre-retransmission for lost packet recovery. Different from many adaptive rate control schemes proposed in the past, the proposed flow control is to ensure continuous, periodic playback of video frames by keeping the receiver buffer queue length at a target value to minimize the probability that player finds an empty buffer. Contrary to the widespread belief that "Retransmission of lost packets is unnecessary for real-time applications," we show the effective use of combining look-ahead pre-retransmission control with proper data preloading and adaptive rate control scheme to improve the real-time playback performance. The performance of the proposed protocol is studied via simulation using actual video traces and actual delay traces collected from the Internet. The simulation results show that R3CP can significantly improve frame playback performance especially for transmission paths with poor packet delivery condition.