Wireless local area networks, in which every station can transmit via any one of the available operating channels, but only one channel at a time, are investigated. Two distributed random access protocols are proposed for these WLANs. The CSMA/CA protocol is similar to the IEEE 802.11 standard but with slight modifications for multiple operating channels. The fuzzy logic controlled protocol employs a simple fuzzy logic controller to tune the size of backoff window. Extensive simulations are provided to evaluate the channel utilization, fairness, and responsiveness of these two protocols. Furthermore, the effects of employing RTS/CTS mechanism with both protocols are considered. Finally, performances of these two protocols are also investigated under conditions of burst traffic and noisy channels. Simulation results show that the fuzzy logic controlled protocol is a great improvement of the CSMA/CA protocol.

This paper proposes cell selection (CS) based on shadowing variation for the forward-link Orthogonal Frequency and Code Division Multiplexing (OFCDM) packet wireless access. We clarify its effects using a broadband propagation channel model in a comparison with fast cell selection (FCS), which tracks the instantaneous fading variation, and with the conventional slow CS, which tracks only the distance-dependent path loss, based on radio link level simulations that take into account time-varying instantaneous fading and shadowing variations. The simulation results show that the achievable throughput with FCS improves slightly in a broadband channel with an increasing number of paths when the average path-loss difference between two cells is greater than 2 dB. Nevertheless, we show that the optimum CS interval becomes approximately 100 msec, because the interval can track the time-varying shadowing variation considering low-to-high mobility up to the maximum Doppler frequency of 200 Hz. Consequently, we show that the throughput by employing the CS based on shadowing variation with the selection interval of 100 msec is increased by approximately 5 and 15% compared to that using the conventional slow CS with the selection interval of 1 sec, for the maximum Doppler frequency of 20 and 200 Hz, respectively.

We propose efficient secret sharing schemes realizing general access structures. Our proposed schemes are perfect secret sharing schemes and include Shamir's (k, n)-threshold schemes as a special case. Furthermore, we show that a verifiable secret sharing scheme for general access structures is realized by one of the proposed schemes.

In this paper, the bit error rate (BER) and the outage probability are presented for a maximal ratio combining (MRC) two-dimensional (2D)-RAKE receiver operating in a correlated frequency-selective Nakagami-m fading environment with multiple access interference. A simple approximated probability distribution function of the signal-to-interference-plus-noise ratio (SINR) is derived for the receiver with multiple correlated antennas and RAKE branches in arbitrary fading environments. The combined effects of spatial and temporal diversity order, average received signal-to-noise ratio, the number of multiple access interference, angular spread, antennae spacing and multi-path Nakagami-m fading environment on the system performance are illustrated. Numerical results indicate that the performance of the 2D-RAKE receiver depends highly on the operating environment and antenna array configuration. The performance can be improved by increasing the spatio-temporal diversity gains and antenna spacing.

In multicarrier code division multiple access (MC-CDMA) systems, the orthogonality among the spreading codes is destroyed because the channels exhibit frequency-selective fading and the despreading stage performs gain control; that is, inter-code interference (ICI) can significantly degrade system performance. This paper proposes an optimum spreading code assignment method that reflects our analysis of ICI for up and downlink MC-CDMA cellular systems over correlated frequency-selective Rayleigh fading channels. At first, we derive theoretical expressions for the desired-to-undesired signal power ratio (DUR) as a quantitative representation of ICI; computer simulation results demonstrate the validity of the analytical results. Next, based on the ICI imbalance among code pairs, we assign specific spreading codes to users to minimize ICI (in short, to maximize the multiplexing performance); our proposed method considers the quality of service (QoS) policy of users or operators. We show that the proposed method yields better performance, in terms of DUR, than the conventional methods. The proposed method can maximize the multiplexing performance of a MC-CDMA cellular system once the channel model, spreading sequence, and combining strategy have been set. Three combining strategies are examined at the despreading stage for the uplink, equal gain combining (EGC), orthogonality restoring combining (ORC), and maximum ratio combining (MRC), while two are considered for the downlink, EGC and MRC.

A new common-bus architecture with temporal and spatial parallel access capabilities under wire-resource constraint is proposed to transfer vast quantities of data between modules inside a VLSI chip. Since bus controllers are distributed into modules, the proposed bus architecture can directly transfer data from one module to another without any central bus control unit like a Direct Memory Access (DMA) controller, which enables to reduce communication steps for data transfer between modules. Moreover, when a start address and the number of block data in both source/destination modules are determined at the first step of a data-transfer scheme, no additional address setting for the data transfer is required in the rest of the scheme, which allows us to use all the wire resources as only the "data bus." Therefore, the bus function is dynamically programmed, which results in achieving high throughput of bus communication. For example, in case of a 64-line common bus, it is evaluated that the maximum data throughput in the proposed architecture with dynamic bus-function programming is four times higher than that in the conventional DMA bus architecture with fixed 32-bit-address/32-bit-data buses.

To fulfill the computing demands in visual media processing, we have been investigating a parallel processing system to improve the processing speed of the visual media related to applications from the point of view of a memory system within a single instruction multiple data (SIMD) computer. In this paper, we have introduced MAMS-PP4, which is similar to a pipelined SIMD architecture type and consists of pq processing elements (PEs) as well as a multi-access memory system (MAMS). MAMS supports simultaneous access to pq data elements within a horizontal (1 pq), a vertical (pq 1) or a block (p q) subarray with a constant interval in an arbitrary position in an M N array of data elements, where the number of memory modules, m, is a prime number greater than pq. MAMS reduces the memory access time for an SIMD computer and also improves the cost and complexity that involved in controlling the large volume of data demanded in visual media applications. PE is designed to be a two-state machine in order to utilize MAMS efficiently. MAMS-PP4 was fabricated into ASIC using TOSHIBA TC240C series library and a test board was used to measure the performance of ASIC. The test board consists of devices such as an MPC860 embedded-PCI board, two ASICs and a FPGA for the control units. Experiment was done on various computer systems in order to compare the performance of MAMS-PP4 using morphological operations as the application. MAMS-PP4 shows a respectful and consistent processing speed.

Spectral-amplitude coding (SAC) techniques in fiber-Bragg-grating (FBG)-based optical code-division multiple-access (OCDMA) systems were investigated in our previous work. This paper adopts the same network architecture to investigate the simultaneous reductions of multiple-access interference (MAI) and optical beat interference (OBI). The MAI is caused by overlapping wavelengths from undesired network coder/decoders (codecs) while the OBI is induced by interaction of simultaneous chips at adjacent gratings. It is proposed that MAI and OBI reductions may be obtained by use of: 1) a source spectrum that is divided into equal chip spacing; 2) coded FBGs characterized by approximately the same number of "0" and "1" code elements; and 3) spectrally balanced photo-detectors. With quasi-orthogonal Walsh-Hadamard coded FBGs, complementary spectral chips is employed as signal pairs to be recombined and detected in balanced photo-detectors, thus achieving simultaneous suppression of both MAIs and OBIs. Simulation results showed that the proposed OCDMA spectral-amplitude coding scheme achieves significant MAI and OBI reductions.

This paper theoretically evaluates the performance of overlapping pulse-position modulation (OPPM) fiber-optic code-division multiple-access (FO-CDMA) systems in the presence of code synchronization errors. The analysis is carried out with a constraint on throughput-pulsewidth product. Discussions on effects of various system parameters, such as timing offset, index of overlap, number of users, are presented. The results show that the OPPM FO-CDMA systems with high index of overlaps have better resistance against imperfect synchronization. In fact, the acceptable performance could be maintained even with timing offsets of up to 30% of chip pulsewidth. On the other hand, strict code synchronization is necessarily required, preferably within a half code chip pulsewidth.

This paper proposes and theoretically evaluates two different schemes of code acquisition for pulse-position modulation (PPM) and overlapping PPM (OPPM) fiber-optic code-division multiple-access (CDMA) systems, namely threshold-based and demodulator-based code acquisition. Single-dwell detector and serial-search algorithm are employed for both schemes. Theoretical analysis is carried out for shot-noise-limited photon-counting receiver. Discussions upon effects of various parameter settings on the performance of code acquisition for PPM/OPPM fiber-optic CDMA systems, such as index of overlap, PPM/OPPM multiplicity, average photon counts per information nat, and darkcurrents, are presented. It is shown that when the threshold is properly selected, the threshold-based code acquisition system offers better performance, in terms of mean number of training frames, than the demodulator-based one.

Supporting quality of service (QoS) capabilities for multi-media applications is one of the major issues in medium access control (MAC) research. In distributed contention-based MAC algorithms, it is a challenging task to support the desired QoS because of the inherent random access characteristics. In this paper, we propose an efficient prioritized fast collision resolution algorithm. The MAC protocol with this new algorithm attempts to provide significantly high throughput performance for data services and support QoS for real-time services. We incorporate the priority algorithm based on service differentiations with the fast collision resolution algorithm, and show that this algorithm can simultaneously achieve high throughput and good QoS support for real-time and data services.

In 2003, Wu proposed a threshold access control scheme based on smart cards. In this letter, we show that the scheme is vulnerable to various attacks.

In this paper, the design of a mobile satellite Internet access (MSIA) system and a mobile broadband satellite access system, called Mobile Broadband Interactive Satellite Multimedia Access Technology System (MoBISAT) are presented. MSIA system provides Internet service, broadcasting, and digital A/V service in both fixed and mobile environments using Ku-band geostationary earth orbit (GEO) satellite. A Ku-band two-way active phased array antenna installed on top of the transportation vehicles can enable the transmission of signals to satellite as well as signal tracking and reception. The forward link and return link are a high speed Time Division Multiplex (TDM) and TDMA transmission media, respectively, both of which carry signaling and user traffic. The MoBISAT, which is a next generation mobile broadband satellite access system, provides both Ku-band satellite TV and Ka-band high-speed Internet to the passengers and crews for land, maritime, and air vehicles. This paper addresses the main technological solutions adopted for the implementation and test results for the MSIA system and the main design features of the MoBISAT system.

This paper presents a novel blind multiple access interference (MAI) suppression filter in DS/CDMA systems. The filter is adaptively updated by parallel projections onto a series of convex sets. These sets are defined based on the received signal as well as a priori knowledge about the desired user's signature. In order to achieve fast convergence and good performance at steady state, the adaptive projected subgradient method (Yamada et al., 2003) is applied. The proposed scheme also jointly estimates the desired signal amplitude and the filter coefficients based on an approximation of an EM type algorithm, following the original idea proposed by Park and Doherty, 1997. Simulation results highlight the fast convergence behavior and good performance at steady state of the proposed scheme.

Parallel memories increase memory bandwidth with several memory modules working in parallel and can be used to feed a processor with only necessary data. The Configurable Parallel Memory Architecture (CPMA) enables a multitude of access formats and module assignment functions to be used within a single hardware implementation, which has not been possible in prior embedded parallel memory systems. This paper focuses on address computation in CPMA, which is implemented using several configurable computation units in parallel. One unit is dedicated for each type of access formats and module assignment functions that the implementation supports. Timing and area estimates are given for a 0.25-micron CMOS process. The utilized resources are shown to be linearly proportional to the number of memory modules.

In 1987, Ito, Saito and Nishizeki proposed a secret sharing scheme realizing general access structures, called the multiple assignment secret sharing scheme (MASSS). In this paper, we propose new MASSS's which are perfect secret sharing schemes and include Shamir's (k,n)-threshold schemes as a special case. Furthermore, the proposed schemes are more efficient than the original MASSS from the viewpoint of the number of shares distributed to each participant.

Shared memory multiprocessors in which each processor has its own TLB must manage consistency among TLBs and a page table. As the large-scale CC-NUMA (cache-coherent non-uniform memory access) shared memory multiprocessors become popular, it is important for TLB consistency management algorithms to be highly scalable. In this paper, we propose a TLB update-hint algorithm as a scalable TLB consistency management solution for CC-NUMA multiprocessors. By using a lazy TLB invalidation approach, we reduced the number of unnecessary processor interruptions and idle-waiting time, and achieved a high level of scalability. Using a shared memory simulator, we evaluated the TLB update-hint algorithm. For performance comparison, we also simulated the TLB shootdown algorithm, one of the most popular TLB consistency algorithms. The simulations demonstrated that the TLB update-hint algorithm scales well in systems with a large number of processors. At 64 node systems, the TLB update-hint algorithm shows 4787% better performance than the TLB shootdown algorithm.

An adaptive filtering algorithm based on the sliding window criterion is known to be very attractive for violent changing environments. In this paper, a new sliding window linearly constrained recursive least squares (SW-LC-RLS) algorithm based on the modified minimum mean squared error (MMSE) structure is devised for the RAKE receiver in direct sequence spread spectrum code-division multiple access (DS-SS CDMA) system over multipath fading channels, where the channel estimation scheme is accomplished at the output of adaptive filter. The proposed SW-LC-RLS algorithm has the advantage of having faster convergence property and tracking ability, and can be applied to the environments, where the narrowband interference is joined suddenly to the system, to achieve desired performance. Via computer simulation, we show that the performance, in terms of mean square errors (MSE), signal to interference plus noise ratio (SINR) and bit error rate (BER), is superior to the conventional LC-RLS and orthogonal decomposition-based LMS algorithms based on the MMSE structure.

This paper considers the problem of providing relative service differentiation in IEEE 802.11 Wireless LAN by using different Medium Access Control (MAC) parameters for different service classes. We present an analytical model which predicts the saturation throughput of IEEE 802.11 Distributed Coordination Function with multiple classes of service. This model allows us to show that relative service differentiation can be achieved by varying the initial contention window alone. In this case, the saturation throughput of a station can be shown to be approximately inversely proportional to the initial contention window size being used by that station. The simulation results validate our analytical model.

This letter shows the performance comparisons of several different rate scheduling schemes for non-real time data service over the uplink of burst switching-based direct sequence code division multiple access (DS/CDMA) system to support the integrated voice/data service. The closed-form solution of optimal scheduling formulation, which minimizes average transmission delay when all of the active data users are transmitting simultaneously, is presented and mathematical analyses with other rate scheduling schemes, which provide efficiency criterion of transmission delay for rate scheduling schemes, are performed. Numerical results show the analyses explicitly.