Recently, in order to improve high speed data transmission and spectral efficiency in wireless communication systems, the combination of OFDM and space-time coding is being actively studied. In order to maximize the system efficiency, the problem of co-channel interference must be solved. One technique to overcome the co-channel interference and to increase the system capacity is to use adaptive antennas. Conventional beamforming techniques for single antenna cannot be applied directly to STBC-OFDM systems, because the signals transmitted from the two transmit antennas are superposed at the receive antenna and the interference between signals of the two transmit antennas occurs. In this paper, we present the MMSE beamforming technique using training sequence for STBC-OFDM systems in reverse link and evaluate the performance by using various parameters such as the number of training blocks, cluster sizes and angle spreads in Two-ray, TU and HT channels. From the simulation results, we show the best cluster sizes and the number of training blocks corresponding to these cluster sizes.

This paper proposes a new effective data transfer method for IEEE 802.11 wireless LANs by integrating priority control and multirate mechanism. The IEEE 802.11 PHY layer supports a multirate mechanism with dynamic rate switching and an appropriate data rate is selected in transmitting a frame. However, the multirate mechanism is used with the CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) protocol, low rate transmissions need much longer time than high rate transmissions to finish sending a frame. As a result, the system capacity is decreased. The proposed method assumes the same number of priority levels as the data rates, and a data rate is associated to a priority level. Priority of a transmission goes up with the used data rate. For this purpose, we have modified the CSMA/CA protocol to support prioritized transmission. By selecting the appropriate priority depending on the data rate and giving more transmission opportunities for high rate transmission, the system capacity is increased. The effect of the proposed mechanism is confirmed by computer simulations.

This paper addresses a robust supervisory control problem for uncertain timed discrete event systems (DESs) modeled as a set of some possible timed models. To avoid the state space explosion problem caused by tick transitions in timed models, the notion of eligible time bounds is presented. Based on the notion and activity (logical) models, this paper shows how the controllability condition of a given language specification is presented as a necessary and sufficient condition for the existence of a robust supervisor to achieve the specification for any timed model in the set.

Multiprocessor architecture becomes common on real-time systems as the workload of real-time systems increases. Recently new deadline-based (EDF-based) multiprocessor scheduling algorithms are devised, and comparative studies on the performance of these algorithms are necessary. In this paper, we compare EDZL, a hybrid of EDF and LLF, with other deadline-based scheduling algorithms such as EDF, EDF-US[m/(2m-1)], and fpEDF. We show EDZL schedules all task sets schedulable by EDF. The experimental results show that the number of preemptions of EDZL is comparable to that of EDF and the schedulable utilization bound of EDZL is higher than those of other algorithms we consider.

This paper proposes a time alignment control (TAC) for reducing an influence of multiple access interference (MAI) due to propagation delays (PDs) in uplink transmission from multiple mobile stations (MSs) to an access point (AP) for an orthogonal frequency division multiple access (OFDMA) based mobile communication system. In addition, this paper presents our evaluation of the proposed TAC as applied to dynamic parameter control orthogonal frequency and time division multiple access (DPC-OF/TDMA) which has been suggested for use in new generation mobile communication system. This paper also proposes several formats for an activation slot (ACTS) in which the GIs are lengthened in order to avoid the MAI because the TAC cannot be performed yet in an initial registration of the MSs. Computer simulation elucidates that lengthening the GIs of data symbols in the ACTS adequately to compensate a maximum delay improves the transmission performance of the ACTS at the initial registration without PDs compensation. The simulation also elucidates that the proposed TAC is performed to reduce the influence of the MAI effectively and that updating the estimates of the PDs every certain period is needed to compensate the PDs accurately under high-mobility environment.

Recently Azou et al. proposed a method of model reduction for discrete systems based on a new impulse response Gramian. The reduced model was derived by first approximating the low-order impulse response Gramian, and then matching some Markov parameters and time-moments of an original model. In this note a modified method is presented so that the reduced model exactly preserves the low-order impulse response Gramian together with a slightly different set of Markov parameters and time-moments of the original model.

This paper introduces the water ring scan method especially designed for the scalable video coding schemes such as fine granularity scalabilities (FGS) on the basis of MPEG-4 part-2 and the H.264. The proposed scanning method can improve the subjective quality of the decoded video by most-preferentially encoding, transmitting and decoding the image information of the region of interest. From the various simulation results of FGS coding schemes with MPEG-4 part 2 and H.264, the proposed scanning method can improve the subjective picture quality about 0.5 dB 3.5 dB better than the widely used raster scan order, especially on the region of interest, without significant loss of the quality in the left-over region.

This paper describes a second-order continuous-time ΔΣ modulator for a W-CDMA receiver, which operates at a supply voltage of 0.9 V, the lowest so far reported for W-CDMA. Inverter-based balanced OTAs without using differential pair are proposed for a low-voltage operation. Circuit parameters are optimized by system simulations. The modulator was implemented in a 0.13-µm CMOS technology. It consumes only 1.5 mW. The measured SNDR is 50.9 dB over a bandwidth of 1.92 MHz.

As the code division multiple access (CDMA) based third generation cellular infrastructure requires high performance signal processing in a baseband modem, an application-specific integrated circuit or a field-programmable gate array has commonly been used for chip rate processing. In this paper, the use of digital signal processors (DSP) is explored for a cdma2000 and a wideband CDMA channel modem with the goal of increasing flexibility. The design concepts of the prototype software-defined radio platform we implemented to estimate the potential and feasibility of commercial SDR platforms are presented. We discuss the hardware and software architecture of the platform, considerations for reconfigurability, and the test results. We also address practical issues for real-time chip rate processing and optimization schemes of DSP software, and provide detailed measurement results of DSP performance.

Recently, Yeh, Shen and Hwang proposed an one-time password authentication scheme, which enhances the S/KEY scheme to resist server spoofing attacks, preplay attacks and off-line dictionary attacks. In this letter, the weaknesses and inconveniences of their scheme are demonstrated.

We propose a new diversity scheme for orthogonal frequency division multiplexing/multi-input multi-output (OFDM/MIMO) systems. The proposed scheme, named turbo layered space-frequency coded OFDM (TLSFC-OFDM), exploits the turbo principle with space hopping (SH). The TLSFC-OFDM system with SH provides a spatial coding so that we can obtain the transmit diversity. We also introduce a successive interference cancellation (SIC) algorithm that requires no ordering and fewer iterations to converge. As a result, this scheme reduces computational complexity. Computer simulation results show that the unordered SIC-based TLSFC-OFDM system outperforms the OFDM/H-BLAST system. It is also shown that the proposed system can operate even with fewer receive antennas than transmit antennas.

The uplink of a space-time block coded multicarrier modulation code division multiple access (MC-CDMA) system equipped with a uniform linear array (ULA) at the base station is studied. A blind decoder that provides closed-form solutions of both transmitted symbol sequences and directions of arrival (DOAs) for all active users in one macrocell is derived without the uplink space-time vector channel estimation. The decoder uses an ESPRIT-like method to separate multiple co-channel users with different impinging DOAs. As a result, the DOAs of multiple users are obtained. In particular, a set of signal spaces, every one of which is spanned by the coded symbol sequences of an individual user, are also obtained. From these signal spaces, the original symbol sequences of multiple users are estimated by exploiting the special structure of space-time block coding (STBC) in combination with the finite alphabet property of transmitted symbols. Performance of the proposed scheme is evaluated by extensive computer simulations.

Polynomial cancellation coding (PCC) was proposed to mitigate the sever inter-carrier-interference (ICI) in an orthogonal frequency division multiplexing (OFDM) system caused by frequency offset. In this paper, we consider the effectiveness of PCC under time-variant multi-path Rayleigh fading analytically and by simulations. We first consider an analytical expression of the signal-to-interference plus noise power ratio (SINR) and then derive an approximation of the bit-error-rate (BER) of the OFDM-PCC system under the assumption that ICI is well approximated by a white Gaussian noise. Since the bandwidth efficiency of OFDM-PCC is half of that of normal OFDM, we compare the BER performance of the scheme with the normal OFDM system of the same bit-rate when low, medium, and high level modulations are used. Our results show that OFDM-PCC performs well even for high modulation level under time-varying multi-path fading.

In this paper, we propose a multiuser detection (MUD) scheme for space-time block coded orthogonal frequency division multiplexing (STBC-OFDM) systems. We derive the optimum weight matrix used to decouple simultaneously signals from active multiple access users using the minimum mean square error (MMSE) multiuser detection method. The proposed scheme provides good performance over different models of the frequency selective fading channel. It is also to show that if the length of the cyclic prefix is larger than that of the channel, the performance of the detector depends on only the total energy extracted from multipath components but not the employed channel model, the number of multipath components or the delay of each multipath component.

Δ-Timed Atomic Broadcast is the broadcast ensuring that all correct processes deliver the same messages in the same order, and that delivery latency of any message broadcast by any correct process is some predetermined time Δ or less. In this paper, we propose a Δ-timed atomic broadcast algorithm in a synchronous system where communication delay is bounded by a known constant d and processes suffer both crash faults and timing faults. The proposed algorithm can tolerate fc crash faults and ft timing faults as long as at least ft + 1 processes are correct, and its maximum delivery latency Δ is (2f' + 7)d where f' is the actual number of (crash or timing) faulty processes. That is, the algorithm attains the early-delivery in the sense that its delivery latency depends on the actual number of faults rather than the maximum number of faults that the algorithm can tolerate. Moreover, the algorithm has a distinct advantage of guaranteeing that timing-faulty processes also deliver the same messages in the same order as the correct processes (Uniformity). We also investigate the maximum number of faulty processes that can be tolerated. We show that no Δ-timed atomic broadcast algorithm can tolerate ft timing faults, if at most ft processes are correct. The impossibility result implies that the proposed algorithm achieves the maximum fault-resilience with respect to the number of faulty processes.

Cryptosystems for smartcard are required to provide protection from Differential Power Analysis (DPA) attack. Self-timed circuit based cryptosystems demonstrate considerable resistance against DPA attack, but they take substantial circuit area. A novel approach offering up to 30% area reduction and maintaining DPA protection level close to DIMS scheme is proposed.

In this study, we propose an object-based multimedia model for specifying the QoS (quality of service) requirements, such as the maximum data-dropping rate or the maximum data-delay rate. We also present a resource allocation model, called the net-profit model, in which the satisfaction of user's QoS requirements is measured by the benefit earned by the system. Based on the net-profit model, the system is rewarded if it can allocate enough resources to a multimedia delivery request and fulfill the QoS requirements specified by the user. At the same time, the system is penalized if it cannot allocate enough resources to a multimedia delivery request. We first investigate the problem of how to allocate resources efficiently, so that the QoS satisfaction is maximized. However, the net-profit may be distributed unevenly among the multimedia delivery requests. Thus, the second problem discusses how to allocate the resource efficiently so that the net-profit difference is minimized between any two multimedia requests. A dynamic programming based algorithm is proposed to find such an optimal solution with the minimum net-profit differences.

This paper presents 160-Gbit/s full channel time-division demultiplexing using a semiconductor optical amplifier hybrid integrated demultiplexer on a planer lightwave circuit. Error-free demultiplexing from a 160-Gbit/s signal to 8 channel 20 Gbit/s signals is successfully demonstrated. Results of a 160-Gbit/s optical time-division-multiplexed full channel OTDM signal transmission experiment using the circuit and successful 80-km transmission are presented.

Pilot-symbol-aided (PSA) channel estimation for OFDM wireless access systems enables the periodic estimation of channel frequency response by generating reference data from the received OFDM signals. The accuracy of this channel estimation can be improved through the average over a certain time period in each subcarrier-channel. However, the accuracy of the channel estimates by the average degrades as the Doppler shift is large due to a decrease in the average section size according to the Doppler shift for the tracking of the time-varying channel. This paper proposes a novel PSA channel estimation method to mitigate the influence of the noises and interferences. This method detects the channel estimates affected by the noises and interferences, and then removes them before the arithmetic or harmonic averaging to avoid propagating the influence of the noises and interferences. This paper also evaluated the proposed channel estimation method by clipping log-likelihood ratio (LLR) data to inspect the influence of the channel estimation on the LLR calculation by computer simulation.

This letter proposes a quadratic optimization decoding (QOD) for space-time block decoding in time-selective Rayleigh fading channels. When channels are fast fading, the simple decoding exploiting the orthogonal structure of the codes can not be used to achieve a proper error performance. In an effort to mitigate the severe performance degradation, in this letter least square decoding and QOD are considered for decoding. Simulation results show that the QOD shows a significant performance improvement compared to the least square and the conventional schemes.