In recent years, the space-time block coding (STBC) method has attracted attention to provide transmission diversity in mobile communication systems. Although the STBC method is very effective in slow fading environments, its performance in fast fading environments has yet to be clearly verified. In this paper we propose a railway radio communication system using space-time coding in cooperation with two base stations. Here, we considered the differential STBC (D-STBC) method in railway communication system to overcome difficulties caused by the fast fading environment. We have compared the performance of STBC and D-STBC method where there is frequency offset between two base stations. Moreover, we have presented the simulation result of overall performance of the system including frequency offset and transmission time delay when operating D-STBC method. The overall evaluation on this paper shows that the D-STBC method is suitable for realizing highly reliable railway communication systems.

We propose a new modulation, phase-silence-shift-keying (PSSK), whose symbol error rate (SER) performance is improved by 6 dB compared with phase-shift-keying (PSK). To prove this, theoretical analysis of probability of error is provided and simulation results are presented.

Time-interleaved (TI) analog-to-digital converters (ADCs) are frequently advocated as a power-efficient solution to realize the high sampling rates required in single-chip transceivers for the emerging communication schemes: ultra-wideband, fast serial links, cognitive-radio and software-defined radio. However, the combined effects of multiple distortion sources due to channel mismatches (bandwidth, offset, gain and timing) severely affect system performance and power consumption of a TI ADC and need to be accounted for since the earlier design phases. In this paper, system-level design of TI ADCs is addressed through a platform-based methodology, enabling effective investigation of different speed/resolution scenarios as well as the impact of parallelism on accuracy, yield, sampling-rate, area and power consumption. Design space exploration of a TI successive approximation ADC is performed top-down via Monte Carlo simulations, by exploiting behavioral models built bottom-up after characterizing feasible implementations of the main building blocks in a 90-nm 1-V CMOS process. As a result, two implementations of the TI ADC are proposed that are capable to provide an outstanding figure-of-merit below 0.15 pJ/conversion-step.

Time variations of wireless multipath channels can lead to severe intercarrier interference (ICI) in orthogonal frequency division multiplex (OFDM) systems, whereas large Doppler frequency spread can provide us with time diversity gain. In order to take advantage of the time diversity and to suppress the interference and noise enhancement at the same time, the receiver normally detects the data successively. In this letter, we propose an improved detection ordering based on the log-likelihood ratio (LLR) rather than the signal-to-noise ratio (SNR) for the successive detector. Using both theoretical analysis and computer simulation, it is shown that this scheme outperforms the traditional successive detection methods.

Probabilistic model checking is an emerging verification technology for probabilistic analysis. Its use has been started not only in computer science but also in interdisciplinary fields. In this paper, we show that probabilistic model checking allows one to analyze the magnetic behaviors of the one-dimensional Ising model, which describes physical phenomena of magnets. The Ising model consists of elementary objects called spins and its dynamics is often represented as the Metropolis method. To analyze the Ising model with probabilistic model checking, we build Discrete Time Markov Chain (DTMC) models that represent the behavior of the Ising model. Two representative physical quantities, i.e., energy and magnetization, are focused on. To assess these quantities using model checking, we devise formulas in Probabilistic real time Computation Tree Logic (PCTL) that represent the quantities. To demonstrate the feasibility of the proposed approach, we show the results of an experiment using the PRISM model checker.

The Chikazawa-Yamagishi scheme is an ID-based key distribution scheme which is based on the RSA cryptosystem. There are several variant schemes to improve the efficiency and the security of the Chikazawa-Yamagishi scheme. Unfortunately, all of the proposed schemes have some weaknesses. First, all the proposed schemes require time synchronization of the communicating parties. Second, none of the proposed schemes provide both forward secrecy and security against session state reveal attacks. In this paper, we suggest an ID-based key distribution scheme which does not require time synchronization and provides both forward secrecy and security against session state reveal attacks.

Conventional symbol time (ST) synchronization algorithms for orthogonal frequency-division multiplexing (OFDM) systems are mostly based on the maximum correlation result of the cyclic prefix. Due to the channel effect, the estimated ST is not accurate enough. Hence, one needs to further identify the channel impulse response (CIR) so as to obtain a better ST estimation. Overall, the required computational complexity is high because it involves time-domain (TD) correlation operations, as well as the fast Fourier transform (FFT) and inverse FFT (IFFT) operations. In this work, without the FFT/IFFT operations and the knowledge of CIR, a low-complexity TD ST estimation is proposed. We first characterize the frequency-domain (FD) interference effect. Based on the derivation, the new method locates the symbol boundary at the sampling point with the minimum interference in the FD (instead of the conventional maximum TD correlation result). Moreover, to reduce the computational complexity, the proposed FD minimum-interference (MI) metric is converted to a low-complexity TD metric by utilizing Parseval's theorem and the sampling theory. Simulation results exhibit good performance for the proposed algorithm in multipath fading channels.

This paper presents experimental evaluations of the effect of time diversity obtained by hybrid automatic repeat request (HARQ) with soft combining in space and path diversity schemes on orthogonal frequency division multiplexing (OFDM)-based packet radio access in a downlink broadband multipath fading channel. The effect of HARQ is analyzed through laboratory experiments employing fading simulators and field experiments conducted in downtown Yokosuka near Tokyo. After confirming the validity of experimental results based on numerical analysis of the time diversity gain in HARQ, we show by the experimental results that, for a fixed modulation and channel coding scheme (MCS), time diversity obtained by HARQ is effective in reducing the required received signal-to-interference plus noise power ratio (SINR) according to an increase in the number of transmissions, K, up to 10, even when the diversity effects are obtained through two-branch antenna diversity reception and path diversity using a number of multipaths greater than 12 observed in a real fading channel. Meanwhile, in combined use with the adaptive modulation and channel coding (AMC) scheme associated with space and path diversity, we clarify that the gain obtained by time diversity is almost saturated at the maximum number of transmissions in HARQ, K ' = 4 in Chase combining and K ' = 2 in Incremental redundancy, since the improvement in the residual packet error rate (PER) obtained through time diversity becomes small owing to the low PER in the initial packet transmission arising from appropriately selecting the optimum MCS in AMC. However, the experimental results elucidate that the time diversity in HARQ with soft combining associated with antenna diversity reception is effective in improving the throughput even in a broadband multipath channel with sufficient path diversity.

A novel transmit diversity scheme with space-time-frequency coded training sequence is proposed to perform channel estimation flexibly over doubly selective channel, which offers a trade-off between maximum delay spread and maximum Doppler spread. Simulation results are presented to verify the effectiveness of the proposed flexible channel estimation method under different propagation conditions.

The end-to-end round trip time (RTT) is one of the most important communication characteristics for Internet applications. From the viewpoint of network operators, RTT may also become one of the important metrics to understand the network conditions. Given this background, we should know how a factor such as a network incident influences RTTs. It is obvious that two or more factors may interfere in the observed delay characteristics, because packet transmission delays in the Internet are strongly dependent on the time-variant condition of the network. In this paper, we propose a modeling method by using mixed distribution which enables us to express delay characteristic more accurately where two or more factors exist together. And, we also propose an inferring method of network behavior by decomposition of the mixed distribution based on modeling results. Furthermore, in experiments we investigate the influence caused by each network impact factor independently. Our proposed method can presume the events that occur in a network from the measurements of RTTs by using the decomposition of the mixed distribution.

In this paper, a novel model of Gaussian pulse propagation in optical fiber is proposed to comprehensively analyze the impact of Group Velocity Dispersion (GVD) on the performance of two-dimensional wavelength hopping/time spreading optical code division multiple access (2-D WH/TS OCDMA) systems. In addition, many noise and interferences, including multiple access interference (MAI), optical beating interference (OBI), and receiver's noise are included in the analysis. Besides, we propose to use the heterodyne detection receiver so that the receiver's sensitivity can be improved. Analytical results show that, under the impact of GVD, the number of supportable users is extremely decreased and the maximum transmission length (i.e. the length at which BER 10-9 can be maintained) is remarkably shortened in the case of normal single mode fiber (ITU-T G.652) is used. The main factor that limits the system performance is time skewing. In addition, we show how the impact of GVD is relieved by dispersion-shifted fiber (ITU-T G.653). For example, a system with 321 Gbit/s users can achieve a maximum transmission length of 111 km when transmitted optical power per bit is -5 dBm.

A time synchronization protocol for WSN is required to compensate time discrepancy. Time discrepancy among sensor nodes inevitably happens in WSN due to several internal and external factors. In order to make WSN's own job done effectively, a time synchronization protocol should be designed to achieve low execution time and low network traffic as well as accurate synchronization. Several synchronization protocols have been proposed to provide accurate time synchronization but do not consider execution time and network traffic for time synchronization. This paper proposes MNTP; it provides rapid and accurate time synchronization in multi-hop communication range. It presents a new broadcast scheme and time stamping mechanism to achieve low execution time and low network traffic along with accurate synchronization. Evaluation results show that MNTP improves synchronization accuracy up to 22% in single-hop and 51% in multi-hop respectively. MNTP also has 67 times and 58 times lower execution time and network traffic when 300 nodes are deployed in 2020 m2 sensor field.

A prototype of a three-axis electro-optic (EO) probe is developed that has the linearity of approximately 0.5 dB in the specific absorption rate (SAR) range of 0.01 to 100 W/kg and the directivities are eight-shaped with cross-axis sensitivity isolation of greater than 30 dB. It is confirmed that electric fields and SAR distributions can be measured using a three-axis EO probe.

Process variation is becoming a primal concern in timing closure of LSI (Large Scale Integrated Circuit) with the progress of process technology scaling. To overcome this problem, SSTA (Statistical Static Timing Analysis) has been intensively studied since it is expected to be one of the most efficient ways for performance estimation. In this paper, we study variation of output transition-time. We firstly clarify that the transition-time variation can not be expressed accurately by a conventional first-order sensitivity-based approach in the case that the input transition-time is slow and the output load is small. We secondly reveal quadratic dependence of the output transition-time to operating margin in voltage. We finally propose a procedure through which the estimation of output transition-time becomes continuously accurate in wide range of input transition-time and output load combinations.

An effective operation mode and a space-time synchronization technique for the spaceborne/airborne hybrid bistatic synthetic aperture radar (SA-BSAR) using sources of opportunity are presented. Our motivation lies in the fact that the existing approaches in the literature, where the transmitter antenna must be steered, can only be used in the hybrid bistatic SAR systems with cooperative transmitter. The presented mode is to widen the receiving beam for the purpose to increase the scene extension in azimuth. The inspiration comes from the much shorter receiving distance as compared to the one in mono-static spaceborne SAR. This means that the receiving gain can be significantly reduced to provide the same signal-to-noise ratio (SNR) with respect to the mono-static case. The feasibility of the wide-beam mode is first preliminarily verified by a quantitative analysis of SNR and a demonstration that the pulse repetition frequency (PRF) used in the spaceborne illuminator can easily satisfy the PRF constraints of the SA-BSAR. The influence on the azimuth ambiguity to signal ratio (AASR) is also discussed and the corresponding broadening factor of the maximum allowable for receiver beamwidth is subsequently derived. Afterwards, the formulae for calculating the overlap time, the scene extension and the azimuth resolution are deduced. As there are no grating lobes in satellite antenna pattern since the non-cooperative illuminator normally operates in the side-looking mode, an existing technique for the space-time synchronization in cooperative hybrid systems can not be directly applied. The modification performed and its underlying principle are presented in detail. The simulation results demonstrate the effectiveness of the wide-beam mode, and show that in most cases a useful scene extension (on the order of at least 1 km) can be achieved with a roughly equivalent azimuth resolution as compared to the one in mono-static spaceborne SAR. In some cases, explicit measures to suppress the azimuth ambiguity must be taken to achieve the expected scene extension.

We propose a polynomial-time algorithm for the scheduling of real-time parallel tasks on multicore processors. The proposed algorithm always finds a feasible schedule using the minimum number of processing cores, where tasks have properties of linear speedup, flexible preemption, arbitrary deadlines and arrivals, and parallelism bound. The time complexity of the proposed algorithm is O(M3log N) for M tasks and N processors in the worst case.

This paper describes new DDRx SDRAM interface architecture suitable for system-on-chip (SOC) implementation. Our test chip fabricated in a 90-nm CMOS process adopts three key schemes and achieves 960 Mb/s/pin operations with 32 bits width. One of new schemes is to suppress timing skew with rising-edge signal transmission I/O circuit and look-up table type impedance calibration circuit. DQS round-trip-time, propagation delay from rising edge of system clock in SOC to arrival of DQS at input PAD of SOC during read operation, becomes longer than one clock cycle time as for DDR2 interface and beyond. Flexible DQS round-trip-time scheme can allow wide range up to N/2 cycles in N bits burst read operation. In addition, full self loop-backed test scheme is also proposed to measure AC timing parameters without high-end tester. The architecture reported in this paper can be continuously adaptive to realize higher data-rate and cost-efficient DDRx-SDRAM interface for various kinds of SOC.

For robust time delay estimation, we propose a new subspace-based algorithm which estimates the phase shift not between the adjacent samples of the dechirped sinusoid but between the dechirped sinusoids in different bands. We analyze the proposed and the conventional method in AWGN channel and verifiy the proposed algorithm through Monte-carlo simulations in AWGN and multi-path channels.

The authors propose a user-centric seamless handover (HO) scheme, which is a kind of a vertical HO from a new perspective, toward a next generation network where heterogeneous access networks converge. The users' experience-oriented scheme allows users to enjoy the optimal service quality for real-time applications in respective access networks. In addition, the scheme sustains on-going sessions during the vertical HO. The proposed scheme consists of two methods -- the bicasting of Different Quality-level Streams (DiffQS) and the Delay Difference Absorption (DDA) method. Initially, the authors propose two plausible methods for the SIP-based bicasting of DiffQS. This document introduces a SIP-capable network element named the HO Assistive Server (HOAS). HOAS controls bicasting of DiffQS and provides users with the optimal service quality for real-time applications via respective access networks as well as avoiding packet loss during HO. The DDA method is also proposed to prevent a service interruption and smoothly continue a real-time service during HO. Evaluation results show that the scheme achieves the seamless service continuity from the users' perspective for HO between cellular and high-speed wireless access via the implementation of a prototype system.

A method is proposed for estimating code and carrier phase parameters of GNSS reflected signals in low SNR (signal-to-noise ratio) environments. Simulation results show that the multipath impact on code and carrier with 0.022 C/A chips delay can be estimated in 0 dB SNR in the condition of 46 MHz sampling rate.