Current collapse phenomenon is a well known obstacle in the AlGaN/GaN HEMTs. In order to improve the surface stability of HEMTs, we have investigated the SiN passivation film deposited by T-CVD, and we found that it improves both gate leakage current and current collapse phenomenon [1]. Moreover, we compared the T-CVD and PE-CVD passivation films, on high electric field DC and RF characteristics. We found that T-CVD SiN passivation film improves BVds-off by 30% because of the reduction of gate leakage current. It also improved ηd in the output power characteristics by load-pull measurement, which indicates the decrease of the current collapse phenomenon. Also we fabricated a multi-fingered 50 W-class AlGaN/GaN HEMT with T-CVD SiN passivation film and achieved 61.2% of high drain efficiency at frequency of 2.14 GHz, which was 3.6 points higher than that with PE-CVD SiN passivation film.

This article proposes a channel estimation method for the downlink channels of a WCDMA system in a high-speed railroad setting. High mobility may cause conventional symbol-level channel estimation to yield severe errors because in conventional methods channel state has to maintain constant within one to several symbol durations. However, in high mobility environment, this assumption may not hold. Errors are particularly more dangerous when using very high spreading factors. In order to counteract the adverse effect of high mobility on channel estimation, we shorten the observation window to that of an N-chip block so that channel conditions or characteristics remain approximately unchanged. We consider channel estimation prior to dispreading the received signal. In other words, channel estimation is done at the chip level rather than the conventional symbol level. The least squares (LS) criterion is employed to acquire channel characteristics for each block of N pilot chips, and the linear interpolation method is used to determine the channel characteristics for each data chip. The LS-based estimator is selected due to its simplicity since it does not need to know channel or noise statistics. An LS-based estimator at the chip level has the further advantage that it is robust against interpath interference (IPI). The uncoded bit error rate (BER) performance of a RAKE receiver using different channel estimation schemes is evaluated and compared through simulations. The proposed scheme is found to be suitable for a high-speed railroad setting.

This paper presents the optimum control interval for intra-cell fractional transmission power control (TPC) for a shared data channel employing frequency domain channel-dependent scheduling and adaptive modulation and coding (AMC) in the Evolved UTRA uplink using single-carrier (SC)-FDMA radio access. The simulation results show that the best attenuation factor in the fractional TPC is approximately 0.6 for achieving the maximum user throughput when the maximum target received signal power, P0 is -60 dBm. Then, we show that the optimum averaging interval for the desired signal level, which corresponds to a substantial control interval for the fractional TPC, is approximately 100-200 msec regardless of the maximum Doppler frequency up to 222 Hz and the distance at the shadowing correlation of 0.5. Throughout the simulation results, we verify that slow intra-cell fractional TPC associated with fast AMC is effective in achieving the maximum cell throughput and cell-edge user throughput.

Codebook based multiple-input multiple-output (MIMO) precoding can significantly improve the system spectral efficiency with limited feedback and has been accepted as one of the most promising techniques for the Evolved UTRA (E-UTRA). Compared with single-user (SU) MIMO, multi-user (MU) MIMO can further improve the system spectral efficiency due to increased multi-user diversity gain. MU-MIMO is preferred for the case of a large number of users,when the total feedback overhead will become a problem. In order to reduce the feedback overhead, feedback of single channel quality indicator (CQI), e.g. rank 1 CQI, is required in E-UTRA currently. The main challenge is how to obtain CQIs of other ranks at Node B for rank adaptation with single CQI feedback. In this paper, an adaptive CQI update scheme at Node B based on statistical characteristics of CQI of various ranks is proposed. To further increase the accuracy of CQI at Node B for data transmission, an adaptive CQI feedback scheme is then proposed in which single CQI with the rank same as previously scheduled is fed back. Simulation results show that our proposed CQI update scheme can achieve 2.5-5% gain compared with the conventional method with fixed backoff. Moreover, with the proposed adaptive feedback scheme, 20-40% performance gain can be obtained and the performance can approach the upper bound.

In the Evolved UTRA (UMTS Terrestrial Radio Access) downlink, Orthogonal Frequency Division Multiplexing (OFDM) based radio access was adopted because of its inherent immunity to multipath interference and flexible accommodation of different spectrum arrangements. This paper presents the optimum adaptive modulation and channel coding (AMC) scheme when resource blocks (RBs) is simultaneously assigned to the same user when frequency and time domain channel-dependent scheduling is assumed in the downlink OFDMA radio access with single-antenna transmission. We start by presenting selection methods for the modulation and coding scheme (MCS) employing mutual information both for RB-common and RB-dependent modulation schemes. Simulation results show that, irrespective of the application of power adaptation to RB-dependent modulation, the improvement in the achievable throughput of the RB-dependent modulation scheme compared to that for the RB-common modulation scheme is slight, i.e., 4 to 5%. In addition, the number of required control signaling bits in the RB-dependent modulation scheme becomes greater than that for the RB-common modulation scheme. Therefore, we conclude that the RB-common modulation and channel coding rate scheme is preferred, when multiple RBs of the same coded stream are assigned to one user in the case of single-antenna transmission.

Frequency-domain equalization (FDE) based on the minimum mean square error (MMSE) criterion can increase the downlink bit error rate (BER) performance of DS-CDMA beyond that possible with conventional rake combining in a frequency-selective fading channel. FDE requires accurate channel estimation. Recently, we proposed a pilot-assisted channel estimation (CE) based on the MMSE criterion. Using MMSE-CE, the channel estimation accuracy is almost insensitive to the pilot chip sequence, and a good BER performance is achieved. In this paper, we propose a channel estimation scheme using one-tap recursive least square (RLS) algorithm, where the forgetting factor is adapted to the changing channel condition by the least mean square (LMS) algorithm, for DS-CDMA with FDE. We evaluate the BER performance using RLS-CE with adaptive forgetting factor in a frequency-selective fast Rayleigh fading channel by computer simulation.

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.

In this letter, we propose a new power allocation scheme for random unitary beamforming assuming a discrete transmission rate with a small amount of feedback information and low latency. Simulation results show that the proposed scheme can improve throughput compared to the conventional power allocation scheme.

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.

Interconnection networks play a crucial role in the performance of massively parallel computers. Hierarchical interconnection networks provide high performance at low cost by exploring the locality that exists in the communication patterns of massively parallel computers. A Tori connected Torus Network (TTN) is a 2D-torus network of multiple basic modules, in which the basic modules are 2D-torus networks that are hierarchically interconnected for higher-level networks. This paper addresses the architectural details of the TTN and explores aspects such as node degree, network diameter, cost, average distance, arc connectivity, bisection width, and wiring complexity. We also present a deadlock-free routing algorithm for the TTN using four virtual channels and evaluate the network's dynamic communication performance using the proposed routing algorithm under uniform and various non-uniform traffic patterns. We evaluate the dynamic communication performance of TTN, TESH, MH3DT, mesh, and torus networks by computer simulation. It is shown that the TTN possesses several attractive features, including constant node degree, small diameter, low cost, small average distance, moderate (neither too low, nor too high) bisection width, and high throughput and very low zero load latency, which provide better dynamic communication performance than that of other conventional and hierarchical networks.

Equivalence checking is one of the most important issues in VLSI design to guarantee that bugs do not enter designs during optimization steps or synthesis steps. In this paper, we propose a new word-level equivalence checking method between two models before and after high-level synthesis or behavioral optimization. Our method converts two given designs into RTL models which have same datapaths so that behaviors by identical control signals become the same in the two designs. Also, functional units become common to the two designs. Then word-level equivalence checking techniques can be applied in bit-level accuracy. In addition, we propose a rule-based equivalence checking method which can verify designs which have complicated control structures faster than existing symbolic simulation based methods. Experimental results with realistic examples show that our method can verify such designs in practical periods.

We fabricated a high-speed wavelength tunable arrayed-waveguide grating (AWG) and an AWG integrated with optical switches using (Pb,La)(Zr,Ti)O3-(PLZT). PLZT has a high electro-optic (EO) coefficient, which means these devices have considerable potential for use in reconfigurable optical add drop multiplexers (ROADMs). The PLZT waveguides in this work have a rib waveguide structure with an effective relative index difference (Δ) of 0.65%. Both AWGs have 8 channels with a frequency spacing of 500 GHz. The fabricated wavelength tunable AWGs allows us to freely shift the output at a particular wavelength to an arbitrary port by applying voltages to 3 mm long electrodes formed on each of the waveguides. We confirmed that the maximum tuning range with driving voltage of 22 V was approximately 32 nm at 1.55 µm. With the integrated 8-ch PLZT waveguide switch array, we could also select the output port by setting the drive voltage applied to the switch array. 2 2 directional coupler switches were used for the switch array. The two devices exhibited insertion losses of 17 dB and 19 dB, adjacent crosstalk of -18.5 dB and -19.7 dB, and a maximum extinction ratio of 19.6 dB and 12.6 dB, respectively. The tuning speed of both devices was 15 ns and their physical sizes were 9.0 23.0 mm and 8.0 29.5 mm, respectively.

A simple exact error rate analysis is presented for random binary direct sequence code division multiple access (DS-CDMA) considering a general pulse shape and flat Nakagami fading channel. First of all, a simple model is developed for the multiple access interference (MAI). Based on this, a simple exact expression of the characteristic function (CF) of MAI is developed in a straight forward manner. Finally, an exact expression of error rate is obtained following the CF method of error rate analysis. The exact error rate so obtained can be much easily evaluated as compared to the only reliable approximate error rate expression currently available, which is based on the Improved Gaussian Approximation (IGA).

Recently, various types of 3-D nonvolatile memory (NVM) devices have been researched to improve the integration density [1]-[3]. The NVM device of pillar structure can be considered as one of the candidates [4],[5]. When this is applied to a NAND flash memory array, bottom end of the device channel is connected to the bulk silicon. In this case, the current in vertical direction varies depending on the thickness of silicon channel. When the channel is thick, the difference of saturation current levels between on/off states of individual device is more obvious. On the other hand, when the channel is thin, the on/off current increases simultaneously whereas the saturation currents do not differ very much. The reason is that the channel potential barrier seen by drain electrons is lowered by read voltage on the opposite sidewall control gate. This phenomenon that can occur in 3-D structure devices due to proximity can be called gate-induced barrier lowering (GIBL). In this work, the dependence of GIBL on silicon channel thickness is investigated, which will be the criteria in the implementation of reliable ultra-small NVM devices.

In digital communication systems employing binary phase-shift keying and non-data-aided carrier phase recovery, a 180 carrier phase ambiguity is inevitable. Here, we propose a simple modification to the standard regular repeat-accumulate (RA) code structure by exploiting the differential encoding inherent to the inner encoder of RA codes resulting in codes that are 180 rotationally invariant. The proposed code structure exhibit performance virtually identical to that of standard regular RA codes with zero carrier phase offset under both zero and 180 carrier phase offsets with negligible additional hardware complexity.

Pairing-based cryptography provides us many novel cryptographic applications such as ID-based cryptosystems and efficient broadcast encryptions. The security problems in ubiquitous sensor networks have been discussed in many papers, and pairing-based cryptography is a crucial technique to solve them. Due to the limited resources in the current sensor node, it is challenged to optimize the implementation of pairings on sensor nodes. In this paper we present an efficient implementation of pairing over MICAz, which is widely used as a sensor node for ubiquitous sensor network. We improved the speed of ηT pairing by using a new efficient multiplication specialized for ATmega128L, called the block comb method and several optimization techniques to save the number of data load/store operations. The timing of ηT pairing over GF(2239) achieves about 1.93 sec, which is the fastest implementation of pairing over MICAz to the best of our knowledge. From our dramatic improvement, we now have much high possibility to make pairing-based cryptography for ubiquitous sensor networks practical.

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.

Biologically-inspired approaches are one of the most promising approaches to realize highly-adaptive distributed systems. Biological systems inherently have self-* properties, such as self-stabilization, self-adaptation, self-configuration, self-optimization and self-healing. Thus, the application of biological systems into distributed systems has attracted a lot of attention recently. In this paper, we present one successful result of bio-inspired approach: we propose distributed algorithms for resource replication inspired by the single species population model. Resource replication is a crucial technique for improving system performance of distributed applications with shared resources. In systems using resource replication, generally, a larger number of replicas lead to shorter time to reach a replica of a requested resource but consume more storage of the hosts. Therefore, it is indispensable to adjust the number of replicas appropriately for the resource sharing application. This paper considers the problem for controlling the densities of replicas adaptively in dynamic networks and proposes two bio-inspired distributed algorithms for the problem. In the first algorithm, we try to control the replica density for a single resource. However, in a system where multiple resources coexist, the algorithm needs high network cost and the exact knowledge at each node about all resources in the network. In the second algorithm, the densities of all resources are controlled by the single algorithm without high network cost and the exact knowledge about all resources. This paper shows by simulations that these two algorithms realize self-adaptation of the replica density in dynamic networks.

This paper proposes joint maximum a posteriori (MAP) detection and spatial filtering for MIMO-OFDM mobile communications; it offers excellent receiver performance even over interference-limited channels. The proposed joint processor consists of a log likelihood generator and a MAP equalizer. The log likelihood generator suppresses cochannel interference by spatially filtering received signals and provides branch metrics of transmitted signal candidates. Using the branch metrics, the MAP equalizer generates log likelihood ratios of coded bits and performs channel decoding based on the MAP criterion. In the first stage, the log likelihood generator performs spatio-temporal filtering (STF) of the received signals prior to the fast Fourier transform (FFT) and is referred to as preFFT-type STF. Estimation of parameters including tap coefficients of the spatio-temporal filters and equivalent channel impulse responses of desired signals is based on the eigenvalue decomposition of an autocorrelation matrix of both the received and transmitted signals. For further improvement, in the second stage, the generator performs spatial filtering (SF) of the FFT output and is referred to as postFFT-type SF. Estimation of both tap coefficients of the spatial filters and channel impulse responses employs the recursive least squares (RLS) with smoothing. The reason for switching from preFFT-type STF into postFFT-type SF is that preFFT-type STF outperforms postFFT-type SF with a limited number of preamble symbols while postFFT-type SF outperforms preFFT-type STF when data symbols can be reliably detected and used for the parameter estimation. Note that there are two major differences between the proposed and conventional schemes: one is that the proposed scheme performs the two-stage processing of preFFT-type STF and postFFT-type SF, while the other is that the smoothing algorithm is applied to the parameter estimation of the proposed scheme. Computer simulations demonstrate that the proposed scheme can achieve excellent PER performance under interference-limited channel conditions and that it can outperform the conventional joint processing of preFFT-type STF and the MAP equalizer.

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.