An algorithm for augmenting a binary linear code is presented. The input to the code augmenting algorithm is (n,k,d) code C and the output is an (n,k^*,d) augmented code C (k^* k) satisfying C C and the Gilbert bound. The algorithm can be considered as an efficient implementation of the proof of Gilbert bound; for a given binary linear code C, the algorithm first finds a coset leader with the largest weight. If the weight of the coset leader is greater than or equal to the minimum distance of C, the coset leader is included to the basis of C.

A performance evaluation method of turbo codes is proposed. For turbo codes with code-matched interleaver, performance is approximately evaluated by using conditional weight enumerating function with input weight greater than two. Performance of turbo codes over an inter-symbol interference (ISI) channel is evaluated by using a combined trellis diagram, into which the trellis diagram of the component code and the trellis diagram of the inter-symbol interference channel are combined. Optimum codes are searched for an ISI channel and for an ISI-free channel with the code matched-interleaver. Simulation results show the evaluated performance is valid.

The effects of noisy estimates of fading on turbo-coded modulation are studied in the presence of flat Rayleigh fading, and the channel capacity of the system is calculated to determine the limit above which no reliable transmission is guaranteed. This limit is then compared to the signal-to-noise ratio required for a turbo-coded modulation scheme to achieve a bit-error-rate of 10^-5. Numerical results are obtained, especially for QAM signals. Our results show that even slightly noisy estimates significantly degrade the theoretical limits related to channel capacities, and that an effective use of capacity-approaching codes can lower the sensitivity to noisy estimates, though noise that exceeds a certain threshold cannot be offset by the performance improvement associated with error-correcting capability.

In this paper, we present a new soft-decision iterative decoding algorithm using an efficient minimum distance search (MDS) algorithm. The proposed MDS algorithm is a top-down and recursive MDS algorithm, which finds a most likely codeword among the codewords at the minimum distance of the code from a given codeword. A search is made in each divided section by a "call by need" from the upper section. As a consequence, the search space and computational complexity are reduced significantly. The simulation results show that the proposed decoding algorithm achieves near error performance to the maximum likelihood decoding for any RM code of length 128 and suboptimal for the (256, 37), (256, 93) and (256, 163) RM codes.

In this paper, we propose a pretty-simple password-authenticated key-exchange protocol, which is proven to be secure in the standard model under the following three assumptions. (1) DDH (Decision Diffie-Hellman) problem is hard. (2) The entropy of the password is large enough to avoid on-line exhaustive search (but not necessarily off-line exhaustive search). (3) MAC is selectively unforgeable against partially chosen message attacks, (which is weaker than being existentially unforgeable against chosen message attacks).

In this paper, a method is proposed to construct an n-out-of-n visual secret sharing scheme for gray-scale images, for short an (n,n)-VSS-GS scheme, which is optimal in the sense of contrast and pixel expansion, i.e., resolution. It is shown that any (n,n)-VSS-GS scheme can be constructed based on the so-called polynomial representation of basis matrices treated in [15],[16]. Furthermore, it is proved that such construction can attain the optimal (n,n)-VSS-GS scheme.

Space-time turbo codes have both advantages of space-time codes and turbo codes, and the space-time turbo code proposed by Su and Geraniotis is known to achieve full coding rate and full antenna diversity. This paper presents some improvements of their space-time turbo code in a two-antenna configuration. We first propose a new condition for full antenna diversity which imposes less constraints on the interleaver. Next, by applying a method used to improve turbo trellis-coded modulation to the space-time turbo code, we propose a new decoding algorithm which utilizes more precise estimates on extrinsic information. Simulation results show that the proposed condition assures full antenna diversity and the new decoding algorithm provides a better performance than that of Su and Geraniotis'.

We investigated the suppression of multiuser interference in uplink multicarrier CDMA systems using the minimum mean squared error combining (MMSEC) method. In MMSEC, many pilot symbols are required to converge the weight vectors, and if we use just a few pilot symbols, the performance cannot be improved very much. We therefore developed a method for calculating weight vectors for MMSEC that uses just a few pilot symbols. The impulse responses of all users are first estimated using the pilot symbols in the time domain and modulated by a discrete Fourier transform. Next, the correlation matrices and correlation vectors are estimated from the impulse responses and the spreading codes of all users. Finally, the weight vectors that are obtained from the correlation matrices and correlation vectors are multiplied by the received signal to suppress the multiuser interference. The results of computer simulations indicated that the bit-error-ratio performance obtained using this method was better than that obtained when using the conventional fading compensation scheme or when using conventional MMSEC with the recursive least squares algorithm.

In this letter, an analysis of bit error probability of 4-state soft decision Viterbi decoding is presented. The bit error probability of recursive systematic convolutional code is also derived.

This letter proposes a method which can embed index data such as memos into JPEG images. The method embeds digital watermarks using the quantitative relation between quantized DCT coefficients in JPEG images. In the method, we can embed extra data to represent index data and can extract the index data without parameters used in embedding process. Furthermore, the method is tolerant of JPEG recompression and prevents the degradation of image quality by rewriting index data.

We propose bit error rate (BER) evaluation methods for a trellis coded modulation (TCM) scheme over a Rayleigh fading channel by using importance sampling (IS). The simulation probability density function for AWGN and Rayleigh fading is separately designed. For efficient simulation of a system model with finite interleaver, frequency of the generation of fading sequences is reduced. The proposed method gives a good BER estimates over a Rayleigh fading channel.

Multiple access interference (MAI) due to many simultaneous users is the main factor that limits the performance of DS-CDMA system. Multiuser detection is a method to avoid performance degradation due to MAI. We propose a blind multiuser detection method based on the algorithm consisting of two-stage decoding process, i.e., linearly constrained constant modulus (LCCM) and iterative least squares (ILS). The computer simulations confirmed that the algorithm is near-far resistant and that the proposed method is effective in the application to the slow fading channels.

The UCHT (Unified Complex Hadamard Transform) has been proposed as a new family of spreading sequences for DS-SSMA systems recently. In this Letter, the periodic autocorrelation (PAC) properties of the Unified Complex Hadamard Transform (UCHT) sequences are analyzed. Upper bounds for the out-of-phase PAC are derived for two groups of the UCHT sequences, namely the HSP-UCHT and the NHSP-UCHT sequences (the later is a more general representation of the well-known Walsh-Hadamard (WH) sequences). A comparison of the two bounds is performed. It turns out that the HSP-UCHT sequences have a lower upper bound for the out-of-phase PAC. This makes the HSP-UCHT sequences more effective than the WH sequences in combating multipath effect for DS-SSMA systems.

This paper addresses the single channel speech enhancement method which utilizes the mean value and variance of the logarithmic noise power spectra. An important issue for single channel speech enhancement algorithm is to determine the trade-off point for the spectral distortion and residual noise. Thus the accurate discrimination between speech spectral and noise components is required. The conventional methods determine the trade-off point using parameters obtained experimentally. As a result spectral discrimination is not adequate. And the enhanced speech is deteriorated by spectral distortion or residual noise. Therefore, a criteria to determine the point is necessary. The proposed method determines the trade-off point of spectral distortion and residual noise level by discrimination between speech spectral and noise components based on statistical criteria. The spectral discrimination is performed using hypothesis testing that utilizes means and variances of the logarithmic power spectra. The discriminated spectral components are divided into speech-dominant spectral components and noise-dominant ones. For the speech-dominant ones, spectral subtraction is performed to minimize the spectral distortion. For the noise-dominant ones, attenuation is performed to reduce the noise level. The performance of the method is confirmed in terms of waveform, spectrogram, noise reduction level and speech recognition task. As a result, the noise reduction level and speech recognition rate are improved so that the method reduces the musical noise effectively and improves the enhanced speech quality.

This paper proposes a design method of 2-D variable IIR digital filters with high frequency tuning accuracy. In the proposed method, a parallel complex allpass structure is used as the prototype structure of the 2-D variable digital filters in order to obtain low sensitivity characteristic. Because the proposed 2-D variable digital filter is composed of first-order complex allpass sections connected in parallel, the proposed variable digital filter possesses several advantages such as low sensitivity characteristic in the passband, simple stability monitoring and high parallelism. In order to improve the frequency tuning accuracy of the proposed variable digital filter, each first-order complex allpass section is substituted by a new first-order complex allpass section with low sensitivity characteristic. Moreover, the coefficient sensitivity analysis of a 2-D parallel complex allpass structure is presented. Numerical examples show that the proposed 2-D variable IIR digital filter has high tuning accuracy under the finite coefficient wordlength.

This article presents a three-step method for designing linear phase FIR filters with signed-powers-of-two (SPT) coefficients. In Step one, a prototype optimal FIR filter is designed by the Remez exchange algorithm. In Step two, a scaling factor is selected by employing simple ad-hoc rules. In Step three, each coefficient of the prototype filter is scaled by the scaling factor and is quantized coarsely as the canonic-signed-digit (CSD) representation. Then, a mixed-integer-linear-programming (MILP) algorithm is applied to three least significant digits (LSDs) of the filter's coefficients to reduce the number of SPT terms. Design examples demonstrate that the proposed algorithm is able to produce linear phase fixed-point FIR filters using fewer SPT terms than the existing methods under the same normalized peak ripple magnitude (NPRM) specification.

A fast capture algorithm of the initial attitude is proposed for the spacecraft using reaction wheels. This algorithm is composed of two steps. The first step deals with absorption of the initial angular momentum of the spacecraft into the reaction wheels and the second step a rest-to-reset large angle maneuver for sun tracking. A criterion of the initial attitude capture and a controller for the fast large angle maneuver method are presented under considering constraints of the reaction wheels. Simulation results show that the proposed algorithm has several advantages of the initial attitude capture and fast large angle maneuver. This algorithm can be applied to the near minimum time rotation control of the spacecraft.

As two simple relaxation oscillators are coupled by periodical and instantaneous switching, the system exhibits rich superstable synchronous phenomena. In order to analyze the phenomena, we derive a hybrid return map of real and binary variables; and give theoretical results for (1) superstability of the synchronous phenomena and (2) period of the synchronous phenomena as a function of the parameters. Using a simple test circuit, typical phenomena are verified in the laboratory.

We present an efficiency improvement on an existing unlinkable divisible e-cash system. In the based e-cash system, an e-coin can be divided to spent, and thus the exact payments are available. Furthermore, to protect customer's privacy, the system also satisfies the unlinkability in all the payments, which is not satisfied in other existing divisible e-cash systems. The unlinkability means the infeasibility of determining whether two payments are made by the same customer. However, in the unlinkable divisible e-cash system, the payment protocol needs O(N) computations, and thus inefficient, where N indicates the divisibility precision. For example, in case of N=100,000, about 200,000 exponentiations are needed for the worst. We improve the payment protocol using the tree approach. In case of N=100,000, the protocol with our improvement needs only about 600 exponentiations for the worst. This good result can be obtained for other N which is more than about 100.

In this paper, we present the theoretical analysis of the bit error rate (BER) performance of Single-Carrier Modulation (SCM) and Orthogonal Frequency Division Multiplexing (OFDM) systems under two types of temporally localized man-made noises (generalized shot noise and bursty noise models) environments. The robustness of OFDM system against these two kinds of man-made noises is discussed and then compared with that of SCM system at the same transmission rate. We show that for OFDM system, the BER performance highly depends on the number of subcarriers and the strength of the man-made noise, i.e., the level of the power spectral density of the man-made noise. In addition to the common knowledge on OFDM, we show that OFDM system is sometime less robust to the man-made noises than SCM system.

To obtain a high throughput for transmission control protocol (TCP) connections over the wireless links, we previously proposed a novel transmission power control method for code division multiple access (CDMA) packet communication systems. By using this transmission power control method, we developed a transmission power control method and a packet multiplexing method to transmit constant bit rate (CBR) and TCP packets over CDMA wireless systems. Our methods can guarantee the quality of service (QoS) for CBR connections and utilize bandwidth effectively without modifying the TCP protocol or using slot assignments. Evaluation of our methods by computer simulation showed that the proposed methods provide a near-maximum throughput and guarantee the packet loss ratio of CBR connections regardless of the number of connections.

A performance comparison is developed between a chaotic communication system and a spread spectrum system with similar features in terms of bandwidth and transceiver structure but based on more conventional Gold sequences. Comparison is made in the presence of noise and multipath contributions which degrade the channel quality. It is shown that, because of its more favourable correlation properties, the chaotic scheme exhibits lower error rates, at a parity of the bandwidth expansion factor. The same favourable correlation properties are also used to explain and show, through a numerical example, the benefits of chaotic segments in a multi-user environment.

Wang et al., in 2000, proposed a generalized group-oriented threshold signature scheme and a generalized authenticated encryption scheme with shared verification. Tseng et al., in 2001, showed that both schemes are insecure, because any attacker is able to reveal the group secret keys from two previously valid threshold signatures. They further presented two improvements on Wang et al.'s schemes to against the attacks. Unfortunately, this paper will show that the improved schemes are still breakable.

When solving combinatorial optimization problems with a binary Hopfield-type neural network, the updating process in neural network is an important step in achieving a solution. In this letter, we propose a new updating procedure in binary Hopfield-type neural network for efficiently solving combinatorial optimization problems. In the new updating procedure, once the neuron is in excitatory state, then its input potential is in positive saturation where the input potential can only be reduced but cannot be increased, and once the neuron is in inhibitory state, then its input potential is in negative saturation where the input potential can only be increased but cannot be reduced. The new updating procedure is evaluated and compared with the original procedure and other improved methods through simulations based on N-Queens problem. The results show that the new updating procedure improves the searching capability of neural networks with shorter computation time. Particularly, the simulation results show that the performance of proposed method surpasses the exiting methods for N-queens problem in synchronous parallel computation model.

One of the reasons of slow convergence in Backpropagation learning is the diminishing value of the derivative of the commonly used activation functions as the nodes approach extreme values, namely, 0 or 1. In this letter, we propose arctangent activation function to accelerate learning speed. Simulation results indicate considerable improvement in convergence performance.

Recent physiological studies on synaptic plasticity have shown that synaptic weights change depending on fine timing of presynaptic and postsynaptic spikes. Here, we show that a phenomenon similar to stochastic resonance with respect to background noise is observed on spike-timing dependent synaptic plasticity (STDP) that can contribute to stable propagation of precisely timed spikes in a multi-layered feedforward neural network.