This paper proposes an OFDM mobile packet transmission scheme that increases throughput by using nonlinear multiuser detection (MUD) and automatic repeat request (ARQ) with metric-combining. The scheme identifies users by detecting user identification (ID) symbols located at the head of a packet, and can separate packets that have collided by using MUD. It also forces the respective transmitters to retransmit the same packets so as to reproduce the collision if the cyclic redundancy check (CRC) detects some errors, and it uses metric-combining to decrease the number of retransmissions. The results of computer simulations show that the proposed scheme can provide twice the throughput of the conventional schemes.

In W-CDMA systems, distributions of the interference power and the total transmit power both measured at base stations are respectively used for capacity analysis in the uplink and downlink. For accurate capacity analysis, these quantities must be in proportion to the traffic amount. However, these quantities are no longer in proportion to the traffic amount since the transmit power control maintains the signal to interference power ratio at a constant level. Although the relationship between these measurements and the traffic amount has been investigated, there are still challenges to calculate the statistics such as the blocking probability or the outage probability accurately. This paper proposes a method to calculate the blocking probability by transforming the distributions of these measurements into distributions that are referred to as "traffic equivalent distributions," where the distributions are automatically adjusted according to the traffic amount. The calculated results show good agreement with the results obtained by dynamic computer simulations in the uplink, and show good agreement in the downlink as well when the traffic load is light. Accurate calculation of the blocking probability using a feedback loop and the observation of the traffic equivalents is also reported.

We show that the necessary and sufficient condition for the existence of a balanced quatrefoil decomposition of the complete multigraph λKn is n 9 and λ(n - 1) 0 (mod 24). Decomposition algorithms are also given.

Similar to direct sequence code division multiple access (DS-CDMA), site diversity can be applied to a multicarrier-CDMA (MC-CDMA) cellular system to improve the bit error rate (BER) performance for a user with weak received signal power, thus resulting in an increased link capacity. In this paper, the downlink site diversity reception using frequency-domain equalization based on minimum mean square error (MMSE) is considered for a MC-CDMA cellular system. A set of active base stations to be involved in the site diversity operation is determined based on the received signal power measurement by a mobile station. Downlink capacity with site diversity is evaluated by computer simulation. The impacts of path loss exponent and shadowing loss standard deviation on the site diversity effect are discussed. Furthermore, the performance improvement by antenna diversity reception is discussed.

In MC-CDMA, the data rate can be increased by reducing the spreading factor SF or by allowing multicode transmission. In this paper, we examine by computer simulations which gives a better bit error rate (BER) performance--lower SF or multicode operation--when high level modulation is used in addition to error control coding. For a coded system in a frequency selective channel, there is a trade-off among frequency diversity gain due to spreading, improved coding gain due to better frequency interleaving effect and orthogonality distortion. It is found that for QPSK, the performance of OFDM (MC-CDMA with SF = 1) is almost the same as that of a fully spread MC-CDMA system. However, for 16QAM and 64QAM, the BER performance is better for lower SF unlike the uncoded system, wherein higher SF gives a better BER.

We consider space division multiplexing in a MIMO-OFDM system for high data rate transmission. Channel estimation is very important for suppressing interference and demultiplexing signals. In a wireless LAN system such as IEEE 802.11a, only a few training symbols are inserted in each subcarrier. First, we propose a channel estimation method for a MIMO-OFDM system with two training symbols per subcarrier. The basic idea is to estimate the time-domain channel responses between the transmit and receive antennas. The array response vectors for each subcarrier are calculated by applying a fast Fourier transform to them. We then can obtain the adaptive weights to cancel the interference. We show that employing training symbols having a lower condition number of the matrix used for the channel estimation improves the estimation accuracy. Furthermore, we show the bit error rate for several signal detection schemes using the above estimated channel. It is shown that an ordered successive detection based on an MMSE criterion has excellent performance, that is, it can achieve higher-speed transmissions with a lower transmit power.

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.

We regard the events of a Markov decision process as the outputs from a Markov information source in order to analyze the randomness of an empirical sequence by the codeword length of the sequence. The randomness is an important viewpoint in reinforcement learning since the learning is to eliminate the randomness and to find an optimal policy. The occurrence of optimal empirical sequence also depends on the randomness. We then introduce the Lempel-Ziv coding for measuring the randomness which consists of the domain size and the stochastic complexity. In experimental results, we confirm that the learning and the occurrence of optimal empirical sequence depend on the randomness and show the fact that in early stages the randomness is mainly characterized by the domain size and as the number of time steps increases the randomness depends greatly on the complexity of Markov decision processes.

A digital signature does not allow any alteration of the document to which it is attached. Appropriate alteration of some signed documents, however, should be allowed because there are security requirements other than that for the integrity of the document. In the disclosure of official information, for example, sensitive information such as personal information or national secrets is masked when an official document is sanitized so that its nonsensitive information can be disclosed when it is demanded by a citizen. If this disclosure is done digitally by using the current digital signature schemes, the citizen cannot verify the disclosed information correctly because the information has been altered to prevent the leakage of sensitive information. That is, with current digital signature schemes, the confidentiality of official information is incompatible with the integrity of that information. This is called the digital document sanitizing problem, and some solutions such as digital document sanitizing schemes and content extraction signatures have been proposed. In this paper, we point out that the conventional digital signature schemes are vulnerable to additional sanitizing attack and show how this vulnerability can be eliminated by using a new digitally signed document sanitizing scheme with disclosure condition control.

In a quasi-synchronous reverse link multicarrier code division multiple access system, the signal detection is vulnerable to the interference due to the insufficient guard interval. A multiuser detection with intersymbol interference cancellation is a potential solution to overcome this problem. In this paper, we proposes a parameter acquisition technique based on a specially designed training sequence for the receiver. The concerned parameters are the transformed signature sequences and the ISI generation sequences. We analyze a criterion for the training sequences to achieve the minimum mean square error and propose the systematic generation of the optimum training sequences. We also propose the noise variance estimator for providing information about noise variance to some classes of multiuser detection. Simulation results prove relevant benefits of the proposed techniques and give useful insights into the system designs.

Analysis of speech acts and discourse structures is essential to a dialogue understanding system because speech acts and discourse structures are closely tied with the speaker's intention. However, it has been difficult to infer a speech act and a discourse structure from a surface utterance because they highly depend on the context of the utterance. We propose a statistical dialogue analysis model to determine discourse structures as well as speech acts using a maximum entropy model. The model can automatically acquire probabilistic discourse knowledge from an annotated dialogue corpus. Moreover, the model can analyze speech acts and discourse structures in one framework. In the experiment, the model showed better performance than other previous works.

For each positive integer r 1, a nondeterministic machine M is r path-bounded if for any input word x, there are r computation paths of M on x. This paper investigates the accepting powers of path-bounded one-way (simple) multihead nondeterministic finite automata. It is shown that for each k 2 and r 1, there is a language accepted by an (r + 1) path-bounded one-way nondeterministic k head finite automaton, but not accepted by any r path-bounded one-way nondeterministic k head finite automaton whether or not simple.

This paper describes the choke-loaded patch array antenna for use in the IMT-2000 repeater systems. The choke structure of the 4-element array is designed by means of an electromagnetic analysis. A high front-to-back (FB) ratio is required for suppressing mutual coupling in order to stop the oscillation caused by the interference waves between a transmitting and receiving antenna. The suppression of the FB ratio by a choke is limited in the case of the 16-element array because its side lobe level is large. In this paper, we examine the effect of suppressing the mutual coupling using a binomial array.

A Shared-State Hidden Markov Model (SS-HMM) has been widely used as an acoustic model in speech recognition. In this paper, we propose a method for constructing SS-HMMs within a practical Bayesian framework. Our method derives the Bayesian model selection criterion for the SS-HMM based on the variational Bayesian approach. The appropriate phonetic decision tree structure of the SS-HMM is found by using the Bayesian criterion. Unlike the conventional asymptotic criteria, this criterion is applicable even in the case of an insufficient amount of training data. The experimental results on isolated word recognition demonstrate that the proposed method does not require the tuning parameter that must be tuned according to the amount of training data, and is useful for selecting the appropriate SS-HMM structure for practical use.

This paper reports the strength of a pseudorandom number generator MUGI, which was published as a stream cipher by Hitachi, Ltd. in 2001, against linear cryptanalysis. MUGI is one of the recommended ciphers of CRYPTREC, which is a project for the e-Government in Japan. It has two internal states called state and buffer, which are updated by a linear function λ and a non-linear function ρ. The non-linear function ρ and the linear function λ have already been analyzed, independently. In this paper, whole MUGI is analyzed by truncated linear cryptanalysis. The analysis of λ function is based on the state variables method. The result is combined to the result of the analysis of ρ function to make a trellis diagram. Viterbi search is conducted on the diagram to find the best possible linear path under 64-bit truncated linear cryptanalysis. As the result, the upper bound of the maximum linear characteristic probability is estimated as less than 2-138. Therefore, MUGI is secure against linear cryptanalysis.

In this paper we propose a new side channel attack, where exponent recodings for public key cryptosystems such as RSA and ECDSA are considered. The known side channel attacks and countermeasures for public key cryptosystems were against the main stage (square and multiply stage) of the modular exponentiation (or the point multiplication on an elliptic curve). We have many algorithms which achieve fast computation of exponentiations. When we compute an exponentiation, the exponent recoding has to be carried out before the main stage. There are some exponent recoding algorithms including conditional branches, in which instructions depend on the given exponent value. Consequently exponent recoding can constitute an information channel, providing the attacker with valuable information on the secret exponent. In this paper we show new algorithms of attack on exponent recoding. The proposed algorithms can recover the secret exponent, when the width-w NAF and the unsigned/signed fractional window representation are used.

Differential power analysis (DPA) might break implementations of elliptic curve cryptosystem (ECC) on memory constraint devices. Goubin proposed a variant of DPA using a point (0,y), which is not randomized in Jacobian coordinates or in an isomorphic class. This point often exists in standardized elliptic curves, and we have to care this attack. In this paper, we propose zero-value register attack as an extension of Goubin's attack. Note that even if a point has no zero-value coordinate, auxiliary registers might take zero value. We investigate these zero-value registers that cannot be randomized by the above randomization. Indeed, we have found several points P = (x,y) which cause the zero-value registers, e.g., (1) 3x2 + a = 0,(2) 5x4 + 2ax2 - 4bx + a2 = 0,(3) P is y-coordinate self-collision point, etc. We demonstrate the elliptic curves recommended in SECG that have these points. Interestingly, some conditions required for zero-value register attack depend on explicit implementation of addition formulae -- in order to resist this type of attacks, we have to care how to implement the addition formulae. Finally, we note that Goubin's attack and the proposed attack assume that a base point P can be chosen by attackers and a secret scalar d is fixed, so that they are not applicable to ECDSA.

This paper proposes fast elliptic curve multiplication algorithms resistant against side channel attacks, based on the Montgomery-type scalar multiplication. The proposed scalar multiplications can be applied to all curves over prime fields, e.g., any standardized curves over finite fields with characteristic larger than 3. The method utilizes the addition formulas xECDBL and xECADD assembled by only x-coordinates of points, and is applicable for any types of curves over finite fields. Then, we encapsulate two addition formulas into one formula xECADDDBL, which accomplishes a faster computation because several auxiliary variables of two formulas can be shared. We also develop a novel addition chain for the new formula, with which we can compute scalar multiplications. The improvement of our scalar multiplications over previous Coron's dummy operation method is about 18% for a 160-bit scalar multiplication. Our method requires no table-up of precomputed points and it is suitable for the implementation on memory constraint computing architectures, e.g., smart cards. Moreover, we optimize the proposed algorithms for parallelized implementations with SIMD operations. Compared with the similar scheme proposed by Fischer et al., our scheme is about 16% faster.

An inverse-RZ modulation scheme for dense WDM systems is proposed. Inverse-RZ signals have tolerances to chromatic dispersion and optical bandwidth limitation. The strongly pre-filtered inverse-RZ signals can be adapted to ultra-dense WDM systems, in which the spectral efficiencies are over 1.0 b/s/Hz. We have confirmed the error-free transmission of pre-filtered and co-polarized 40-Gb/s inverse-RZ signals where the channel intervals were 37.5 GHz.

This paper presents a computer-aided design procedure of simulated annealing algorithm to optimize dual-wideband microstrip line filters with symmetrical at least 500 MHz bandwidths respectively. This method demonstrates the superiority of designing microstrip line dual-band filters. The value of characteristic impedances and electrical lengths of transmission lines synthesizing 2.4 GHz and 5.2 GHz dualband filters with a single input and a single output are adjusted to the annealing process by the optimization algorithm. The fabricated dual-wideband spectral transmittance and reflectance curves of these filters applying this method all effectively achieved desired high performances and resulted in a lower cost dual-band filters and open the way to commercial mass production. The method is applicable not only in 2.4 GHz and 5.2 GHz, but can be applied to any other multi-frequency bands.