A new technique based on the auto-correlation function is described for the estimation of the Doppler spread in mobile communication systems. We first propose to divide a uncertainty region of Doppler spread into multiple frequency bins. Based on the given multiple bins the correlator compares the estimated value at a certain time index to the theoretical exact value and then decides which bin the Doppler spread is estimated in. The certain time index can be optimized to give the largest decision region among multiple bins. We derive the optimum time index algorithm to give the largest decision region for each bin based on Rayleigh fading channel. We also apply the same Doppler spread estimator to the Rician case with the slight transformation of the received signal. We show that the proposed technique is not affected significantly by the Rician factor and the SNR degradation with the reasonable number of samples for estimation which is not the case of other estimators given in the literature.

In this paper, we propose pilot-assisted decision feedback channel estimation (PA-DFCE) for space-time coded transmit diversity (STTD) in orthogonal frequency division multiplexing (OFDM). Two transmit channels are simultaneously estimated by transmitting the STTD encoded pilot. To improve the tracking ability of the channel estimation against fast fading, decision feedback is also used in addition to pilot. For noise reduction and preventing the error propagation, windowing of the estimated channel impulse response in the time-delay domain is applied. The average bit error rate (BER) performance of OFDM with STTD is evaluated by computer simulation. It is found that the use of PA-DFCE can achieve a degradation in the required Eb/N0 from ideal CE of as small as 0.6 dB for an average BER = 10-3 and requires about 2.4 dB less Eb/N0 compared to differential STTD that requires no CE.

Since the interference is quite related to the performance of CDMA cellular systems, it is necessary to estimate Other-Cell-Interference Factor (OCIF). Here, starting from OCIF calculation for an aeronautical communication system, we investigate the forward link performance of data packet transmission in which the capacity, throughput and delay of the system are measured. To the numerical results, one can see that the performance of the aeronautical communication system is worse than that for terrestrial cellular systems and also depends logarithmically on both the cell radius and height.

In the Multi Carrier (MC)-CDMA system, the frequency diversity gain is obtained by its being spread in the frequency domain. The frequency interleaving technique can improve the frequency diversity gain. In this paper, the bit error rate (BER) performance in the MC-CDMA system which adopts the frequency interleaving scheme in the frequency selective fading channel is evaluated by computer simulation. In this simulation, orthogonal restoration combining (ORC) and minimum mean square error combining (MMSEC) are considered as frequency equalization combining techniques. This paper shows that BER performance with the frequency interleaver is better than without it in various environments.

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.

In this paper, we consider a mobile system consisting of a single isolated circular cell with K independent users simultaneously sharing the channel using binary DS-CDMA to establish a full duplex channel with the base station. Both coherent and differential detection RAKE receivers with Maximal Ratio Combining (MRC) techniques are considered. The performance of two uplink/downlink receivers in Nakagami wideband fading channel is studied. Our approach relies on the use of total instantaneous interference power calculations instead of the use of average power approximations. We analyzed and derived new exact formulae for bit error probabilities for the considered system, and presented a set of numerical results both for the exact formulae and Gaussian approximation. The performance comparisons suggest that the exact formulae provide superior performance to Gaussian approximation especially at low number of users and either high fading parameters of the desired user or low fading parameters of the MAIs.

Adaptive routing algorithms, which dynamically select the route of a packet, have been widely studied for interconnection networks in massively parallel computers. An output selection function (OSF), which decides the output channel when some legal channels are free, is essential for an adaptive routing. In this paper, we propose a simple and efficient OSF called minimal multiplexed and least-recently-used (MMLRU). The MMLRU selection function has the following simple strategies for distributing the traffic: 1) each router locally grasps the congestion information by the utilization ratio of its own physical channels; 2) it is divided into the two selection steps, the choice from available physical channels and the choice from available virtual channels. The MMLRU selection function can be used on any type of network topology and adaptive routing algorithm. Simulation results show that the MMLRU selection function improves throughput and latency especially when the number of dimension becomes larger or the number of nodes per dimension become larger.

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 paper addresses the problem of channel estimation for multiple-input and multiple-output orthogonal frequency division multiplexing (MIMO-OFDM)-based wireless networks with frequency-reuse. Firstly, the optimal time-domain training sequences are derived for the multiple cells with the same frequency group and a set of suitable sequences are also presented for practical implementation. Secondly, a low complexity iterative algorithm is combined with the time domain channel estimation to suppress the co-channel interferences (CCIs). The channel estimation method is applied to synchronous and asynchronous cellular and the MSE performance of the estimator is also analyzed. Simulation results demonstrate that the presented channel estimation approach can substantially suppress CCIs and outperform the conventional LS MIMO-OFDM channel estimation over multipath fading channels in multicell environments.

This paper proposes a scattered-pilot-OFDM reception scheme employing turbo inter-carrier interference (ICI) cancellation in the fast varying fading environments of mobile communications. In the OFDM transmission, the orthogonality among the subcarriers cannot hold due to large Doppler shift, and the OFDM signal suffers from severe degradation due to ICI. The proposed receiver carries out two modes: (i) a coherent detection (CD) mode, and (ii) a turbo ICI cancellation (TC) mode. Initially, the receiver performs the CD mode. When any decision errors are detected, it shifts from the CD mode to the TC one that carries out both the ICI cancellation and the channel estimation by using the decoder output (the log likelihood ratio). In addition, the iteration of the TC mode can improve the accuracy of the channel estimation and ICI cancellation ability. Computer simulations following specifications for the mobile reception mode in the digital terrestrial television broadcasting demonstrate that the receiver can effectively cancel ICI due to the fast varying fading, and that its average BER performance is much better than that of CD.

Isogeny for elliptic curve cryptosystems was initially used for efficient improvement of order counting methods. Recently, Smart proposed a countermeasure using isogeny for resisting a refined differential power analysis by Goubin (Goubin's attack). In this paper, we examine a countermeasure using isogeny against zero-value point (ZVP) attack that is generalization of Goubin's attack. We show that some curves require higher order of isogeny to prevent ZVP attack. Moreover, we prove that the class of curves that satisfies (-3/p) = 1 and whose order is odd cannot be mapped by isogeny to curves with a = -3 and secure against ZVP attack. We point out that three SECG curves are in this class. In the addition, we compare some efficient algorithms that are secure against both Goubin's attack and ZVP attack, and present the most efficient method of computing a scalar multiplication for each curve from SECG. Finally, we discuss another improvement for an efficient scalar multiplication, namely the usage of a point (0,y) for a base point of curve parameters. We are able to improve about 11% for double-and-add-always method, when the point (0,y) exists in an underlying curve or its isogeny.

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.

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.

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.

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.

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.

SFLASH was chosen as one of the final selection of the NESSIE project in 2003. It is one of the most efficient digital signature scheme and is suitable for implementation on memory-constrained devices such as smartcards. Side channel attacks (SCA) are a serious threat to memory-constrained devices. If the implementation on them is careless, the secret key may be revealed. In this paper, we experimentally analyze the effectiveness of a side channel attack on SFLASH. There are two different secret keys for SFLASH, namely the proper secret key (s,t) and the random seed Δ used for the hash function SHA-1. Whereas many papers discussed the security of (s,t), little is known about that of Δ. Steinwandt et al. proposed a theoretical DPA for finding Δ by observing the XOR operations. We propose another DPA on Δ using the addition operation modulo 232, and present an experimental result of the DPA. After obtaining the secret key Δ, the underlying problem of SFLASH can be reduced to the C* problem broken by Patarin. From our simulation, about 1408 pairs of messages and signatures are needed to break SFLASH. Consequently, SHA-1 must be carefully implemented in order to resist SCA on SFLASH.

In wide-area wireless access systems such as satellite communications systems and stratospheric platform systems, electric power supplies for radio communications are realized using solar photovoltaic cells and/or fuel cells. However, the on-board weight limits restrict the number of cells that can be equipped. In addition, the transmission power of such systems is limited taking account of issues and regulations on sharing the same frequency band with other systems. Hence, both the frequency band and electric power is limited, which are crucial radio resources for those systems. Although radio channel allocation methods taking account of the frequency constraint only or the power constraint only have been proposed, radio channel allocation methods taking account of both constraints simultaneously have been insufficiently studied. This paper proposes a radio channel allocation method that provides global optimum allocation results by utilizing the linear programming method. The proposed method has features such that the method first allocates radio channels in proportion to the traffic demand distributed over the service coverage area and then maximizes the total radio channels allocated to systems. Numerical results are presented for a stratospheric platform system that covers an area of Japan, as an example, to demonstrate that the proposed method optimally allocates radio channels taking account of both constraints while efficiently allocating excess resources. In addition, whether a system reaches either the frequency or power limit can be estimated, by investigating the radio channel allocation results. Furthermore, enhanced linear programming models based on a method aiming at practical use of the radio channel allocation results in operation are also introduced. The enhanced model is demonstrated to work effectively to avoid unbalanced radio channel allocations over geographical areas. The proposed method and linear programming models are useful not only for making pre-plans but also for determining the amount of necessary frequency and power resources in designing systems.

This paper presents a performance comparison between carrier interferometry coded OFDM (CI/COFDM) and typical coded OFDM (COFDM) using multiple receive antennas. The feature of CI/COFDM with diversity reception is to combine the time diversity benefit introduced by channel coding with the frequency and space diversity benefits created by frequency-domain equalization (FDE). Simulation results for QPSK showed that, at the perfect channel estimation, CI/COFDM outperforms COFDM under frequency and time selective fading channels because of the powerful time diversity benefit introduced by channel coding with time interleaving. However, at the imperfect channel estimation, this advantage of CI/COFDM over COFDM becomes very limited.

We propose a practical blind channel identification algorithm based on the principal component analysis. The algorithm estimates (1) the channel order, (2) the noise variance, and then identifies (3) the channel impulse response, from the autocorrelation of the channel output signal without using the eigenvalue and singular-value decomposition. The special features of the proposed algorithm are (1) practical method to find the channel order and (2) reduction of computational complexity. Numerical examples show the effectiveness of the proposed algorithm.