Publishing type：Research paper (international conference proceedings)   Kind of work：Joint Work  

Publishing type：Research paper (international conference proceedings)   Kind of work：Joint Work  

Publishing type：Research paper (international conference proceedings)   Kind of work：Joint Work  

Publishing type：Research paper (scientific journal)   Kind of work：Joint Work  

Publishing type：Research paper (scientific journal)   Kind of work：Joint Work  

Publishing type：Research paper (scientific journal)  

Publishing type：Research paper (scientific journal)  

Publishing type：Research paper (scientific journal)  

DOI： 10.1016/j.sigpro.2018.10.017

Publishing type：Research paper (international conference proceedings)  

DOI： 10.1109/GCCE46687.2019.9015305

Publishing type：Research paper (scientific journal)  

Publishing type：Research paper (international conference proceedings)  

DOI： 10.1109/ISPACS48206.2019.8986334

Publishing type：Research paper (scientific journal)  

© 2018 Wiley Periodicals, Inc. Proportional–integral–derivative (PID) controllers have been widely used for process systems. However, a good control result is not always obtained with fixed PID gains when a controlled object has nonlinearity. This paper proposes a design method for a nonlinear PID controller that utilizes a neural network to overcome the problem. In the proposed controller, PID gains are tuned online by a neural network and a controlled object is manipulated by the PID controller with the tuned PID gains. The neural network is learned by an offline learning algorithm based on the Extended Fictitious Reference Iterative Tuning (E-FRIT) and the backpropagation. E-FRIT is a method that tunes control parameters directly by using operating data and evaluates not only a controlled output but also the difference of manipulated variable. Simulation examples are provided to show the effectiveness of the proposed method. Moreover, the experimental result of a level control of a tank system is also given to demonstrate the performance of the proposed method.

DOI： 10.1002/eej.23141

Publishing type：Research paper (scientific journal)  

In this paper, we design a no-overloading error feedback quantizer based on a ΔΣ modulator, composed of an error feedback filter and a static quantizer. To guarantee no-overloading in the quantizer, we impose an l∞ norm constraint on the feedback signal in the quantizer. Then, for a prescribed l∞ norm constraint on the error at the system output induced by the quantizer, we design the error feedback filter that requires the minimum number of bits that achieves the constraint. Next, for a fixed number of bits for the quantizer, we investigate the achievable minimum l∞ norm of the error at the system output with the no-overloading quantizer. Numerical examples are provided to validate our analysis and synthesis.

DOI： 10.1109/TCSI.2017.2758801

Publishing type：Research paper (scientific journal)  

In this paper, we design noise shaping filters for quantization noise reduction in ΔΣ modulators. The design problem for the noise shaping infinite impulse response (IIR) filter in the feedback of the ΔΣ modulator is in non-convex form, which is hard to solve in general. We propose an iterative linear matrix inequality (LMI) algorithm to find a near optimal noise shaping IIR filter by solving the non-convex design problem. The least-squares and the hybrid design are two methods, either of which can serve as a starting point for our iterative LMI algorithm to obtain a near optimal solution. We also compare and analyze the performances of some other noise shaping techniques with our proposed algorithm. A design example for a bandpass ΔΣ modulator based radio frequency transmitter is provided which takes into account the non-ideal behavior of the output analog filter.

DOI： 10.1016/j.sigpro.2017.10.018

Publishing type：Research paper (scientific journal)  

In this study, a simple and high performance precoding scheme for downlink multi-user MIMO (DL MU-MIMO) systems is investigated. Sorted QR decomposition algorithm is adapted for zero-forcing (ZF) Tomlinson-Harashima precoding (THP) with user-stream permutation in order to select a permutation pattern which maximizes a sum-rate of the DL MU-MIMO system. In addition, allocating unequal number of bits to each user-stream improves the DL performance of the ZF-THP in the MU-MIMO scenario. Numerical results show that the proposed ZF-THP scheme outperforms a conventional MMSE-THP scheme.

DOI： 10.1587/comex.2018XBL0070

Publishing type：Research paper (international conference proceedings)  

This paper proposes a computationally efficient algorithm for downlink (DL) multiuser (MU) MIMO systems using a new zero-forcing (ZF) Tomlinson-Harashima precoding (THP) scheme maximizing a sum-rate. The ZF-THP is more computationally efficient than minimum mean square error (MMSE) THP
however, its bit error rate performance is inferior to the one. User stream permutations, which determine the order of the interference cancellation at the transmitter, is incorporated into the ZF-THP
then, we can select a permutation pattern so as to maximize the sum-rate. Since searching for an optimal permutation pattern is computationally cumbersome, we propose an efficient algorithm, which produces almost optimal user stream permutation, for the ZF-THP. We also propose to allocate unequal number of bits to each user to improve DL performance of the MU-MIMO system. Numerical results are provided to show that the proposed ZF-THP scheme outperforms the conventional MMSE-THP scheme.

DOI： 10.1109/SPAWC.2017.8227798

Publishing type：Research paper (scientific journal)  

Quantization is a fundamental process in digital signal processing, Delta Sigma Smodulators are often utilized for quantization, which can be easily implemented with static uniform quantizers and error feedback filters. In this paper, we analyze the mean squared quantization error of the quantizer with error feedback including the Delta Sigma modulators. First, we study the quantizer with an ideal optimal error feedback filter that minimizes the mean squared error (MSE) of quantization. We show that the amplitude response of the optimal error feedback filter can be parameterized by one parameter. This parameterization enables us to find the optimal error feedback filter numerically. Second, the relationship between the number of bits used for the quantizer and the achievable MSE is clarified by using the optimal error feedback filter. This makes it possible to investigate the efficiency of the quantizer with the optimal error feedback filter in terms ofMSE. Then, ideal optimal error feedback filters are approximated by practically implementable filters using the Yule-Walker method and the linear matrix inequality-basedmethod. Numerical examples are provided for demonstrating our analysis and synthesis.

DOI： 10.1109/TSP.2017.2745450

Publishing type：Research paper (international conference proceedings)  

In this paper, we propose two types of noise shaping infinite impulse response (IIR) filters for ΔΣ modulators based on H2 and H∞ norms of the in-band quantization noise. We utilize the hybrid-design and the least-squares approximation method to obtain noise shaping IIR filters by taking an indirect approach for solving the non-convex optimization problem. In both methods, we also incorporate the non-ideal behavior of the output lowpass filter into the design criteria of our noise shaping IIR filter as a weighting function. To demonstrate the effectiveness of proposed noise shaping IIR filters, we compare it with existing design methods.

DOI： 10.1109/ISCAS.2017.8050767

Publishing type：Research paper (scientific journal)  

Noise shaping filters for quantizers with error feedback are designed to mitigate the effects of quantization errors. In this paper, we prove that if the transfer function from the quantization error to the signal-of-interest has minimum phase and there is no constraint on the feedback signal, then the scaled inverse of the transfer function is the optimal noise shaping filter. Next, we design a noise shaping filter to minimize the variance or the l(2) norm of the error in the signal-ofinterest under a constraint on the variance or the l(2) norm of the feedback signal, which can be expressed as bilinear matrix inequalities (BMIs). Although the BMIs are not convex, we prove that the minimization of the error variance under the constraint on the variance of the feedback signal can be cast into a convex optimization problem. This enables us to design optimal noise shaping filters using numerical methods. We formulate our design problem as a convex optimization problem using extended linear matrix inequalities to obtain noise shaping filters, except for the special case. Examples are provided to demonstrate the effectiveness of the designed noise shaping filters.

DOI： 10.1109/TCSI.2016.2627031

Publishing type：Research paper (scientific journal)  

DOI： 10.1186/s13634-017-0454-3

Publishing type：Research paper (scientific journal)  

DOI： 10.1109/APSIPA.2017.8282157

Publishing type：Research paper (international conference proceedings)  

To develop reliable communication systems, we propose a single user multiple-input multiple-output communication system using zero-forcing (ZF) Tomlinson-Harashima precoding (THP) with adaptive bit allocation. Eigenbeam space division multiplexing (E-SDM), which utilizes a singular-value decomposition (SVD) and a water filling technique, is known to be an optimal linear precoding MIMO scheme. However, the SVD of a MIMO channel matrix requires a heavy computational cost. On the other hand, the ZF-THP is a non-linear precoding scheme which is computationally efficient while showing allowable bit error rate (BER) performance. In order to improve further in terms of the BER, we propose to allocate bits adaptively to minimize the upper-bound of the theoretical BER for every transmission in a fading environment. Numerical examples provide a comparison with the E-SDM method.

Publishing type：Research paper (international conference proceedings)  

Delta-Sigma modulators are often utilized to convert analog signals into digital signals. The quantization error of a Delta-Sigma modulator can be reduced by oversampling. However, oversampling increases the number of bits per time if the same number of bits are assigned to each output of the quantizer. Due to limited bandwidth, the rate-distortion relation is necessary to balance the rate and the distortion. In this paper, we analyze the relationship between the rate and the distortion of an optimal scalar Delta-Sigma modulator that minimizes the variance of the error in the output of the system connected to the Delta-Sigma modulator. Numerical exam pies are provided to show rate-distortion relations of the optimal Delta-Sigma modulators.

DOI： 10.1109/ICASSP.2017.7953024

Publishing type：Research paper (scientific journal)  

Optimal finite impulse response (FIR) error feedback filters for noise shaping in Delta Sigma modulators are designed by using weighting functions based on the system norms. We minimize the weighted norms of the quantization error in the output of a Delta Sigma modulator, which corresponds to the minimization of the system norm. Three norms, the H-2 system norm, the H-infinity system norm, and the l(1) norm of the impulse response of the system, are adopted. The optimization problem for three types of FIR filters are evaluated by using linear matrix inequalities (LMIs) and then solved numerically via semi-definite programming. Design examples are provided to demonstrate the effectiveness of our proposed methods.

DOI： 10.1186/s13634-016-0326-2

Publishing type：Research paper (international conference proceedings)  

In this paper, we propose a design procedure to obtain infinite impulse response (IIR) error feedback filters for Delta Sigma modulators. It is known that the design of optimal finite impulse response (FIR) error feedback filters for Delta Sigma modulators can be cast into a convex optimization problem, while the design of IIR error feedback filters cannot. To design IIR error feedback filters, we first obtain the optimal FIR error feedback filter and then approximate it with an IIR filter. As an example, we utilize the H-infinity norm of the filter to design the optimal filter. We provide design examples to show the effectiveness of IIR error feedback filters for Delta Sigma modulator designed by our proposed method.

DOI： 10.1109/APSIPA.2016.7820914

Publishing type：Research paper (scientific journal)  

In this paper, we propose a zero-forcing (ZF) Tomlinson-Harashima precoding (THP) with substream permutations based on the bit rate maximization for single-user MIMO (SU-MIMO) systems. We study the effect of substream permutations on the ZF-THP SU-MIMO systems, when the mean squared error (MSE) and the bit rate are adopted for the selection of the permutation matrix as criteria. Based on our analysis, we propose a method to increase the bit rate by substream permutations, and derive QR and Cholesky decomposition-based algorithms which realize the proposed method. Furthermore, to improve the error rate performance, we apply zero transmission to subchannels with low signal-to-noise ratios. Numerical examples are provided to demonstrate the effectiveness of the proposed THP MIMO system.

DOI： 10.1587/transfun.E98.A.1095

Other URL： http://dblp.uni-trier.de/db/journals/ieicet/ieicet98a.html#journals/ieicet/KinjoO15

Publishing type：Research paper (international conference proceedings)  

We consider MISO wiretap channels with multiple eavesdroppers. Under the deterministic channel uncertainties, the beamformer and the covariance of the artificial noise are jointly designed to minimize the transmit power subject to SINR constraints. Our design problems are resolved by using a semidefinite program, which can be numerically solved. Simulation results are provided to see the effects of channel uncertainties on the transmit power.

DOI： 10.1109/ICASSP.2015.7178544

Publishing type：Research paper (international conference proceedings)  

Optimal error feedback filters for AID converters of remote sensors in networked control systems are studied. It is shown that if the transfer function from the quantization error to the output of interest has a minimum phase, then its inverse is the optimal error feedback filter in terms of any kind of norms of errors. For general LTI systems, the design of error feedback filters is formulated as an optimization problem, which can be numerically solved. A design example is provided to demonstrate the effectiveness of our proposed method.

DOI： 10.1109/ICASSP.2015.7178695

Publishing type：Research paper (scientific journal)  

In this paper, we consider a data-embedding method in the discrete wavelet transform (DWT) domain of digital images. A bit pattern embedding method with the human visual system (HVS) and correlation-based detection have been proposed by Kino and Wada based on the watermarking scheme, which is referred to as the conventional method. In the conventional method, the capacity of embedding data depends on the potential correlation between the DWT coefficients of the host image and the watermark signal. We propose a new embedding and detection algorithm not affected by the potential correlation in order to increase the capacity of embedding data. The data-embedding and detection in the proposed method are performed by considering the absolute value of the correlation between the watermarked DWT coefficients and the watermark signal. In simulations, we show that the capacity of the embedding data in the proposed method is increased compared with the conventional method at the same PSNR value (image quality).

DOI： 10.1002/ecj.11597

Publishing type：Research paper (scientific journal)  

Most design problems of digital filters (or differentiators) are formulated with a set of grid point in the frequency region (frequency sampling). These problems are usually difficult to solve, and often require iterative optimization. The objective of this paper is to provide an efficient and simplified design approach to multi-band filters (including low-pass filters or high-pass filters) as well as full-band differentiators. The proposed method does not require frequency sampling and iterative optimization to compute the coefficients of the filters or that of the differentiators. The magnitude and phase specifications are simultaneously approximated, and the errors in the specified frequency bands are controlled by using frequency-weighting factors. In addition, a maximum pole radius, which corresponds to a stability margin, can be specified to robustly ensure the stability of the filters or the differentiators. To evaluate the efficiency of proposed method, we compare the proposed method with several established methods. Simulation results show that, although the propose method does not utilize frequency sampling and iterative optimization, the designed filters and differentiators have sufficient performance.

DOI： 10.1109/TIE.2013.2290765

Publishing type：Research paper (scientific journal)  

In this paper, preamble design for estimation and compensation of channel distortion parameters (or channel impairments) in orthogonal frequency-division multiplexing (OFDM) transmission over peak-limited channels is studied. Specifically, the designed preamble considers the estimation of frequency selective channels, carrier frequency offset (CFO), in-phase/quadraturephase (I/Q) imbalance together with the minimization of peak-to-average power ratio (PAPR) of the transmitted signals. In the proposed design, we employ adaptive Markov chain Monte Carlo (AMCMC) techniques to select preamble sequence that minimizes the channel estimate mean-squared error while suppressing the effect of the I/Q mismatch. AMCMC algorithm is also deployed to select phase information to the designed preamble in order to minimize the PAPR of the oversampled preamble signals in time domain. To estimate CFO, maximum likelihood-based scheme that utilizes two successive OFDM preambles is employed, and the CFO is estimated by considering phase rotation between two consecutive received OFDM preambles. Numerical simulations are provided to verify the efficacy of the proposed design. © The Authors, 2014.

DOI： 10.1017/ATSIP.2014.2

Publishing type：Research paper (international conference proceedings)  

We consider physical-layer security of a wireless LAN where multiple receivers collude to eavesdrop the information from the basestation to the intended receiver. To enhance the physical-layer security, we design the interference signals to combat the eavesdropping. Our design problems are resolved using semidefinite relaxation problems, which can be numerically solved efficiently by the existing convex optimization solvers. Simulation results are provided to demonstrate the efficiency of the proposed designs.

DOI： 10.1109/ICASSP.2014.6854766

Publishing type：Research paper (scientific journal)  

Low complexity equalizers are investigated for single carrier transmissions over doubly selective channels. The ZF equalizer that utilizes circulant matrices constructed from time-averaged channel coefficients exhibits good performance, keeping small computational complexity.

DOI： 10.1587/comex.3.80

CiNii Article

Publishing type：Research paper (international conference proceedings)  

In this paper, we consider load control in a multiple residence setup where energy consumption scheduling (ECS) devices in smart meters are employed for Demand-side management (DSM). Several residential end-users share the same energy source and each residential user has non adjustable loads, adjustable loads and a storage device. The ECS devices interact automatically by running an iterative distributed algorithm to find the optimal energy consumption schedule for each end-user, in order to reduce the total energy cost as well as the peak-to-average-ratio (PAR) in load demand in the system. Simulation results are provided to evaluate the performance of the proposed game theoretic based distributed DSM technique.

DOI： 10.1109/Kaleidoscope.2014.6858476

Publishing type：Research paper (scientific journal)  

In this article, challenges regarding the provision of channel state information (CSI) in non-contiguous orthogonal frequency division multiplexing (NC-OFDM) cognitive radio (CR) systems are addressed. We propose a novel scheme that utilizes cross entropy (CE) optimization together with an analytical pilot power distribution technique to design pilot symbols that minimizes the channel estimate mean squared error (MSE) of frequency-selective channels. The optimal selection of pilot subcarriers is a combinatorial problem that requires heavy computations. To reduce the computational complexity, the CE optimization is utilized to determine the position of pilot subcarriers. Then, for a given pilot placement obtained by the CE algorithm, a closed form expression to obtain optimal pilot power distribution is employed. Simulation results indicate that, the proposed pilot symbol design provides better channel estimate MSE as well as the bit error rate (BER) performance when compared with the conventional equal powered pilot design.

DOI： 10.1186/1687-6180-2013-51

Publishing type：Research paper (international conference proceedings)  

Highly correlated Multiple-Input Multiple-Output (MIMO) channels induce severe ill-conditioned MIMO channel matrices. MIMO receivers suffer from performance degradation due to the noise enhancement caused by the severe channel matrices. Precoding schemes, such as spatial Tomlinson-Harashima precoding (THP), can mitigate the performance degradation
however, the effect upon the highly correlated channels is insufficient. One way to avoid the noise enhancement is to turn off a few substreams related to the poor MIMO channels, which becomes possible by introducing an appropriate permutation matrix. We formulate a bit rate on the zero-forcing THP (ZF-THP) MIMO system, which shows that there is an optimum permutation matrix which maximizes the bit rate. We propose to apply a modified version of the sorted decomposition (SQRD) algorithm, which was originally developed for Vertical Bell Labs Layered Space Time (V-BLAST) receiver to carry out the QR decomposition of a MIMO channel matrix with permutation in its column direction simultaneously, to find a feasible permutation and create a precoder. Finally, it results in developing an efficient scheme to turn off the substreams. The proposed scheme is evaluated by computer simulations to verify its efficiency upon the correlated MIMO channels. © 2013 IEEE.

DOI： 10.1109/ISPACS.2013.6704557

Publishing type：Research paper (international conference proceedings)  

In this paper, training design for estimation of frequency selective channels, carrier frequency offset (CFO) and in-phase/quadrature-phase (I/Q) imbalance in multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) systems is proposed. We utilize convex optimization technique to optimize power of all active subcarriers, then we employ adaptive Markov chain Monte Carlo (AMCMC) techniques to select training sequence that minimizes the channel estimate mean squared error (MSE) while suppressing the effect of the I/Q imbalance. To estimate CFO, maximum likelihood (ML) based scheme that utilizes two successive OFDM training symbols is deployed. Numerical simulations are provided to corroborate the efficacy of the proposed design. © 2013 IEEE.

DOI： 10.1109/ISCIT.2013.6645883

Publishing type：Research paper (international conference proceedings)  

This paper presents an efficient peak to average power ratio (PAPR) reduction schemes for a multiple input multiple output (MIMO) orthogonal frequency division multiplexing (OFDM) based cognitive radio (CR) network. The proposed scheme makes use of a constrained adaptive Markov chain Monte Carlo (CAMCMC) technique to select peak reduction tones (PRT) for the tone reservation (TR) technique. The CAMCMC algorithm exploits non contiguous (NC) subcarriers available for a secondary user (SU) and make appropriate selection of the PRT set to reduce the PAPR of a MIMO-OFDM based CR system. To the selected PRT set, adaptive amplitude clipping (AAC) technique as well as convex optimization based schemes are employed to reduce the PAPR of the SU with multiple transmit antennas. Numerical simulations are provided to corroborate the efficacy of the designed PRT set with the AAC algorithm and the convex optimization technique.

DOI： 10.1109/ISPACS.2013.6704576

Publishing type：Research paper (scientific journal)  

This paper treats the minimization problem for roundoff noise in two-dimensional (2-D) recursive digital filters with an error feedback (EF). The EF is known as an available scheme for the reduction of roundoff noise, and this can be achieved by extracting a roundoff error from the quantizer and feeding it back to the adder. Since the EF circuit is an additional device of the digital filter, the bit length of the coefficients should be as short as possible for the fast arithmetic and low-cost realization. In this paper, we propose an algorithm to optimize the EF coefficients in a discrete space. The proposed algorithm is based on the branch and bound method by estimating the lower bound of the optimal solution, and the optimality of the obtained solution can be guaranteed. Finally, we give a numerical example to demonstrate the effectiveness of the proposed method, and show the optimal solution can be found without using the round-robin algorithm.

DOI： 10.1007/s00034-011-9376-8

Publishing type：Research paper (scientific journal)  

Publishing type：Research paper (scientific journal)  

In this article, we address the challenges regarding the provision of channel state information as well as reducing peak-to-average power ratio (PAPR) of a multiple input multiple output orthogonal frequency multiplexing (MIMO-OFDM) system. The mean squared error (MSE) of the channel estimate is adopted as the optimization criterion to design pilot symbols for channel estimation in MIMO-OFDM systems with null subcarriers. We design the placement and power distribution to the pilot symbols for multiple transmit antennas to minimize the MSE of the least square (LS) channel estimates. To reduce interference of the pilot symbols transmitted from different antennas, an algorithm to guarantee that pilot symbols are disjoint from any other transmitter pilot set is proposed. To efficiently reduce the PAPR of the MIMO-OFDM signals, a method that mixes dummy symbols and phase information of the pilot symbols is presented. Simulation results based on IEEE 802.16e are presented to illustrate the superior performance of our proposed channel estimation method over the existing standard and the partially equi-spaced pilot symbols. We also demonstrate that, by mixing the dummy symbols and phase information of the pilot symbols, the PAPR of the MIMO-OFDM signals can significantly be reduced.

DOI： 10.1186/1687-1499-2012-201

Publishing type：Research paper (international conference proceedings)  

In this paper, training symbol designs for estimation of frequency selective channels and compensation of in-phase (I) and quadrature (Q) imbalances on OFDM transmitters and receivers are studied. We utilize cross entropy (CE) optimization techniques together with convex optimization to design training sequence that minimizes the channel estimate mean squared error (MSE) as well as estimating the effect of I/Q mismatch while lowering the peak power of the training signals. The proposed design provide better channel estimate MSE and bit error rate (BER) performances. The efficacies of the proposed designs are corroborated by analysis and simulation results.

DOI： 10.1109/VETECS.2012.6239897

Publishing type：Research paper (international conference proceedings)  

Wireless communications are susceptible to eavesdropping. To interfere the eavesdropper having multiple antennas, secret information signals and interference signals that are transmitted from multiple antennas are designed. It is shown that our design can be cast into a semidefinite program, which can be numerically solved efficiently. Simulation results are provided to demonstrate the efficiency of the proposed design.

DOI： 10.1109/ICASSP.2012.6288597

Publishing type：Research paper (scientific journal)  

In this paper, we consider a design problem of two-dimensional (2-D) error feedback (EF) circuit for reduction of a roundoff noise in 2-D infinite impulse response (IIR) digital filters. Though the EF is effective for reduction of the roundoff noise, the number of shifters in the EF circuit should be small for the fast arithmetic and low-cost realization. Canonical signed digit (CSD) number is efficient to represent the EF coefficients with finite word-length because it can be represented with less non-zero bits (shifters) than the one with the generally used binary number. In this paper, we propose an algorithm to optimize the EF coefficients with CSD coefficients. The proposed algorithm is based on the branch and bound method by estimating the lower bound of the optimal solution. The good point of the proposed method is that the optimality of the obtained solution can be guaranteed. Finally, we give a numerical example to demonstrate the effectiveness of the EF with CSD coefficients. Also, we show that the number of shifters can be efficiently reduced by using the CSD coefficients. © 2012 The Institute of Electrical Engineers of Japan.

DOI： 10.1541/ieejeiss.132.1806

Publishing type：Research paper (international conference proceedings)  

A low-complexity linear minimum mean square error (LMMSE) based channel estimator is proposed for orthogonal frequency-division multiplexing (OFDM) systems over frequency-selective channels. Using the law of a large number, we approximate the LMMSE estimator to reduce the numerical complexity of the channel estimation. Our estimator exhibits comparable performance with the LMMSE estimator at low SNR but suffers the performance floor due to the approximation, which are verified by numerical simulations.

Other URL： http://dblp.uni-trier.de/db/conf/apsipa/apsipa2012.html#conf/apsipa/OhnoMM12

Publishing type：Research paper (international conference proceedings)  

In this paper, a low complexity equalizer for single carrier transmissions over doubly selective channels is developed. The main attribute of the proposed equalizer is its low complexity compared to the existing equalizers, which is attained at the expense of a slight performance loss. Simulation results show that the performance of our proposed low complexity equalizer is as good as that of the conventional equalizers with extensive computation complexity.

DOI： 10.1109/ICASSP.2012.6288594

Publishing type：Research paper (international conference proceedings)  

In this paper, challenges regarding the provision of channel state information (CSI) in non-contiguous orthogonal frequency division multiplexing (NC-OFDM) cognitive radio (CR) systems are addressed. We propose a novel scheme that utilizes convex optimization together with the cross entropy (CE) optimization to design pilot symbols that minimizes the channel estimate mean squared error (MSE) of frequency selective channels. Simulation results indicate that, the proposed pilot symbol design provides better channel estimate MSE as well as the bit error rate (BER) performance when compared with the equal powered pilot design.

Other URL： http://dblp.uni-trier.de/db/conf/eusipco/eusipco2012.html#conf/eusipco/ManassehON12

Publishing type：Research paper (international conference proceedings)  

In this paper, preamble design for estimation of frequency selective channels and In-phase/Quadrature-phase (I/Q) imbalance in multiple input multiple output orthogonal frequency division multiplexing (MIMO-OFDM) systems is proposed. First we utilize convex optimization to optimize power of all active subcarriers, then we employ cross entropy (CE) optimization techniques to select optimal preamble sequence that minimizes the channel estimate mean squared error (MSE) while suppressing the effect of I/Q mismatch. To mitigate inter-antenna interferences, disjoint preamble sequences are utilized for each transmit antenna. An algorithm to guarantee that the preamble sequences are disjoint for each transmitter is proposed. Numerical simulations are provided to verify the advantages and effectiveness of the proposed preamble sequences over the conventional sequences.

Other URL： http://dblp.uni-trier.de/db/conf/apsipa/apsipa2012.html#conf/apsipa/ManassehON12

Publishing type：Research paper (scientific journal)  

In this paper, challenges regarding the provision of channel state information (CSI) and carrier frequency synchronization for orthogonal frequency division multiplexing (OFDM) systems with null subcarriers are addressed. We propose novel maximum likelihood (ML) based schemes that estimate the aggregate effects of the CFO and channel by using two successive OFDM preambles. In the presented scheme, CFO is estimated by considering the phase rotation between two consecutive received OFDM preambles. Both single input single output (SISO) as well as multiple input multiple output (MIMO) OFDM systems are considered. The mean squared errors (MSE) of the channel and CFO are used to evaluate the performance of our proposed scheme. By using two successive OFDM preambles, the estimation of channel and the estimation of CFO are decoupled, which leads to a simple estimation method. Simulation results show that the BER performance of the proposed estimators is comparable to that of known channel state information and the CFO MSE performance achieves the Cramer-Rao bound (CRB) of the fully loaded OFDM system.

DOI： 10.1587/transfun.E94.A.2271

Other URL： http://dblp.uni-trier.de/db/journals/ieicet/ieicet94a.html#journals/ieicet/ManassehON11

Publishing type：Research paper (scientific journal)  

For copyright protection, a watermark signal is embedded in host images with a secret key, and a correlation is applied to judge the presence of watermark signal in the watermark detection. This paper treats a discrete wavelet transform (DWT)-based image watermarking method under specified false positive probability. We propose a new watermarking method to improve the detection performance by using not only positive correlation but also negative correlation. Also we present a statistical analysis for the detection performance with taking into account the false positive probability and prove the effectiveness of the proposed method. By using some experimental results, we verify the statistical analysis and show this method serves to improve the robustness against some attacks.

DOI： 10.1587/transfun.E94.A.661

Other URL： http://dblp.uni-trier.de/db/journals/ieicet/ieicet94a.html#journals/ieicet/NakamotoSMHO11

Publishing type：Research paper (scientific journal)  

Publishing type：Research paper (international conference proceedings)  

Training sequences for estimation of channel parameters have been well designed under the condition that the number of the unknown channel parameters is not greater than the number of the known symbols. Without the condition, we develop a method to obtain the optimal number of known pilot symbols for pilot-aided linear channel estimation in orthogonal frequency division multiplexing (OFDM). We derive an expression for channel estimation error covariance from which performance measures can be numerically evaluated to obtain the optimal number of pilot symbols. Using capacity lower bound as a performance measure, we demonstrate that for channels with exponential power profiles, it is better to estimate a truncated version of the channel response, which reveals the disadvantage of the existing design based on the condition.

DOI： 10.1109/ICASSP.2011.5946252

Publishing type：Research paper (international conference proceedings)  

We develop a closed-form approximation algorithm for designing IIR digital filters with linear phase and guaranteed stability. In this algorithm, the stopband and passband edge frequency can be specified in the frequency domain and the filter coefficients can be easily obtained for approximating the given frequency response. Additionally, there is a property that the maximum of absolute values of all poles can be controlled, so we can avoid the risk that the designed filters under finite word-length effects become unstable. Finally, a numerical example is given to illustrate the utility of the proposed method.

DOI： 10.1109/ICASSP.2011.5946814

Publishing type：Research paper (scientific journal)  

In this article, design of preamble for channel estimation and pilot symbols for pilot-assisted channel estimation in orthogonal frequency division multiplexing system with null subcarriers is studied. Both the preambles and pilot symbols are designed to minimize the l(2) or the l(infinity) norm of the channel estimate mean-squared errors (MSE) in frequency-selective environments. We use convex optimization technique to find optimal power distribution to the preamble by casting the MSE minimization problem into a semidefinite programming problem. Then, using the designed optimal preamble as an initial value, we iteratively select the placement and optimally distribute power to the selected pilot symbols. Design examples consistent with IEEE 802.11a as well as IEEE 802.16e are provided to illustrate the superior performance of our proposed method over the equi-spaced equi-powered pilot symbols and the partially equi-spaced pilot symbols.

DOI： 10.1186/1687-1499-2011-2

Publishing type：Research paper (international conference proceedings)  

We study the application of turbo equalization in orthogonal frequency division multiplexing (OFDM) system over doubly-selective channel. Maximum A Posteriori (MAP) equalization that exploits characteristics of the banded channel structure in OFDM is used in our turbo equalization. In settings similar to Japan's ISDB-T standards, we find that banded channel structure that accounts for only 1 neighboring subcarrier (on one side) is sufficient to ensure a good equalization performance. At moderate Doppler frequency, turbo equalization does not enhance the system performance significantly. Furthermore, the effects of 1) initial channel estimation with turbo equalization; 2) channel re-estimation with turbo equalization are investigated, with the latter conferring greater bit error-rate (BER) enhancement than the former. Channel estimation has a more prominent effect on performance than turbo equalization, as verified by numerical simulations.

DOI： 10.1109/WCINS.2010.5541906

Publishing type：Research paper (international conference proceedings)  

We prove that discrete Fourier basis channel model is optimal for least squares estimation of channel coefficients used for low-complexity equalization of OFDM transmission over doubly selective channels. We show that regardless of channel statistics, the channel model order is determined by the number of coefficients used in the equalization. Our theoretical findings are numerically validated by using linear zero forcing equalizers and non-linear maximum likelihood equalizer with pilot-aided channel estimation.

DOI： 10.1109/ICASSP.2010.5496097

Publishing type：Research paper (international conference proceedings)  

In this paper, we propose a fast arithmetic error feedback (EF) structure with shift operation circuits and shared multiplier. For optimization of EF network, we show the design method based on the Lagrange multiplier method for designing the shared multiplier and the branch and bound based algorithm for optimization of the shift operation circuits. The branch and bound method can reduce the calculation cost because the sub-trees can be cut based on the lower bound estimation. Finally, we present a numerical example by designing the EF with shift operation circuits and shared multiplier. From the results, we demonstrate the effectiveness of the proposed method and show the calculation time is a few seconds.

Publishing type：Research paper (international conference proceedings)  

In this paper, pilot design for channel estimation in multiple input multiple output orthogonal frequency division multiplexing (MIMO-OFDM) systems with null subcarriers is considered, where the mean square error (MSE) is chosen as our optimization criterion. We design the placement of pilot symbols and their powers for multiple transmit antennas to minimize the MSE of the least square (LS) channel estimates. To reduce the interference of pilot symbols from other transmit antennas, an algorithm that ensures that the pilot symbols are disjoint from the ones of any other antenna is proposed. Simulation results based on IEEE 802.16e are presented to illustrate the superior performance of our proposed method over the existing standard and the partially equi-spaced pilot symbols.

Other URL： http://dblp.uni-trier.de/db/conf/eusipco/eusipco2010.html#conf/eusipco/ManassehON10

Publishing type：Research paper (scientific journal)   Kind of work：Single Work  

Alamouti's orthogonal space-time block code (OSTBC) is a simple yet important technique to take advantage of transmit diversity to mitigate fading channel effects. In this paper, we analyze the effects of time-selective channels and channel estimation errors on the bit error rate (BER) performance of Alamouti's OSTBC. We develop an analytical expression of the BER performance for the linear decoding with minimum mean squared error (MMSE) channel estimates in place of the true channel. Based on the expression. we derive a BER performance limit in decision-directed mode where the channel is tracked with Kalman filtering Numerical examples are provided to validate our analysis and to see the impact of time-selective fading and channel estimation errors on the BER performance.

DOI： 10.1587/transfun.E92.A.2844

Other URL： http://dblp.uni-trier.de/db/journals/ieicet/ieicet92a.html#journals/ieicet/Ohno09

Publishing type：Research paper (scientific journal)  

Orthogonal Frequency Division Multiplexing (OFDM) transmission is robust to frequency-selective channels but sensitive to time-selective channels. Time variations of channels generate inter-carrier interference (ICI), which degrades system performance. In this paper, we develop frequency-domain Viterbi-type algorithm to effectively suppress the ICI, by exploiting the property of ICI terms in OFDM symbols. Null subcarriers, which are embedded in OFDM symbols for the reduction of interferences from/to adjacent bands. are exploited to reduce the complexity of the algorithm. Then, an approximate BER expression of Viterbi-type equalizer is derived. Simulations are provided to show the Viterbi-type equalizer works well in time- and frequency-selective (doubly selective) channel with affordable complexity.

DOI： 10.1587/transfun.E92.A.1905

Other URL： http://dblp.uni-trier.de/db/journals/ieicet/ieicet92a.html#journals/ieicet/OhnoT09

Publishing type：Research paper (international conference proceedings)  

Antenna selection is a simple yet efficient technique to obtain diversity advantage from Multi-Input and Multi-Output (MIMO) communication systems. Without specifying system factors such as antenna selection criterion, coding and decoding, equalization, and channel statistics, we evaluate diversity orders of antenna selections over frequency-selective MIMO channels. We show that the diversity order of any configuration approximately scales in the numbers of transmit and receive antennas. Numerical simulations are provided to validate our analysis.

DOI： 10.1109/ICASSP.2009.4960208

Publishing type：Research paper (scientific journal)  

We study the bit-error rate (BER) for different code lengths and number of users in CDMA system with linear minimum mean squared error (MMSE) and non-linear equalizations. We first show that for a fix channel and a fix number of users, BER of each symbol after linear equalization degrades with a decrease in the code length. Then, we prove that for a fix code length, the BER averaged over random channels improves with a decrease in the number of users. Furthermore, in the non-linear serial-interference cancellation (SIC) scheme, we prove analytically that the BER improves with each step of symbol cancellation for any channel not just at high signal-to-interference noise ratio (SINR) but at all range of SINR. Simulation results are presented to substantiate our theoretical findings.

DOI： 10.1093/ietfec/e91-a.8.2008

Publishing type：Research paper (international conference proceedings)  

For Orthogonal Frequency Division Multiplexing (OFDM) system, training preamble for channel estimation is studied when null subcarriers are embedded in each OFDM symbol. The optimal training preamble that minimize the mean squared channel estimation error is numerically found by casting the design problem into a semidefinite programming problem. A design example under the same setting as IEEE802.11a is provided to verify the efficacy of our proposal as well as to show that the preamble of IEEE802.11a is almost optimal.

Publishing type：Research paper (international conference proceedings)  

Maximum Likelihood (ML) equalization for Orthogonal Frequency Division Multiplexing (OFDM) over time- and frequency- selective channels is analyzed in this paper. An approximate expression for bit error rate (BER) performance of the ML equalizer with a limited number of taps is developed, which subsumes the matched filter bound (MFB) equalization performance as a special case, and which saves on intensive time-consuming empirical simulations. Numerical simulations validate our approximate expression.

DOI： 10.1109/ICASSP.2008.4518293

Publishing type：Research paper (scientific journal)  

To describe joint time- and frequency-selective (doublyselective) channels in mobile broadband wireless communications, we propose to use the finite parameter model based on the same Bessel functions for each tap (Bessel model). An expression of channel estimation mean squared error (MSE) based on the finite parameter models in Orthogonal Frequency Division Multiplexing (OFDM) systems is derived. Then, our Bessel model is compared with commonly used finite parameter models in terms of the channel estimation MSE. Even if the channel taps have different channel correlations and some of the taps do not coincide with the Bessel function, the channel estimation MSE of the Bessel model is shown to be comparable or outperform existing models as validated by Monte-Carlo simulations over an ensemble of channels in typical urban and suburban environments.

DOI： 10.1093/ietfec/e90-a.11.2395

Publishing type：Research paper (scientific journal)  

We present universal BER performance ordering for different antenna sizes in Multiple-Input Multiple-Output (MIMO) wireless systems with zero-forcing (ZF) equalization, which hold for all SNR. We first show that when the number of transmit antennas is fixed, BER of each symbol degrades with a decrease in the number of receive antennas even if the received SNR is kept constant. Then, we prove that when the number of receive antennas is fixed, the average BER improves with a decrease in the number of transmit antennas, which shows the tradeoff between BER and bandwidth efficiency in MIMO with ZF equalization. Furthermore, discussions on the relation of BER with the same change in the number of receive and transmit antennas show that there is no universal order in BER for any particular channel. These highlight the advantage and the limit of MIMO with ZF equalization.

DOI： 10.1109/TWC.2007.060064

Publishing type：Research paper (scientific journal)  

Simple but efficient antenna selection schemes are proposed for the downlink of Orthogonal Frequency Division Multiplexing (OFDM) transmission with multiple transmit antennas over frequency selective fading channels, where transmit antennas are selected at the mobile terminal and the base station is informed of the selected antennas through feedback channel. To obtain the optimal antenna selection, channel frequency responses are required and performances have to be evaluated at all the subcarriers. To reduce the computational complexity at mobile terminal, time-domain channels are utilized for antenna selection in place of channel frequency responses. Our scheme does not guarantee the optimal antenna selection but is shown by numerical simulations to yield reasonable selections. Moreover, by using a specially designed pilot OFDM preamble, an antenna selection without channel estimation is developed. Efficiencies of our suboptimal antenna selections with less computational complexities are verified by numerical simulations.

DOI： 10.1093/ietfec/e90-a.8.1594

Publishing type：Research paper (international conference proceedings)  

The presence of null subcarriers in Orthogonal Frequency Division Multiplexing system complicates the design of both training preamble for channel estimation and pilot symbols for pilot-aided channel estimation that minimize the mean square error (MSE) of estimates of frequency- selective channels. In this paper, we numerically find optimal preambles, casting the MSE minimization problem into a semidefinite programming problem. Then, based on the optimal preamble, we also design pilot symbols for pilot-aided channel estimation. A design example under the same setting as IEEE802.11 a is provided to verify the efficacy of our proposal.

DOI： 10.1109/ICASSP.2007.366527

Publishing type：Research paper (scientific journal)  

Orthogonal Frequency Division Multiple Access (OFDMA) allows multiple users to make use of the same bandwidth as the sin-le-user OFDM for data transmission and is a promising candidate to be used for future cellular systems. A key issue at hand is the rate-adaptive resource allocation problem. In this paper, we propose two basic subcarrier allocation schemes with low complexities based on the magnitude of the channel frequency responses. The proposed algorithms ensure a fair resource allocation in terms of the number of subcarriers with affordable bit-rates. Through extensive discussions and Monte Carlo simulations, a comprehensive comparison with previously derived subcarrier allocation schemes is performed which depicts the pros and cons of our proposed algorithms.

DOI： 10.1093/ietfec/e89-a.11.3131

Publishing type：Research paper (scientific journal)   Kind of work：Single Work  

We study uncoded bit-error-rate (BER) performances of single-carrier block transmissions, zero-padded (ZP), and cyclic-prefixed (CP) transmission, when linear equalizers are applied and the BERs are averaged over one block. We show analytically that the BER of ZP transmission with linear equalization degrades as the bandwidth efficiency increases, i.e., there is a tradeoff between BER and bandwidth efficiency in ZP transmission. It is also proven that when minimum mean-squared-error (MMSE) equalization is adopted, ZP transmission outperforms CP transmission and uncoded orthogonal frequency-division multiplexing (OFDM) on the average over random channels. However, the difference between the ZP and the CP transmission becomes smaller as the block size gets larger, since the average BER performance of the ZP transmission degrades, while the average BER performance of CP transmission improves, as a function of the block size. Numerical examples are provided to validate our theoretical findings and to compare the block transmission systems.

DOI： 10.1109/TSP.2006.879321

Publishing type：Research paper (scientific journal)  

The minimization problem of an L-2-sensitivity measure subject to L-2-norm dynamic-range scaling constraints is formulated for a class of two-dimensional (2-D) state-space digital filters. First. the problem is converted into an unconstrained optimization problem by using linear-algebraic techniques. Next, the unconstrained optimization problem is solved by applying an efficient quasi-Newton algorithm with closed-form formula for gradient evaluation. The coordinate transformation matrix obtained is then used to synthesize the optimal 2-D state-space filter structure that minimizes the L-2-sensitivity measure subject to L-2-norm dynamic-range scaling constraints. Finally, a numerical example is presented to illustrate the utility of the proposed technique.

DOI： 10.1093/ietfec/e89-a.7.1987

Publishing type：Research paper (scientific journal)  

Multicarrier multiple access with channel knowledge and prescribed power at the transmitters is shown to maximize the sum-rate for circulant intersymbol-interference (ISI) channels. A low-complexity iterative algorithm is derived for optimal subcarrier allocation to multiple users, while power is loaded per user by specializing an existing iterative algorithm to circulant ISI channels. It is analytically shown that each subcarrier should be allocated to the user having relatively better subcarrier gain and that different users may share certain subcarriers.

DOI： 10.1109/JSAC.2005.864006

Publishing type：Research paper (scientific journal)  

In the downlink of multiuser OFDM (Orthogonal Frequency Division Multiplexing) systems, subcarriers of one OFDM symbol can be allocated to multiple users to enable multiplexing. System performance can be enhanced by appropriate subcarrier allocation and bit-loading. In this paper, while guaranteeing a fixed BER performance, we impose a fixed transmitted power constraint on each user to achieve a fair resource allocation. Based on this condition, we propose an algorithm to maximize the sum-rate of the system. This sum-rate optimization is a discrete optimization problem which is hard to solve by conventional methods. We relax the problem into a continuous convex optimization problem so that the subcarrier allocation to each user can be determined numerically. Using the subcarrier allocation for the continuous problem, bits are then loaded to each user. The effectiveness of the proposed method is substantiated by numerical simulations. (c) 2005 Wiley Periodicals, Inc.

DOI： 10.1002/ecjc.20198

Publishing type：Research paper (international conference proceedings)  

Multiple-Input Multiple-Output (MIMO) wireless systems offer multiplexing gain and diversity gain. While it is known that the diversity order of MIMO transmissions with N-t transmit and N-r receive antennas over i.i.d. Rayleigh fading channels is N-r - N-t + 1 at high signal-to-noise ratio (SNR), the same conclusion cannot in general be drawn for bit-error rate (BER) gain for all SNR and other channels. We show a BER performance ordering for different transmit antenna sizes in MIMO wireless systems with practical zero-forcing (ZF) equalization, which hold for all SNR. We prove that when the number of receive antennas is fixed, the average BER improves with a decrease in the number of transmit antennas, which shows the tradeoff between BER and bandwidth efficiency in MIMO with ZF equalization.

Publishing type：Research paper (scientific journal)  

Other URL： http://dblp.uni-trier.de/db/conf/eusipco/eusipco2006.html#conf/eusipco/OhnoT06

Publishing type：Research paper (international conference proceedings)  

We study the bit-error rate (BER) of CDMA system focusing on minimum mean squared error successive interference cancellation (MMSE-SIC). Based on the novel method we developed for MIMO system in [7] that do not require the channel probability density functions, we prove analytically that in the non-linear MMSE-SIC scheme, the BER improves with each step of symbol cancellation for any channel that holds not just at high signal-to-noise (SNR) and signal-to-interference noise ratio (SINR) but for all range of SNR and SINR. Simulation results are presented to substantiate our theoretical findings.

DOI： 10.1109/APCCAS.2006.342534

Publishing type：Research paper (international conference proceedings)  

In Orthogonal Frequency Division Multiplexing (OFDM) transmissions over doubly-selective channels, pilot-aided channel estimation is a practical approach to obtain channel estimates. In this paper, we apply Bessel model for channel modeling based on which pilot-aided channel estimation is performed. The utilization of Bessel model enables the estimation of intercarrier interference (ICI) effect of the neighboring subcarriers. A low complexity Viterbi-type Maximum Likelihood (ML) equalizer then uses the Bessel model-based channel estimates for equalization, displaying good bit-error rate (BER) performance. Monte-Carlo simulations show that the combination of Viterbi-type ML equalization and Bessel model-based estimates yields improved BER performance over other combinations of linear equalization and finite parameter model-based estimates.

DOI： 10.1109/GLOCOM.2006.542

Publishing type：Research paper (scientific journal)  

To take intercarrier interference (ICI) attributed to time variations of the channel into consideration, the time- and frequency-selective (doubly-selective) channel is parameterized by a finite parameter model. By capitalizing on the finite parameter model to approximate the doubly-selective channel, a Kalman filter is developed for channel estimation. The ICI suppressing, reduced-complexity Viterbi-type Maximum Likelihood (RML) equalizer is incorporated into the Kalman filter for recursive channel tracking and equalization to improve the system performance. An enhancement in the channel tracking ability is validated by theoretical analysis, and a significant improvement in BER performance using the channel estimates obtained by the recursive channel estimation method is verified by Monte-Carlo simulations.

DOI： 10.1093/ietfec/e88-a.11.3076

Publishing type：Research paper (scientific journal)  

In Orthogonal Frequency Division Multiplexing (OFDM), the composite time signal exhibits a high peak-to-average power ratio (PAPR). Due to non-linearities of the transmit power amplifiers, this high PAPR generates in-band distortion, out of band noise (OBN) or spectral spreading, which degrades the bit-error rate (BER) performance. In this paper, we propose a simple way to combat this problem without sacrificing channel estimation and frequency-offset tracking accuracy, by designing a sub-optimal configuration of the pilot tones. The effectiveness of the newly designed pilot tones in reducing PAPR is validated by Monte-Carlo simulations. The corresponding improvement in BER is also verified by simulations under IEEE 802.11 a standard settings, by using the channel with perfect CSI and the designed pilot-aided estimated channel for coherent detection.

DOI： 10.1093/ietfec/e88-a.8.2117

Publishing type：Research paper (international conference proceedings)  

Multipath propagation and mobility-induced Doppler effects contribute to the time- and frequency- selectivity (doubly-selectivity) in channels. To describe the doubly-selective channel, we propose a finite parameter model composed of kernels expressed as a function of the channel correlation based on Karhunen-Loeve (KL) expansion. The proposed model is optimal in the MMSE sense, provided that the instantaneous channel correlation is matched. It can also adapt to changes in the velocity of the terminal. Even when the channel correlation is unavailable and can only be assumed from Jakes' model, the channel estimation mean square error (MSE) of our proposed model is shown to be comparable or outperforms the oversampled basis expansion model (OBEM), polynomial model (PM) and Slepian model as validated by Monte-Carlo simulations on Rayleigh fading channels. In the application in an OFDM system, the proposed model confers significant BER performance gain over OBEM and PM.

DOI： 10.1109/GLOCOM.2005.1578424

Publishing type：Research paper (international conference proceedings)  

Orthogonal Frequency Division Multiplexing (OFDM) transmission is robust to frequency-selective channels but sensitive to time-selective channels. Time variations of channels destroy the orthogonality between subcarriers, resulting in a considerable performance loss due to intercarrier interference (ICI) between subcarriers. In this paper, we propose a Viterbi-type algorithm to effectively suppress the ICI, by exploiting the property of ICI terms and null subcarriers embedded in OFDM symbols for the reduction of interferences from/to adjacent bands. Simulations show that the proposed equalizer works well with affordable complexity and outperforms linear equalizers.

DOI： 10.1109/ICASSP.2005.1415843

Publishing type：Research paper (international conference proceedings)  

To take intercarrier interference (ICI) attributed to time variations of the channel into consideration, we propose to parameterize the doubly-selective channel by the oversampled polynomial model (OPM). The OPM outperforms the oversampled basis expansion model (OBEM). On top of that, we. also propose to incorporate the effective ICI suppressing, reduced-complexity Viterbi-type Maximum Likelihood (RML) equalizer into Kalman filter for recursive channel tracking and equalization. An enhancement in the channel tracking ability and a significant improvement in BER performance are validated by numerical simulations.

Publishing type：Research paper (international conference proceedings)  

We study uncoded bit-error rate (BER) performance of single-carrier block transmissions, zero-padded (ZP) transmission and cyclic-prefixed (CP) transmission, when minimum mean-squared error (MMSE) equalizers are applied. We show analytically that the BER of ZP transmission with MMSE equalization degrades as the bandwidth efficiency increases, i.e. there is a trade-off between BER and bandwidth efficiency in ZP transmission. It is also proven that ZP transmission outperforms CP transmission and uncoded orthogonal frequency-division multiplexing (OFDM) on the average. Numerical examples are provided to validate our theoretical findings and to compare the block transmission systems.

DOI： 10.1109/GLOCOM.2005.1578040

Publishing type：Research paper (international conference proceedings)  

In Orthogonal Frequency Division Multiplexing (OFDM), the composite time signal exhibits a high Peak-to- Average Power ratio (PAPR). Due to non-linearities of the transmit power amplifiers, this high PAPR generates spectral spreading, in-band distortion and out of band noise (OBN) which degrades the bit-error rate (BER). In this paper, we propose a way to combat this problem without sacrificing channel estimation and frequency-offset tracking accuracy, by designing a sub-optimal configuration of the pilot tones. The effectiveness of the proposed method to reduce PAPR and improve BER will be validated by computer simulations based on the conditions in IEEE 802.11a standard. © 2005 IEEE.

DOI： 10.1109/ISCAS.2005.1466010

Publishing type：Research paper (international conference proceedings)  

Orthogonal Frequency Division Multiple Access (OFDMA) allows multiple users to make use of the same bandwidth as the single-user OFDM for data transmission and is a promising candidate to be used for future cellular systems. A key issue at hand is the rate-adaptive resource allocation problem. In this paper, we propose two subcarrier allocation schemes with low complexities based on the magnitude of the channel frequency responses. The proposed methods ensure a fair resource allocation in terms of the number of subcarriers with affordable bit-rates. Monte-Carlo simulations show that our proposed methods achieve a higher overall bit-rate for all users compared to previously derived subcarrier allocation schemes.

Publishing type：Research paper (scientific journal)  

The location, number, and power of pilot symbols embedded in multicarrier block transmissions over rapidly fading channels, are important design parameters affecting not only channel estimation performance, but also channel capacity. Considering orthogonal frequency-division multiplexing (OFDM) systems with decoupled information-bearing symbols from pilot symbols transmitted over wireless frequency-selective Rayleigh-fading channels, we show that equispaced and equipowered pilot symbols are optimal in terms of minimizing the mean-square channel estimation error. We also design the number of pilots, and the power distributed between information bearing and pilot symbols, using as criterion a lower bound on the average capacity. Numerical results corroborate our theoretical findings.

DOI： 10.1109/TIT.2004.833365

Publishing type：Research paper (international conference proceedings)  

We study bit-error rate (BER) performance of single-carrier (SC) zero-padded (ZP) transmissions with zero-forcing (ZF) equalization. Padded redundant zeros mitigate multipath effects but reduce the bandwidth efficiency. We show analytically that the BER improves as the bandwidth efficiency decreases, i.e., there is a clear trade-off between BER and bandwidth efficiency. We also demonstrate that ZP outperforms SC cyclic prefixed (CP) transmission.

DOI： 10.1109/ICASSP.2004.1327002

Publishing type：Research paper (international conference proceedings)  

Channels with long impulse response often arise in high-rate digital transmissions due to severe multipath. This necessitates sophisticated equalization at the receiver. On the other hand, exploiting the available multipath diversity improves bit error performance. To collect the full multipath diversity, computationally cumbersome maximum likelihood sequence estimation (MLSE) is required. Although the Viterbi algorithm (VA) for MLSE is more efficient than the exhaustive ML search, its complexity increases exponentially with the channel length, which varies with the propagation environment. Since the computational power of the receiver is limited, VA becomes infeasible for long channels. In this paper, we develop a transmission capable of handling relatively long channels. The transmitter controls the computational complexity of MLSE at the receiver by periodically inserting zeros within information-bearing symbols, depending on the channel length and the computational power of the receiver. The optimal MLSE with reduced complexity becomes available at the expense of reduced data rate.

DOI： 10.1109/ICC.2004.1313037

Publishing type：Research paper (international conference proceedings)  

In mobile radio applications, the relative motion between the receiver and the transmitter gives rise to a time-varying channel and as a result, channel estimation is essential to acquire the channel so as to coherently decode the received signals. For Space-Time Block Code(STBC) transmitted over time-selective slowly fading channels, a novel Kalman filter-based channel estimation method, based on the incorporation of the quasi-ML symbol decoding mechanism will be developed throughout this paper, complete with computer simulations to see the influence of various parameters like the fading rate, SNR and pilot symbol placement on the BER performance. Evaluation of the results reveals that Kalman filtering, coupled with adequate pilot symbol placement, provides an optimum BER performance with little loss in channel capacity.

Publishing type：Research paper (international conference proceedings)  

Coherent decoding of orthogonal space-time block codes (OSTBC) takes advantage of transmit/receive diversity to mitigate fading channel effects. Thanks to the orthogonality of OSTBC, a low-complexity linear maximum likelihood (NIL) decoding becomes available if the receiver knows the channel state information (CSI). For OSTBC over time-selective channels, we develop an analytical expression of the bit error rate (BER) performance for the linear decoding with minimum mean squared error (MMSE) channel estimates in place of the true channel. Based on the expression, we derive a BER performance limit in decision-directed mode where the channel is tracked with Kalman filtering. Numerical examples are provided to see the impact of time-selective fading on the BER performance.

DOI： 10.1109/GLOCOM.2004.1378479

Publishing type：Research paper (scientific journal)  

High data rates give rise to frequency-selective propagation, whereas carrier frequency-offsets and mobility-induced Doppler shifts introduce time-selectivity in wireless links. To mitigate the resulting time- and frequency-selective (or doubly selective) channels, optimal training sequences have been designed only for special cases: pilot symbol assisted modulation (PSAM) for time-selective channels and pilot tone-assisted orthogonal frequency division multiplexing (OFDM) for frequency-selective channels. Relying on a basis expansion channel model, in this paper, we design low-complexity optimal PSAM for block transmissions over doubly selective channels. The optimality in designing our PSAM parameters consists of maximizing a tight lower bound on the average channel capacity that is shown to be equivalent to the minimization of the minimum mean-square channel estimation error. Numerical results corroborate our theoretical designs.

DOI： 10.1109/TSP.2003.810304

CiNii Article

Publishing type：Research paper (scientific journal)  

Publishing type：Research paper (international conference proceedings)  

Standardized wireless transmissions include fixed-size fragments per packet. Relying on this structure, we develop two schemes for distributing redundancy across fragments, in order to improve performance of packet transmissions over frequency-selective fading channels. We prove that both schemes guarantee symbol detectability, which implies that they both enable the full multipath diversity. We test their relative merits, and compare them with competing alternatives using simulations.

DOI： 10.1109/ICASSP.2003.1202583

Publishing type：Research paper (scientific journal)  

The adoption of orthogonal frequency-division multiplexing by wireless local area networks and audio/video broad. casting standards testifies to the importance of recovering block precoded transmissions propagating through frequency-selective finite-impulse response (FIR) channels. Existing block transmission standards invoke bandwidth-consuming error control codes to mitigate channel fades, and training sequences to identify the FIR channels. To enable block-by-block receiver processing, we design redundant precoders with cyclic prefix and superimposed training sequences for optimal channel estimation and guaranteed symbol detectability, regardless of the underlying frequency-selective FIR channels. Numerical results are presented to access the performance of the designed training and precoding schemes.

DOI： 10.1109/TCOMM.2002.806547

CiNii Article

Publishing type：Research paper (scientific journal)  

DOI： 10.1109/TWC.2002.804183

Publishing type：Research paper (international conference proceedings)  

High data rates give rise to frequency-selective propagation, while carrier frequency-offsets and mobility-induced Doppler shifts introduce time-selectivity in wireless links. To mitigate the resulting time- and frequency-selective (or doubly-selective) channels, an optimal training strategy is designed in this paper for block transmissions over doubly-selective channels, relying on a basis expansion channel model. The optimality in designing our PSAM parameters consists of maximizing a tight lower bound on the average channel capacity, that is also shown to be equivalent to the minimization of the minim-am mean-square channel estimation error. Numerical results corroborate our theoretical designs.

DOI： 10.1109/ICASSP.2002.5744900

Publishing type：Research paper (scientific journal)  

Publishing type：Research paper (international conference proceedings)  

Without channel state information available, universal designs of wireless OFDM transmitters and receivers offer performance that is robust to the underlying channel fades and the adopted signal constellation. We show that judiciously designed precoders with redundancy twice as long as the channel order are necessary and sufficient for guaranteed constellation- and channel-irrespective symbol recovery as well as for blind channel identifiability. Simulated comparisons of the novel precoders with zero-padded and cyclic-prefixed block transmissions delineate their relative merits.

Publishing type：Research paper (international conference proceedings)  

The adoption of orthogonal frequency-division multiplexing (OFDM) by wireless local area networks and audio/video broadcasting standards testifies to the importance of recovering block precoded transmissions propagating through frequency-selective FIR channels. Existing block transmission standards invoke bandwidth-consuming error control codes to mitigate channel fades and training sequences to identify the FIR channels. To enable low-complexity block-by-block receiver processing, we design redundant precoders with cyclic prefix (CP) and superimposed training sequences for optimal channel estimation and guaranteed symbol recovery regardless of the underlying FIR frequency-selective channels. Numerical results are presented to access the performance of the designed training and precoding schemes.

DOI： 10.1109/ICASSP.2001.940481

Publishing type：Research paper (international conference proceedings)  

The adoption of orthogonal frequency-division multiplexing (OFDM) by wireless local area networks and audio/video broadcasting standards testifies to the importance of recovering block precoded transmissions propagating through frequency-selective FIR channels. Existing block transmission standards invoke bandwidth-consuming error control codes to mitigate channel fades and training sequences to identify the FIR channels. For low-complexity block-by-block processing, redundant precoders with cyclic prefix (CP) along with superimposed training sequence are optimally designed to enable least squares estimation of time-varying FIR channels and symbol recovery regardless of the underlying FIR frequency-selective multipath channel. Numerical results are also presented to access the performance of the proposed affine precoders.

DOI： 10.1109/ICC.2001.937171

Publishing type：Research paper (scientific journal)  

Publishing type：Research paper (scientific journal)  

Publishing type：Research paper (scientific journal)  

Publishing type：Research paper (scientific journal)  

Publishing type：Research paper (international conference proceedings)  

Adaptive algorithms for blind equalization of multichannel FIR systems are developed based on the periodic AR model matching of multichannel FIR systems. An LMS algorithm is first presented. Then, an order and time- recursive circular lattice algorithm is applied. The algorithms directly equalize the channel output without the identification of the channel. Simulation results are presented to see the potential of the proposed algorithms.

DOI： 10.1109/ISCAS.1999.778786

Publishing type：Research paper (international conference proceedings)  

In order to attain multichannel blind deconvolution of linear time-invariant nonminimum-phase dynamic systems, Inouye and Habe proposed in 1995 a single-stage maximization criterion. The criterion function is the sum of squared forth-order cumulants of the equalizer outputs, and the coefficients of the equalizer are determined at once. On the other hand, one of possible approaches for multichannel blind deconvolution is to construct an equalizer based on the system identified by higher-order cumulant-matching. In this paper, it is shown that the single-stage maximization criterion is equivalent to a least-squares fourth-order cumulant-matching criterion after multichannel pre-whitening of channel outputs. This result provides us with an important interpretation of the single-stage maximization criterion.

DOI： 10.1109/ICASSP.1998.681559

Publishing type：Research paper (international conference proceedings)  

We give an adaptive algorithm for the single-stage maximization criterion to attain multichannel blind deconvolution. This criterion determines the coefficients of equalizers for all channels simultaneously. However, the original maximization criterion has many constraints so that it is difficult to directly implement it as a numerical algorithm. By exploiting pre-whitening and lattice representations of paraunitary systems, we can reduce the original maximization problem into a simple constraint-free one and then present a stochastic gradient algorithm.

Publishing type：Research paper (scientific journal)  

Other URL： http://dblp.uni-trier.de/db/conf/eusipco/eusipco1998.html#conf/eusipco/OhnoIT98

Publishing type：Research paper (scientific journal)  

Publishing type：Article, review, commentary, editorial, etc. (scientific journal)  

We propose a closed-form design algorithm for linear phase IIR digital niters. The band-pass characteristic including the low-pass and high-pass characteristic can be given and the weighting of pass-band can be specified. By using the proposed method, the given band-pass response can be approximated without repeating procedure under specified maximum pole radius. Finally, a numerical example is given to illustrate the effectiveness of the proposed method.

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Publishing type：Article, review, commentary, editorial, etc. (scientific journal)  

Wireless communications are susceptible to eavesdropping. To interfere the eavesdropper having multiple antennas, secret information signals and interference signals that are transmitted from multiple antennas are designed. It is shown that our design is cast into a semidefinite program, which can be numerically solved efficiently. Simulation results are provided to demonstrate the efficiency of the design.

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Publishing type：Article, review, commentary, editorial, etc. (scientific journal)  

This paper treats a discrete wavelet transform(DWT)-based image watermarking with considering the false positive probability and bit-pattern embedding. We propose an iterative embedding algorithm of watermarking signals which are K sets pseudo-random numbers generated by a secret key. In the detection, K correlations between the watermarked DWT coefficients and watermark signals are computed by using the secret key. L correlations are made available for the judgment of the watermark presence with specified false positive probability, and the other K - L correlations are corresponding to the bit-pattern signal. In the experiment, we show the detection results with specified false positive probability and the bit-pattern recovery, and the comparison of the proposed method against JPEG compression, scaling down and cropping. © 2012 The Institute of Electrical Engineers of Japan.

DOI： 10.1541/ieejeiss.132.932

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Publishing type：Article, review, commentary, editorial, etc. (scientific journal)  

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Publishing type：Article, review, commentary, editorial, etc. (scientific journal)  

盗聴局が知り得ない正規局間の通信路情報を用いて秘密情報の伝送を行う無線ステガノグラフィ方式が提案されている.本論文は,その無線ステガノグラフィ方式では通信の秘匿性と秘密性が保障されないことを示す.

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Publishing type：Article, review, commentary, editorial, etc. (scientific journal)  

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Other URL： http://hdl.handle.net/2433/141125

Publishing type：Article, review, commentary, editorial, etc. (scientific journal)  

The watermark systems should be robust against common image processing operations, such as noise adding and JPEG compression. The correlation-based watermark systems are known as robust against common image processing operations. However, the correlation is usually one-sided correlation, so when the correlation between the discrete cosine transform (DCT) coefficients of original image and key sequence is negative value, it makes the robustness weak. In this paper, we propose the two-sided correlation based image watermark methods. In this method, we take into account the negative correlation in the detection process. By using this method, if the correlation between the DCT coefficients and key sequence is negative value, the robustness of the watermark is retained. Finally, we show the numerical example to illustrate the effectiveness of the proposed watermark systems. From the simulation results, we can see the robustness against JPEG compression, Gaussian noise and cropping is improved.

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Publishing type：Article, review, commentary, editorial, etc. (scientific journal)  

Digital watermarking is a copyright protection method by embedding watermark data in the digital contents. In this work, we propose the correlation-based image watermark method based on smoothing. First, the original image is smoothed by the 2-D digital filter and we obtain the smoothing image in advance. Next, we decide the weighting function of watermark signal based on the smoothing image. Thus, the watermark signal is embedded so that the image is smoothed. If the image contains a Gaussian noise, the noise will be removed by the watermark signal. Finally, we show the image is smoothed when the watermark is embedded and the proposed watermarking method is effective for noise reduction. Also we confirm the robustness of the proposed watermarking method against JPEG compression.

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Publishing type：Article, review, commentary, editorial, etc. (scientific journal)  

Single carrier cyclic prefixed transmissions between a base station having multiple antennas and a terminal having one antenna are considered. We propose two antenna selections based on the channel norm and on the worst channel gain. Our selection schemes do not always yield the optimal selection but have less computational complexities. The channel norm criterion has the least complexity, while the channel gain criterion exhibits almost the same performance with the optimal selection.

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Publishing type：Article, review, commentary, editorial, etc. (scientific journal)  

In the downlinks of both singe-carrier cyclic prefixed (SC-CP) and orthogonal frequency division multiplexing (OFDM) systems with multiple transmit antennas, an efficient transmit antenna selection scheme with low complexity is proposed. This scheme is based on the application of the max-min criterion to choose the maximum of the minimum among the channel frequency responses available. Numerical simulations are performed which show that the solution from the max-min criterion almost coincides with the optimal solution. Furthermore, extensive comparisons with other schemes based on channel norm criterion and bit-error rate criterion are made which demonstrate the superiority of the proposed scheme. Our results also show that for transmit antenna selection with/without space-time block coding, SC-CP outperforms OFDM.

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Publishing type：Article, review, commentary, editorial, etc. (scientific journal)  

To describe joint time- and frequency-selective channels in mobile broadband wireless communications, we propose to use the finite parameter model based on the same Bessel functions for each tap (Bessel model). An expression of channel estimation mean squared error (MSE) based on the finite parameter models in Orthogonal Frequency Division Multiplexing (OFDM) systems is derived. Then, our Bessel model is compared with commonly used finite parameter models in terms of the channel estimation MSE. Even if the channel taps have different channel correlations and some of the taps do not coincide with the Bessel function, the channel estimation MSE of the Bessel model is shown to be comparable or outperform the oversampled basis expansion model (OBEM), polynomial model (PM) and Slepian model as validated by simulations in typical suburban environment. The application of the Bessel model in an OFDM system also results in BER performance gain over OBEM, PM and Slepian model.

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Publishing type：Article, review, commentary, editorial, etc. (scientific journal)  

In a single user OFDM (Orthogonal Frequency Division Multiplexing) system, adaptive modulation schemes can be applied to each subcarrier according to the subcarrier gain, to enhance the system performance. On the other hand, in a multiuser OFDM system, the adaptive modulation schemes after allocating the subcarriers to each user improve the bit rate and the bit error rate of the system. In this paper, we propose two low-complexity algorithms that allocate the subcarriers as fairly as possible to maximize the total bit rate under a bit error rate requirement. Numerical examples are provided to compare the proposed algorithms with existing ones and to evaluate their performances.

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Publishing type：Article, review, commentary, editorial, etc. (scientific journal)  

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DOI： 10.11509/isciesci.46.7_394

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Publishing type：Article, review, commentary, editorial, etc. (scientific journal)  

An algorithm for blind identification of multichannel (single-input and multiple-output) FIR systems is proposed. The proposed algorithm is based oil subspace approach to blind identification. which requires so-called noise space spanned by some eigenvectors of correlation matt ices of observations. It is shown that a subspace of the noise space can be obtained by one-step scalar-valued linear prediction and then the subspace is sufficient for blind identification. To acquire the subspace. the proposed algorithm utilizes one-step scalar-valued linear prediction in place of a singular- (or eigen-) value decomposition and hence it is computationally efficient. Computer simulations are presented to compare the proposed algorithm with the original one.

Publishing type：Article, review, commentary, editorial, etc. (scientific journal)  

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Publishing type：Article, review, commentary, editorial, etc. (scientific journal)  

Thanks to the development of communication technology and inexpensive Intemet connections，the number of users of Internet has been increasing exponentially. The Internet is now an indispensable and elementary tool for many people. A lot of commercial and non-commercial useful services can be found out there. However，it should be noticed that the Internet is inherently insecure. Actually，the number of inci dent on the Internet has been also increasing. To warn the risks of using the Internet，this paper reports some incident happened at a lower site of Shimane University's information network. Some lesson can belearned from the incident. The opmion on network system administration is also provided. ネットワークの整備が進み，インターネット上で，電子メールのような必要不可欠なサービスが展開され，その利用者が爆発的に増加している．一方，インターネット利用者の増加に伴い，インターネットを悪用したシステムヘの不正アクセス・不正利用も増加している．本稿では，ネットワークシステムの危険性を喚起するため，著者が遭遇したネットワークシステムヘの不正アクセス・不正利用のいくつかの事例を紹介する．また，その経験から得られた教訓を述べるとともに，ネットワークシステム管理経験者として，ネットワークシステム管理に関する意見を述べる．

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Publishing type：Article, review, commentary, editorial, etc. (scientific journal)  

An adaptive algorithm for blind source separation is developed based on genetic algorithms. It utilizes pre-whitening and a parameterization of separation matrices to reduce the computational burden. As a object function, criteria for blind source separation can be employed. Simulation results are presented to see the performance of the proposed algorithm

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Publishing type：Article, review, commentary, editorial, etc. (scientific journal)  

Publishing type：Article, review, commentary, editorial, etc. (scientific journal)  

The perfect reconstruction (PR) filter banks have now a great importance in multirate signal processing. Many attempts have been made to generalize from the results of one-dimensional system. In this paper, the multi-dimensional systems are studied from stochastic point of view. At first the multi-dimensional cyclostationary process is introduced to analyze filter banks since the output of a filter bank for a stationary input is cyclostationary. Then the spectral correlation density of this output is presented. Using these results we propose a criterion to construct an optimal multi-dimensional PR filter banks which minimizes the averaged mean squared reconstruction error when some subband signals are dropped.

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Publishing type：Article, review, commentary, editorial, etc. (scientific journal)  

Many communication signals exhibit cyclostationary property. Recently, Gardner and Chen introduced the estimate of the time difference of arrival (TDOA) of a cyclostationary signal at two separate sensors by exploiting the cyclic spectrum of this signal. But their method uses only a single slice from the cyclic spectrum. In this paper, we derive the maximum likehood estimator of the TDOA, using the stationary representation of a cyclostationary signal. We estimate the cyclic spectrum by the multivariate AR model fitting. As this method uses all parts of the cyclic spectrum, the performance of this method is better than the previous ones. This was clarified by the results of the computer simulation.

DOI： 10.5687/iscie.7.304

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