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IJAL. 01/2011; 2:16-29.
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S.C. Pan,
C.C. Chi, C.C. Ko,
H.H. Kuo,
F.M. Huang,
S.C. Lee,
M.H. Lin,
H.Y. Huang,
C.H. Hsieh,
S.M. Jeng,
H.J. Tao,
Min Cao,
Y.J. Mii
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ABSTRACT: The combination of self-formed barrier (SFB) and extreme low-k (ELK) dielectric is an attractive candidate for interconnect integration beyond 28nm-node regarding to low RC delay and Cu filling. Attempt is made to understand the formation mechanism of SFB through combinations with various ELK dielectrics in this study. In terms of wiring and dielectric reliabilities, the combination of Mn<sub>x</sub>O<sub>y</sub> SFB and ELK with the lowest k value in this study could have the best reliability results if a suitable patterning approach is selected. Furthermore, the property requirements for interface on which SFB will form are studied by an additional thin CVD dielectric liner deposited on the patterned ELK dual-damascene structure. It is found that the suitable interface for SFB formation with high corrosion resistance and robust electro-migration (EM) performance would need higher carbon content and the existence of nitrogen.
Interconnect Technology Conference (IITC), 2010 International; 07/2010
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ABSTRACT: Owing to the loss of subcarrier orthogonalities in high-speed applications, the use of conventional frequency-domain-based channel estimation in high mobility orthogonal frequency division multiple access (OFDMA) systems such as mobile WiMax may give rise to an unacceptable high channel estimation error floor. To alleviate this problem, the authors develop some basis expansion model (BEM)-based estimation schemes for the OFDMA uplink. Specifically, the authors express the time-varying channel as a superposition of a small number of complex exponential basis functions spanning the entire Doppler range, and then formulate least square (LS) and linear minimum mean square error (LMMSE) algorithms to estimate the basis coefficients for two different types of pilot patterns. The authors also derive the respective Cramer-Rao lower bounds for these estimators. It has been shown that the time domain BEM using a pilot scheme where pilots are placed over time axis will give better performance under a high Doppler scenario. Lastly, using simulation results, the proposed algorithms have been found to have better estimation accuracy over current frequency domain estimation techniques. This is in addition to the advantage that the proposed algorithms have in general a lower computational complexity.
IET Communications 03/2010; · 0.83 Impact Factor
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ABSTRACT: Basis expansion model (BEM) based estimation is studied for highly time varying OFDMA uplink channel. We consider two popular pilot patterns-pilot placed over frequency-time grid as proposed in IEEE 802.16e standard and pilot placed on the time axis. In a high Doppler spread scenario, taking intercarrier interference (ICI) impact from data block into account, least square (LS) estimator is developed for both time and frequency domain pilot aided estimation. From the performance analysis and simulation result, it is shown that, time domain LS (LS-T) estimator performs better than the current frequency domain estimator. The computational complexity for time domain LS is also discussed.
Communications Theory Workshop, 2009. AusCTW 2009. Australian; 03/2009
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ABSTRACT: Follower partial band jamming has been recognised as an efficient strategy to degrade the performance of frequency hopping (FH) systems. To alleviate this detrimental effect, a novel detection scheme for slow FH/M-ary frequency shift keying systems, which use two receive antennas in a quasi-static flat fading channel, is proposed. Specifically, using the principle of vector similarity, an area-based vector similarity metric is formulated to give an estimate of the unknown spatial correlation of the received jamming components at the two receive antennas. The jamming signal can then be removed in the symbol detection process. The improved performance of the new scheme is verified by using an analysis under a jamming dominant environment as well as simulated bit error rate results.
IET Signal Processing 01/2009; · 0.56 Impact Factor
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ABSTRACT: A new scheme for MIMO-OFDMA (Multiple Input Multiple Output-Orthogonal Frequency Division Multiple Access) downlink resource allocation is presented in this paper. It has an objective of maximizing the total system capacity while having constraints on the total available power and proportional fairness. Dominant eigen-channels obtained from MIMO state matrix are used to formulate this scheme with low complexity. It gives a tradeoff strategy among system capacity, computational complexity and proportional fairness. Simulation results show that this new adaptive allocation scheme can provide much better capacity gain than static allocation methods while achieve near optimal fairness of rate distribution with a linear complexity. As a result, it may be suitable for fulfilling diverse QoS requirements of MIMO-OFDMA systems.
Communications, 2008. APCC 2008. 14th Asia-Pacific Conference on; 11/2008
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ABSTRACT: In this paper, resource allocation problems in a downlink relay-assistant OFDMA cellular system are studied. Amplify-and-forward relaying strategy is utilized at each relay station to assist signal transmission while half-duplex transmission protocol is adopted. A joint optimization of subcarrier-pairing in two consecutive time slots and power allocation on each subcarrier in each time slot is investigated through a capacity maximization problem constrained on independently available power at BS and RS. Some fundamental principles to achieve optimality are given while a few subcarrier and power allocation algorithms are proposed. The performance of these resource allocation algorithms is evaluated by computer simulation.
Communications, 2008. APCC 2008. 14th Asia-Pacific Conference on; 11/2008
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ABSTRACT: A blind joint timing and frequency synchronization algorithm is derived for orthogonal frequency-division multiplexing (OFDM) signals transmitting over multipath fading channels. The proposed estimator requires neither the knowledge of the signal-to-noise ratio (SNR) nor the power delay profile of the fading channels, except that the maximum delay spread L should be known and should be shorter than the guard interval. In the proposed estimator, the transmitted time-domain OFDM samples are modeled as an independent Gaussian random sequence, and the received time-domain OFDM samples are partitioned into a few subsets in such a way that the neighboring entries in each subset are uncorrelated. This is achieved by picking out OFDM samples that are spaced L samples apart into each subset. By modeling the channel gains as unknown deterministic variables, a joint symbol timing and frequency offset (FO) estimation algorithm is derived based on these subsets. Simulation results show that, in terms of lock-in probability, the proposed blind ST estimator achieves better performance than the estimators studied by Van De Beek and Speth , as well as the approximate maximum-likelihood estimator described by Lee and Cheun when the SNR is sufficiently high. For FO estimation, in terms of mean square error (MSE), the proposed estimator outperforms the estimators studied by Van De Beek and Speth at medium and high SNR. In terms of bit error rate (BER), the proposed estimator is superior to the estimators studied by Van De Beek and Speth Furthermore, the simulation results show that the performance of the proposed estimator is tolerant to the variation in the number of delay paths. This means that the proposed estimator will still adequately perform, even when the number of delay paths is time varying and is not estimated at the receiver. Finally, simulation results show that the use of virtual subcarriers has negligible performance loss in the proposed estimator.
IEEE Transactions on Vehicular Technology 10/2008; · 1.92 Impact Factor
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ABSTRACT: We propose an estimator that will jointly estimate the channel and the carrier frequency offset (CFO) for orthogonal frequency-division multiplexing (OFDM) systems based on the maximum-likelihood criterion. The proposed estimator uses only one received OFDM symbol to perform estimation and does not require the knowledge of power delay profile, except that the number of delay paths should be known. The search range for the CFO is first partitioned into a number of subranges. The expectation-maximization (EM) and Newton's methods are then used within each subrange in an alternative manner to jointly obtain the channel and the CFO estimates. By assuming that the transmitted time-domain OFDM signals can be modeled as Gaussian random variables, we show that the EM method has closed-form expressions and, thus, can be efficiently implemented. We then study the mean-square-error (MSE) performance of the estimator under various signal-to-noise (SNRs) ratios and various numbers of cyclic prefix samples through simulations. Finally, we compare the performance of our proposed estimator with that of some existing estimators in terms of bit error rate (BER). Our simulation results show that the proposed estimator performs better than the reported estimator, which sequentially estimates the channel and the CFO. By suitably choosing the number of subranges, our estimator also performs better than a reported estimator, which makes use of pilot symbols to obtain the initial CFO and channel estimates and then uses a decision-directed technique for overall eventual estimation.
IEEE Transactions on Vehicular Technology 10/2008; · 1.92 Impact Factor
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ABSTRACT: The jamming robustness of frequency hopping (FH) systems with M-ary frequency shift keying (MFSK) modulation may be potentially neutralized by a follower partial-band jammer. In this paper, a maximum likelihood (ML)-based algorithm that uses a two-element array is proposed for joint follower jamming rejection and symbol detection in slow FH/MFSK systems over quasi-static flat Rayleigh fading channels. The algorithm is derived by treating both the received jamming components and the unknown data symbols as deterministic quantities to be jointly estimated in an integrated ML operation. In addition, an approximate expression for the symbol error rate (SER) of the proposed scheme is derived when BFSK signaling is employed in a jamming dominant scenario. Analytical and simulated results show that the proposed approach is able to remove jamming and outperform the conventional and sample matrix inversion (SMI)-based beam-formers in the presence of a follower partial-band jammer.
IEEE Transactions on Communications 10/2008; · 1.68 Impact Factor
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ABSTRACT: This paper investigates inter-carrier interference (ICI) suppression for the uplink of an orthogonal frequency division multiple access (OFDMA) system in a time and frequency selective Rayleigh fading channel. In such a doubly selective channel, channel variation within each OFDMA block disrupts the orthogonality among subcarriers, leading to ICI. We formulate the signal model for OFDMA uplink in a doubly selective Rayleigh fading channel, and then propose a MMSE and a MMSE successive detection (MMSE-SD) scheme to suppress ICI. It is shown that the MMSE scheme is the optimal linear scheme in terms of achievable data rate, while the MMSE-SD scheme is able to further remove ICI and exploit the Doppler diversity embedded in time-varying channels. Simulation results are presented to illustrate the performance improvements that can be obtained from using the proposed suppression schemes.
Vehicular Technology Conference, 2008. VTC Spring 2008. IEEE; 06/2008
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ABSTRACT: In this paper, the use of Multi-antenna based Dynamic Subcarrier Sharing (MDSS) algorithms for maximizing the total capacity of an uplink OFDMA (Orthogonal Frequency Division Multiple Access) system are investigated. By having multiple antennas at the base station (BS), we formulate a capacity optimization problem with the introduced group constraint (GPC). This optimization problem is under constraints on transmitted power of each user, bit error rate (BER), equal fairness and restricted subcarrier sharing GPC. We then propose two MDSS algorithms to solve this problem with relatively low computational complexity. Simulation results show that the proposed algorithms, as new methods to adaptively allocate resources to users, can enhance the system capacity and rate fairness among user.
Information, Communications & Signal Processing, 2007 6th International Conference on; 01/2008
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ABSTRACT: This paper presents a new scheme for MIMO-OFDMA (multiple input multiple output-orthogonal frequency division multiple access) downlink resource allocation. This scheme intends to simultaneously enhance capacity and proportional rate fairness among users. A suboptimal power allocation method is also described in detail in this paper. The system objective is to maximize the total capacity subject to constraints on total available power, BER and rate proportions. Based on MIMO channel state information, eigen-channels are intelligently used to determine subcarrier allocation and power allocation, which forms the foundation of the new scheme. Simulation results show that this new adaptive allocation scheme can achieve good tradeoff between capacity and fairness as well as provide much better capacity gain than the conventional static FDMA allocation method.
Communications and Information Technologies, 2007. ISCIT '07. International Symposium on; 11/2007
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ABSTRACT: A new scheme is proposed in this paper for OFDMA (orthogonal frequency division multiple access) downlink resource allocation. It seeks to maximize the total system capacity subject to constraints on total available power, BER and rate proportions. Future communication systems should take both capacity and fairness into account together. Therefore, the new scheme has a novel step to rearrange the subcarriers among users with least capacity loss and most fairness gain to achieve a certain tradeoff between enhanced rate (Hui and Zhou, 2006) and proportional fairness (Shen et al., 2003). This tradeoff can be easily controlled by the introduced tradeoff-factor (TF). Simulation results show that this new capacity and fairness controllable scheme may be suitable for future quality of service (QoS) adjustable OFDMA systems.
Vehicular Technology Conference, 2007. VTC-2007 Fall. 2007 IEEE 66th; 11/2007
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ABSTRACT: In this paper, a block-spread interleaved frequency division multiple access (BS-IFDMA) scheme is proposed. It is capable of allocating users in block-level code-spreading process for multiple access on top of the IFDMA multiple access scheme for better user allocation. Orthogonality among users in block spreading is achieved when the block spreading, carried out in the time domain, is over a few IFDMA block symbols with negligible channel variation (i.e. transmission over quasi time-invariant channel). Given the same number of supported users and same system bandwidth, by allocating more users in the block level code spreading domain, more frequency diversity can be achieved for the proposed scheme since more subcarriers can be allocated to individual user in the frequency domain compared to the conventional IFDMA scheme.
Personal, Indoor and Mobile Radio Communications, 2007. PIMRC 2007. IEEE 18th International Symposium on; 10/2007
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ABSTRACT: In this paper, multiple access interference (MAI) suppression in synchronous interleaved orthogonal frequency division multiple access (OFDMA) system is investigated. Based on the signature vector formulated for each user, a new user detector is proposed. It is shown that when matched filtering is employed to suppress MAI in the proposed detector, under imperfect power control, strong users can severely degrade the performance of its neighboring users. To improve the system performance, we derive a subspace zero-forcing and a subspace MMSE MAI suppression scheme for the proposed detector. We show that only the knowledge of the desired user's CFO is required in MAI suppression and the system can be rendered almost MAI-free provided that the CFO estimation is accurate enough.
Vehicular Technology Conference, 2006. VTC-2006 Fall. 2006 IEEE 64th; 10/2006
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ABSTRACT: We propose a new method for direction of arrival (DOA) estimation in the presence of multipath propagation and mutual coupling for a frequency hopping (FH) system. With the use of pilot symbols and assuming perfect time-frequency synchronization for a linear array, we take mutual coupling and multipath propagation into account, and derive a maximum likelihood (ML) estimator for both the mutual coupling matrix and DOA estimation. We then formulate an iterative alternating minimization (AM) algorithm for finding the mutual coupling and DOA parameters in an alternate manner. Simulation results illustrating the performance of the algorithm and comparison with the Cramer-Rao bound (CRB) are also presented.
IEEE Transactions on Aerospace and Electronic Systems 05/2005; · 1.10 Impact Factor
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ABSTRACT: A maximum likelihood (ML)-based algorithm for frequency estimation and synchronization of frequency hopping signals is proposed in this paper. By using a two-hop signal model that incorporates the unknown hop transition time, the likelihood function of the received frequency hopping signal is formulated. A new iterative method is then derived to estimate the hopping frequencies and hop transition time. Without using any pilot signal or sync bit, the new algorithm is able to implement synchronization and frequency estimation at the same time. Unlike the time-frequency distribution (TFD) and the wavelet-based algorithms in papers by Barbarossa and Scaglione (1997) and by Khalil and Hippenstiel (1996), the new ML algorithm does not require the selection of a kernel or mother wavelet function. In addition, compared with the TFD-based algorithm, it has a better performance with a lower implementation complexity.
IEEE Transactions on Signal Processing 03/2005; · 2.63 Impact Factor
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ABSTRACT: In this paper, we deal with the problem of document image rectification from images captured by digital cameras. The improvement on the resolution of digital camera sensors has brought more and more applications for non-contact text capture. Unfortunately, perspective distortion coupled with resulting images makes it harder to properly identify the contents of captured texts using the traditional optical character recognition (OCR) system. We propose in this work a new technique, which is capable of removing distortion and recovering the fronto-parallel view of text with a single image. Different from reported approaches in the literature, the image rectification is carried out using character boundary and tip point, which are extracted from character strokes based on multiple fuzzy sets and morphological operators. The algorithm needs neither camera calibration nor high-contrast document boundary. Experimental results show our rectification process is fast and robust.
Image Processing, 2004. ICIP '04. 2004 International Conference on; 11/2004
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ABSTRACT: We propose a new method for DOA estimation in the presence of multipath propagation and mutual coupling (MC) for a frequency hopping (FH) system. We take MC and multipath propagation into account, and derive a maximum likelihood (ML) estimator for both MC matrix and DOA estimation. We then formulate an iterative alternating minimization (AM) algorithm for finding the MC and DOA parameters in an alternate manner. Simulation results illustrating the performance of the algorithm and comparison with the Cramer-Rao bound (CRB) are also presented.
Communications, 2004 IEEE International Conference on; 07/2004
