Now showing 1 - 10 of 62
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    An efficient suboptimum detector based on linear prediction in Rayleigh flat-fading channels
    It is well known that the optimum detector based on linear prediction, for frequency-nonselective (flat) fading channels, requires MP states for M-ary signalling, where P is the order of the prediction filter required to decorrelate the received signal. In this paper, we first derive a suboptimal linear predictive-based (SLP) detector for M-ary PSK by making a high SNR assumption. We then demonstrate using the concept of isometry that for M-ary PSK signalling, only MP-1 states are required for this SLP detector. The complexity reduction is achieved with no loss in symbol-error-rate performance compared to the case where MP states are used. As a corollary, we also show that for a first-order prediction filter and M-ary PSK signalling, the SLP detector is identical to the conventional differential detector. © 2001 Elsevier Science B.V.
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    Leverage weighted decision directed channel tracking for OFDM systems
    Decision directed channel tracking (DDCT) in OFDM systems can suffer from error propagation at high fade rates, due to the combined effect of rapid variation of the channel, long frame length and frequency selectivity of the channel. Conventional estimators like the 2D-minimum mean square error (MMSE) channel estimator and the expectation maximization (EM) based Kalman channel estimator [3] show poor performance when they are applied to DDCT over large frame lengths, due to the error propagation induced by wrong symbol decisions. The poor symbols decisions usually act like leverage points in the regression matrix, and can be identified using the hat matrix as a leverage diagnostic. We use extreme value theory (EVT) on the hat matrix to define a channel estimator, which, in addition to exploiting time and frequency correlation of the channel, downweighs leverage points before utilizing them in the estimator structure. The proposed EVT-leverage weighted (LW) estimator reduces error propagation in the frame since it downweighs possible wrong decisions before using them in the channel estimator structure. The proposed EVT-LW estimator has a significantly better error rate performance when compared to both the 2D-MMSE estimator [2] and the EM based Kalman estimator [3]. © 2006 IEEE.
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    Studying the effect of delay diversity on a DS-CDMA downlink
    (01-01-2005)
    Jagannathan, Sumanth
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    In this paper, we study the error rate performance of a direct sequence CDMA downlink similar to the CDMA2000 standard. While analytical computation of the error rate is possible in DS-CDMA systems under certain simplifying assumptions, computer simulations are perhaps the only means to quantify the error rate for realistic channel models. Since the delay spread of the multipath channel varies with distance, we investigate the error performance as a function of the distance from the base station. The computer simulations reveal certain interesting results which to our knowledge have not explicitly appeared in open literature. We compare the RAKE error rate performance under situations of no handoff, soft handoff, independent fading, correlated fading, chip-spaced sampling, sub-chip sampling, and pilot channel cancellation at the mobile receiver. These results indicate that narrow-band DS-CDMA downlink has certain severe shortcomings, and that it may be vital to ensure line of sight propagation for the “near” users, and exploit base-station (macro) diversity for the “far” users (in soft handoff) to ensure adequate error rate performance. © 2005 Taylor & Francis Group, LLC.
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    Efficient synchronization and frequency tracking for cellular Reuse-I OFDMA systems
    (01-01-2007)
    Bhargava, Y.
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    Orthogonal Frequency Division Multiplexing (OFDM) systems are highly sensitive to frequency synchronization errors which cause inter-carrier interference, leading to degradation in the system performance. In this paper, a pragmatic timing and frequency synchronization scheme for reuse-I OFDMA systems that is compatible with IEEE 802.16d/e wireless MAN standard is proposed. The algorithm uses cyclic prefix correlation technique for estimating and tracking the OFDMA frame boundary and fractional frequency offset. Once the preamble is captured, the integral part of the frequency offset and cell-specific preamble code can be estimated. This novel scheme is computationally efficient and robust to co-channel interference and multipath fading. Simulation results show that the proposed algorithm gives excellent performance even at very low signal to interference noise ratios. © 2007 by the IETE.
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    Impulsive interference cancellation in uplink macro-diversity combining
    (27-11-2007)
    Jose, Jubin
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    Decision feedback equalizers (DFEs) are designed to deal with AWGN noise, and hence, generally perform very poorly in the presence of impulsive noise. Extreme value theory (EVT) is used to modify the DFE structure to handle impulsive noise. The received measurements are modified using EVT based weights before passing it to the equalizer. A modified maximal ratio combining (MRC) scheme which also uses EVT is developed to further improve the error-rate performance in the presence of impulsive interference. The proposed method performs much better than the conventional DFE-MRC technique which uses the simple MRC in conjunction with the DFE at low signal-tointerference (SIR) ratios. © 2007 IEEE.
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    A new diagonally layered spatial multiplexing scheme with partial channel knowledge
    (01-12-2008)
    Srinivas, K. V.
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    Vertical layering with successive interference cancellation (SIC) at the receiver (e.g. V-BLAST) is a popular spatial multiplexing scheme that achieves high data rates over multi-antenna wireless channels. However, SIC suffers from low diversity gains and recently, with limited feedback, a diversity optimal SIC receiver has been proposed based on greedy QR decomposition (GQR-SIC). But the low diversity gain of the first decoded layer degrades the overall diversity gain. In this paper, we present a new diagonally layered spatial multiplexing scheme that employs GQR-SIC receiver but achieves significantly higher diversity gains. In the proposed scheme, the diversity gain of a weak layer is improved through interleaving the co-ordinates of the symbols transmitted over the weak layer and a strong layer. In this work, we focus on spatial multiplexing with only two layers and analyze the diversity multiplexing tradeoff to show that the proposed scheme achieves maximum diversity gain. When compared with other diagonally layered schemes, the proposed scheme has low decoding complexity. © 2008 IEEE.
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    Extreme value theory based decision directed OFDM channel tracking
    Decision directed channel tracking (DDCT) at high fade rates in OFDM based systems is addressed in this paper. Channel estimation in DDCT can be formulated as a linear errors-in-variables regression problem. While most of the errors in the regression matrix (equalization errors) are Gaussian in nature, few of the detected symbols can have high error due to the frequency and time selective fading. These poor symbol decisions behave like outliers in the regression matrix and give rise to contaminated Gaussian noise distributions. Classical estimators like total least squares (TLS) and the expectation-maximization (EM) based estimators exhibit poor performance in the presence of such outliers. We propose the Huber's M (HM) estimator and an extreme value theory (EVT) based M estimator for the DDCT problem. The proposed HM and EVT-HM estimators are robust to outliers and have an efficiency greater than 95% in purely Gaussian noise. The error rate performance of the proposed HM and EVT-HM estimators are compared with that of the TLS estimator, and the EM based estimator proposed in [4]. © 2006 IEEE.
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    H∞ channel estimation for cellular OFDM
    (27-11-2007)
    Lakshminarayanan, R.
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    Co-channel interference (CCI) on pilot subcarriers can severely degrade the channel estimation accuracy in reuse-1 cellular OFDM systems. Currently most OFDM channel estimation schemes use a Kalman filter, which implicitly assumes that the CCI is Gaussian. However, when only one or two strong CCI signals are present, the Gaussianity assumption of the disturbance sequence is usually not valid. We propose H∞ based channel estimation for such CCI limited OFDM systems. Simulated performance results of normalized mean squared error as well as singular value plots indicate that the H∞ based algorithm yields a significantly superior performance when compared to the Kalman filter. © 2007 IEEE.
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    Single snapshot spatial smoothing with improved effective array aperture
    (01-12-2009)
    Thakre, Arpita
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    Haardt, Martin
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    Spatial smoothing is a widely used preprocessing scheme for direction-of-arrival (DOA) estimation of more than one source from a single snapshot, although the effective array aperture gets reduced by this process. In this paper we propose a preprocessing scheme applicable for DOA estimation algorithms that exploit the shift invariance property of the array steering matrix and call it spatial smoothing with improved aperture (SSIA). SSIA, when applied to a noise corrupted data vector, improves the effective array aperture significantly as opposed to conventional spatial smoothing. Simulations confirm the significant performance gain provided by SSIA in conjunction with Unitary ESPRIT. © 2009 IEEE.
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    Performance enhancement of space-time trellis codes when encountering AWGN and ricean channels
    (01-07-2005)
    Milleth, J. Klutto
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    Space-time trellis codes (STTCs) have been designed for quasi-static Rayleigh channels. When these codes are directly used for additive white Gaussian noise (AWGN) channels or Ricean channels with a high K factor, their frame error-rate (FER) performance is no longer optimal. We propose open-loop (no feedback) preprocessing methods for improving the error-rate performance of these codes when used in channels with a high K. For K = 0 (Rayleigh channels), these open-loop schemes generally do not alter the FER performance of the STTCs. The preprocessing operation at the transmitter is either to rotate the constellation points in one of the transmit antennas relative to the other by some angle, or suitably divide (unequally) the total power between the two transmit antennas. The angle of rotation and power division is optimized by maximizing the dfree of the STTCs. Simulation results show that for AWGN or Ricean fading channels with a large K, the FER performance can be significantly improved by doing either rotation or power division at the transmitter. While the FER performance is nearly unchanged for quasi-static Rayleigh channels, the improvement in performance is between 1 to 3 dB for Ricean or AWGN channels. © 2005 IEEE.