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ABSTRACT: In frequency-selective channels linear receivers enjoy significantly-reduced complexity compared with maximum likelihood receivers at the cost of performance degradation which can be in the form of a loss of the inherent frequency diversity order or reduced coding gain. This paper demonstrates that the minimum mean-square error symbol-by-symbol linear equalizer incurs no diversity loss compared to the maximum likelihood receivers. In particular, for a channel with memory ν, it achieves the full diversity order of (ν+1) while the zero-forcing symbol-by-symbol linear equalizer always achieves a diversity order of one.
IEEE Transactions on Communications 10/2011; · 1.68 Impact Factor
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ABSTRACT: In this paper, we design MMSE-optimal training sequences for multi-user MIMO-OFDM systems with an arbitrary number of transmit antennas and an arbitrary number of training symbols. It addresses spectrally-efficient uplink transmission scenarios where the users overlap in time and frequency and are separated using spatial processing at the base station. The robustness of the proposed training sequences to residual carrier frequency offset and phase noise is evaluated. This analysis reveals an interesting design tradeoff between the peak-to-average power ratio of a training sequence and the increase in channel estimation mean squared error over the ideal case when these two impairments are not present.
IEEE Transactions on Wireless Communications 08/2011; · 2.59 Impact Factor
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ABSTRACT: We investigate the performance of orthogonal frequency division multiplexing (OFDM)-based dual-hop amplify-and-forward (AF) relay networks in the presence of phase noise (PHN). We show that the use of an AF relay may not be beneficial compared to a direct transmission in the presence of PHN. Using outage probability analysis, an upper bound on the allowable PHN level is derived which ensures that the dual-hop outperforms the direct transmission. To improve dual-hop transmission performance in the presence of PHN, we propose a reduced-complexity joint channel and PHN estimator using full-pilot OFDM symbols for AF relay transmission. The proposed approach achieves a lower mean squared error compared to the conventional channel estimator. In addition, we derive a joint data detection and PHN estimation scheme for comb-type data OFDM symbols by modifying the maximum ratio combining metric to account for the effect of PHN at the destination node.
IEEE Transactions on Communications 06/2011; · 1.68 Impact Factor
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ABSTRACT: We present a new parallel algorithm for computing the optimum coefficients of the fractionally-spaced FIR MMSE-DFE based on the Householder orthogonal transformation which avoids the back-substitution computational bottleneck. The resulting significant reductions in the algorithm's execution time are demonstrated through a real-time FPGA implementation.
IEEE Communications Letters 12/2010; · 0.98 Impact Factor
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ABSTRACT: The maximum likelihood estimate of the impulse response of a frequency-selective channel in the presence of phase noise and I/Q imbalance is derived. The complexity of the joint estimator is reduced using approximate cost functions for both phase noise and I/Q imbalance. The proposed estimator is first applied to OFDM transmission in a single-input single-output system and then generalized to multi-input multi-output OFDM systems. The bit error rate performance of popular space-time codes for two and four transmit antennas is evaluated under zero-forcing, minimum mean squared error and maximum likelihood detection rules. An expression for the residual inter-carrier interference variance after phase noise and I/Q imbalance compensation is derived and compared to the uncompensated case. Significant improvement in signal-to-interference-noise is obtained with the proposed algorithm.
IEEE Transactions on Wireless Communications 12/2010; · 2.59 Impact Factor
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ABSTRACT: A practical approach for detecting packet-based orthogonal-frequency-division multiplexing (OFDM) signals in the presence of phase noise is presented. An OFDM packet consists of several OFDM symbols with full-pilot symbols at the beginning followed by consecutive data symbols. Based on the full-pilot OFDM symbol, a frequency-domain joint phase noise and channel vector estimator is first derived. It is shown that the phase noise vector can be estimated by maximizing a constrained quadratic form without requiring knowledge of the channel vector. This estimated phase noise vector is then used to compute the least squares channel estimator. Assuming that the channel is constant during each packet, the estimated channel is used in subsequent data OFDM symbols for equalization and data detection. Since phase noise changes from one OFDM symbol to the next, the scattered pilots in each data OFDM symbol are used to non-iteratively estimate and mitigate the phase noise induced interference. A significant improvement in the signal-to-interference-plus-noise ratio is achieved using the proposed algorithm.
IEEE Transactions on Signal Processing 12/2010; · 2.63 Impact Factor
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ABSTRACT: Direct-conversion multiple-input multiple-output orthogonal frequency division multiplexing (MIMO-OFDM) transceivers enjoy high data rates and reliability at practical implementation complexity. However, analog front-end impairments such as I/Q imbalance and high mobility requirements of next-generation broadband wireless standards result in performance-limiting inter-carrier interference (ICI). In this paper, we study the effects of ICI due to these impairments for OFDM with space frequency block codes and spatial multiplexing, derive a generalized linear model and propose a non-iterative reduced-complexity digital baseband joint compensation scheme. Furthermore, we present a pilot scheme for joint estimation of the channel and the I/Q imbalance parameters and evaluate its performance through simulations. Our proposed scheme is effective in estimating and compensating for frequency-independent and frequency-dependent transmit and receive I/Q imbalances even in the presence of a residual frequency offset.
IEEE Transactions on Wireless Communications 06/2010; · 2.59 Impact Factor
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ABSTRACT: Orthogonal frequency-division multiplexing (OFDM) combines the advantages of high performance and relatively low implementation complexity. However, for reliable coherent detection of the input signal, the OFDM receiver needs accurate channel information. When the channel exhibits fast time variation as it is the case with several recent OFDM-based mobile broadband wireless standards (e.g., WiMAX, LTE, DVB-H), channel estimation at the receiver becomes quite challenging for two main reasons: 1) the receiver needs to perform this estimation more frequently and 2) channel time-variations introduce intercarrier interference among the OFDM subcarriers which can degrade the performance of conventional channel estimation algorithms significantly. In this paper, we propose a new pilot-aided algorithm for the estimation of fast time-varying channels in OFDM transmission. Unlike many existing OFDM channel estimation algorithms in the literature, we propose to perform channel estimation in the frequency domain, to exploit the structure of the channel response (such as frequency and time correlations and bandedness), optimize the pilot group size and perform most of the computations offline resulting in high performance at substantial complexity reductions.
IEEE Transactions on Signal Processing 05/2010; · 2.63 Impact Factor
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ABSTRACT: In this paper, the bit error rate performances of multi-input multi-output orthogonal frequency division multiplexing transmission schemes for two and four transmit antennas are studied and compared in the presence of phase noise, I/Q imbalance and channel estimation errors. It is shown that for matched rates and same decoding complexity, maximum diversity schemes such as the Alamouti code, perform better than the maximum capacity schemes such as spatial multiplexing. The simulation environment comply with IEEE 802.11n standard.
Acoustics Speech and Signal Processing (ICASSP), 2010 IEEE International Conference on; 04/2010 · 4.63 Impact Factor
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ABSTRACT: In our previous work, digital baseband compensation of joint Tx/Rx I/Q imbalance in mobile SC-FDE transceivers was investigated. In this sequel, the training sequence design for joint channel and I/Q imbalance parameter estimation in mobile SC-FDE transceivers is discussed in greater detail.
Acoustics Speech and Signal Processing (ICASSP), 2010 IEEE International Conference on; 04/2010 · 4.63 Impact Factor
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ABSTRACT: In this sequel to [1], the joint Tx/Rx I/Q imbalance model of mobile SC-FDE transceivers is extended to include the effect of residual carrier frequency offset. Furthermore, the equivalent channel model for multiple-antenna spatially-multiplexed SC-FDE transceivers in the presence of I/Q imbalance and mobility is derived and an effective digital baseband compensation scheme is proposed, and its performance is evaluated including the effects of channel estimation.
Acoustics Speech and Signal Processing (ICASSP), 2010 IEEE International Conference on; 04/2010 · 4.63 Impact Factor
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ABSTRACT: In this paper, we propose a robust non-iterative pilot-aided channel estimator for multi-input multi-output orthogonal frequency division multiplexing systems in the presence I/Q imbalance and phase noise. We show that by exploiting the phase noise and I/Q imbalance properties, the joint cost function for estimating them reduces to separate cost functions, each of which can be readily solved. Mean-squared-error performance of the proposed estimator is evaluated for IEEE 802.11n wireless local area network applications.
Acoustics Speech and Signal Processing (ICASSP), 2010 IEEE International Conference on; 04/2010 · 4.63 Impact Factor
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ABSTRACT: Single-carrier frequency-division multiple-access (SC-FDMA) has been adopted in the uplink of the LTE standard due to its lower peak-to-average-power ratio (PAPR) compared to orthogonal frequency-division multiple access (OFDMA). Recent activities in the LTE-advanced (LTE-A) standardization have focused on effective uplink transmit diversity schemes with low PAPR but without considering the performance degradation due to Doppler despite the fact that LTE-A is required to support high mobility. In this paper, we present an open-loop space-frequency block coding (SFBC) scheme for the SC-FDMA uplink and demonstrate its robustness to high Doppler and large multipath delay spread while enjoying full spatial diversity, low PAPR and practical decoding complexity by a suitable design of the frequency span of each SFBC codeword. In this paper, we study the design tradeoffs involved in SFBC design for mobile SC-FDMA.
Acoustics Speech and Signal Processing (ICASSP), 2010 IEEE International Conference on; 04/2010 · 4.63 Impact Factor
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ABSTRACT: In this paper, we derive the maximum likelihood (ML) channel estimate in the presence of phase noise (PHN) and I/Q imbalance for OFDM systems. Since it is estimated jointly with I/Q and PHN, the channel estimate is more robust to these RF impairments compared to the conventional schemes. The complexity of the joint estimator is reduced using an approximate cost functions for both PHN and I/Q imbalance. The performance of the proposed estimator in terms of both channel mean squared error (MSE) and bit error rate (BER) is also presented and compared to the case of perfect channel knowledge.
Global Telecommunications Conference, 2009. GLOBECOM 2009. IEEE; 01/2010
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ABSTRACT: A new method for detecting packet-based OFDM signals in the presence of phase noise is presented. The scattered pilots in each data OFDM symbol are used to non-iteratively estimate and mitigate the phase noise induced inter-carrier interference (ICI). A significant improvement in the bit error rate (BER) is achieved using this technique, especially for systems suffering from high phase noise.
Global Telecommunications Conference, 2009. GLOBECOM 2009. IEEE; 01/2010
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ABSTRACT: Effective intercarrier interference (ICI) mitigation for multiple-input-multiple-output orthogonal frequency-division multiplexing (MIMO-OFDM) requires accurate channel estimation, which is very challenging due to the large number and fast time-varying nature of the channel parameters to be estimated using scattered pilots. We present a novel space-frequency block coding OFDM (SFBC-OFDM) scheme for doubly selective channels and a reduced-complexity channel-estimation algorithm, which exploits the banded and sparse structure of the channel in the frequency and time domains, respectively. Furthermore, we design a finite-impulse-response minimum-mean-square-error (FIR-MMSE) ICI cancellation algorithm for mobile SFBC-OFDM and demonstrate its effectiveness for digital video broadcasting-handheld (DVB-H) systems.
IEEE Transactions on Vehicular Technology 07/2009; · 1.92 Impact Factor
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ABSTRACT: We design a novel class of space-time codes, called linear diversity-embedding space-time block codes (LDE-STBC) where a high-rate STBC is linearly superimposed on a high-diversity STBC without requiring channel knowledge at the transmitter. In applying this scheme to multimedia wireless communications, each traffic type constitutes a transmission layer that operates at a suitable rate-diversity tradeoff point according to its quality-of-service requirements. This, in turn, provides an unequal-error-protection (UEP) capability to the different information traffic types and allows a form of wireless communications where the high-rate STBC opportunistically takes advantage of good channel realizations while the embedded high-diversity STBC ensures that at least part of the information is decoded reliably. We investigate transceiver design issues specific to LDE-STBC including reduced-complexity coherent decoding and effective schemes to vary the coding gain to further enhance UEP capabilities of the code. Furthermore, we investigate the application of LDE-STBC to wireless multicasting and demonstrate its performance advantage over conventional equal-error-protection STBC.
IEEE Transactions on Communications 07/2009; · 1.68 Impact Factor
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ABSTRACT: Direct conversion orthogonal frequency division multiplexing (OFDM) systems suffer from transmit and receive analog processing impairments such as in-phase/quadrature (I/Q) imbalance causing inter-carrier interference (ICI) among sub-carriers. Another source of performance-limiting ICI in OFDM systems is Doppler spread due to mobility. However, the nature of ICI due to each of them is quite different. Unlike previous work which considered these two impairments separately, we develop a unified mathematical framework to characterize, estimate, and jointly mitigate ICI due to I/Q imbalance and high mobility. Based on our general model, we derive a closed-form expression for the degradation in signal-to-interference-plus-noise ratio (SINR) due to the impairments. Moreover, we exploit the special ICI structure to design efficient OFDM channel estimation and digital baseband compensation schemes for joint transmit/receive frequency-independent (FI) and frequency-dependent (FD) I/Q imbalances under high-mobility conditions.
IEEE Journal of Selected Topics in Signal Processing 07/2009; · 2.88 Impact Factor
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ABSTRACT: We propose a new full-rate space-time block code (STBC) for two transmit antennas which can be designed to achieve maximum diversity or maximum capacity while enjoying optimized coding gain and reduced-complexity maximum-likelihood (ML) decoding. The maximum transmit diversity (MTD) construction provides a diversity order of 2N<sub>r</sub> for any number of receive antennas N<sub>r</sub> at the cost of channel capacity loss. The maximum channel capacity (MCC) construction preserves the mutual information between the transmit and the received vectors while sacrificing diversity. The system designer can switch between the two constructions through a simple parameter change based on the operating signal-to-noise ratio (SNR), signal constellation size and number of receive antennas. Thanks to their special algebraic structure, both constructions enjoy low-complexity ML decoding proportional to the square of the signal constellation size making them attractive alternatives to existing full-diversity full-rate STBCs in [6], [3] which have high ML decoding complexity proportional to the fourth order of the signal constellation size. Furthermore, we design a differential transmission scheme for our proposed STBC, derive the exact ML differential decoding rule, and compare its performance with competitive schemes. Finally, we investigate transceiver design and performance of our proposed STBC in spatial multiple-access scenarios and over frequency-selective channels.
IEEE Transactions on Wireless Communications 05/2009; · 2.59 Impact Factor
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ABSTRACT: Direct-conversion orthogonal frequency division multiplexing (OFDM) systems suffer from transmit and receive analog processing impairments such as in-phase/ quadrature (I/Q) imbalance causing intercarrier interference (ICI) between the sub-carriers. Another source of performance-limiting ICI, but with a different nature, in OFDM systems is Doppler spread due to mobility. Unlike previous work which considered these two problems separately, we develop a generalized analytical framework to characterize, estimate and jointly mitigate ICI due to both I/Q imbalance and high mobility. Based on our general model, we exploit the special ICI structure to design efficient channel and I/Q imbalance parameter estimation and digital baseband compensation schemes for joint transmit/receive frequency-independent and frequency-dependent I/Q imbalance under high-mobility conditions. Moreover, we extend the model, compensation and channel estimation methods to the multiple input multiple output (MIMO) case, spatial multiplexing (SM) in particular.
Information Sciences and Systems, 2009. CISS 2009. 43rd Annual Conference on; 04/2009
