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Papers by Ben LU

Research paper thumbnail of Bayesian Monte Carlo multiuser receiver for space-time coded multicarrier CDMA systems

IEEE Journal on Selected Areas in Communications, 2001

We consider the design of optimal multiuser receivers for space-time block coded (STBC) multicarr... more We consider the design of optimal multiuser receivers for space-time block coded (STBC) multicarrier code-division multiple-access (MC-CDMA) systems in unknown frequency-selective fading channels. Under a Bayesian framework, the proposed multiuser receiver is based on the Gibbs sampler, a Markov chain Monte Carlo (MCMC) method for numerically computing the marginal a posteriori probabilities of different users' data symbols. By exploiting the orthogonality property of the STBC and the multicarrier modulation, the computational complexity of the receiver is significantly reduced. Furthermore, being a soft-input soft-output algorithm, the Bayesian Monte Carlo multiuser detector is capable of exchanging the so-called extrinsic information with the maximum a posteriori (MAP) outer channel code decoders of all users, and successively improving the overall receiver performance. Several practical issues, such as testing the convergence of the Gibbs sampler in fading channel applications, resolving the phase ambiguity as well as the antenna ambiguity, and adapting the proposed receiver to multirate MC-CDMA systems, are also discussed. Finally, the performance of the Bayesian Monte Carlo multiuser receiver is demonstrated through computer simulations. Index Terms-Frequency-selective fading, Gibbs sampler, Monte Carlo signal processing, multicarrier CDMA (MC-CDMA), multiuser detection, space-time code (STC), turbo processing. I. INTRODUCTION T HE INTEREST in code-division multiple-access (CDMA) technology has been increasing over the past decades. Recently, as promising candidates for the third-generation (3G) wideband CDMA (WCDMA) systems, different multicarrier CDMA (MC-CDMA) systems have been proposed and investigated [1], [2]. In particular, in [3], an MC-CDMA system which transmits parallel multiple narrowband direct-sequence (DS) waveforms on different frequency carriers is proposed. In contrast, by combining the orthogonal frequency-division multiplexing (OFDM) technique with CDMA, another type of MC-CDMA system which transmits spread signals on different frequency subcarriers is proposed in [4]. The scheme proposed in [3] is more compatible with the currently deployed second-generation (2G) CDMA cellular systems. On the other hand, without compromising the system performance, the scheme in [4] can further reduce the receiver complexity by taking advantage of the special structure of the OFDM multicarrier modulation. In this paper, we follow the OFDM Manuscript

Research paper thumbnail of Method of manufacturing a golf club shaft

Research paper thumbnail of A Space-Time Trellis Code Design Method for OFDM Systems

Wireless Personal Communications, 2003

We introduce a new design method for space-time trelliscodes (STTC's) in orthogonal frequency-div... more We introduce a new design method for space-time trelliscodes (STTC's) in orthogonal frequency-division multiplexing(OFDM) systems with frequency-selective fading. First, byanalyzing the pairwise error probability (PEP), we conclude thatlarge effective length and random interleaving are twocritical principles in designing robust space-time codes (STC's)for OFDM systems. Then, based on the analogy between the proposedSTC design principles for multiple-antenna OFDM systems and thetrellis-coded modulation (TCM) code design criteria forsingle-antenna flat-fading channels, we develop a new STTC designmethod. At each trellis stage, this method converts the singleoutput code symbol of a traditional TCM code into several STTCcode symbols, which are to be simultaneously transmitted frommultiple transmitter-antennas, and hence results in a new class ofSTTC's. In this way, the effective lengths that have beenoptimized for traditional TCM codes are preserved in the resultingSTTC's; together with a random interleaver, the proposed new classof STTC's can robustly and efficiently exploit both the spatialand the frequency-selective fading diversity resources inmultiple-antenna OFDM systems. Finally, the excellent performanceof the proposed STTC's are demonstrated through computersimulations.

Research paper thumbnail of Space-time code design in OFDM systems

Abstract We consider a space-time coded (STC) orthogonal frequency-division multiplexing (OFDM) s... more Abstract We consider a space-time coded (STC) orthogonal frequency-division multiplexing (OFDM) system in frequency-selective fading channels. By analyzing the pairwise error probability (PEP), we show that STC-OFDM systems can potentially provide a diversity ...

Research paper thumbnail of Iterative receivers for multiuser space-time coding systems

IEEE Journal on Selected Areas in Communications, 2000

Space-time coding (STC) techniques, which combine antenna array signal processing and channel cod... more Space-time coding (STC) techniques, which combine antenna array signal processing and channel coding techniques, are very promising approaches to substantial capacity increase in wireless channels. Multiuser detection techniques are powerful signal processing methodologies for interference suppression in CDMA systems. In this paper, by drawing analogies between a synchronous CDMA system and an STC multiuser system, we study the applications of some multiuser detection methods to STC multiuser systems. Specifically, we show that the so-called "turbo multiuser detection" technique, which performs soft interference cancellation and decoding iteratively, can be applied to STC multiuser systems in flat-fading channels. Iterative multiuser receiver and its projection-based variants are developed for both the space-time block coding (STBC) system and the space-time trellis coding (STTC) system. During iterations, extrinsic information is computed and exchanged between a soft multiuser demodulator and a bank of MAP decoders, to achieve successively refined estimates of the users' signals.

Research paper thumbnail of Design of Multilayer Coded Modulation for Nonergodic Block-Fading Channels

IEEE Transactions on Communications, 2007

Multilayer coded modulation (MLyC) in single-antenna and multiple-antenna wireless fading channel... more Multilayer coded modulation (MLyC) in single-antenna and multiple-antenna wireless fading channels is investigated. While the MLyC has been known to achieve ergodic fading channel capacity, the problem of interest here is to design practical and efficient MLyC for nonergodic block-fading channels. We develop a design methodology which maximizes the information rate of MLyC, subject to an upper bound on the decoding error probability. Moreover, we show that the MLyC performs closer to channel capacity when the overall diversity order of the channel is higher and proper spatial interleaving is employed. The relationship between the MLyC scheme and other transmission techniques, such as multilevel coded modulation and bit-interleaved coded modulation (BICM), is also discussed. It is shown through simulation examples that in realistic block-fading channels, the MLyC with practical coded modulation outperforms the BICM without turbo receiver iteration. We propose the MLyC as a promising alternative to BICM, especially when turbo iterative processing is not desired, e.g., for progressive layered media transmission

Research paper thumbnail of Iterative Receivers for Multiuser Space-Time Coding Systems

Space-time coding (STC) techniques, which combine antenna array signal processing and channel cod... more Space-time coding (STC) techniques, which combine antenna array signal processing and channel coding techniques, are very promising approaches to substantial capacity increase in wireless channels. Multiuser detection techniques are powerful signal processing methodologies for interference suppression in CDMA systems. In this paper, by drawing analogies between a synchronous CDMA system and an STC multiuser system, we study the applications of some multiuser detection methods to STC multiuser systems. Specifically, we show that the so-called "turbo multiuser detection" technique, which performs soft interference cancellation and decoding iteratively, can be applied to STC multiuser systems in flat-fading channels. Iterative multiuser receiver and its projection-based variants are developed for both the space-time block coding (STBC) system and the space-time trellis coding (STTC) system. During iterations, extrinsic information is computed and exchanged between a soft multiuser demodulator and a bank of MAP decoders, to achieve successively refined estimates of the users' signals.

Research paper thumbnail of Low Complexity Turbo Equalization with Binary Precoding

We consider the design of convolutionally coded systems and low complexity receivers for communic... more We consider the design of convolutionally coded systems and low complexity receivers for communicating through intersymbol interference (ISI) channels when iterative equalization and decoding is employed in the receiver. We first introduce a binary precoding technique that makes a non-recursive ISI channel `appear' recursive to the outer code and, hence, provides excellent bit error rate (BER) performance. Then, a low

Research paper thumbnail of Bayesian Monte Carlo multiuser receiver for space-time coded multicarrier CDMA systems

IEEE Journal on Selected Areas in Communications, 2001

We consider the design of optimal multiuser receivers for space-time block coded (STBC) multicarr... more We consider the design of optimal multiuser receivers for space-time block coded (STBC) multicarrier code-division multiple-access (MC-CDMA) systems in unknown frequency-selective fading channels. Under a Bayesian framework, the proposed multiuser receiver is based on the Gibbs sampler, a Markov chain Monte Carlo (MCMC) method for numerically computing the marginal a posteriori probabilities of different users' data symbols. By exploiting the orthogonality property of the STBC and the multicarrier modulation, the computational complexity of the receiver is significantly reduced. Furthermore, being a soft-input soft-output algorithm, the Bayesian Monte Carlo multiuser detector is capable of exchanging the so-called extrinsic information with the maximum a posteriori (MAP) outer channel code decoders of all users, and successively improving the overall receiver performance. Several practical issues, such as testing the convergence of the Gibbs sampler in fading channel applications, resolving the phase ambiguity as well as the antenna ambiguity, and adapting the proposed receiver to multirate MC-CDMA systems, are also discussed. Finally, the performance of the Bayesian Monte Carlo multiuser receiver is demonstrated through computer simulations.

Research paper thumbnail of LDPC-based space-time coded OFDM systems over correlated fading channels: Performance analysis and receiver design

IEEE Transactions on Communications, 2002

We consider a space-time coded (STC) orthogonal frequency-division multiplexing (OFDM) system wit... more We consider a space-time coded (STC) orthogonal frequency-division multiplexing (OFDM) system with multiple transmitter and receiver antennas over correlated frequencyand time-selective fading channels. It is shown that the product of the time-selectivity order and the frequency-selectivity order is a key parameter to characterize the outage capacity of the correlated fading channel. It is also observed that STCs with large effective lengths and ideal built-in interleavers are more effective in exploiting the natural diversity in multiple-antenna correlated fading channels. We then propose a low-density parity-check (LDPC)-code-based STC-OFDM system. Compared with the conventional space-time trellis code (STTC), the LDPC-based STC can significantly improve the system performance by exploiting both the spatial diversity and the selective-fading diversity in wireless channels. Compared with the recently proposed turbo-code-based STC scheme, LDPC-based STC exhibits lower receiver complexity and more flexible scalability. We also consider receiver design for LDPC-based STC-OFDM systems in unknown fast fading channels and propose a novel turbo receiver employing a maximum a posteriori expectation-maximization (MAP-EM) demodulator and a soft LDPC decoder, which can significantly reduce the error floor in fast fading channels with a modest computational complexity. With such a turbo receiver, the proposed LDPC-based STC-OFDM system is a promising solution to highly efficient data transmission over selective-fading mobile wireless channels.

Research paper thumbnail of Iterative receivers for space-time block-coded OFDM systems in dispersive fading channels

IEEE Transactions on Wireless Communications, 2002

We consider the design of iterative receivers for space-time block-coded orthogonal frequency-div... more We consider the design of iterative receivers for space-time block-coded orthogonal frequency-division multiplexing (STBC-OFDM) systems in unknown wireless dispersive fading channels, with or without outer channel coding. First, we propose a maximum-likelihood (ML) receiver for STBC-OFDM systems based on the expectation-maximization (EM) algorithm. By assuming that the fading processes remain constant over the duration of one STBC code word and by exploiting the orthogonality property of the STBC as well as the OFDM modulation, we show that the EM-based receiver has a very low computational complexity and that the initialization of the EM receiver is based on the linear minimum mean square error (MMSE) channel estimate for both the pilot and the data transmission. Since the actual fading processes may vary within one STBC code word, we also analyze the effect of a modeling mismatch on the receiver performance and show both analytically and through simulations that the performance degradation due to such a mismatch is negligible for practical Doppler frequencies. We further propose a turbo receiver based on the maximum a posteriori-EM algorithm for STBC-OFDM systems with outer channel coding. Compared with the previous noniterative receiver employing a decision-directed linear channel estimator, the iterative receivers proposed here significantly improve the receiver performance and can approach the ML performance in typical wireless channels with very fast fading, at a reasonable computational complexity well suited for real-time implementations

Research paper thumbnail of Factor-Graph-Based Soft Self-Iterative Equalizer for Multipath Channels

Eurasip Journal on Wireless Communications and Networking, 2005

We consider factor-graph-based soft self-iterative equalization in wireless multipath channels. S... more We consider factor-graph-based soft self-iterative equalization in wireless multipath channels. Since factor graphs are able to characterize multipath channels to per-path level, the corresponding soft self-iterative equalizer possesses reduced computational complexity in sparse multipath channels. The performance of the considered self-iterative equalizer is analyzed in both single-antenna and multiple-antenna multipath channels. When factor graphs of multipath channels have no cycles or mild cycle conditions, the considered self-iterative equalizer can converge to optimum performance after a few iterations; but it may suffer local convergence in channels with severe cycle conditions.

Research paper thumbnail of Analysis and Design of Finite-Length LDPC Codes

IEEE Transactions on Vehicular Technology, 2007

We consider the performance analysis and code construction of finite-length low-density parity-ch... more We consider the performance analysis and code construction of finite-length low-density parity-check (LDPC) codes. First, by convergence analysis based on the extrinsic information evolution, we analyze the performance of both regular and irregular finite-length LDPC codes under iterative decoding. Next, by focusing on a special class of LDPC codes, namely, systematic irregular repeat-accumulate (IRA) codes, we propose a design procedure to construct finite-length LDPC codes. In addition to giving rise to a simple encoding structure, the special structure of IRA codes can be exploited to introduce unequal protection with cycle control for different types of nodes in the factor-graph code representation. We propose a modified bit-filling algorithm that leads to the construction of a set of finite-length IRA codes with low error floors.

Research paper thumbnail of Performance analysis and design optimization of LDPC-coded MIMO OFDM systems

IEEE Transactions on Signal Processing, 2004

We consider the performance analysis and design optimization of low-density parity check (LDPC) c... more We consider the performance analysis and design optimization of low-density parity check (LDPC) coded multiple-input multiple-output (MIMO) orthogonal frequency-division multiplexing (OFDM) systems for high data rate wireless transmission. The tools of density evolution with mixture Gaussian approximations are used to optimize irregular LDPC codes and to compute minimum operational signal-to-noise ratios (SNRs) for ergodic MIMO OFDM channels. In particular, the optimization is done for various MIMO OFDM system configurations, which include a different number of antennas, different channel models, and different demodulation schemes; the optimized performance is compared with the corresponding channel capacity. It is shown that along with the optimized irregular LDPC codes, a turbo iterative receiver that consists of a soft maximum a posteriori (MAP) demodulator and a belief-propagation LDPC decoder can perform within 1 dB from the ergodic capacity of the MIMO OFDM systems under consideration. It is also shown that compared with the optimal MAP demodulator-based receivers, the receivers employing a low-complexity linear minimum mean-square-error soft-interference-cancellation (LMMSE-SIC) demodulator have a small performance loss ( 1dB) in spatially uncorrelated MIMO channels but suffer extra performance loss in MIMO channels with spatial correlation. Finally, from the LDPC profiles that already are optimized for ergodic channels, we heuristically construct small block-size irregular LDPC codes for outage MIMO OFDM channels; as shown from simulation results, the irregular LDPC codes constructed here are helpful in expediting the convergence of the iterative receivers.

Research paper thumbnail of Blind Bayesian multiuser receiver for space-time coded MC-CDMA system over frequency-selective fading channel

We consider the design of a blind optimal multiuser receiver for space-time block coded (STBC) mu... more We consider the design of a blind optimal multiuser receiver for space-time block coded (STBC) multi-carrier code-division multiple-access (MC-CDMA) systems in unknown frequency-selective fading channels. A novel blind Bayesian multiuser detector is derived for joint estimation of unknown fading channel and symbols. Such a detector is based on the Bayesian inference of all unknown quantities. The Gibbs sampler, a Markov chain Monte Carlo (MCMC) method, is then used for Bayesian computation. Moreover, being soft-input and soft-output, the proposed Bayesian multiuser detector fits well into the turbo receiver framework and it exchanges the extrinsic information with the MAP channel decoder to successively refine its performance. Finally, its performance over random generated slow and fast frequency-selective fading channel is demonstrated through simulation examples

Research paper thumbnail of Bayesian blind turbo receiver for coded OFDM systems with frequency offset and frequency-selective fading

IEEE Journal on Selected Areas in Communications, 2001

The design of a blind receiver for coded orthogonal frequency-division multiplexing communication... more The design of a blind receiver for coded orthogonal frequency-division multiplexing communication systems in the presence of frequency offset and frequency-selective fading is investigated. The proposed blind receiver iterates between a Bayesian demodulation stage and a maximum a posteriori channel decoding stage. The extrinsic a posteriori probabilities of data symbols are iteratively exchanged between these two stages to achieve successively improved performance. The Bayesian demodulator computes the a posteriori data symbol probabilities, based on the received signals (without knowing or explicitly estimating the frequency offset and the fading channel states), by using Markov chain Monte Carlo (MCMC) techniques. In particular, two MCMC methods-the Metropolis-Hastings algorithm and the Gibbs sampler-are studied for this purpose. Computer simulation results show that the proposed Bayesian blind turbo receiver can achieve good performance and is robust against modeling mismatch.

Research paper thumbnail of Throughput of CDMA data networks with multiuser detection, ARQ, and packet combining

IEEE Transactions on Wireless Communications, 2004

In code-division multiple-access (CDMA) packet data networks, the throughput depends on physical-... more In code-division multiple-access (CDMA) packet data networks, the throughput depends on physical-layer receiver algorithms and medium access control (MAC) layer protocols. Taking a holistic approach, we investigate the throughput of a CDMA network employing linear multiuser detection, type-I automatic retransmission request (ARQ), and packet combining. In particular, the following two models of CDMA data networks are considered. 1) Fixed-access CDMA data networks, by which we mean that the number of active users is fixed. The corresponding throughput is analyzed by using a one-dimensional Markov chain, and conditions for achieving the optimal throughput are explored. 2) Random-access CDMA data networks, in which we take into account the random arrivals/departures of users. By viewing the fixed-access network studied in the first case as a snapshot of the random-access network, we exploit the results therein to analyze the random-access network, under a processor-sharing system model. Moreover, we identify some important properties of the corresponding throughput and devise a simple recursive algorithm to find the throughput-optimal admission region. Some recent advances on large-system performance of various multiuser detection algorithms are employed in our study. The results in this paper quantitatively characterize the potential for network throughput gain by employing multiuser detection and packet combining, in both fixed-access and random-access CDMA packet data networks.

Research paper thumbnail of Bayesian blind turbo receiver for coded OFDM systems with frequency offset and frequency-selective fading

The design of a blind receiver for coded orthogonal frequency-division multiplexing (OFDM) commun... more The design of a blind receiver for coded orthogonal frequency-division multiplexing (OFDM) communication systems in the presence of frequency offset and frequency-selective fading is investigated. The proposed blind receiver iterates between a Bayesian demodulation stage and a maximum a posteriori (MAP) channel decoding stage; and the extrinsic a posteriori probabilities of data symbols are iteratively exchanged between these two stages to achieve successively improved performance. The Bayesian demodulator computes the a posteriori data symbol probabilities, based on the received signals (without knowing or explicitly estimating the frequency offset and the fading channel states), by using Markov chain Monte Carlo (MCMC) techniques. In particular, two MCMC methods, namely, the Metropolis-Hastings algorithm and the Gibbs sampler, are studied for this purpose. Computer simulation results show that the proposed Bayesian blind turbo receiver can achieve good performance and it is robust against modeling mismatch

Research paper thumbnail of Performance analysis and design optimization of LDPC coded MIMO OFDM systems

In this paper, the performance of a self-powered unipolar gate driver supply circuit for power de... more In this paper, the performance of a self-powered unipolar gate driver supply circuit for power devices is studied, with the aim of analyzing the viabi lity of using such circuits in high voltage applications with discrete component s. A simplified model of the circuit, capturing the essential features, is propo sed, from which practical design guidelines are provided to optimize the overall circuit performance. These design guidelines allow a proper component selection that can result in significant improvements in the circuit performance. Experim ental results of typical parameters characterizing the turn-on and turn-off tran sients, including the turn-on and turnoff energy loss, are provided for a wide r ange of current values and different gate resistances. The results are compared to those obtained using a conventional gate driver power supply.

Research paper thumbnail of A cross-layer TCP modelling framework for MIMO wireless systems

IEEE Transactions on Wireless Communications, 2006

We propose a general framework based in the Gilbert model for cross-layer analysis of TCP and UDP... more We propose a general framework based in the Gilbert model for cross-layer analysis of TCP and UDP over MIMO wireless systems. Our framework takes into consideration diverse system characteristics often difficult to express as a Gilbert model such as fading, space-time transmission schemes, modulation, channel coding and ARQ. We apply our framework to analyze the TCP performance of two representative MIMO systems, namely, the BLAST system and the orthogonal spacetime block coded (STBC) system. In particular, we investigate the optimal information rate that maximizes the TCP throughput, the effect of Doppler on the optimal TCP throughput and the optimal channel coding rate for various modulations. We provide simulations results from the ns-2 network simulator to demonstrate the accuracy of the proposed analytical framework in characterizing the TCP performance. We further apply the framework to two additional cross-layer applications: the analysis of the buffer occupancy on the base station, and the analysis of CBR video transmission over MIMO systems. We show that while the optimal rate for maximum TCP throughput is far from the channel capacity, the optimal rate for error and delay-tolerant video transmission requires much higher rates, and so the physical layer should be aware and adapt to the type of application in order to increase the system performance. We also show that mobility benefits systems with larger buffers, especially for TCP, as the ARQ scheme is able to recover the shorter burst errors. In general, our investigation shows that the type of application plays a crucial role in the optimization of a wireless system, and that our modelling framework is useful for the cross-layer analysis and design of those systems.

Research paper thumbnail of Bayesian Monte Carlo multiuser receiver for space-time coded multicarrier CDMA systems

IEEE Journal on Selected Areas in Communications, 2001

We consider the design of optimal multiuser receivers for space-time block coded (STBC) multicarr... more We consider the design of optimal multiuser receivers for space-time block coded (STBC) multicarrier code-division multiple-access (MC-CDMA) systems in unknown frequency-selective fading channels. Under a Bayesian framework, the proposed multiuser receiver is based on the Gibbs sampler, a Markov chain Monte Carlo (MCMC) method for numerically computing the marginal a posteriori probabilities of different users' data symbols. By exploiting the orthogonality property of the STBC and the multicarrier modulation, the computational complexity of the receiver is significantly reduced. Furthermore, being a soft-input soft-output algorithm, the Bayesian Monte Carlo multiuser detector is capable of exchanging the so-called extrinsic information with the maximum a posteriori (MAP) outer channel code decoders of all users, and successively improving the overall receiver performance. Several practical issues, such as testing the convergence of the Gibbs sampler in fading channel applications, resolving the phase ambiguity as well as the antenna ambiguity, and adapting the proposed receiver to multirate MC-CDMA systems, are also discussed. Finally, the performance of the Bayesian Monte Carlo multiuser receiver is demonstrated through computer simulations. Index Terms-Frequency-selective fading, Gibbs sampler, Monte Carlo signal processing, multicarrier CDMA (MC-CDMA), multiuser detection, space-time code (STC), turbo processing. I. INTRODUCTION T HE INTEREST in code-division multiple-access (CDMA) technology has been increasing over the past decades. Recently, as promising candidates for the third-generation (3G) wideband CDMA (WCDMA) systems, different multicarrier CDMA (MC-CDMA) systems have been proposed and investigated [1], [2]. In particular, in [3], an MC-CDMA system which transmits parallel multiple narrowband direct-sequence (DS) waveforms on different frequency carriers is proposed. In contrast, by combining the orthogonal frequency-division multiplexing (OFDM) technique with CDMA, another type of MC-CDMA system which transmits spread signals on different frequency subcarriers is proposed in [4]. The scheme proposed in [3] is more compatible with the currently deployed second-generation (2G) CDMA cellular systems. On the other hand, without compromising the system performance, the scheme in [4] can further reduce the receiver complexity by taking advantage of the special structure of the OFDM multicarrier modulation. In this paper, we follow the OFDM Manuscript

Research paper thumbnail of Method of manufacturing a golf club shaft

Research paper thumbnail of A Space-Time Trellis Code Design Method for OFDM Systems

Wireless Personal Communications, 2003

We introduce a new design method for space-time trelliscodes (STTC's) in orthogonal frequency-div... more We introduce a new design method for space-time trelliscodes (STTC's) in orthogonal frequency-division multiplexing(OFDM) systems with frequency-selective fading. First, byanalyzing the pairwise error probability (PEP), we conclude thatlarge effective length and random interleaving are twocritical principles in designing robust space-time codes (STC's)for OFDM systems. Then, based on the analogy between the proposedSTC design principles for multiple-antenna OFDM systems and thetrellis-coded modulation (TCM) code design criteria forsingle-antenna flat-fading channels, we develop a new STTC designmethod. At each trellis stage, this method converts the singleoutput code symbol of a traditional TCM code into several STTCcode symbols, which are to be simultaneously transmitted frommultiple transmitter-antennas, and hence results in a new class ofSTTC's. In this way, the effective lengths that have beenoptimized for traditional TCM codes are preserved in the resultingSTTC's; together with a random interleaver, the proposed new classof STTC's can robustly and efficiently exploit both the spatialand the frequency-selective fading diversity resources inmultiple-antenna OFDM systems. Finally, the excellent performanceof the proposed STTC's are demonstrated through computersimulations.

Research paper thumbnail of Space-time code design in OFDM systems

Abstract We consider a space-time coded (STC) orthogonal frequency-division multiplexing (OFDM) s... more Abstract We consider a space-time coded (STC) orthogonal frequency-division multiplexing (OFDM) system in frequency-selective fading channels. By analyzing the pairwise error probability (PEP), we show that STC-OFDM systems can potentially provide a diversity ...

Research paper thumbnail of Iterative receivers for multiuser space-time coding systems

IEEE Journal on Selected Areas in Communications, 2000

Space-time coding (STC) techniques, which combine antenna array signal processing and channel cod... more Space-time coding (STC) techniques, which combine antenna array signal processing and channel coding techniques, are very promising approaches to substantial capacity increase in wireless channels. Multiuser detection techniques are powerful signal processing methodologies for interference suppression in CDMA systems. In this paper, by drawing analogies between a synchronous CDMA system and an STC multiuser system, we study the applications of some multiuser detection methods to STC multiuser systems. Specifically, we show that the so-called "turbo multiuser detection" technique, which performs soft interference cancellation and decoding iteratively, can be applied to STC multiuser systems in flat-fading channels. Iterative multiuser receiver and its projection-based variants are developed for both the space-time block coding (STBC) system and the space-time trellis coding (STTC) system. During iterations, extrinsic information is computed and exchanged between a soft multiuser demodulator and a bank of MAP decoders, to achieve successively refined estimates of the users' signals.

Research paper thumbnail of Design of Multilayer Coded Modulation for Nonergodic Block-Fading Channels

IEEE Transactions on Communications, 2007

Multilayer coded modulation (MLyC) in single-antenna and multiple-antenna wireless fading channel... more Multilayer coded modulation (MLyC) in single-antenna and multiple-antenna wireless fading channels is investigated. While the MLyC has been known to achieve ergodic fading channel capacity, the problem of interest here is to design practical and efficient MLyC for nonergodic block-fading channels. We develop a design methodology which maximizes the information rate of MLyC, subject to an upper bound on the decoding error probability. Moreover, we show that the MLyC performs closer to channel capacity when the overall diversity order of the channel is higher and proper spatial interleaving is employed. The relationship between the MLyC scheme and other transmission techniques, such as multilevel coded modulation and bit-interleaved coded modulation (BICM), is also discussed. It is shown through simulation examples that in realistic block-fading channels, the MLyC with practical coded modulation outperforms the BICM without turbo receiver iteration. We propose the MLyC as a promising alternative to BICM, especially when turbo iterative processing is not desired, e.g., for progressive layered media transmission

Research paper thumbnail of Iterative Receivers for Multiuser Space-Time Coding Systems

Space-time coding (STC) techniques, which combine antenna array signal processing and channel cod... more Space-time coding (STC) techniques, which combine antenna array signal processing and channel coding techniques, are very promising approaches to substantial capacity increase in wireless channels. Multiuser detection techniques are powerful signal processing methodologies for interference suppression in CDMA systems. In this paper, by drawing analogies between a synchronous CDMA system and an STC multiuser system, we study the applications of some multiuser detection methods to STC multiuser systems. Specifically, we show that the so-called "turbo multiuser detection" technique, which performs soft interference cancellation and decoding iteratively, can be applied to STC multiuser systems in flat-fading channels. Iterative multiuser receiver and its projection-based variants are developed for both the space-time block coding (STBC) system and the space-time trellis coding (STTC) system. During iterations, extrinsic information is computed and exchanged between a soft multiuser demodulator and a bank of MAP decoders, to achieve successively refined estimates of the users' signals.

Research paper thumbnail of Low Complexity Turbo Equalization with Binary Precoding

We consider the design of convolutionally coded systems and low complexity receivers for communic... more We consider the design of convolutionally coded systems and low complexity receivers for communicating through intersymbol interference (ISI) channels when iterative equalization and decoding is employed in the receiver. We first introduce a binary precoding technique that makes a non-recursive ISI channel `appear' recursive to the outer code and, hence, provides excellent bit error rate (BER) performance. Then, a low

Research paper thumbnail of Bayesian Monte Carlo multiuser receiver for space-time coded multicarrier CDMA systems

IEEE Journal on Selected Areas in Communications, 2001

We consider the design of optimal multiuser receivers for space-time block coded (STBC) multicarr... more We consider the design of optimal multiuser receivers for space-time block coded (STBC) multicarrier code-division multiple-access (MC-CDMA) systems in unknown frequency-selective fading channels. Under a Bayesian framework, the proposed multiuser receiver is based on the Gibbs sampler, a Markov chain Monte Carlo (MCMC) method for numerically computing the marginal a posteriori probabilities of different users' data symbols. By exploiting the orthogonality property of the STBC and the multicarrier modulation, the computational complexity of the receiver is significantly reduced. Furthermore, being a soft-input soft-output algorithm, the Bayesian Monte Carlo multiuser detector is capable of exchanging the so-called extrinsic information with the maximum a posteriori (MAP) outer channel code decoders of all users, and successively improving the overall receiver performance. Several practical issues, such as testing the convergence of the Gibbs sampler in fading channel applications, resolving the phase ambiguity as well as the antenna ambiguity, and adapting the proposed receiver to multirate MC-CDMA systems, are also discussed. Finally, the performance of the Bayesian Monte Carlo multiuser receiver is demonstrated through computer simulations.

Research paper thumbnail of LDPC-based space-time coded OFDM systems over correlated fading channels: Performance analysis and receiver design

IEEE Transactions on Communications, 2002

We consider a space-time coded (STC) orthogonal frequency-division multiplexing (OFDM) system wit... more We consider a space-time coded (STC) orthogonal frequency-division multiplexing (OFDM) system with multiple transmitter and receiver antennas over correlated frequencyand time-selective fading channels. It is shown that the product of the time-selectivity order and the frequency-selectivity order is a key parameter to characterize the outage capacity of the correlated fading channel. It is also observed that STCs with large effective lengths and ideal built-in interleavers are more effective in exploiting the natural diversity in multiple-antenna correlated fading channels. We then propose a low-density parity-check (LDPC)-code-based STC-OFDM system. Compared with the conventional space-time trellis code (STTC), the LDPC-based STC can significantly improve the system performance by exploiting both the spatial diversity and the selective-fading diversity in wireless channels. Compared with the recently proposed turbo-code-based STC scheme, LDPC-based STC exhibits lower receiver complexity and more flexible scalability. We also consider receiver design for LDPC-based STC-OFDM systems in unknown fast fading channels and propose a novel turbo receiver employing a maximum a posteriori expectation-maximization (MAP-EM) demodulator and a soft LDPC decoder, which can significantly reduce the error floor in fast fading channels with a modest computational complexity. With such a turbo receiver, the proposed LDPC-based STC-OFDM system is a promising solution to highly efficient data transmission over selective-fading mobile wireless channels.

Research paper thumbnail of Iterative receivers for space-time block-coded OFDM systems in dispersive fading channels

IEEE Transactions on Wireless Communications, 2002

We consider the design of iterative receivers for space-time block-coded orthogonal frequency-div... more We consider the design of iterative receivers for space-time block-coded orthogonal frequency-division multiplexing (STBC-OFDM) systems in unknown wireless dispersive fading channels, with or without outer channel coding. First, we propose a maximum-likelihood (ML) receiver for STBC-OFDM systems based on the expectation-maximization (EM) algorithm. By assuming that the fading processes remain constant over the duration of one STBC code word and by exploiting the orthogonality property of the STBC as well as the OFDM modulation, we show that the EM-based receiver has a very low computational complexity and that the initialization of the EM receiver is based on the linear minimum mean square error (MMSE) channel estimate for both the pilot and the data transmission. Since the actual fading processes may vary within one STBC code word, we also analyze the effect of a modeling mismatch on the receiver performance and show both analytically and through simulations that the performance degradation due to such a mismatch is negligible for practical Doppler frequencies. We further propose a turbo receiver based on the maximum a posteriori-EM algorithm for STBC-OFDM systems with outer channel coding. Compared with the previous noniterative receiver employing a decision-directed linear channel estimator, the iterative receivers proposed here significantly improve the receiver performance and can approach the ML performance in typical wireless channels with very fast fading, at a reasonable computational complexity well suited for real-time implementations

Research paper thumbnail of Factor-Graph-Based Soft Self-Iterative Equalizer for Multipath Channels

Eurasip Journal on Wireless Communications and Networking, 2005

We consider factor-graph-based soft self-iterative equalization in wireless multipath channels. S... more We consider factor-graph-based soft self-iterative equalization in wireless multipath channels. Since factor graphs are able to characterize multipath channels to per-path level, the corresponding soft self-iterative equalizer possesses reduced computational complexity in sparse multipath channels. The performance of the considered self-iterative equalizer is analyzed in both single-antenna and multiple-antenna multipath channels. When factor graphs of multipath channels have no cycles or mild cycle conditions, the considered self-iterative equalizer can converge to optimum performance after a few iterations; but it may suffer local convergence in channels with severe cycle conditions.

Research paper thumbnail of Analysis and Design of Finite-Length LDPC Codes

IEEE Transactions on Vehicular Technology, 2007

We consider the performance analysis and code construction of finite-length low-density parity-ch... more We consider the performance analysis and code construction of finite-length low-density parity-check (LDPC) codes. First, by convergence analysis based on the extrinsic information evolution, we analyze the performance of both regular and irregular finite-length LDPC codes under iterative decoding. Next, by focusing on a special class of LDPC codes, namely, systematic irregular repeat-accumulate (IRA) codes, we propose a design procedure to construct finite-length LDPC codes. In addition to giving rise to a simple encoding structure, the special structure of IRA codes can be exploited to introduce unequal protection with cycle control for different types of nodes in the factor-graph code representation. We propose a modified bit-filling algorithm that leads to the construction of a set of finite-length IRA codes with low error floors.

Research paper thumbnail of Performance analysis and design optimization of LDPC-coded MIMO OFDM systems

IEEE Transactions on Signal Processing, 2004

We consider the performance analysis and design optimization of low-density parity check (LDPC) c... more We consider the performance analysis and design optimization of low-density parity check (LDPC) coded multiple-input multiple-output (MIMO) orthogonal frequency-division multiplexing (OFDM) systems for high data rate wireless transmission. The tools of density evolution with mixture Gaussian approximations are used to optimize irregular LDPC codes and to compute minimum operational signal-to-noise ratios (SNRs) for ergodic MIMO OFDM channels. In particular, the optimization is done for various MIMO OFDM system configurations, which include a different number of antennas, different channel models, and different demodulation schemes; the optimized performance is compared with the corresponding channel capacity. It is shown that along with the optimized irregular LDPC codes, a turbo iterative receiver that consists of a soft maximum a posteriori (MAP) demodulator and a belief-propagation LDPC decoder can perform within 1 dB from the ergodic capacity of the MIMO OFDM systems under consideration. It is also shown that compared with the optimal MAP demodulator-based receivers, the receivers employing a low-complexity linear minimum mean-square-error soft-interference-cancellation (LMMSE-SIC) demodulator have a small performance loss ( 1dB) in spatially uncorrelated MIMO channels but suffer extra performance loss in MIMO channels with spatial correlation. Finally, from the LDPC profiles that already are optimized for ergodic channels, we heuristically construct small block-size irregular LDPC codes for outage MIMO OFDM channels; as shown from simulation results, the irregular LDPC codes constructed here are helpful in expediting the convergence of the iterative receivers.

Research paper thumbnail of Blind Bayesian multiuser receiver for space-time coded MC-CDMA system over frequency-selective fading channel

We consider the design of a blind optimal multiuser receiver for space-time block coded (STBC) mu... more We consider the design of a blind optimal multiuser receiver for space-time block coded (STBC) multi-carrier code-division multiple-access (MC-CDMA) systems in unknown frequency-selective fading channels. A novel blind Bayesian multiuser detector is derived for joint estimation of unknown fading channel and symbols. Such a detector is based on the Bayesian inference of all unknown quantities. The Gibbs sampler, a Markov chain Monte Carlo (MCMC) method, is then used for Bayesian computation. Moreover, being soft-input and soft-output, the proposed Bayesian multiuser detector fits well into the turbo receiver framework and it exchanges the extrinsic information with the MAP channel decoder to successively refine its performance. Finally, its performance over random generated slow and fast frequency-selective fading channel is demonstrated through simulation examples

Research paper thumbnail of Bayesian blind turbo receiver for coded OFDM systems with frequency offset and frequency-selective fading

IEEE Journal on Selected Areas in Communications, 2001

The design of a blind receiver for coded orthogonal frequency-division multiplexing communication... more The design of a blind receiver for coded orthogonal frequency-division multiplexing communication systems in the presence of frequency offset and frequency-selective fading is investigated. The proposed blind receiver iterates between a Bayesian demodulation stage and a maximum a posteriori channel decoding stage. The extrinsic a posteriori probabilities of data symbols are iteratively exchanged between these two stages to achieve successively improved performance. The Bayesian demodulator computes the a posteriori data symbol probabilities, based on the received signals (without knowing or explicitly estimating the frequency offset and the fading channel states), by using Markov chain Monte Carlo (MCMC) techniques. In particular, two MCMC methods-the Metropolis-Hastings algorithm and the Gibbs sampler-are studied for this purpose. Computer simulation results show that the proposed Bayesian blind turbo receiver can achieve good performance and is robust against modeling mismatch.

Research paper thumbnail of Throughput of CDMA data networks with multiuser detection, ARQ, and packet combining

IEEE Transactions on Wireless Communications, 2004

In code-division multiple-access (CDMA) packet data networks, the throughput depends on physical-... more In code-division multiple-access (CDMA) packet data networks, the throughput depends on physical-layer receiver algorithms and medium access control (MAC) layer protocols. Taking a holistic approach, we investigate the throughput of a CDMA network employing linear multiuser detection, type-I automatic retransmission request (ARQ), and packet combining. In particular, the following two models of CDMA data networks are considered. 1) Fixed-access CDMA data networks, by which we mean that the number of active users is fixed. The corresponding throughput is analyzed by using a one-dimensional Markov chain, and conditions for achieving the optimal throughput are explored. 2) Random-access CDMA data networks, in which we take into account the random arrivals/departures of users. By viewing the fixed-access network studied in the first case as a snapshot of the random-access network, we exploit the results therein to analyze the random-access network, under a processor-sharing system model. Moreover, we identify some important properties of the corresponding throughput and devise a simple recursive algorithm to find the throughput-optimal admission region. Some recent advances on large-system performance of various multiuser detection algorithms are employed in our study. The results in this paper quantitatively characterize the potential for network throughput gain by employing multiuser detection and packet combining, in both fixed-access and random-access CDMA packet data networks.

Research paper thumbnail of Bayesian blind turbo receiver for coded OFDM systems with frequency offset and frequency-selective fading

The design of a blind receiver for coded orthogonal frequency-division multiplexing (OFDM) commun... more The design of a blind receiver for coded orthogonal frequency-division multiplexing (OFDM) communication systems in the presence of frequency offset and frequency-selective fading is investigated. The proposed blind receiver iterates between a Bayesian demodulation stage and a maximum a posteriori (MAP) channel decoding stage; and the extrinsic a posteriori probabilities of data symbols are iteratively exchanged between these two stages to achieve successively improved performance. The Bayesian demodulator computes the a posteriori data symbol probabilities, based on the received signals (without knowing or explicitly estimating the frequency offset and the fading channel states), by using Markov chain Monte Carlo (MCMC) techniques. In particular, two MCMC methods, namely, the Metropolis-Hastings algorithm and the Gibbs sampler, are studied for this purpose. Computer simulation results show that the proposed Bayesian blind turbo receiver can achieve good performance and it is robust against modeling mismatch

Research paper thumbnail of Performance analysis and design optimization of LDPC coded MIMO OFDM systems

In this paper, the performance of a self-powered unipolar gate driver supply circuit for power de... more In this paper, the performance of a self-powered unipolar gate driver supply circuit for power devices is studied, with the aim of analyzing the viabi lity of using such circuits in high voltage applications with discrete component s. A simplified model of the circuit, capturing the essential features, is propo sed, from which practical design guidelines are provided to optimize the overall circuit performance. These design guidelines allow a proper component selection that can result in significant improvements in the circuit performance. Experim ental results of typical parameters characterizing the turn-on and turn-off tran sients, including the turn-on and turnoff energy loss, are provided for a wide r ange of current values and different gate resistances. The results are compared to those obtained using a conventional gate driver power supply.

Research paper thumbnail of A cross-layer TCP modelling framework for MIMO wireless systems

IEEE Transactions on Wireless Communications, 2006

We propose a general framework based in the Gilbert model for cross-layer analysis of TCP and UDP... more We propose a general framework based in the Gilbert model for cross-layer analysis of TCP and UDP over MIMO wireless systems. Our framework takes into consideration diverse system characteristics often difficult to express as a Gilbert model such as fading, space-time transmission schemes, modulation, channel coding and ARQ. We apply our framework to analyze the TCP performance of two representative MIMO systems, namely, the BLAST system and the orthogonal spacetime block coded (STBC) system. In particular, we investigate the optimal information rate that maximizes the TCP throughput, the effect of Doppler on the optimal TCP throughput and the optimal channel coding rate for various modulations. We provide simulations results from the ns-2 network simulator to demonstrate the accuracy of the proposed analytical framework in characterizing the TCP performance. We further apply the framework to two additional cross-layer applications: the analysis of the buffer occupancy on the base station, and the analysis of CBR video transmission over MIMO systems. We show that while the optimal rate for maximum TCP throughput is far from the channel capacity, the optimal rate for error and delay-tolerant video transmission requires much higher rates, and so the physical layer should be aware and adapt to the type of application in order to increase the system performance. We also show that mobility benefits systems with larger buffers, especially for TCP, as the ARQ scheme is able to recover the shorter burst errors. In general, our investigation shows that the type of application plays a crucial role in the optimization of a wireless system, and that our modelling framework is useful for the cross-layer analysis and design of those systems.