R-3 Repository :: Browsing by Author "Borran, Mohammad Jaber"

Browsing by Author "Borran, Mohammad Jaber"

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Channel Estimation and Signal Detection for Multi-Carrier CDMA Systems with Pulse-Shaping Filter
(2003-05-20) Borran, Mohammad Jaber; Varshney, Prabodh; Vilpponen, Hannu; Papadimitriou, Panayiotis; Center for Multimedia Communications (http://cmc.rice.edu/)
We consider the problem of digital communication in a fading environment using the Multi-Carrier CDMA technology. By incorporating the effect of the pulse-shaping filter in the channel estimation and signal detection modules, we develop new estimator and detector structures which significantly outperform the commonly used time-domain equalizers and matched filter detector. Our simulation results demonstrate the elimination of some of the error floors which one would experience if usual time-domain techniques were used to cancel the effect of nonideal pulse-shaping filter.
Constellations for Imperfect Channel State Information at the Receiver
(2002-10-20) Borran, Mohammad Jaber; Sabharwal, Ashutosh; Aazhang, Behnaam; Center for Multimedia Communications (http://cmc.rice.edu/)
We consider the problem of digital communication in a Rayleigh flat fading environment using a multiple antenna system. We assume that the transmitter doesn't know the channel coefficients, and that the receiver has only an estimate of them with some known estimation error. Inspired by the Stein's lemma, we propose to use the Kullback-Leibler distance between distributions assigned to the transmitted symbols as a performance criterion. Using this performance criterion, we derive a design criterion based on maximizing the minimum KL distance between constellation points. As an example, we design constellations for a single transmit antenna system using the above criterion, and through simulation, show that the new constellations can provide a substantial improvement in the performance over existing and widely-used constellations.
Design of Coded Modulation Schemes for Orthogonal Transmit Diversity
(2001-06-20) Borran, Mohammad Jaber; Memarzadeh, Mahsa; Aazhang, Behnaam; Center for Multimedia Communications (http://cmc.rice.edu/)
In this paper, we propose a technique to decouple the problems of spatial and temporal diversity gain maximization involved in the design of space-time codes in fast Rayleigh fading channels by using orthogonal transmit diversity systems. We will also introduce the idea of constellation expansion (in dimension or size) to design coded modulation schemes with maximum temporal diversity gain for orthogonal transmit systems. The Chernoff upper bound for the error probability is used to show that the code design criterion reduces to the maximization of Hamming and product distances in the expanded constellation. The proposed technique is demonstrated by designing multilevel and multiple trellis coded modulation schemes for orthogonal transmit diversity systems. These codes are shown to have better performance compared to the coding schemes designed for single transmit and receive antennas.
Design of Coded Modulation Schemes for Orthogonal Transmit Diversity
(2001-05-20) Borran, Mohammad Jaber; Memarzadeh, Mahsa; Aazhang, Behnaam; Center for Multimedia Communications (http://cmc.rice.edu/)
In this paper, we propose a technique to decouple the problems of maximizing spatial diversity gain, temporal diversity gain, and coding gain, involved in the design of space-time codes in Rayleigh fading channels. For this purpose, we will use a set of concatenated codes with orthogonal transmit diversity system as their inner code. In the case of slowly fading channels, no temporal diversity is available, and the proposed technique decouples the problems of maximizing spatial diversity gain and coding gain. The design criterion for the outer codewill be shown to be based on the maximization of the free Euclidean distance. For fast fading channels, by using the new idea of constellation expansion (in dimension or size), we decouple the code design problem into two simpler problems, namely maximizing the spatial diversity gain, and maximizing the temporal diversity and coding gain. In this case, the code design criteria for the outer encoder will be shown to reduce to the maximization of Hamming and product distances in the expanded constellation. The proposed techniques are illustrated by designing multilevel and multiple trellis coded modulation schemes for orthogonal transmit diversity systems. These codes are shown to have better performance compared to some existing space-time trellis codes with the same complexities.
An Efficient Detection Technique for Synchronous CDMA Communication Systems Based on the Expectation Maximization Algorithm
(2000-09-20) Borran, Mohammad Jaber; Nasiri-Kenari, Masoumeh; Center for Multimedia Communications (http://cmc.rice.edu/)
Maximum likelihood detection of superimposed signals in code-division multiple access (CDMA) communication systems has a computational complexity that is exponential in the number of users, and its implementation is practically prohibitive even for a moderate number of users. Applying the expectation maximization algorithm to this problem, we decompose the multiuser detection problem into a series of single-user problems, and thus present an iterative computationally efficient algorithm for detection of superimposed signals in synchronous direct-sequence CDMA communication systems. The resulting structure includes the well-known multistage detector as one of its special cases. With a proper choice of its parameters, the new detector can achieve the advantages of both the multistage and conventional detector and have good performance for both strong and weak users.
EM-Based Multiuser Detection in Fast Fading Multipath Environments
(2002-08-01) Borran, Mohammad Jaber; Aazhang, Behnaam; Center for Multimedia Communications (http://cmc.rice.edu/)
We address the problem of multiuser detection in fast fading multipath environments for DS-CDMA systems. In fast fading scenarios, temporal variations of the channel cause significant performance degradation even with the RAKE receiver. We use a previously introduced Time-Frequency (TF) RAKE receiver based on a canonical formulation of the channel and signals to simultaneously combat fading and multipath effects. This receiver uses the Doppler spread caused by rapid time-varying channel as another means of diversity. In dealing with multiaccess interference and as an attempt to avoid the prohibitive computational complexity of the optimum Maximum-Likelihood (ML) detector, we use the Expectation Maximization (EM) algorithm to derive an approximate ML detector. The new detector turns out to have an iterative structure very similar to the well-known multistage detector, but because of using the EM algorithm, it has better convergence properties than the multistage detector.
Multilevel Codes and Iterative Multistage Decoding: Rate Design Rules and Practical Considerations
(2000-09-20) Borran, Mohammad Jaber; Aazhang, Behnaam; Center for Multimedia Communications (http://cmc.rice.edu/)
It is well known that MultiLevel Coding (MLC) and MultiStage Decoding (MSD) suffice to approach capacity if the rates at different levels are chosen appropriately. In most of the practical cases, however, the rate design rule for MSD doesn't leave any room for coding at higher levels of the MLC scheme, which is very important in fading environments. In this paper, the rate design rule for multilevel coding using iterative multistage decoding is investigated, and it is shown that using iterative MSD with updated a priori probabilities of the constellation points, a broader subregion of the capacity region of the MLC scheme can be achieved.
Non-coherent and Partially Coherent Space-Time Constellations
(2003-06-01) Borran, Mohammad Jaber; Center for Multimedia Communications (http://cmc.rice.edu/)
With the rapid growth of wireless networks and multimedia applications, next generation cellular systems are expected to support data rates that are orders of magnitude higher than those currently available. Due to the limited amount of battery power in the mobile handsets, more power efficient signaling techniques need to be developed. Outdoor cellular systems are also required to be able to operate in rapidly fading environments. Exploiting multiple transmit and receive antennas to meet some or all of the above requirements have been recently proposed and extensively investigated. Nevertheless, designing signal constellations and codes that meet all of the above requirements and have practical design and decoding complexities still remains a challenge. In this work, we consider the code and constellation design problem for digital communication in a Rayleigh fading environment using a multiple-antenna system. We assume that the the channel coefficients are not known at the transmitter, and are only partially known at the receiver. Inspired by the Steinâ s lemma, we propose to use the Kullback-Leibler distance between conditional distributions to design space-time constellations. We show that this distance, while being relatively easy to derive and work with, provides an efficient performance and design criterion. Using the KL-based design criterion, we construct codes and constellations for multiple-antenna systems which can be decoded non-coherently or in the presence of channel estimation errors, and thus are suitable for fast block fading scenarios. We also show that the new constellations are more efficient than the existing designs for non-coherent systems at low signal-to-noise ratios or high spectral efficiencies. The new partially coherent constellations, on the other hand, provide significant performance improvements when the estimation variance is comparable to the reciprocal of the signal-to-noise ratio. We also propose a recursive construction for real unitary constellations with low decoding complexity, derive a KL-based design criterion and construction method for partially coherent coded modulation, and design partially coherent constellations for a multi-carrier system in a multipath environment. We show that, in the presence of channel estimation errors, the proposed codes and constellations achieve significant performance improvement over the conventional coding and modulation techniques.
Non-coherent and partially coherent space-time constellations
(2004) Borran, Mohammad Jaber; Aazhang, Behnaam
With the rapid growth of wireless networks and multimedia applications, next generation cellular systems are expected to support data rates that are orders of magnitude higher than those currently available. Due to the limited amount of battery power in the mobile handsets, more power efficient signaling techniques need to be developed. Outdoor cellular systems are also required to be able to operate in rapidly fading environments. Exploiting multiple transmit and receive antennas to meet some or all of the above requirements have been recently proposed and extensively investigated. Nevertheless, designing signal constellations and codes that meet all of the above requirements and have practical design and decoding complexities still remains a challenge. In this work, we consider the code and constellation design problem for digital communication in a Rayleigh fading environment using a multiple-antenna system. We assume that the channel coefficients are not known at the transmitter, and are only partially known at the receiver. Inspired by the Stein's lemma, we propose to use the Kullback-Leibler distance between conditional distributions to design space-time constellations. We show that this distance, while being relatively easy to derive and work with, provides an efficient performance and design criterion. Using the KL-based design criterion, we construct codes and constellations for multiple-antenna systems which can be decoded non-coherently or in the presence of channel estimation errors, and thus are suitable for fast block fading scenarios. We also show that the new constellations are more efficient than the existing designs for non-coherent systems at low signal-to-noise ratios or high spectral efficiencies. The new partially coherent constellations, on the other hand, provide significant performance improvements when the estimation variance is comparable to the reciprocal of the signal-to-noise ratio. We also propose a recursive construction for real unitary constellations with low decoding complexity, derive a KL-based design criterion and construction method for partially coherent coded modulation, and design partially coherent constellations for a multi-carrier system in a multipath environment. We show that, in the presense of channel estimation errors, the proposed codes and constellations achieve significant performance improvement over the conventional coding and modulation techniques.
On Design Criteria and Construction of Non-coherent Space-Time Constellations
(2002-07-20) Borran, Mohammad Jaber; Sabharwal, Ashutosh; Aazhang, Behnaam; Johnson, Don; Center for Multimedia Communications (http://cmc.rice.edu/)
We consider the problem of digital communication in a Rayleigh flat fading environment using a multiple antenna system, when both the transmitter and the receiver are unaware of the channel coefficients. Using Stein's lemma to bound the pairwise error probabilities, we propose to use the Kullback-Leibler distance between distributions assigned to the transmitted symbols to design space-time constellations for non-coherent communication. We show that optimal codes according to the proposed criterion can be obtained by partitioning the signal space into appropriate subsets and using unitary designs inside each subset. Actual constellation design is performed by solving an optimization problem to find the size and location of the subsets. Our simulation results show that the new constellations can provide a substantial improvement in the performance over known unitary space-time codes. Furthermore, the proposed constellations can be decoded using a simple low-complexity decoder.
On Design Criteria and Construction of Non-coherent Space-Time Constellations
(2003-10-20) Borran, Mohammad Jaber; Sabharwal, Ashutosh; Aazhang, Behnaam; Center for Multimedia Communications (http://cmc.rice.edu/)
We consider the problem of digital communication in a Rayleigh flat fading environment using a multiple-antenna system, when the channel state information is available neither at the transmitter nor at the receiver. It is known that at high SNR, or when the coherence interval is much larger than the number of transmit antennas, a constellation of unitary matrices can achieve the capacity of the non-coherent system. However, at low SNR, high spectral efficiencies, or for small values of coherence interval, the unitary constellations lose their optimality and fail to provide an acceptable performance. In this work, inspired by the Stein's lemma, we propose to use the Kullback-Leibler distance between conditional distributions to design space-time constellations for non-coherent communication. In fast fading, i.e., when the coherence interval is equal to one symbol period and the unitary construction provides only one signal point, the new design criterion results in PAM-type constellations with unequal spacing between constellation points. We also show that in this case, the new design criterion is equivalent to design criteria based on the exact pairwise error probability or the Chernoff information. When the coherence interval is larger than the number of transmit antennas, the resulting constellations overlap with the unitary constellations at high SNR, but at low SNR they have a multilevel structure and show significant performance improvement over unitary constellations of the same size. The performance improvement becomes especially more significant when a large number of receive antennas are used. This property, together with the facts that the proposed constellations eliminate the need for training sequences and are most suitable for low SNR, makes them a good candidate for uplink communication in wireless systems.
Partially Coherent Constellations for Multiple-Antenna Systems
(2003-11-20) Borran, Mohammad Jaber; Sabharwal, Ashutosh; Aazhang, Behnaam; Varshney, Prabodh; Center for Multimedia Communications (http://cmc.rice.edu/)
We consider the problem of digital communication in a Rayleigh flat fading environment using a multiple antenna system. We assume that the transmitter doesnâ t know the channel coefficients, and that the receiver has only an estimate of them. We further assume that the transmitter and receiver know the statistics of the estimation error. We will refer to this system as a partially coherent system. In an earlier work, we had derived a design criterion for the partially coherent constellations based on maximizing the minimum KL distance between conditional distributions. We had also designed single transmit antenna constellations using this criterion, which showed substantial improvement in the performance over existing and widely-used constellations. In this work, using the KL-based design criterion, we design partially coherent constellations for multiple antenna systems, and evaluate their performance through simulation. We show that, even with only a few percent channel estimation error, the new constellations achieve significant performance gains over the conventional constellations and existing multiple antenna techniques. The proposed constellations are multi-level, with multi-dimensional spherical constellations at each level. We also propose a recursive construction for the constituent spherical subsets of the multiple-antenna partially coherent constellations.
Partially Coherent Multiple Antenna Systems: Design Criterion and Construction Methods for Constellations and Coded Modulation
(2004-01-15) Borran, Mohammad Jaber; Sabharwal, Ashutosh; Aazhang, Behnaam; Center for Multimedia Communications (http://cmc.rice.edu/)
We consider multiple-antenna communication systems in Rayleigh fading channel, where the transmitter does not know the channel coefficients and the receiver has only an estimate of them. We further assume that the transmitter and receiver know the statistics of the estimation error. We refer to this system as partially coherent system, for which we derive the expressions for the optimal detector and study the code and constellation design problems. Finding the Chernoff bound intractable, and inspired by Steinâ s Lemma, we propose to use the Kullback-Leibler (KL) distance between conditional distributions to design space-time codes and constellations for partially coherent systems. We show that the KL distance is relatively easy to derive and work with, and furthermore, provides an efficient design criterion. Using the KL-based design criterion, we construct constellations for multiple-antenna systems which can be decoded in the presence of channel estimation errors, and thus are suitable for fading scenarios with short coherence intervals. The proposed constellations are multi-level, with multi-dimensional spherical constellations at each level. We also propose a recursive construction for the constituent spherical subsets of the multiple-antenna partially coherent constellations. The new partially coherent constellations provide significant performance improvement over the conventional single-antenna PSK and QAM constellations and multiple-antenna techniques such as Bell Labâ s Space-Time (BLAST) architecture and Orthogonal Transmit Diversity (OTD) schemes, when the estimation variance is comparable to the reciprocal of the signal-to-noise ratio. More specifically, we show that by using the proposed constellations, the error floors due to the estimation errors can be reduced by as much as one order of magnitude. Next, we bring coding into the picture and derive a KL-based design criterion for partially coherent coded modulation. We also propose a construction method for partially coherent coded modulation using the idea of mapping by set partitioning. We show that, in the presence of channel estimation errors, the proposed codes provide substantial performance improvement over the conventional coded modulation techniques, and the gains in this case are even larger than the gains obtained in the case of uncoded systems.
Power-Efficient Non-coherent Space-Time Constellations
(2003-07-20) Borran, Mohammad Jaber; Sabharwal, Ashutosh; Aazhang, Behnaam; Center for Multimedia Communications (http://cmc.rice.edu/)
We consider the problem of digital communication in a Rayleigh flat-fading environment using a multiple-antenna system, when the channel state information is available neither at the transmitter nor at the receiver. It is known that at high SNR, or when the coherence interval is much larger than the number of transmit antennas, a constellation of unitary matrices can achieve the capacity of the non-coherent system. However, at low SNR, high spectral efficiencies, or for small values of coherence interval, the unitary constellations lose their optimality and fail to provide an acceptable performance. In an earlier work, using a design criterion based on the KL distance between distributions, we have shown, for the case of a single transmit antenna, that at low SNR, a multilevel constellation can provide better error rate performance than the unitary constellation of the same size. In this work, we extend those single antenna results to the case of multiple transmit antennas. The interesting result of this work is that the same idea of multilevel constellations can be generalized to design low-power non-coherent constellations for multiple antennas. The resulting constellations overlap with the unitary constellations at high SNR, but at low SNR they have a multilevel structure and show significant performance improvement over unitary constellations of the same size. The performance improvement becomes especially more significant when a large number of receive antennas are used. This property, together with the facts that the proposed constellations eliminate the need for training sequences and are most suitable for low SNR, makes them a good candidate for uplink communication in wireless systems.
A Recursive Construction for Low-Complexity Non-coherent Constellations
(2004-04-01) Borran, Mohammad Jaber; Sabharwal, Ashutosh; Aazhang, Behnaam; Center for Multimedia Communications (http://cmc.rice.edu/)
It is known that at high signal to noise ratio (SNR), or for large coherence interval (T), a constellations of unitary matrices can achieve the capacity of the non-coherent multiple-antenna system in block Rayleigh flat-fading channel. For a single transmit antenna system, a unitary constellation is simply a collection of T-dimensional unit vectors. Nevertheless, except for a few special cases, the optimal constellations are obtained only through exhaustive or random search, and their decoding complexity is exponential in the rate of the constellation and the length of the coherence interval, T. In this work, we propose a recursive construction method for real-valued single transmit antenna non-coherent constellations, in which a T-dimensional unitary constellation is constructed by using a number of (T-1)-dimensional unitary or spherical constellations as its equi-latitude subsets. Comparison of the minimum distances achieved by the proposed constructions with the best known packings in G(T, 1) [1] shows that, for practical values of T, the recursive constellations are close to optimal. We also propose a simple low-complexity decoding algorithm for the single-antenna recursive constellations. The complexity of the proposed decoder is linear in the total number of the two-dimensional constituent subsets, which is usually much smaller than the number of the constellation points. Nevertheless, the performance of the suboptimal decoder is similar to the optimal decoder. A comparison of the error rate performance of the recursive constellations with the complex-valued systematic designs of [2] shows that the proposed real-valued constellations have similar performance to the complex-valued systematic designs. The recursive designs also show a significant gain over the low-complexity PSK constellations of [3].
Wavelet-Based Denoising Using Hidden Markov Models
(2001-05-20) Borran, Mohammad Jaber; Nowak, Robert David; Center for Multimedia Communications (http://cmc.rice.edu/)
Hidden Markov models have been used in a wide variety of wavelet-based statistical signal processing applications. Typically, Gaussian mixture distributions are used to model the wavelet coefficients and the correlation between the magnitudes of the wavelet coefficients within each scale and/or across the scales is captured by a Markov tree imposed on the (hidden) states of the mixture. This paper investigates correlations directly among the wavelet coefficient amplitudes (sign Ã magnitude), instead of magnitudes alone. Our theoretical analysis shows that the coefficients display significant correlations in sign as well as magnitude, especially near strong edges. We propose a new wavelet-based HMM structure based on mixtures of one-sided exponential densities that exploits both sign and magnitude correlations. We also investigate the application of this for denoising the signals corrupted by additive white Gaussian noise. Using some examples with standard test signals, we show that our new method can achieve better mean squared error, and the resulting denoised signals are generally much smoother.

Browsing by Author "Borran, Mohammad Jaber"

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