Rice Wireless

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https://hdl.handle.net/1911/21945
Formerly the Center for Multimedia Communications, Rice Wireless is part of the university's Electrical and Computer Engineering Department. More information about the group can be found at http://wireless.rice.edu/.

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Browsing Rice Wireless by Author "Borran, Mohammad Jaber"

Now showing 1 - 5 of 5
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    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.
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    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.
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    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.
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    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.
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    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.