Browsing by Author "Muharemovic, Tarik"

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    Antenna Packing in Low Power Systems: Communication Limits and Array Design
    (2008) Muharemovic, Tarik; Sabharwal, Ashutosh; Aazhang, Behnaam; Center for Multimedia Communications (http://cmc.rice.edu/)
    In this paper, we study design of transceiver antenna arrays and its impact on spectral efficiency of low-power systems. Our primary motivation is construction of practical and portable multi-antenna configurations with a very small and a-priori fixed volume for placing antennas. Using spectral efficiency as a target metric for array optimization, we show that any array configuration, transmit or receive, can be characterized via a parameter that we interpret as "effective degrees of freedom." For any array configuration, effective degrees of freedom describes an equivalent uncorrelated array, which results in the same low-power behavior of spectral efficiency. Joint optimization of transmit and receive antenna configurations decouples into maximizing effective degrees of freedom for transmitter and receiver separately. To achieve this goal, we introduce and study a theoretical benchmark of "limiting degrees of freedom," which is the least upper bound on effective degrees of freedom, evaluated over all configurations with finite number of antennas. Limiting degrees of freedom therefore describes the best possible performance for any transceiver array which confines its elements inside a given space. We compute a closed-form expression for limiting degrees of freedom of a circular geometry. Finally, we present numerical procedure and examples for designing linear and square arrays with non-uniform spacing, which typically exhibit significant spectral efficiency gains over uniform arrays.
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    An Approach to Capacity Analysis of Coarsely Coordinated Low Power Multiple Access Systems
    (2004-07-01) Muharemovic, Tarik; Sabharwal, Ashutosh; Aazhang, Behnaam; Center for Multimedia Communications (http://cmc.rice.edu/)
    We consider multiaccess problem in low power systems, where we allow each user to select its own data rate and transmit power locally and independently from other users. Here, every user has a set of low power codebooks, labeled a policy, which accomodates a range of small spectral efficiencies, while treating instantaneous data rates of other users as an unknown compound parameter. Even with such coarse user coordination, multiuser detection enables a system which is superior to any classic orthogonal division system. First we fully characterise the set of achievable policies, after which we demonstrate that in multiantenna systems, policies are be viewed as awarding protected receiver spatial dimensions to each user.
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    Bit Interleaved Space-Time Codes
    (2001-09-20) Muharemovic, Tarik; Gatherer, Alan; Ebel, Will; Srihosour, Srinath; Hocevar , Dale; Huang, Everest; Center for Multimedia Communications (http://cmc.rice.edu/)
    We develop a new construction criterion for BSPK space-time codes which simultaneously reflects both "coding" and "diversity" advantage. Furthermore, we propose a new method of improving the performance of space-time codes, naemly bit interleaving. Subsequently we provide a computationally effective, iterative way of decoding bit interleaved space-time codes. At the end we note the 2dB gain due to the new approach. We note the sixteen state code which operates close to 3dB from the outage probability.
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    Capacity of low power multiuser systems with antenna arrays
    (2005) Muharemovic, Tarik; Aazhang, Behnaam; Sabharwal, Ashutosh
    In this thesis, we study wireless multiuser communication systems in the regime of low spectral efficiencies, where users and the multiple access point are equipped with antenna arrays. Our first contribution is to develop a generic mathematical framework which captures tradeoffs between fundamental parameters of a low power multiuser system: spectral efficiency and energy per information bit, of each user. Using the framework that we developed we next consider variable data rate multiple access problem, in low power systems, where we remove the usual assumption of tight user coordination, and we allow users to select their own data rates and trans mit powers, without coordinating, and without negotiating with the access point. Here, every user has a set of low power codebooks, that we name the policy, which accommodates a range of small spectral efficiencies, but particular data rates of other users are assumed to be an unknown---compound parameter---at each mobile. In antenna-array transmission and reception, we demonstrate an elegant interpretation of users policies, where each policy is represented by partitioning spatial dimensions into blocks, and each block is dedicated to a different user. Finally, we address the paradigm of statistically correlated antenna arrays, where we derive the effective number of uncorrelated receive spatial dimensions, which we partition to represent users policies. As more correlated antennas are packed into a limited area we show that effective receive dimensionality converges to a finite limit which we evaluate for some simple geometries.
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    Hybrid Linear-Iterative Detection Algorithms for MIMO CDMA
    (2002-05-20) Muharemovic, Tarik; Onggosanusi, Eko; Dabak, Anand; Aazhang, Behnaam; Center for Multimedia Communications (http://cmc.rice.edu/)
    We assume a CDMA downlink multiuser scenario, where the base station is equipped with multiple transmit antennas, and each user has multiple receive antennas. For this setup we propose a new, computationally effective MIMO multiuser detector for frequency selective channels. We exploit the fact that the interference mainly comes from the spatial domain, while the residual interference is due to temporal dispersion and multiuser interference. This motivates us to suppress the residual interference in a linear fashion, while combating the spatial interference iteratively. This approach is an efficient performance-complexity tradeoff between full iterative and linear detectors.
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    Improving Performance of a Space-Time Turbo Code in a Rayleigh Faded Channel
    (2001-10-20) Huang, Everest; Gatherer, Alan; Muharemovic, Tarik; Hocevar , Dale; Center for Multimedia Communications (http://cmc.rice.edu/)
    Starting with an ordinary parallel concatenated turbo encoder in a block fading Rayleigh channel, we introduce bit-interleaving of the outputs, not for time diversity, but to increase the coding gain. We develop a guideline metric for designing QPSK space-time codes which reveals both the coding and diversity advantage of the system, as well as shows that it is unnecessary to guarantee full space diversity to achieve good performance in Rayleigh fading. Using channel a priori information in the MAP interations for decoding, we achieve frame error rates within 1 dB of the outage probability with only modest increases in decoding complexity.
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    Information Theoretic Optimality of Orthogonal Space-Time Codes and Concatenated Code Construction
    (2000-06-20) Muharemovic, Tarik; Aazhang, Behnaam; Muharemovic, Tarik; Aazhang, Behnaam; Center for Multimedia Communications (http://cmc.rice.edu/)
    We demonstrate that the information theoretic capacity of multiple-transmit antenna systems is not reduced by using space-time orthogonal transmit schemes as means of exploiting the transmit diversity. Then, we consider concatenation of a channel code with space-time orthogonal transmit scheme, where data stream is first encoded with the channel code, and then transmitted using orthogonal space-time transmit scheme. Using this orthogonal space time transmit scheme the elements in the rpoduct criterion for code construction are made more uniform and hence better gain is achieved. We dreive code construction criterion for designing the channel code, taking into consideration coherence time of the channel.
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    Information theory of transmit diversity and space-time code design
    (2000) Muharemovic, Tarik; Aazhang, Behnaam
    We restate the achievable information rates for multiple transmit multiple receive antenna systems in fast fading channels. Then we consider non-ergodic channel, where we evaluate asymptotic expression for the outage probability and note its striking similarity with the error probability. After that, we consider a simple transmit diversity technique [7], and evaluate its achievable information rate. We note that in the case of a single receiver antenna there is no loss in capacity if one reserves to this technique as a mean of exploiting transmit diversity. However, we note that in the case of multiple receivers there is a penalty in capacity. Also, we demonstrate that CDMA orthogonalisation between antennas results in capacity loss. Then we derive performance criterion for codes concatenated to the simple diversity technique. After that, we propose some high-rate space time codes which use multiple antennas in order to increase the rate rather than improving communication reliability. At the end we note how some distance spectrum trimming can increase a performance of space-time codes without increasing computational complexity.
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    Information Theory of Transmit Diversity and Space-Time Code Design
    (2000-05-20) Muharemovic, Tarik; Center for Multimedia Communications (http://cmc.rice.edu/)
    We restate the achievable information rates for multiple transmit multiple receive antenna systems in fast fading channels. Then we consider non-ergodic channel, where we evaluate asymptotic expression for the outage probability and note its striking similarity with the error probability. After that, we consider a simple transmit diversity technique Alamouti, and evaluate its achievable information rate. We note that in the case of a single receiver antenna there is no loss in capacity if one reserves to this technique as a mean of exploiting transmit diversity. However, we note that in the case of multiple receivers there is a penalty in capacity. Also, we demonstrate that CDMA orthogonalisation between antennas results in capacity loss. Then we derive performance criterion for codes concatenated to the simple diversity technique. After that, we propose some high-rate space time codes which use multiple antennas in order to incerase the rate rather than improving communication reliability. At the end we note how some distance spectrum trimming can increase a performance of space-time codes without increasing computational complexity.
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    On Limits of Antenna Packing in Low Power Systems
    (2004-09-01) Muharemovic, Tarik; Sabharwal, Ashutosh; Aazhang, Behnaam; Center for Multimedia Communications (http://cmc.rice.edu/)
    We evaluate spectral efficiency limits of low power portable communication devices which are only constrained by fixed transceiver aperture G "subset&equal" real3 for mounting antennas. A new notion of antenna-space capacity of G naturally emerges from analysis of the achievable spectral efficiency. Antenna-space capacity measures the maximum growth of spectral efficiency that antenna configuration can achieve, given arbitrary number of antennas. Thus, the notion of antenna-space capacity also has a practical utility as a benchmark for design of actual antenna configurations. As a concrete example, we compute the antenna-space capacity of a circular geometry, where antennas can only be placed on the circumference of a circle. We show that uniform antenna placement on the circle is suboptimal antenna configuration for any fixed number of antennas, but comes arbitrarily close to optimum as the number of antennas grows unboundedly.
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    On Outage and Capacity for Orthogonal Transmit Diversity
    (2001-09-20) Muharemovic, Tarik; Aazhang, Behnaam; Center for Multimedia Communications (http://cmc.rice.edu/)
    Assuming that only the receiver has perfect channel state information, we analyze the orthogonal transmit diversity (OTD). First, we consider the case of fast fading or ergodic channels. We demonstrate that for a single receive antenna, OTD does not result in a capacity loss. This is not the case when there are multiple receive antennas. Then, we consider block fading or non-ergodic channels. We propse a new, simple, and asymptotically tight upper bound for the outage probability in multiple transmit and single receive antenna systems. Assuming equal power distribution, we also demonstrate that for non-ergodic channels and a single receive antenna, OTD preserves the achievable outage probability. However, in the case of multiple receive antennas, OTD increases the achievable outage probability. In short, regardless of the nature of fading, OTD is optimal only in the case of a single receive antenna.
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    On the outage proabability of a class of signaling schemes for multiple antennas
    (2002-07-20) Mukkavilli, Krishna Kiran; Muharemovic, Tarik; Sabharwal, Ashutosh; Aazhang, Behnaam; Center for Multimedia Communications (http://cmc.rice.edu/)
    In this work, we investigate large SNR behavior of the outage probability for a class of signaling schemes designed for flat fading Rayleigh multiple antennas systems. Outage probability is important in a practical system since it reflects the frame error rates achievable for a given rate of transmission. With an assumption of short-term power constraint, which diminishes the effectiveness of time power control, our analysis characterizes the diversity advantage of different schemes. Our main result is the characterization of a large class of signaling schemes, all of which are guaranteed to have the identical rate of decay of outage probability with SNR given by the maximum spatial diversity available in the system. We show that several important multiple antenna signaling schemes like space time coding and various beamforming techniques, designed with different amount of channel state information at the transmitter are included in the proposed class of signaling schemes. Such a classification of schemes, based on similar error performance, helps in addressing practical issues like tradeoffs in feedback resources and decoding complexity required by each scheme. Finally, the techniques developed in this paper can also be used to derive the diversity gain of an arbitrary signaling scheme.
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    On the Outage Theorem and its Converse for Discrete Memoryless Channels
    (2002-01-15) Muharemovic, Tarik; Aazhang, Behnaam; Center for Multimedia Communications (http://cmc.rice.edu/)
    We prove both the outage theorem and its converse for block fading discrete memoryless channel. If Pout is the outage probability for the rate R then any transmission rate below R together with any average error probability greater than Pout is achievable. Conversely, for any code with rate slightly greater than R the error probability is greater than any number smaller than Pout, for large codeword lengths. Two cases are considered depending on whether the transmitter has channel state information (CSI) or not.
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    Robust Slope Region for Wideband CDMA With Multiple Antennas
    (2003-04-01) Muharemovic, Tarik; Aazhang, Behnaam; Center for Multimedia Communications (http://cmc.rice.edu/)
    We analyze a low power, wideband CDMA system with multiple antennas. The joint "slope region" - of transmission rates at minimum energy per bit - is derived as a function of proportion between vanishing rates for multiple users. Vertices of the slope region may be achieved by a matched filter linear interface, followed by successive interference cancellation. We introduce and evaluate "robust slope region," the largest region which is inside every other slope region and is therefore independent from the relative proportion between transmission rates. Furthermore, we find the "robust slope," maximum slope which can always be guaranteed to all users.
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    Space-Time Codes with Bit Interleaving
    (2001-11-20) Muharemovic, Tarik; Gatherer, Alan; Ebel, Will; Srihosour, Srinath; Hocevar , Dale; Huang, Everest; Center for Multimedia Communications (http://cmc.rice.edu/)
    We develop a new construction criterioin for BSPK space-time codes. Based on this criterion, we propose a new method of improving the performance of space-time codes, namely bit interleaving. Then, we provide a computationally effective, iterative way of decoding bit interleaved space-time codes. At the end, we note the 2dB gain due to the new approach. A proposed 16 state bit interleaved code operates close to 3dB from the outage probability.