19 research outputs found

    Power Control in Two-Tier Femtocell Networks

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    In a two tier cellular network -- comprised of a central macrocell underlaid with shorter range femtocell hotspots -- cross-tier interference limits overall capacity with universal frequency reuse. To quantify near-far effects with universal frequency reuse, this paper derives a fundamental relation providing the largest feasible cellular Signal-to-Interference-Plus-Noise Ratio (SINR), given any set of feasible femtocell SINRs. We provide a link budget analysis which enables simple and accurate performance insights in a two-tier network. A distributed utility-based SINR adaptation at femtocells is proposed in order to alleviate cross-tier interference at the macrocell from cochannel femtocells. The Foschini-Miljanic (FM) algorithm is a special case of the adaptation. Each femtocell maximizes their individual utility consisting of a SINR based reward less an incurred cost (interference to the macrocell). Numerical results show greater than 30% improvement in mean femtocell SINRs relative to FM. In the event that cross-tier interference prevents a cellular user from obtaining its SINR target, an algorithm is proposed that reduces transmission powers of the strongest femtocell interferers. The algorithm ensures that a cellular user achieves its SINR target even with 100 femtocells/cell-site, and requires a worst case SINR reduction of only 16% at femtocells. These results motivate design of power control schemes requiring minimal network overhead in two-tier networks with shared spectrum.Comment: 29 pages, 10 figures, Revised and resubmitted to the IEEE Transactions on Wireless Communication

    Information Theoretic Optimality of Orthogonal Space-Time Codes and Concatenated Code Construction

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    Conference PaperWe 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.Noki

    On the Outage Theorem and its Converse for Discrete Memoryless Channels

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    Journal PaperWe prove both the outage theorem and its converse for block fading discrete memoryless channel. If <i>P<sub>out</sub></i> is the outage probability for the rate <i>R</i> then any transmission rate below <i>R</i> together with any average error probability greater than <i>P<sub>out</sub></i> is achievable. Conversely, for any code with rate slightly greater than <i>R</i> the error probability is greater than any number smaller than <i>P<sub>out</sub></i>, for large codeword lengths. Two cases are considered depending on whether the transmitter has channel state information (CSI) or not.Noki

    Robust Slope Region for Wideband CDMA With Multiple Antennas

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    Conference PaperWe 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.Noki

    Capacity of low power multiuser systems with antenna arrays

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    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

    Information theory of transmit diversity and space-time code design

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    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

    An Approach to Capacity Analysis of Coarsely Coordinated Low Power Multiple Access Systems

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    Conference paperWe 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 <i>policy</i>, 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.Noki

    On Limits of Antenna Packing in Low Power Systems

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    Conference PaperWe evaluate spectral efficiency limits of low power portable communication devices which are only constrained by fixed transceiver aperture G "subset&equal" real<sup>3</sup> for mounting antennas. A new notion of <i>antenna-space capacity</i> 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.NokiaNokia/Texas Instrument