10,100 research outputs found

    On Optimal Power Control for Delay-Constrained Communication Over Fading Channels

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    Power and delay trade-offs in fading channels

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    Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2000.Includes bibliographical references (p. 193-198).Energy is a constrained resource in mobile wireless networks. In such networks, communication takes place over fading channels. By varying the transmission rate and power based on the current fading level, a user in a wireless network can more efficiently utilize the available energy. For a given average transmission rate, information theoretic arguments provide the optimal power allocation. However, such an approach can lead to long delays or buffer overflows. These delays can be reduced but at the expense of higher transmission power. The trade-offs between the required power and various notions of delay are analyzed in this thesis. We consider a user communicating over a fading channel. Arriving data for this user is stored in buffer until it is transmitted. We develop several buffer control problems which fit into a common mathematical framework. In each of these problems, the goal is to both minimize the average transmission power as well as the average "buffer cost". In two specific examples, this buffer cost corresponds to the probability of buffer overflow or the average buffer delay. These buffer control problems are analyzed using dynamic programming techniques. Several structural characteristics of optimal policies are given. The relationship of this model to the delay-limited capacity and outage capacity of fading channels is discussed. We then analyze the asymptotic performance in two cases - the probability of buffer overflow case and the average delay case. In both cases, we bound the asymptotic performance and provide simple policies which are asymptotically optimal or nearly optimal. Finally we extend this analysis to a model with multiple users communicating over a multiple-access channel to a common receiver. The single user results for the probability of buffer overflow case are generalized to this multiple user situation. Extensions to other multi-user models are also discussed.by Randall A. Berry.Ph.D

    Polar Codes over Fading Channels with Power and Delay Constraints

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    The inherent nature of polar codes being channel specific makes it difficult to use them in a setting where the communication channel changes with time. In particular, to be able to use polar codes in a wireless scenario, varying attenuation due to fading needs to be mitigated. To the best of our knowledge, there has been no comprehensive work in this direction thus far. In this work, a practical scheme involving channel inversion with the knowledge of the channel state at the transmitter, is proposed. An additional practical constraint on the permissible average and peak power is imposed, which in turn makes the channel equivalent to an additive white Gaussian noise (AWGN) channel cascaded with an erasure channel. It is shown that the constructed polar code could be made to achieve the symmetric capacity of this channel. Further, a means to compute the optimal design rate of the polar code for a given power constraint is also discussed.Comment: 6 pages, 6 figure

    Distortion Minimization in Gaussian Layered Broadcast Coding with Successive Refinement

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    A transmitter without channel state information (CSI) wishes to send a delay-limited Gaussian source over a slowly fading channel. The source is coded in superimposed layers, with each layer successively refining the description in the previous one. The receiver decodes the layers that are supported by the channel realization and reconstructs the source up to a distortion. The expected distortion is minimized by optimally allocating the transmit power among the source layers. For two source layers, the allocation is optimal when power is first assigned to the higher layer up to a power ceiling that depends only on the channel fading distribution; all remaining power, if any, is allocated to the lower layer. For convex distortion cost functions with convex constraints, the minimization is formulated as a convex optimization problem. In the limit of a continuum of infinite layers, the minimum expected distortion is given by the solution to a set of linear differential equations in terms of the density of the fading distribution. As the bandwidth ratio b (channel uses per source symbol) tends to zero, the power distribution that minimizes expected distortion converges to the one that maximizes expected capacity. While expected distortion can be improved by acquiring CSI at the transmitter (CSIT) or by increasing diversity from the realization of independent fading paths, at high SNR the performance benefit from diversity exceeds that from CSIT, especially when b is large.Comment: Accepted for publication in IEEE Transactions on Information Theor
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