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42nd Annual Conference on Information Sciences and Systems

作者： Lin, Che Raghavan, Vasanthan Veeravalli, Venugopal V. Univ Illinois Urbana IL 61801 USA

ISBN: (纸本)9781424422463

Space-time codes have been designed over the last decade to enhance the reliability of open-loop fading wireless systems. However, recent technological advances have enabled the possibility of low-rate feedback from the receiver to the transmitter. The focus of this work is on the implications of this feedback in a point-to-point MIMO system with a coherent receiver and a general model for spatial correlation that is known at the transmitter. We assume that B bits of quantized channel information are available at the transmitter. coding over space-time is then studied by considering the family of linear dispersion (LD) codes that meets an additional orthogonal condition, which leads to a decoding complexity comparable to orthogonal space-time block codes (OSTBC). Our results show that, when B is small, a space-time coding scheme that is equivalent to beamforming and does not code across time is optimal in a weak sense in that it maximizes the average received SNR. As B increases, this weak optimality transitions to optimality in a strong sense characterized by the maximization of the average mutual information. Hence, from a system designer's perspective, our work suggests that beamforming may not only be attractive from a low-complexity viewpoint, but also from an information-theoretic viewpoint.

关键词： adaptive coding diversity methods fading channels feedback communication MIMO systems multiplexing quantization

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Quantized Multimode Precoding in Spatially Correlated Multiantenna Channels

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IEEE TRANSACTIONS ON SIGNAL PROCESSING 2008年第12期56卷 6017-6030页

作者： Raghavan, Vasanthan Veeravalli, Venugopal V. Sayeed, Akbar M. Univ Illinois Coordinated Sci Lab Urbana IL 61801 USA Univ Illinois Dept Elect & Comp Engn Urbana IL 61801 USA Univ Wisconsin Dept Elect & Comp Engn Madison WI 53706 USA

Multimode precoding, where the number of independent data streams is adapted optimally, can be used to maximize the achievable throughput in multiantenna communication systems. Motivated by standardization efforts embraced by the industry, the focus of this work is on systematic precoder design with realistic assumptions on the spatial correlation, channel state information (CSI) at the transmitter and the receiver, and implementation complexity. For the spatial correlation of the channel matrix, we assume a general channel model, based on physical principles, that has been verified by many recent measurement campaigns. We also assume a coherent, linear minimum mean-square error (MMSE) receiver and knowledge of the spatial statistics at the transmitter along with the presence of an ideal, low-rate feedback link from the receiver to the transmitter. The reverse link is used for codebook-index feedback and the goal of this work is to construct precoder codebooks, adaptable in response to the statistical information, such that the achievable throughput is significantly enhanced over that of a fixed, nonadaptive, independent and identically distributed (i.i.d.) codebook design. We illustrate how a codebook of semiunitary precoder matrices localized around some fixed center on the Grassmann manifold can be skewed in response to the spatial correlation via low-complexity maps that can rotate and scale submanifolds on the Grassmann manifold. The skewed codebook in combination with a low-complexity statistical power allocation scheme is then shown to bridge the gap in performance between a perfect CSI benchmark and an i.i.d. codebook design.

关键词： adaptive coding channel state information (CSI) at transmitter limited feedback communication low-complexity signaling multimode signaling multiple-input multiple-output (MIMO) systems quantized feedback

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Space-time code selection for transmit antenna diversity systems

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IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY 2008年第1期57卷 620-629页

作者： Mavares, Dimas T. Torres, Rafael P. Univ Polytech Antonio Jose Sucre UNEXPO Lara 3001 Venezuela Univ Cantabria Dept Commun Engn Cantabria 39005 Spain

In this correspondence, a space-time code selection approach for transmit antenna diversity (TAD) systems is proposed. The receiver compares the equivalent single-input-single-output channel envelopes with a set of predetermined threshold levels, decides the antennas to be used, and sends this information to the transmitter, which adjusts the space-time code to the number of selected antennas using a small amount of feedback information. Threshold levels are found out-of-line based on the Doppler frequency in such a way that channel state information outdate at the transmitter is taken into account. Analysis and simulation results show that the system performs well for a wide range of Doppler frequencies, outperforming transmit antenna selection techniques and adaptive TAD systems.

关键词： adaptive coding adaptive transmission diversity methods multiple-input-multiple-output (MIMO) systems space-time coding transmit antenna diversity (TAD) Wireless local area network (LAN)

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Low-complexity adaptive transmission for cognitive radios in dynamic spectrum access networks

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IEEE JOURNAL ON SELECTED AREAS IN COMMUNICATIONS 2008年第1期26卷 83-94页

作者： Pursley, Michael B. Royster, Thomas C. Clemson Univ Clemson SC 29631 USA

Cognitive radios that are employed in a network with dynamic frequency assignments must operate efficiently in the presence of uncertainties and variations in the propagation characteristics of the network's communication links. A low-complexity adaptive transmission protocol is described and evaluated for use in cognitive radio networks whose links have unknown and possibly time-varying propagation losses as a result of such phenomena as slow fading or variations in shadowing. The cognitive radios are required to derive only simple statistics in the receivers in order to provide the information that is needed by our protocol;no estimates or measurements of received power or channel gain are used. The protocol's primary mechanism for responding to changes in propagation loss is to adjust the modulation and coding. Because of disruptions that can be caused by higher levels of interference to other radios in the network, the transmitter power is increased only if the most powerful combination of coding and modulation is inadequate. We employ finite-state Markov models for slowly varying channels, and we demonstrate that for such channels our protocol performs nearly as well as an ideal protocol that is told the exact value of the propagation loss for each packet transmission. Thus, the additional complexity that is required to enable cognitive radios to obtain precise channel-gain estimates is not justified and would lead to only negligible improvement in throughput. The throughput of our adaptive transmission protocol is compared with an upper bound that is derived from information theory for a hypothetical ideal protocol that is given perfect channel-state information, and some preliminary results on learning the adaptation decision intervals are included.

关键词： adaptive modulation adaptive coding cognitive radio dynamic spectrum access packet radio

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Compressive sampling and lossy compression

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IEEE SIGNAL PROCESSING MAGAZINE 2008年第2期25卷 48-56页

作者： Goyal, Vivek K. Fletcher, Alyson K. Rangan, Sundeep MIT Cambridge MA 02139 USA

Recent results in compressive sampling have shown that sparse signals can be recovered from a small number of random measurements. This property raises the question of whether random measurements can provide an efficient representation of sparse signals in an information-theoretic sense. Through both theoretical and experimental results, we show that encoding a sparse signal through simple scalar quantization of random measurements incurs a significant penalty relative to direct or adaptive encoding of the sparse signal. Information theory provides alternative quantization strategies, but they come at the cost of much greater estimation complexity.

关键词： estimation theory signal sampling compressive sampling estimation complexity information-theoretic sense lossy compression scalar quantization sparse signals estimation theory scalar quantization signals adaptive coding random measurement lossy compression signal sampling

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Rule-selection and action-selection have a shared neuroanatomical basis in the human prefrontal and parietal cortex

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CEREBRAL CORTEX 2008年第10期18卷 2275-2285页

作者： Rowe, J. Hughes, L. Eckstein, D. Owen, A. M. Univ Cambridge Dept Clin Neurosci Cambridge CB2 2QQ England MRC Cognit & Brain Sci Unit Cambridge CB2 7EF England MRC Behav & Clin Neurosci Inst Cambridge CB2 2EB England

The human capacity for voluntary action is one of the major contributors to our success as a species. In addition to choosing actions themselves, we can also voluntarily choose behavioral codes or sets of rules that can guide future responses to events. Such rules have been proposed to be superordinate to actions in a cognitive hierarchy and mediated by distinct brain regions. We used event-related functional magnetic resonance imaging to study novel tasks of rule-based and voluntary action. We show that the voluntary selection of rules to govern future responses to events is associated with activation of similar regions of prefrontal and parietal cortex as the voluntary selection of an action itself. The results are discussed in terms of hierarchical models and the adaptive coding potential of prefrontal neurons and their contribution to a global workspace for nonautomatic tasks. These tasks include the choices we make about our behavior.

关键词： action adaptive coding fMRI prefrontal cortex rule selection

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adaptive coding for mobile ad hoc networks with frequency-hop transmission and soft-decision decoding

Adaptive coding for mobile ad hoc networks with frequency-ho...

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International Conference on Wireless Networks, Communications and Mobile Computing

作者： M.B. Pursley J.S. Skinner Clemson University Clemson SC USA

Alternative protocols are presented for adapting the code from packet to packet in a mobile, wireless, ad hoc network that employs frequency hopping, coherent or noncoherent demodulation, and soft-decision decoding on channels with time-varying partial-band interference. The protocols are implemented in subsystems that are external to the demodulator and soft-decision decoder, so they can be employed with a wide range of codes and decoding algorithms. No channel measurements are needed and no external side information is supplied to the receiver. Performance results are provided for packet transmissions that employ turbo product codes. Protocol performance is compared with capacity limits and with an ideal protocol that uses perfect channel state information for the most recent packet transmission to select the code that maximizes the expected throughput.

关键词： adaptive coding Mobile ad hoc networks Frequency Decoding Demodulation Wireless application protocol Ad hoc networks Interference Product codes Channel state information

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An adaptive fast multiple reference frames selection algorithm for H.264/AVC

An adaptive fast multiple reference frames selection algorit...

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33rd IEEE International Conference on Acoustics, Speech and Signal Processing

作者： Wu, Peng Xiao, Chung-Bai Beijing Univ Technol Coll Comp Sci & Technol Beijing Peoples R China

ISBN: (纸本)9781424414833

To make full use of the temporal correlation of video sequences, H.264/AVC adopts multiple reference frames to enhance the coding quality. While the performance being improved, the complexity of computation has been increased linearly, too. In this paper, we study motion characteristic of video sequences and spatial correlation in video frames first, and then we propose an adaptive fast multiple reference frames selection algorithm. It can decrease the number of reference frames for motion compensation, and reduce the complexity of coding adaptively according to the features of video sequences. The results show that our algorithm achieves 45% coding time saving on average with unnoticeable quality loss.

关键词： video coding H.264/AVC multiple reference frames adaptive coding motion compensation

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adaptive coding in Retinal Ganglion Cells

Adaptive Coding in Retinal Ganglion Cells

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International Conference on Neural Networks and Brain (ICNNB)

作者： Xin Jin Hai-Qing Gong Pei-Ji Liang Ai-Hua Chen Departmnent of Biomedical Engineering Shanghai Jiaotong University Shanghai China Shanghai Institutes for Biological Sciences Chinese Academy and Sciences Shanghai China

Sensory systems show many aspects of adaptation. The chicken retinal ganglion cells examined in our study showed typical light adaptation and contrast adaptation. The prolonged presence of contrast or luminance stimuli reduced the firing rate of retinal ganglion cells during the adaptation phase. However, further experimental results suggest that the information about stimulation might be stored in the retinal neural network in spite of the reduction in firing rate during the adaptation process. We found that the application of background light may result in some modification for retina sensitivity, in an illumination level dependent manner. This may indicate a novel adaptive mechanism underlying the phenomena of that contrast discrimination improved when the background illumination was increased. Inspired by the physiological mechanisms, a simple and biophysically realistic model was then employed, where the spiking neuron could change their membrane properties, especially the ion channel activity, during the process of adaptation or synaptic modification. Our analyses reveal that activity-dependent regulation of membrane properties contributes to sensitivity adaptation, which may be potentially useful for neural signal detection and information processing

关键词： adaptive coding Retina Lighting Biomembranes Neural networks Neurons Mechanical factors Information analysis Signal analysis Signal detection

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adaptive coding for high-rate direct-sequence spread spectrum

Adaptive coding for high-rate direct-sequence spread spectru...

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International Conference on Wireless Networks, Communications and Mobile Computing

作者： M.B. Pursley T.C. Royster IV Clemson University Clemson SC USA

High-rate direct-sequence (DS) spread spectrum is a modulation technique in which most or all of the spreading is provided by nonbinary data modulation. For applications to mobile ad hoc wireless networks, the limited processing gain of high-rate DS spread spectrum gives only modest protection against multiple-access or multipath interference, but increased interference rejection capability can be obtained from error-control coding. In this paper, we evaluate the performance of high-rate DS spread spectrum with turbo product codes. The utility of the Gaussian approximation for the design of adaptive coding systems is illustrated. An adaptive coding algorithm is described and its performance is compared with that of fixed-rate codes for channels with unknown propagation loss. The algorithm is based on statistics obtained in the receiver, and no external side information is employed.

关键词： adaptive coding Spread spectrum communication Multiple access interference Wireless networks Protection Product codes Gaussian approximation Performance loss Propagation losses Statistics

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