86,848 research outputs found
Equivalent models for multi-terminal channels
The recently introduced network equivalence results are used to create bit-pipe models that can replace multi-terminal channels within a discrete memoryless network. The goal is to create a set of simple “components” or “blocks” that can be substituted for the channel in such a way that the resulting network is capable of emulating the operation of the original one. We develop general upper and lower bounding models for the multiple access channel and for a class of broadcast channels. These bounds are sharp in the sense that there exists networks where the original channel can achieve the maximum sum rate permissible through the upper or lower bounding models. This approach provides a simple method for analyzing the capacity of large networks, which we illustrate with an example
Error Rate Analysis of GF(q) Network Coded Detect-and-Forward Wireless Relay Networks Using Equivalent Relay Channel Models
This paper investigates simple means of analyzing the error rate performance
of a general q-ary Galois Field network coded detect-and-forward cooperative
relay network with known relay error statistics at the destination. Equivalent
relay channels are used in obtaining an approximate error rate of the relay
network, from which the diversity order is found. Error rate analyses using
equivalent relay channel models are shown to be closely matched with simulation
results. Using the equivalent relay channels, low complexity receivers are
developed whose performances are close to that of the optimal maximum
likelihood receiver.Comment: 28 pages, 10 figures. This work has been submitted to the IEEE for
possible publication. Copyright may be transferred without notice, after
which this version may no longer be accessibl
The Multi-way Relay Channel
The multiuser communication channel, in which multiple users exchange
information with the help of a relay terminal, termed the multi-way relay
channel (mRC), is introduced. In this model, multiple interfering clusters of
users communicate simultaneously, where the users within the same cluster wish
to exchange messages among themselves. It is assumed that the users cannot
receive each other's signals directly, and hence the relay terminal in this
model is the enabler of communication. In particular, restricted encoders,
which ignore the received channel output and use only the corresponding
messages for generating the channel input, are considered. Achievable rate
regions and an outer bound are characterized for the Gaussian mRC, and their
comparison is presented in terms of exchange rates in a symmetric Gaussian
network scenario. It is shown that the compress-and-forward (CF) protocol
achieves exchange rates within a constant bit offset of the exchange capacity
independent of the power constraints of the terminals in the network. A finite
bit gap between the exchange rates achieved by the CF and the
amplify-and-forward (AF) protocols is also shown. The two special cases of the
mRC, the full data exchange model, in which every user wants to receive
messages of all other users, and the pairwise data exchange model which
consists of multiple two-way relay channels, are investigated in detail. In
particular for the pairwise data exchange model, in addition to the proposed
random coding based achievable schemes, a nested lattice coding based scheme is
also presented and is shown to achieve exchange rates within a constant bit gap
of the exchange capacity.Comment: Revised version of our submission to the Transactions on Information
Theor
Dynamic Resource Allocation in Cognitive Radio Networks: A Convex Optimization Perspective
This article provides an overview of the state-of-art results on
communication resource allocation over space, time, and frequency for emerging
cognitive radio (CR) wireless networks. Focusing on the
interference-power/interference-temperature (IT) constraint approach for CRs to
protect primary radio transmissions, many new and challenging problems
regarding the design of CR systems are formulated, and some of the
corresponding solutions are shown to be obtainable by restructuring some
classic results known for traditional (non-CR) wireless networks. It is
demonstrated that convex optimization plays an essential role in solving these
problems, in a both rigorous and efficient way. Promising research directions
on interference management for CR and other related multiuser communication
systems are discussed.Comment: to appear in IEEE Signal Processing Magazine, special issue on convex
optimization for signal processin
Power-Constrained Sparse Gaussian Linear Dimensionality Reduction over Noisy Channels
In this paper, we investigate power-constrained sensing matrix design in a
sparse Gaussian linear dimensionality reduction framework. Our study is carried
out in a single--terminal setup as well as in a multi--terminal setup
consisting of orthogonal or coherent multiple access channels (MAC). We adopt
the mean square error (MSE) performance criterion for sparse source
reconstruction in a system where source-to-sensor channel(s) and
sensor-to-decoder communication channel(s) are noisy. Our proposed sensing
matrix design procedure relies upon minimizing a lower-bound on the MSE in
single-- and multiple--terminal setups. We propose a three-stage sensing matrix
optimization scheme that combines semi-definite relaxation (SDR) programming, a
low-rank approximation problem and power-rescaling. Under certain conditions,
we derive closed-form solutions to the proposed optimization procedure. Through
numerical experiments, by applying practical sparse reconstruction algorithms,
we show the superiority of the proposed scheme by comparing it with other
relevant methods. This performance improvement is achieved at the price of
higher computational complexity. Hence, in order to address the complexity
burden, we present an equivalent stochastic optimization method to the problem
of interest that can be solved approximately, while still providing a superior
performance over the popular methods.Comment: Accepted for publication in IEEE Transactions on Signal Processing
(16 pages
Contacts and Edge State Equilibration in the Fractional Quantum Hall Effect
We develop a simple kinetic equation description of edge state dynamics in
the fractional quantum Hall effect (FQHE), which allows us to examine in detail
equilibration processes between multiple edge modes. As in the integer quantum
Hall effect (IQHE), inter-mode equilibration is a prerequisite for quantization
of the Hall conductance. Two sources for such equilibration are considered:
Edge impurity scattering and equilibration by the electrical contacts. Several
specific models for electrical contacts are introduced and analyzed. For FQHE
states in which edge channels move in both directions, such as , these
models for the electrical contacts {\it do not} equilibrate the edge modes,
resulting in a non-quantized Hall conductance, even in a four-terminal
measurement. Inclusion of edge-impurity scattering, which {\it directly}
transfers charge between channels, is shown to restore the four-terminal
quantized conductance. For specific filling factors, notably and
, the equilibration length due to impurity scattering diverges in the
zero temperature limit, which should lead to a breakdown of quantization for
small samples at low temperatures. Experimental implications are discussed.Comment: 14 pages REVTeX, 6 postscript figures (uuencoded and compressed
Quantum to Classical Transition of the Charge Relaxation Resistance of a Mesoscopic Capacitor
We present an analysis of the effect of dephasing on the single channel
charge relaxation resistance of a mesoscopic capacitor in the linear low
frequency regime. The capacitor consists of a cavity which is via a quantum
point contact connected to an electron reservoir and Coulomb coupled to a gate.
The capacitor is in a perpendicular high magnetic field such that only one
(spin polarized) edge state is (partially) transmitted through the contact. In
the coherent limit the charge relaxation resistance for a single channel
contact is independent of the transmission probability of the contact and given
by half a resistance quantum. The loss of coherence in the conductor is modeled
by attaching to it a fictitious probe, which draws no net current. In the
incoherent limit one could expect a charge relaxation resistance that is
inversely proportional to the transmission probability of the quantum point
contact. However, such a two terminal result requires that scattering is
between two electron reservoirs which provide full inelastic relaxation. We
find that dephasing of a single edge state in the cavity is not sufficient to
generate an interface resistance. As a consequence the charge relaxation
resistance is given by the sum of one constant interface resistance and the
(original) Landauer resistance. The same result is obtained in the high
temperature regime due to energy averaging over many occupied states in the
cavity. Only for a large number of open dephasing channels, describing
spatially homogenous dephasing in the cavity, do we recover the two terminal
resistance, which is inversely proportional to the transmission probability of
the QPC. We compare different dephasing models and discuss the relation of our
results to a recent experiment.Comment: 10 pages, 8 figure
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