91 research outputs found
A Simple Cooperative Diversity Method Based on Network Path Selection
Cooperative diversity has been recently proposed as a way to form virtual
antenna arrays that provide dramatic gains in slow fading wireless
environments. However most of the proposed solutions require distributed
space-time coding algorithms, the careful design of which is left for future
investigation if there is more than one cooperative relay. We propose a novel
scheme, that alleviates these problems and provides diversity gains on the
order of the number of relays in the network. Our scheme first selects the best
relay from a set of M available relays and then uses this best relay for
cooperation between the source and the destination. We develop and analyze a
distributed method to select the best relay that requires no topology
information and is based on local measurements of the instantaneous channel
conditions. This method also requires no explicit communication among the
relays. The success (or failure) to select the best available path depends on
the statistics of the wireless channel, and a methodology to evaluate
performance for any kind of wireless channel statistics, is provided.
Information theoretic analysis of outage probability shows that our scheme
achieves the same diversity-multiplexing tradeoff as achieved by more complex
protocols, where coordination and distributed space-time coding for M nodes is
required, such as those proposed in [7]. The simplicity of the technique,
allows for immediate implementation in existing radio hardware and its adoption
could provide for improved flexibility, reliability and efficiency in future 4G
wireless systems.Comment: To appear, IEEE JSAC, special issue on 4
Secured Communication over Frequency-Selective Fading Channels: a practical Vandermonde precoding
In this paper, we study the frequency-selective broadcast channel with
confidential messages (BCC) in which the transmitter sends a confidential
message to receiver 1 and a common message to receivers 1 and 2. In the case of
a block transmission of N symbols followed by a guard interval of L symbols,
the frequency-selective channel can be modeled as a N * (N+L) Toeplitz matrix.
For this special type of multiple-input multiple-output (MIMO) channels, we
propose a practical Vandermonde precoding that consists of projecting the
confidential messages in the null space of the channel seen by receiver 2 while
superposing the common message. For this scheme, we provide the achievable rate
region, i.e. the rate-tuple of the common and confidential messages, and
characterize the optimal covariance inputs for some special cases of interest.
It is proved that the proposed scheme achieves the optimal degree of freedom
(d.o.f) region. More specifically, it enables to send l <= L confidential
messages and N-l common messages simultaneously over a block of N+L symbols.
Interestingly, the proposed scheme can be applied to secured multiuser
scenarios such as the K+1-user frequency-selective BCC with K confidential
messages and the two-user frequency-selective BCC with two confidential
messages. For each scenario, we provide the achievable secrecy degree of
freedom (s.d.o.f.) region of the corresponding frequency-selective BCC and
prove the optimality of the Vandermonde precoding. One of the appealing
features of the proposed scheme is that it does not require any specific
secrecy encoding technique but can be applied on top of any existing powerful
encoding schemes.Comment: To appear in EURASIP journal on Wireless Communications and
Networking, special issue on Wireless Physical Security, 200
Secret Sharing over Fast-Fading MIMO Wiretap Channels
Secret sharing over the fast-fading MIMO wiretap channel is considered. A
source and a destination try to share secret information over a fast-fading
MIMO channel in the presence of a wiretapper who also makes channel
observations that are different from but correlated to those made by the
destination. An interactive authenticated unrestricted public channel is also
available for use by the source and destination in the secret sharing process.
This falls under the "channel-type model with wiretapper" considered by
Ahlswede and Csiszar. A minor extension of their result (to continuous channel
alphabets) is employed to evaluate the key capacity of the fast-fading MIMO
wiretap channel. The effects of spatial dimensionality provided by the use of
multiple antennas at the source, destination, and wiretapper are then
investigated.Comment: Revision submitted to EURASIP Journal on Wireless Communications and
Networking, Special Issue on Wireless Physical Layer Security, Sept. 2009.
v.3: Fixes to proofs. Matthieu Bloch added as co-author for contributions to
proof
On the Transmit Beamforming for MIMO Wiretap Channels: Large-System Analysis
With the growth of wireless networks, security has become a fundamental issue
in wireless communications due to the broadcast nature of these networks. In
this work, we consider MIMO wiretap channels in a fast fading environment, for
which the overall performance is characterized by the ergodic MIMO secrecy
rate. Unfortunately, the direct solution to finding ergodic secrecy rates is
prohibitive due to the expectations in the rates expressions in this setting.
To overcome this difficulty, we invoke the large-system assumption, which
allows a deterministic approximation to the ergodic mutual information.
Leveraging results from random matrix theory, we are able to characterize the
achievable ergodic secrecy rates. Based on this characterization, we address
the problem of covariance optimization at the transmitter. Our numerical
results demonstrate a good match between the large-system approximation and the
actual simulated secrecy rates, as well as some interesting features of the
precoder optimization.Comment: Published in Lecture Notes in Computer Science 8317, pp. 90-102,
2014. (Proceedings of International Conference on Information-Theoretic
Security (ICITS), Singapore, November 2013
Deletion of the gabra2 gene results in hypersensitivity to the acute effects of ethanol but does not alter ethanol self administration
Human genetic studies have suggested that polymorphisms of the GABRA2 gene encoding the GABA(A) α2-subunit are associated with ethanol dependence. Variations in this gene also convey sensitivity to the subjective effects of ethanol, indicating a role in mediating ethanol-related behaviours. We therefore investigated the consequences of deleting the α2-subunit on the ataxic and rewarding properties of ethanol in mice. Ataxic and sedative effects of ethanol were explored in GABA(A) α2-subunit wildtype (WT) and knockout (KO) mice using a Rotarod apparatus, wire hang and the duration of loss of righting reflex. Following training, KO mice showed shorter latencies to fall than WT littermates under ethanol (2 g/kg i.p.) in both Rotarod and wire hang tests. After administration of ethanol (3.5 g/kg i.p.), KO mice took longer to regain the righting reflex than WT mice. To ensure the acute effects are not due to the gabra2 deletion affecting pharmacokinetics, blood ethanol concentrations were measured at 20 minute intervals after acute administration (2 g/kg i.p.), and did not differ between genotypes. To investigate ethanol's rewarding properties, WT and KO mice were trained to lever press to receive increasing concentrations of ethanol on an FR4 schedule of reinforcement. Both WT and KO mice self-administered ethanol at similar rates, with no differences in the numbers of reinforcers earned. These data indicate a protective role for α2-subunits, against the acute sedative and ataxic effects of ethanol. However, no change was observed in ethanol self administration, suggesting the rewarding effects of ethanol remain unchange
Secrecy capacity of a class of orthogonal relay eavesdropper channels
The secrecy capacity of relay channels with orthogonal components is studied
in the presence of an additional passive eavesdropper node. The relay and
destination receive signals from the source on two orthogonal channels such
that the destination also receives transmissions from the relay on its channel.
The eavesdropper can overhear either one or both of the orthogonal channels.
Inner and outer bounds on the secrecy capacity are developed for both the
discrete memoryless and the Gaussian channel models. For the discrete
memoryless case, the secrecy capacity is shown to be achieved by a partial
decode-and-forward (PDF) scheme when the eavesdropper can overhear only one of
the two orthogonal channels. Two new outer bounds are presented for the
Gaussian model using recent capacity results for a Gaussian multi-antenna
point-to-point channel with a multi-antenna eavesdropper. The outer bounds are
shown to be tight for two sub-classes of channels. The first sub-class is one
in which the source and relay are clustered and the and the eavesdropper
receives signals only on the channel from the source and the relay to the
destination, for which the PDF strategy is optimal. The second is a sub-class
in which the source does not transmit to the relay, for which a
noise-forwarding strategy is optimal.Comment: Submitted to Eurasip Journal on Wireless Communications and
Networking special issue on Wireless physical layer security, Dec. 2008,
Revised Jun. 200
Genomic Analysis of Individual Differences in Ethanol Drinking: Evidence for Non-Genetic Factors in C57BL/6 Mice
Genetic analysis of factors affecting risk to develop excessive ethanol drinking has been extensively studied in humans and animal models for over 20 years. However, little progress has been made in determining molecular mechanisms underlying environmental or non-genetic events contributing to variation in ethanol drinking. Here, we identify persistent and substantial variation in ethanol drinking behavior within an inbred mouse strain and utilize this model to identify gene networks influencing such “non-genetic” variation in ethanol intake. C57BL/6NCrl mice showed persistent inter-individual variation of ethanol intake in a two-bottle choice paradigm over a three-week period, ranging from less than 1 g/kg to over 14 g/kg ethanol in an 18 h interval. Differences in sweet or bitter taste susceptibility or litter effects did not appreciably correlate with ethanol intake variation. Whole genome microarray expression analysis in nucleus accumbens, prefrontal cortex and ventral midbrain region of individual animals identified gene expression patterns correlated with ethanol intake. Results included several gene networks previously implicated in ethanol behaviors, such as glutamate signaling, BDNF and genes involved in synaptic vesicle function. Additionally, genes functioning in epigenetic chromatin or DNA modifications such as acetylation and/or methylation also had expression patterns correlated with ethanol intake. In verification for the significance of the expression findings, we found that a histone deacetylase inhibitor, trichostatin A, caused an increase in 2-bottle ethanol intake. Our results thus implicate specific brain regional gene networks, including chromatin modification factors, as potentially important mechanisms underlying individual variation in ethanol intake
Basis of the Gabamimetic Profile of Ethanol
This article summarizes the proceedings of a symposium held at the 2005 Research Society on Alcoholism meeting. The initial presentation by Dr. Wallner provided evidence that selected GABAA receptors containing the δ subunit display sensitivity to low intoxicating ethanol concentrations and this sensitivity is further increased by a mutation in the cerebellar α6 subunit, found in alcohol-hypersensitive rats. Dr. Mameli reported that ethanol affects γ-aminobutyric acid (GABA) function by affecting neural circuits that influence GABA release. Dr. Parsons presented data from electrophysiological and microdialysis investigations that ethanol is capable of releasing GABA from presynaptic terminals. Dr. Morrow demonstrated that systemic ethanol increases neuroactive steroids in brain, the absence of which alters various functional responses to ethanol. Dr. Criswell presented evidence that the ability of ethanol to increase GABA was apparent in some, but not all, brain regions indicative of regional specificity. Further, Dr. Criswell demonstrated that neurosteroids alone and when synthesized locally by ethanol act postsynaptically to enhance the effect of GABA released by ethanol in a region specific manner. Collectively, this series of reports support the GABAmimetic profile of acutely administered ethanol being dependent on several specific mechanisms distinct from a direct effect on the major synaptic isoforms of GABAA receptors
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