118 research outputs found
Delay Constrained Throughput Analysis of a Correlated MIMO Wireless Channel
The maximum traffic arrival rate at the network for a given delay guarantee
(delay constrained throughput) has been well studied for wired channels.
However, few results are available for wireless channels, especially when
multiple antennas are employed at the transmitter and receiver. In this work,
we analyze the network delay constrained throughput of a multiple input
multiple output (MIMO) wireless channel with time-varying spatial correlation.
The MIMO channel is modeled via its virtual representation, where the
individual spatial paths between the antenna pairs are Gilbert-Elliot channels.
The whole system is then described by a K-State Markov chain, where K depends
upon the degree of freedom (DOF) of the channel. We prove that the DOF based
modeling is indeed accurate. Furthermore, we study the impact of the delay
requirements at the network layer, violation probability and the number of
antennas on the throughput under different fading speeds and signal strength.Comment: Submitted to ICCCN 2011, 8 pages, 5 figure
The Linear Model under Mixed Gaussian Inputs: Designing the Transfer Matrix
Suppose a linear model y = Hx + n, where inputs x, n are independent Gaussian
mixtures. The problem is to design the transfer matrix H so as to minimize the
mean square error (MSE) when estimating x from y. This problem has important
applications, but faces at least three hurdles. Firstly, even for a fixed H,
the minimum MSE (MMSE) has no analytical form. Secondly, the MMSE is generally
not convex in H. Thirdly, derivatives of the MMSE w.r.t. H are hard to obtain.
This paper casts the problem as a stochastic program and invokes gradient
methods. The study is motivated by two applications in signal processing. One
concerns the choice of error-reducing precoders; the other deals with selection
of pilot matrices for channel estimation. In either setting, our numerical
results indicate improved estimation accuracy - markedly better than those
obtained by optimal design based on standard linear estimators. Some
implications of the non-convexities of the MMSE are noteworthy, yet, to our
knowledge, not well known. For example, there are cases in which more pilot
power is detrimental for channel estimation. This paper explains why
Projection-Based and Look Ahead Strategies for Atom Selection
In this paper, we improve iterative greedy search algorithms in which atoms
are selected serially over iterations, i.e., one-by-one over iterations. For
serial atom selection, we devise two new schemes to select an atom from a set
of potential atoms in each iteration. The two new schemes lead to two new
algorithms. For both the algorithms, in each iteration, the set of potential
atoms is found using a standard matched filter. In case of the first scheme, we
propose an orthogonal projection strategy that selects an atom from the set of
potential atoms. Then, for the second scheme, we propose a look ahead strategy
such that the selection of an atom in the current iteration has an effect on
the future iterations. The use of look ahead strategy requires a higher
computational resource. To achieve a trade-off between performance and
complexity, we use the two new schemes in cascade and develop a third new
algorithm. Through experimental evaluations, we compare the proposed algorithms
with existing greedy search and convex relaxation algorithms.Comment: sparsity, compressive sensing; IEEE Trans on Signal Processing 201
Asymptotic Analysis of SU-MIMO Channels With Transmitter Noise and Mismatched Joint Decoding
Hardware impairments in radio-frequency components of a wireless system cause
unavoidable distortions to transmission that are not captured by the
conventional linear channel model. In this paper, a 'binoisy' single-user
multiple-input multiple-output (SU-MIMO) relation is considered where the
additional distortions are modeled via an additive noise term at the transmit
side. Through this extended SU-MIMO channel model, the effects of transceiver
hardware impairments on the achievable rate of multi-antenna point-to-point
systems are studied. Channel input distributions encompassing practical
discrete modulation schemes, such as, QAM and PSK, as well as Gaussian
signaling are covered. In addition, the impact of mismatched detection and
decoding when the receiver has insufficient information about the
non-idealities is investigated. The numerical results show that for realistic
system parameters, the effects of transmit-side noise and mismatched decoding
become significant only at high modulation orders.Comment: 16 pages, 7 figure
Le paradoxe français
C’est l’historien belge Maxime Steinberg qui a parlé d’un « paradoxe français » au regard du bilan de la Shoah en Europe occidentale. Dans un article de 1993, il se demande pourquoi la France, pays ayant eu un gouvernement antisémite et collaborateur, connaît un taux d’extermination des juifs plus faible (25 %) que les Pays-Bas (80 %) et la Belgique (45 %) alors que ces pays n’ont pas eu de tel gouvernement. Cette comparaison est plus pertinente qu’avec l’Italie (16 %) dans la mesure où ces trois pays ont été occupés dans la même période (mai-juin 1940) et la « solution finale » décidée au même moment (juin 1942). Cette forte proportion de survie des juifs en France – l’une des plus élevées d’Europe – n’exonère en rien les crimes de Vichy. Tout sentiment de satisfaction serait indécent au regard des 80 000 morts de la Shoah. Mais qui travaille sur le génocide ne peut que s’interroger sur cette singularité du cas français. [Premier paragraphe de l'article
Analysis of Sparse Representations Using Bi-Orthogonal Dictionaries
The sparse representation problem of recovering an N dimensional sparse
vector x from M < N linear observations y = Dx given dictionary D is
considered. The standard approach is to let the elements of the dictionary be
independent and identically distributed (IID) zero-mean Gaussian and minimize
the l1-norm of x under the constraint y = Dx. In this paper, the performance of
l1-reconstruction is analyzed, when the dictionary is bi-orthogonal D = [O1
O2], where O1,O2 are independent and drawn uniformly according to the Haar
measure on the group of orthogonal M x M matrices. By an application of the
replica method, we obtain the critical conditions under which perfect
l1-recovery is possible with bi-orthogonal dictionaries.Comment: 5 pages, 2 figures. The main result and numerical examples have been
revise
On the Optimal Precoding for MIMO Gaussian Wire-Tap Channels
We consider the problem of finding secrecy rate of a multiple-input
multiple-output (MIMO) wire-tap channel. A transmitter, a legitimate receiver,
and an eavesdropper are all equipped with multiple antennas. The channel states
from the transmitter to the legitimate user and to the eavesdropper are assumed
to be known at the transmitter. In this contribution, we address the problem of
finding the optimal precoder/transmit covariance matrix maximizing the secrecy
rate of the given wiretap channel. The problem formulation is shown to be
equivalent to a difference of convex functions programming problem and an
efficient algorithm for addressing this problem is developed.Comment: Published in Proceedings of the Tenth International Symposium on
Wireless Communication Systems (ISWCS 2013), Ilmenau, Germany, August 201
- …