17 research outputs found
Optimal Use of Current and Outdated Channel State Information - Degrees of Freedom of the MISO BC with Mixed CSIT
We consider a multiple-input-single-output (MISO) broadcast channel with
mixed channel state information at the transmitter (CSIT) that consists of
imperfect current CSIT and perfect outdated CSIT. Recent work by Kobayashi et
al. presented a scheme which exploits both imperfect current CSIT and perfect
outdated CSIT and achieves higher degrees of freedom (DoF) than possible with
only imperfect current CSIT or only outdated CSIT individually. In this work,
we further improve the achievable DoF in this setting by incorporating
additional private messages, and provide a tight information theoretic DoF
outer bound, thereby identifying the DoF optimal use of mixed CSIT. The new
result is stronger even in the original setting of only delayed CSIT, because
it allows us to remove the restricting assumption of statistically equivalent
fading for all users
Aiming Perfectly in the Dark - Blind Interference Alignment through Staggered Antenna Switching
We propose a blind interference alignment scheme for the vector broadcast
channel where the transmitter is equipped with M antennas and there are K
receivers, each equipped with a reconfigurable antenna capable of switching
among M preset modes. Without any knowledge of the channel coefficient values
at the transmitters and with only mild assumptions on the channel coherence
structure we show that MK/M+K-1 degrees of freedom are achievable. The key to
the blind interference alignment scheme is the ability of the receivers to
switch between reconfigurable antenna modes to create short term channel
fluctuation patterns that are exploited by the transmitter. The achievable
scheme does not require cooperation between transmit antennas and is therefore
applicable to the MxK X network as well. Only finite symbol extensions are
used, and no channel knowledge at the receivers is required to null the
interference.Comment: 27 pages, 15 figure
Multiple Unicast Capacity of 2-Source 2-Sink Networks
We study the sum capacity of multiple unicasts in wired and wireless multihop
networks. With 2 source nodes and 2 sink nodes, there are a total of 4
independent unicast sessions (messages), one from each source to each sink node
(this setting is also known as an X network). For wired networks with arbitrary
connectivity, the sum capacity is achieved simply by routing. For wireless
networks, we explore the degrees of freedom (DoF) of multihop X networks with a
layered structure, allowing arbitrary number of hops, and arbitrary
connectivity within each hop. For the case when there are no more than two
relay nodes in each layer, the DoF can only take values 1, 4/3, 3/2 or 2, based
on the connectivity of the network, for almost all values of channel
coefficients. When there are arbitrary number of relays in each layer, the DoF
can also take the value 5/3 . Achievability schemes incorporate linear
forwarding, interference alignment and aligned interference neutralization
principles. Information theoretic converse arguments specialized for the
connectivity of the network are constructed based on the intuition from linear
dimension counting arguments.Comment: 6 pages, 7 figures, submitted to IEEE Globecom 201
Feasibility Conditions for Interference Alignment
The degrees of freedom of MIMO interference networks with constant channel
coefficients are not known in general. Determining the feasibility of a linear
interference alignment solution is a key step toward solving this open problem.
Our approach in this paper is to view the alignment problem as a system of
bilinear equations and determine its solvability by comparing the number of
equations and the number of variables. To this end, we divide interference
alignment problems into two classes - proper and improper. An interference
alignment problem is called proper if the number of equations does not exceed
the number of variables. Otherwise, it is called improper. Examples are
presented to support the intuition that for generic channel matrices, proper
systems are almost surely feasible and improper systems are almost surely
infeasible.Comment: 6 pages. Submitted to IEEE Globecom March 31 200
A vueltas con el exilio [De Juan José Domenchina a Gerardo Deniz]
Previous work showed that the X network with M transmitters, N receivers has
MN/(M+N-1) degrees of freedom. In this work we study the degrees of freedom of
the X network with secrecy constraints, i.e. the X network where some/all
messages are confidential. We consider the network where all
messages are secured and show that N(M-1)/(M+N-1) degrees of freedom can be
achieved. Secondly, we show that if messages from only M-1 transmitters are
confidential, then MN/(M+N-1) degrees of freedom can be achieved meaning that
there is no loss of degrees of freedom because of secrecy constraints. We also
consider the achievable secure degrees of freedom under a more conservative
secrecy constraint. We require that messages from any subset of transmitters
are secure even if other transmitters are compromised, i.e., messages from the
compromised transmitter are revealed to the unintended receivers. We also study
the achievable secure degrees of freedom of the K user Gaussian interference
channel under two different secrecy constraints where 1/2 secure degrees of
freedom per message can be achieved. The achievable scheme in all cases is
based on random binning combined with interference alignment.Comment: To appear in Proceedings of 46th Annual Allerton Conference on
Communication, Control and Computing, Sept. 200