2,045 research outputs found
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
Cyclic Interference Alignment and Cancellation in 3-User X-Networks with Minimal Backhaul
We consider the problem of Cyclic Interference Alignment (IA) on the 3-user
X-network and show that it is infeasible to exactly achieve the upper bound of
degrees of freedom for the lower bound of n=5
signalling dimensions and K=3 user-pairs. This infeasibility goes beyond the
problem of common eigenvectors in invariant subspaces within spatial IA.
In order to gain non-asymptotic feasibility with minimal intervention, we
first investigate an alignment strategy that enables IA by feedforwarding a
subset of messages with minimal rate. In a second step, we replace the proposed
feedforward strategy by an analogous Cyclic Interference Alignment and
Cancellation scheme with a backhaul network on the receiver side and also by a
dual Cyclic Interference Neutralization scheme with a backhaul network on the
transmitter side.Comment: 8 pages, short version submitted to ISIT 201
Degrees of Freedom of Two-Hop Wireless Networks: "Everyone Gets the Entire Cake"
We show that fully connected two-hop wireless networks with K sources, K
relays and K destinations have K degrees of freedom both in the case of
time-varying channel coefficients and in the case of constant channel
coefficients (in which case the result holds for almost all values of constant
channel coefficients). Our main contribution is a new achievability scheme
which we call Aligned Network Diagonalization. This scheme allows the data
streams transmitted by the sources to undergo a diagonal linear transformation
from the sources to the destinations, thus being received free of interference
by their intended destination. In addition, we extend our scheme to multi-hop
networks with fully connected hops, and multi-hop networks with MIMO nodes, for
which the degrees of freedom are also fully characterized.Comment: Presented at the 2012 Allerton Conference. Submitted to IEEE
Transactions on Information Theor
Degrees of Freedom of Uplink-Downlink Multiantenna Cellular Networks
An uplink-downlink two-cell cellular network is studied in which the first
base station (BS) with antennas receives independent messages from its
serving users, while the second BS with antennas transmits
independent messages to its serving users. That is, the first and second
cells operate as uplink and downlink, respectively. Each user is assumed to
have a single antenna. Under this uplink-downlink setting, the sum degrees of
freedom (DoF) is completely characterized as the minimum of
,
, , and , where denotes
. The result demonstrates that, for a broad class of network
configurations, operating one of the two cells as uplink and the other cell as
downlink can strictly improve the sum DoF compared to the conventional uplink
or downlink operation, in which both cells operate as either uplink or
downlink. The DoF gain from such uplink-downlink operation is further shown to
be achievable for heterogeneous cellular networks having hotspots and with
delayed channel state information.Comment: 22 pages, 11 figures, in revision for IEEE Transactions on
Information Theor
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