162 research outputs found
A Generalized Cut-Set Bound for Deterministic Multi-Flow Networks and its Applications
We present a new outer bound for the sum capacity of general multi-unicast
deterministic networks. Intuitively, this bound can be understood as applying
the cut-set bound to concatenated copies of the original network with a special
restriction on the allowed transmit signal distributions. We first study
applications to finite-field networks, where we obtain a general outer-bound
expression in terms of ranks of the transfer matrices. We then show that, even
though our outer bound is for deterministic networks, a recent result relating
the capacity of AWGN KxKxK networks and the capacity of a deterministic
counterpart allows us to establish an outer bound to the DoF of KxKxK wireless
networks with general connectivity. This bound is tight in the case of the
"adjacent-cell interference" topology, and yields graph-theoretic necessary and
sufficient conditions for K DoF to be achievable in general topologies.Comment: A shorter version of this paper will appear in the Proceedings of
ISIT 201
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
Information-theoretic secrecy for wireless networks
The aim of information-theoretic secrecy is to ensure that an eavesdropper who listens to the wireless transmission of a message can only collect an arbitrarily small number of information bits about this message. In contrast to cryptography, there are no assumptions on the computational power of the eavesdropper. Information-theoretically secret communication has been studied for many particular wireless network topologies. In the main part of this thesis, we consider such communication for arbitrary acyclic wireless network topologies. We provide lower and upper bounds on the strong perfect secrecy capacity for the case when the channels of the network are either Gaussian or deterministic. These results are based on the recent understanding of the capacity of wireless networks (without secrecy constraints) by Avestimehr, Diggavi and Tse. As a side result, we give inner and outer bounds on the capacity region for the multisource problem in arbitrary wireless networks with Gaussian or deterministic signal interaction. For linear deterministic signal interaction, we find the exact capacity region. For Gaussian signal interaction, we are able to bound the gap between the two bounds on the capacity region. This gap depends only on the network topology, but not on the signal-to-noise ratio (SNR), which leads to an approximation of the capacity region for the high SNR regime. We further consider a particular network topology, called the fan-network, in which we assume that an eavesdropper has physical access to every node in a subset of the relay nodes. We give a general upper bound on the perfect secrecy capacity, and we characterize the perfect secrecy capacity for two special cases. In the second part of the thesis, we consider interactive secrecy, i.e., secrecy in the presence of a public feedback link from the destination to the source. We focus on the problem of secret key generation rather than secret communication. The benefit of public discussion for secret key generation in a broadcast channel was first shown by Maurer. We extend his ideas to a relay network called the line network, leading to a lower bound on the strongly secret key capacity for this network topology. Finally, we introduce a new channel coding setup called the interference-multiple access (IMA) channel. This channel is a variant of the interference channel where one of the receivers is required to decode the messages from both transmitters. We derive an inner bound on the capacity region of the IMA channel, as well as an outer bound for the so-called structured IMA channel. In a semi-deterministic version of the structured IMA channel, the bounds match, providing a characterization of the capacity region. In the Gaussian case, we obtain a 1 bit-approximation of the capacity region. We also show an inner bound on the equivocation-capacity region for the IMA channel, where we require that part of the private message for one receiver is kept information-theoretically secret from the other receiver
Degrees of Freedom of Full-Duplex Multiantenna Cellular Networks
We study the degrees of freedom (DoF) of cellular networks in which a full
duplex (FD) base station (BS) equipped with multiple transmit and receive
antennas communicates with multiple mobile users. We consider two different
scenarios. In the first scenario, we study the case when half duplex (HD)
users, partitioned to either the uplink (UL) set or the downlink (DL) set,
simultaneously communicate with the FD BS. In the second scenario, we study the
case when FD users simultaneously communicate UL and DL data with the FD BS.
Unlike conventional HD only systems, inter-user interference (within the cell)
may severely limit the DoF, and must be carefully taken into account. With the
goal of providing theoretical guidelines for designing such FD systems, we
completely characterize the sum DoF of each of the two different FD cellular
networks by developing an achievable scheme and obtaining a matching upper
bound. The key idea of the proposed scheme is to carefully allocate UL and DL
information streams using interference alignment and beamforming techniques. By
comparing the DoFs of the considered FD systems with those of the conventional
HD systems, we establish the DoF gain by enabling FD operation in various
configurations. As a consequence of the result, we show that the DoF can
approach the two-fold gain over the HD systems when the number of users becomes
large enough as compared to the number of antennas at the BS.Comment: 21 pages, 16 figures, a shorter version of this paper has been
submitted to the IEEE International Symposium on Information Theory (ISIT)
201
On Linear Operator Channels over Finite Fields
Motivated by linear network coding, communication channels perform linear
operation over finite fields, namely linear operator channels (LOCs), are
studied in this paper. For such a channel, its output vector is a linear
transform of its input vector, and the transformation matrix is randomly and
independently generated. The transformation matrix is assumed to remain
constant for every T input vectors and to be unknown to both the transmitter
and the receiver. There are NO constraints on the distribution of the
transformation matrix and the field size.
Specifically, the optimality of subspace coding over LOCs is investigated. A
lower bound on the maximum achievable rate of subspace coding is obtained and
it is shown to be tight for some cases. The maximum achievable rate of
constant-dimensional subspace coding is characterized and the loss of rate
incurred by using constant-dimensional subspace coding is insignificant.
The maximum achievable rate of channel training is close to the lower bound
on the maximum achievable rate of subspace coding. Two coding approaches based
on channel training are proposed and their performances are evaluated. Our
first approach makes use of rank-metric codes and its optimality depends on the
existence of maximum rank distance codes. Our second approach applies linear
coding and it can achieve the maximum achievable rate of channel training. Our
code designs require only the knowledge of the expectation of the rank of the
transformation matrix. The second scheme can also be realized ratelessly
without a priori knowledge of the channel statistics.Comment: 53 pages, 3 figures, submitted to IEEE Transaction on Information
Theor
Slepian-Wolf Coding Over Cooperative Relay Networks
This paper deals with the problem of multicasting a set of discrete
memoryless correlated sources (DMCS) over a cooperative relay network.
Necessary conditions with cut-set interpretation are presented. A \emph{Joint
source-Wyner-Ziv encoding/sliding window decoding} scheme is proposed, in which
decoding at each receiver is done with respect to an ordered partition of other
nodes. For each ordered partition a set of feasibility constraints is derived.
Then, utilizing the sub-modular property of the entropy function and a novel
geometrical approach, the results of different ordered partitions are
consolidated, which lead to sufficient conditions for our problem. The proposed
scheme achieves operational separation between source coding and channel
coding. It is shown that sufficient conditions are indeed necessary conditions
in two special cooperative networks, namely, Aref network and finite-field
deterministic network. Also, in Gaussian cooperative networks, it is shown that
reliable transmission of all DMCS whose Slepian-Wolf region intersects the
cut-set bound region within a constant number of bits, is feasible. In
particular, all results of the paper are specialized to obtain an achievable
rate region for cooperative relay networks which includes relay networks and
two-way relay networks.Comment: IEEE Transactions on Information Theory, accepte
Design of linear Boolean network codes for combination networks
Network coding is a promising technique to improve the throughput of communication networks. In the thesis, we investigate linear Boolean network codes on a special class of multicast networks, called combination networks. Using companion matrices of primitive polynomials over finite fields, we design a class of symmetric linear Boolean network codes from Reed-Solomon codes for single-source combination networks. We also prove that, for some cases, the linear Boolean network codes are optimal in the sense of minimum network uses. In the thesis, we further consider two-source network coding problem for combination networks and other specific networks. We develop a method to evaluate an outer bound for these two-source networks. By designing linear Boolean network codes which achieve extreme points of rate regions, we show that the outer bound is actually tight for some of these networks
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