21,292 research outputs found
Speeding up Future Video Distribution via Channel-Aware Caching-Aided Coded Multicast
Future Internet usage will be dominated by the consumption of a rich variety
of online multimedia services accessed from an exponentially growing number of
multimedia capable mobile devices. As such, future Internet designs will be
challenged to provide solutions that can deliver bandwidth-intensive,
delay-sensitive, on-demand video-based services over increasingly crowded,
bandwidth-limited wireless access networks. One of the main reasons for the
bandwidth stress facing wireless network operators is the difficulty to exploit
the multicast nature of the wireless medium when wireless users or access
points rarely experience the same channel conditions or access the same content
at the same time. In this paper, we present and analyze a novel wireless video
delivery paradigm based on the combined use of channel-aware caching and coded
multicasting that allows simultaneously serving multiple cache-enabled
receivers that may be requesting different content and experiencing different
channel conditions. To this end, we reformulate the caching-aided coded
multicast problem as a joint source-channel coding problem and design an
achievable scheme that preserves the cache-enabled multiplicative throughput
gains of the error-free scenario,by guaranteeing per-receiver rates unaffected
by the presence of receivers with worse channel conditions.Comment: 11 pages,6 figures,to appear in IEEE JSAC Special Issue on Video
Distribution over Future Interne
Finite Length Analysis of Caching-Aided Coded Multicasting
In this work, we study a noiseless broadcast link serving users whose
requests arise from a library of files. Every user is equipped with a cache
of size files each. It has been shown that by splitting all the files into
packets and placing individual packets in a random independent manner across
all the caches, it requires at most file transmissions for any set of
demands from the library. The achievable delivery scheme involves linearly
combining packets of different files following a greedy clique cover solution
to the underlying index coding problem. This remarkable multiplicative gain of
random placement and coded delivery has been established in the asymptotic
regime when the number of packets per file scales to infinity.
In this work, we initiate the finite-length analysis of random caching
schemes when the number of packets is a function of the system parameters
. Specifically, we show that existing random placement and clique cover
delivery schemes that achieve optimality in the asymptotic regime can have at
most a multiplicative gain of if the number of packets is sub-exponential.
Further, for any clique cover based coded delivery and a large class of random
caching schemes, that includes the existing ones, we show that the number of
packets required to get a multiplicative gain of is at least
. We exhibit a random placement and an efficient clique cover based
coded delivery scheme that approximately achieves this lower bound. We also
provide tight concentration results that show that the average (over the random
caching involved) number of transmissions concentrates very well requiring only
polynomial number of packets in the rest of the parameters.Comment: A shorter version appeared in the 52nd Annual Allerton Conference on
Communication, Control, and Computing (Allerton), 201
On Coding for Cooperative Data Exchange
We consider the problem of data exchange by a group of closely-located
wireless nodes. In this problem each node holds a set of packets and needs to
obtain all the packets held by other nodes. Each of the nodes can broadcast the
packets in its possession (or a combination thereof) via a noiseless broadcast
channel of capacity one packet per channel use. The goal is to minimize the
total number of transmissions needed to satisfy the demands of all the nodes,
assuming that they can cooperate with each other and are fully aware of the
packet sets available to other nodes. This problem arises in several practical
settings, such as peer-to-peer systems and wireless data broadcast. In this
paper, we establish upper and lower bounds on the optimal number of
transmissions and present an efficient algorithm with provable performance
guarantees. The effectiveness of our algorithms is established through
numerical simulations.Comment: Appeared in the proceedings of the 2010 IEEE Information Theory
Workshop (ITW 2010, Cairo
Non-linear index coding outperforming the linear optimum
The following source coding problem was introduced by Birk and Kol: a sender
holds a word , and wishes to broadcast a codeword to
receivers, . The receiver is interested in , and has
prior \emph{side information} comprising some subset of the bits. This
corresponds to a directed graph on vertices, where is an edge iff
knows the bit . An \emph{index code} for is an encoding scheme
which enables each to always reconstruct , given his side
information. The minimal word length of an index code was studied by
Bar-Yossef, Birk, Jayram and Kol (FOCS 2006). They introduced a graph
parameter, \minrk_2(G), which completely characterizes the length of an
optimal \emph{linear} index code for . The authors of BBJK showed that in
various cases linear codes attain the optimal word length, and conjectured that
linear index coding is in fact \emph{always} optimal.
In this work, we disprove the main conjecture of BBJK in the following strong
sense: for any and sufficiently large , there is an
-vertex graph so that every linear index code for requires codewords
of length at least , and yet a non-linear index code for
has a word length of . This is achieved by an explicit
construction, which extends Alon's variant of the celebrated Ramsey
construction of Frankl and Wilson.
In addition, we study optimal index codes in various, less restricted,
natural models, and prove several related properties of the graph parameter
\minrk(G).Comment: 16 pages; Preliminary version appeared in FOCS 200
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