21,292 research outputs found

    Speeding up Future Video Distribution via Channel-Aware Caching-Aided Coded Multicast

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    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

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    In this work, we study a noiseless broadcast link serving KK users whose requests arise from a library of NN files. Every user is equipped with a cache of size MM 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 N/MN/M 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 FF scales to infinity. In this work, we initiate the finite-length analysis of random caching schemes when the number of packets FF is a function of the system parameters M,N,KM,N,K. 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 22 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 43g\frac{4}{3}g is at least O((N/M)g)O((N/M)^g). 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

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    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

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    The following source coding problem was introduced by Birk and Kol: a sender holds a word x∈{0,1}nx\in\{0,1\}^n, and wishes to broadcast a codeword to nn receivers, R1,...,RnR_1,...,R_n. The receiver RiR_i is interested in xix_i, and has prior \emph{side information} comprising some subset of the nn bits. This corresponds to a directed graph GG on nn vertices, where iji j is an edge iff RiR_i knows the bit xjx_j. An \emph{index code} for GG is an encoding scheme which enables each RiR_i to always reconstruct xix_i, 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 GG. 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 ϵ>0\epsilon > 0 and sufficiently large nn, there is an nn-vertex graph GG so that every linear index code for GG requires codewords of length at least n1−ϵn^{1-\epsilon}, and yet a non-linear index code for GG has a word length of nϵn^\epsilon. 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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