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Hari Hara Suthan – ECE

Hari Hara Suthan C received his doctorate in Electronics and Communication Engineering  (ECE). His research work was supervised by Dr. Prasad Krishnan. Here’s a summary of  Hari Hara Suthan’s thesis, Coded Caching: Low Subpacketization Schemes and Improved Secretive Schemes.

Next generation wireless networks (5G and beyond) present the challenges of serving clients via channels which are not traditional point-to-point communication channels. The study of efficient high throughput communication techniques for broadcast channels, interference channels, multiple access channels, device-to-device (D2D) communication, all obtain relevance in the present wireless communication scenario. The technique of utilizing local storage (which has become quite affordable, thanks to advances in hardware design), either on the client’s device or in a nearby location, for aiding communication services on a broadcast channel was introduced formally in the landmark paper by AliNiesen, under the title of Coded Caching. In this paper, it was shown that a combination of (a) carefully utilizing the local storage or cache available to individual clients, and (b) coded transmissions during the delivery phase, brings tremendous gains in the rate of delivery of information of a broadcast channel. 

To obtain this large gain in the rate, the coded caching schemes in the landmark paper and state of the art literature, require that the files at the server be divisible into a large number of parts (this number is called subpacketization). In fact, most schemes require the subpacketization to be growing asymptotically as exponential in some r th root of the number of users K, with r being some positive integer. On the other extreme, few schemes having subpacketization linear in K are known; however they require extremely large number of users to exist, or they offer only little gain in rate. Because of these reasons the state of the art coded caching schemes are impractical for applications, even though they are theoretically sound. 

The first objective of this thesis is to propose a new coded caching scheme with low subpacketization and moderate rate gains utilizing projective geometries over finite fields. The proposed scheme achieve the asymptotic subpacketization, which is subexponential in K or exponential in O((log K) 2 ) (thus improving on the root-of-K exponent) and has a small cache requirement. As a special case of our proposed scheme, we get a new linear subpacketization scheme, which has a more flexible range of parameters than existing linear subpacketization schemes. Extending our techniques, we also obtain low subpacketization schemes for other multi-receiver settings such as D2D communications and distributed computing. We show numerical comparisons of our scheme with existing popular and state-of-the-art schemes, and see that our schemes achieve lower and practically relevant levels of subpacketization in many situations at the cost of higher rate (or equivalently, a lesser caching gain). 

In another line of work, secretive coded caching was proposed in the literature, where the cache content and the transmissions are required to be such that each user can decode only the requested file by that user, and no information about other files. In non-secretive coded caching, it was shown in the literature that commonality among the user demands help in reducing the rate of communication. The direct adaptation of this result to secretive coded caching is not possible because of the presence of an unique key in each transmission. The second objective of this thesis is to propose an improved (reduced rate) perfect secretive coded caching scheme by exploiting commonality among the user demands. 

The issues addressed in this thesis are summarized as follows: 

  • Based on the ideas from Projective geometries over finite fields, this thesis leverages the set containment and the subspace containment properties, to develop subexponential and linear subpacketization coded caching schemes for practical number of users and low cache sizes. 
  • This thesis extends the low subpacketization coded caching scheme developed (in this thesis) for the broadcast channel to – Coded caching in D2D networks. – Distributed computing framework. 
  • By studying the precise leakage properties of a modified scheme of a state of the art secretive coded caching scheme, this thesis develops an improved (reduced rate) secretive coded caching scheme by exploiting commonality of the user demands. 

 

Keywords: Coded caching, subpacketization, secrecy, common demands, broadcast network, D2D network, distributed computing, Projective geometries over finite fields, bipartite graph, line graphs of bipartite graphs.