Papers by Hooshang Ghasemi
In this paper, we address the matrix completion problem and propose a novel algorithm based on a ... more In this paper, we address the matrix completion problem and propose a novel algorithm based on a smoothed rank func-tion (SRF) approximation. Among available algorithms like FPCA and OptSpace, there is no solution that can simulta-neously cover wide range of easy and hard problems. This new algorithm provides accurate results in almost all scena-rios with a reasonable run time. It especially has low execu-tion time in hard problems where other methods need long time to converge. Furthermore, when the rank is known in advance and is high, our method is very faster than previous methods for the same accuracy. The main idea of the algo-rithm is based on a continuous and differentiable approxi-mation of the rank function and then, using gradient projec-tion approach to minimize it. Index Terms — Matrix completion, nuclear norm,
2017 IEEE International Symposium on Information Theory (ISIT), Jun 1, 2017
Coded caching is a technique that promises huge reductions in network traffic in content-delivery... more Coded caching is a technique that promises huge reductions in network traffic in content-delivery networks. However, the original formulation and several subsequent contributions in the area, assume that the file requests from the users are synchronized, i.e., they arrive at the server at the same time. In this work, we formulate and study the coded caching problem when the file requests from the users arrive at different times. We assume that each user also has a prescribed deadline by which they want their request to be completed. In the offline case, we assume that the server knows the arrival times before starting transmission and in the online case, the user requests are revealed to the server over time. We present a linear programming formulation for the offline case that minimizes the overall transmission rate from the server subject to the constraint that each user meets his/her deadline. While the online case is much harder, we introduce a novel heuristic for it and show that under certain conditions, with high probability the request of each user can be satisfied with her/his deadline. Our simulation results indicate that in the presence of mild asynchronism, much of the benefit of coded caching can still be leveraged.
Caching is often used in content delivery networks as a mechanism for reducing network traffic. R... more Caching is often used in content delivery networks as a mechanism for reducing network traffic. Recently, the technique of coded caching was introduced whereby coding in the caches and coded transmission signals from the central server were considered. Prior results in this area demonstrate that carefully designing the placement of content in the caches and designing appropriate coded delivery signals from the server allow for a system where the delivery rates can be significantly smaller than conventional schemes. However, matching upper and lower bounds on the transmission rate have not yet been obtained.
The original formulation of the coded caching problem assumes that the file requests from the use... more The original formulation of the coded caching problem assumes that the file requests from the users are synchronized, i.e., they arrive at the server at the same time. Several subsequent contributions work under the same assumption. Furthermore, the majority of prior work does not consider a scenario where users have deadlines. In our previous work we formulated the asynchronous coded caching problem where user requests arrive at different times. Furthermore, the users have specified deadlines. We proposed a linear program for obtaining its optimal solution. However, the size of the LP (number of constraints and variables) grows rather quickly with the number of users and cache sizes. In this work, we explore a dual decomposition based approach for solving the LP under consideration. We demonstrate that the dual function can be evaluated by equivalently solving a number of minimum cost network flow algorithms. Minimum cost network flow algorithms have been the subject of much invest...
Coded caching is a technique for reducing peak data rate in content delivery systems that employ ... more Coded caching is a technique for reducing peak data rate in content delivery systems that employ caching. The original formulation of the coded caching problem assumes that the file requests from the users are synchronous, i.e., they arrive at the same time. In this work, we consider the asynchronous setting where the file requests are revealed to the server in an online fashion. We propose a novel online algorithm for this problem building on our prior work for the offline setting (where the server knows the request arrival times and deadlines in advance). Our simulation results demonstrate that our proposed online algorithm allows for a natural tradeoff between the feasibility of the schedule and the rate gains of coded caching.
Coded caching is a technique that promises huge rate savings in certain canonical content distrib... more Coded caching is a technique that promises huge rate savings in certain canonical content distribution scenarios over the Internet. In the coded caching setting, previous contributions have demonstrated a constant multiplicative gap between the achievable rate and corresponding lower bound on the rate, independent of the problem parameters. Our prior work demonstrated that good lower bounds on the coded caching rate can be obtained by equivalently considering a combinatorial problem on a directed tree. In this work, we study certain structural properties of our algorithm that allow us to analytically quantify improvements on the rate lower bound. This analysis allows us to obtain a multiplicative gap of at most four between the achievable rate and our lower bound. To our best knowledge, this is the best known multiplicative gap known for this problem.
2017 51st Asilomar Conference on Signals, Systems, and Computers, Oct 1, 2017
The original formulation of the coded caching problem assumes that the file requests from the use... more The original formulation of the coded caching problem assumes that the file requests from the users are synchronized, i.e., they arrive at the server at the same time. Several subsequent contributions work under the same assumption. Furthermore, the majority of prior work does not consider a scenario where users have deadlines. In our previous work we formulated the asynchronous coded caching problem where user requests arrive at different times. Furthermore, the users have specified deadlines. We proposed a linear program for obtaining its optimal solution. However, the size of the LP (number of constraints and variables) grows rather quickly with the number of users and cache sizes. In this work, we explore a dual decomposition based approach for solving the LP under consideration. We demonstrate that the dual function can be evaluated by equivalently solving a number of minimum cost network flow algorithms. Minimum cost network flow algorithms have been the subject of much investigation and current solvers routinely solve instances with millions of nodes in minutes. Our proposed approach leverages these fast solvers and allows us to solve several large scale instances of the asynchronous coded caching problem with manageable time and memory complexity.
IEEE/ACM Transactions on Networking
2015 IEEE International Symposium on Information Theory (ISIT), 2015
Content delivery networks often employ caching to reduce transmission rates from the central serv... more Content delivery networks often employ caching to reduce transmission rates from the central server to the end users. Recently, the technique of coded caching was introduced whereby coding in the caches and coded transmission signals from the central server are considered. Prior results in this area demonstrate that (a) carefully designing placement of content in the caches and (b) designing appropriate coded delivery signals allow for a system where the delivery rates can be significantly smaller than conventional schemes. However, matching upper and lower bounds on the transmission rates have not yet been obtained. In this work, we derive tighter lower bounds on coded caching rates than were known previously. We demonstrate that this problem can equivalently be posed as one of optimally labeling the leaves of a directed tree. Several examples that demonstrate the utility of our bounds are presented.
2011 IEEE International Conference on Acoustics, Speech and Signal Processing (ICASSP), 2011
In this paper, we address the matrix completion problem and propose a novel algorithm based on a ... more In this paper, we address the matrix completion problem and propose a novel algorithm based on a smoothed rank function (SRF) approximation. Among available algorithms like FPCA and OptSpace, there is no solution that can simultaneously cover wide range of easy and hard problems. This new algorithm provides accurate results in almost all scenarios with a reasonable run time. It especially has low execution time in hard problems where other methods need long time to converge. Furthermore, when the rank is known in advance and is high, our method is very faster than previous methods for the same accuracy. The main idea of the algorithm is based on a continuous and differentiable approximation of the rank function and then, using gradient projection approach to minimize it.
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Papers by Hooshang Ghasemi