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Locally vs. Globally Optimized Flow-Based Content Distribution to Mobile Nodes
Mhameed Aezladen, Reuven Cohen and Danny Raz
Submitted for journal publication. An earlier version was presented in: Infocom’2009
bibtex
Abstract: The paper deals with efficient distribution of timely information to flows of mobile devices. We consider the case where a set of Information Dissemination Devices (IDDs) broadcast a limited amount of information to passing mobile nodes that are moving along well-defined paths. This is the case, for example, in intelligent transportation systems. We develop a novel model that captures the main aspects of the problem, and define a new optimization problem we call MBMAP (Maximum Benefit Message Assignment Problem). We study the computational complexity of this problem in the global and local cases, and provide new approximation algorithms.

Cross-Layer Hybrid FEC/ARQ Reliable Multicast with Adaptive Modulation and Coding in Broadband Wireless Networks
Reuven Cohen, Guy Grebla and Liran Katzir
Published in: research/ACM Transactions on Networking, Vol. 18, No. 6, Dec. 2010, pp. 1908 — 1920. An earlier version was presented in: Infocom’2009
bibtex
Abstract: In this paper we define and address a {\em new problem} that arises when a base station in a broadband wireless network wishes to multicast information to a large group of nodes and to guarantee some level of reliability using Application layer FEC codes. Every data block to be multicast is translated into a sequence of $K+n$ packets, from which every receiver must receive at least $K$ in order to correctly decode the block. The new problem is to determine which PHY layer MCS (Modulation and Coding Scheme) the base station should use for each packet. We present several variants of this problem, which differ in the number of ARQ (Automatic Repeat reQuest) rounds during which the delivery of a data block must be completed. Most of these variants are shown to be NP-hard. However, we present optimal solutions for practical instances, where the number of MCSs is small, and efficient approximations and heuristics for the general case of each variant.

“Not All At Once!” — A Generic Scheme for Estimating the Number of Affected Nodes
Reuven Cohen and Alexander Landau
Submitted for journal publication. An earlier version was presented in: Infocom’2009 mini-conference
bibtex
Abstract: We present a generic scheme for estimating the size of a group of nodes affected by the same event in a large-scale network, such as a grid, a sensor network or a wireless broadband access network, while receiving only a small number of feedback messages from this group. Using the proposed scheme, a centralized gateway analyzes the transmission times of these feedback messages, defines a likelihood function for them, and then uses the Newton-Raphson method to find the number of affected nodes for which this function is maximized. We present complete mathematical analysis for the precision of the proposed algorithm and provide tight upper and lower bounds for the estimation error. These bounds allow us to improve the precision of our estimation, and to bring the error very close to 0.

Topology Maintenance in Asynchronous Sensor Networks
Reuven Cohen and Boris Kapchits
Published in: research/ACM Transactions on Networking, Vol. 19, No. 1, Feb. 2011 . An earlier version was presented in: SECON’2008.
bibtex
Abstract: In most sensor networks the nodes are static. Nevertheless, the node connectivity is subject to changes because of disruptions in wireless connectivity, transmission power changes, or loss of synchronization between neighboring nodes. Hence, even after a sensor is aware of its immediate neighbors, it must continuously maintain its view, a process we call topology maintenance. This work is the first to distinguish between neighbor discovery during sensor network initialization and topology maintenance. Whereas many works focus on the former task, we focus on the latter. We view topology maintenance as a joint task of all the connected sensors. Each sensor employs a simple protocol in a coordinate effort to reduce power consumption without increasing the time required to detect hidden sensors.

Computational Analysis and Efficient Algorithms for Micro and Macro OFDMA Scheduling
Reuven Cohen and Liran Katzir
Published in: research/ACM Transactions on Networking, Vol. 18, No. 1, February 2010. An earlier version was presented in: Infocom’2008
bibtex
Abstract: OFDMA is one of the most important modulation and access methods for the future mobile networks. Before transmitting a frame on the downlink, an OFDMA base station has to invoke an algorithm that determines which of the pending packets will be transmitted, what modulation should be used for each of them, and how to construct the complex OFDMA frame matrix as a collection of rectangles that fit into a single matrix with fixed dimensions. This paper proposes a scheme that solves this intricate OFDMA scheduling problem by breaking it down into two sub-problems, referred to as macro and micro scheduling.We analyze the computational complexity of these sub-problems and develop efficient algorithms for solving them.

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