Reuven Cohen

An efficient approximation for the generalized assignment problem

We present a simple family of algorithms for solving the Generalized Assignment Problem (GAP). Our technique is based on a novel combinatorial translation of any algorithm for the knapsack problem into an approximation algorithm for GAP. If the approximation ratio of the knapsack algorithm is α and its running time is O(f(N)), our algorithm guarantees a (1 + α)-approximation ratio, and it runs in O(M ċ f(N) + M ċ N), where N is the number of items and M is the number of bins. Not only does our technique comprise a general interesting framework for the GAP problem; it also matches the best combinatorial approximation for this problem, with a much simpler algorithm and a better running time.

Restricted dynamic Steiner trees for scalable multicast in datagram networks

The paper addresses the issue of minimizing the number of nodes involved in routing over a multicast tree and in the maintenance of such a tree in a datagram network. It presents a scheme where the tree routing and maintenance burden is laid only upon the source node and the destination nodes associated with the multicast tree. The main concept behind this scheme is to view each multicast tree as a collection of unicast paths and to locate only the multicast source and destination nodes on the junctions of their multicast tree. The paper shows that despite this restriction, the cost of the created multicast trees is not necessarily higher than the cost of the trees created by other algorithms that do not impose the restriction and therefore require all nodes along the data path of a tree to participate in routing over the tree and in the maintenance of the tree.

An optimal wake-up scheduling algorithm for minimizing energy consumption while limiting maximum delay in a mesh sensor network

This paper presents an algorithm for maximizing the lifetime of a sensor network while guaranteeing an upper bound on the end-to-end delay. We prove that the proposed algorithm is optimal and requires simple computing operations that can be implemented by simple devices. To the best of our knowledge, this is the first paper to propose a sensor wake-up frequency that depends on the sensors location in the routing paths. Using simulations, we show that the proposed algorithm significantly increases the lifetime of the network while guaranteeing a maximum on the end-to-end delay.

A unicast-based approach for streaming multicast

Network layer multicast is know as the most efficient way to support multicast sessions. However, for security, QoS and other considerations, most of the real-time application protocols can be better served by upper layer (transport or application) multicast. We propose a scheme called M-RTP for multicast RTP sessions. The idea behind this scheme is to set up the multicast RTP session over a set of unicast RTP sessions, established between the various participants (source and destinations) of the multicast session. We then address the issue of finding a set of paths with maximum bottleneck for an M-RTP session. We show that this problem is NP-complete, and propose several heuristics to solve it.

Computational analysis and efficient algorithms for micro and macro OFDMA downlink scheduling

Orthogonal frequency-division multiple access (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. We propose efficient algorithms, with performance guarantee, that solve this intricate OFDMA scheduling problem by breaking it down into two subproblems, referred to as macro and micro scheduling. We analyze the computational complexity of these subproblems and develop efficient algorithms for solving them.

High-speed Internet access through unidirectional geostationary satellite channels

One of the proposed solutions for increasing the speed of Internet access is to connect the home user to a direct satellite channel, at a speed 20 times faster than that of an average telephone modem. Communication over satellite links is often characterized by sporadic high bit-error rates and burst losses. This is especially true when working in the Ka band, where weather conditions greatly affect link availability. Under such conditions, the TCP protocol that is predominantly used by data applications degrades dramatically in performance. Using simulations, this paper studies the performance of TCP under different network conditions. Several modifications, that take advantage of the special properties of the satellite channel, are proposed, and a new sender algorithm which can efficiently handle burst losses is presented. The main attractiveness of the proposed new sender algorithm is that it can be implemented only at the satellite ground station, rather than at every server in the world.

Continuous neighbor discovery in asynchronous sensor networks

In most sensor networks, the nodes are static. Nevertheless, node connectivity is subject to changes because of disruptions in wireless communication, 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 continuous neighbor discovery. In this work, we distinguish between neighbor discovery during sensor network initialization and continuous neighbor discovery. We focus on the latter and view it as a joint task of all the nodes in every connected segment. Each sensor employs a simple protocol in a coordinate effort to reduce power consumption without increasing the time required to detect hidden sensors.

Cross-layer hybrid FEC/ARQ reliable multicast with adaptive modulation and coding in broadband wireless networks

In this paper, we define and address a 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 forward error correction (FEC) codes. Every data block to be multicast is translated into a sequence of packets, from which every receiver must receive at least in order to correctly decode the block. The new problem is to determine which PHY-layer modulation and coding scheme (MCS) the base station should use for each packet. We present several variants of this problem, which differ in the number of automatic repeat request (ARQ) 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.

PCRTT Enhancement for Off-Line Video Smoothing

An enhancement of the Piecewise Constant Rate Transmission and Transport (PCRTT) algorithm for reducing the burstiness of a video stream, based on smoothing constant interval is proposed. The new algorithm, called e-PCRTT, relies on geometrical consideration rather than traditional rate-control analysis. E-PCRTT is shown to construct transmission rate-plans with smaller buffer sizes, as compared to the original PCRTT. Alternatively, for the same buffer size, e-PCRTT reduces the number of bandwidth changes as compared to PCRTT. In addition, e-PCRTT produces a rate-plan that requires a smaller initial playback delay.

Computational Analysis and Efficient Algorithms for Micro and Macro OFDMA Scheduling

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. We propose efficient, and theoretically best possible, algorithms 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.