Creating and sharing knowledge for telecommunications

Complexity of Covering Problems on Hypergraphs

on 21-10-2016

... Bruno Coutinho (Instituto de Telecomunicações)

Date & time: 21/10/2016 at 16:00.
Location: Room P3.10, Mathematics Building, Instituto Superior Técnico, Lisbon.

A hypergraph is a natural generalization of a graph, here an edge (often called hyperedge) can simultaneously onnect any number of vertices. The fact that hyperedges can connect more than two vertices facilitates a more faithful representation of many real-world networks. For example, given a set of proteins and a set of protein complexes, the corresponding hypergraph naturally captures the information on proteins that interact together within a protein complex. For a biochemical reaction system, the hypergraph representation indicates which biomolecules participate in a particular reaction. Collaboration networks can also be represented by a hypergraph, where vertices represent individuals and hyperedges connect individuals who were involved in a specific collaboration, e.g. a scientific paper, a patent, a consulting task, or an art performance.

The core of a graph - defined as the remainder of the greedy leaf removal procedure where leaves (vertices of degree one) and their neighbors are removed iteratively from the graph - has been related to the conductor-insulator transition, structural controllability, and many combinatorial optimization problems. In fact, the size of the core is related to a fundamental combinatorial issue: the computational complexity of the minimum vertex cover (MVC) problem. The MVC aims to find the smallest set of vertices in a graph (or hypergraph) so that every edge (or hyperedge) is incident to at least one node in the set.

I will talk about two generalizations of the core in graphs to hypergraphs, one associated with the edge-cover problem and another associated with the vertex-cover problem. We found that these two cores tend to be very small in real-world hypergraphs. This result indicates that the hyperedge and vertex cover problems in real-world hypergraphs can actually be solved in polynomial time.

Physics of Information Seminar

Supported with funding from FCT, FEDER and EU FP7, namely via projects PEst-OE/EEI/LA0008/2013, UID/EEA/50008/2013, IT/QuSim, CQVibes, and PAPETS (323901).
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Advanced Antenna Systems for 21st Century Satellite Communication Payloads

on 27-09-2016

... Sept 21st century has so far seen several new satellite services such as local-channel broadcast for direct broadcast satellite service (DBS), high capacity K/Ka-band personal communication satellite (PCS) service, hosted payloads, mobile satellite services using very large deployable reflectors, high power hybrid satellites etc. All these satellite services are driven by the operators need to reduce the cost of satellite and pack more capability into the satellite. Antenna sub-system design, mechanical packaging on the spacecraft, and RF performance become very critical for these satellites. This talk will cover recent developments in the areas of antenna systems for FSS, BSS, PCS, & MSS satellite communications. System requirements that drive the antenna designs will be presented initially with brief introduction to satellite communications. Reflector and array antenna designs will be covered in this talk.

Advanced antenna system designs for contoured beams, multiple beams, and reconfigurable beams will be presented. Contoured beam antennas using dual-gridded reflectors, shaped single reflectors, and shaped Gregorian reflectors will discussed. Multiple beam antenna (MBA) concepts and their advantages compared to conventional contoured beams will be introduced. Various designs of the MBA for DBS, PCS, and MSS services will be discussed along with practical examples. Recent advances in feed technology and reflector technology will be addressed and few examples. Advances in multi-band antennas covering multiple bands will be presented. Topics such as antenna designs for high capacity satellites, large deployable mesh reflector designs, low PIM designs, and power handling issues will be included. Advanced high power test methods for the satellite payloads will be addressed. Brief introductions to TT&C antennas, passive inter modulation products (PIM) and multipaction for satellite payloads will be given. Future trends in the satellite antennas will be discussed. At the end of this talk, engineers will be exposed to typical requirements, designs, hardware, software, and test methods for various satellite antennas.
Tuesday, September 27, 14:30
Instituto Superior Técnico
Room LT2, Torre Norte, 4th Floor
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