Sharif Digital Repository

Many forms of diversity have been exploited in communication systems to combat fading nature of wireless channels. Some examples are time diversity, frequency diversity, space diversity and multiuser diversity. However, another form of diversity inherent in any wireless network, which is usually not exploited, is the diversity due to the shared medium of propagation. When in a network one node transmits to the other node, all the other nodes may hear this transmission and some of them can decode the data as well. This data can be used later to enhance the end-to-end network reliability. Previous works mostly consider some special cases of relay nodes configurations, not the general case. In... 

This paper studies a simultaneous weekly assignment and scheduling decisionmaking problem in operating theaters with elective patients. Because of limited recourses in hospitals, considering assignment and scheduling decisions simultaneously can help mangers exploit the available resources more efficiently and make the work-load uniformly distributed during the planning horizon. This procedure can significantly reduce hospital costs and increase satisfaction of patients and personnel. This paper formulates the mentioned problem as a Mixed Integer Linear Program (MILP) considering applicable assumptions like finite recovery beds and limitation of equipment. Since the problem is NP-hard, in... 

The railroad blocking problem is an important issue at the tactical level of railroad freight transportation. This problem consists of determining paths between the origins and destinations of each shipment to minimize the operating and user costs while satisfying the railroad supply and demand restrictions. A mixed-integer program (MIP) is developed to find the optimal paths, and a new heuristic is developed to solve the proposed model. This heuristic decomposes the model into two sub-problems of manageable size and then provides feasible solutions. We discuss the performance of the proposed heuristic for a set of instances with up to 90 stations. A comparison with the CPLEX MIP solver... 

We develop a heuristic procedure for solving the discrete time/resource trade-off problem in the field of project scheduling. In this problem, a project contains activities interrelated by finish-start-type precedence constraints with a time lag of zero, which require one or more constrained renewable resources. Each activity has a specified work content and can be performed in different modes, i.e. with different durations and resource requirements, as long as the required work content is met. The objective is to schedule each activity in one of its modes in order to minimize the project makespan. We use a scatter search algorithm to tackle this problem, using path relinking methodology as... 

Ant colony algorithms are a group of heuristic optimization algorithms that have been inspired by ants foraging for food. In these algorithms there are some agents, the ants, that for finding the suitable solution, search the solution space. Ant colony algorithms have some parameters like relative pheromone importance on trail and pheromone decay coefficient that convergence and efficiency of algorithms is highly related to them. Usually desirable value of these parameters regarding the problem is determined through trial and error. Some approaches proposed to adapt parameter of these algorithms for optimizing them. The most important feature of the proposed algorithms are complication and... 

In this paper, a design of the supply chain distributer-retailer network for a seasonal multiple-product location allocation-inventory control problem in a planning horizon consisting of multiple periods is modeled. The distance between the distributers and retailers are assumed to be Euclidean and square Euclidean while retailers purchase the products from the distributers under all-unit and incremental quantity discount policies. Furthermore, the products are delivered in packets of known size of items and in case of shortage, a fraction of demand is considered backorder and a fraction lost sale. Besides, the distributers store the manufactured products in their warehouses before... 

Train scheduling has been one of the significant issues in the railway industry in recent years since it has an important role in efficacy of railway infrastructure. In this paper, the timetabling problem of a multiple-tracked railway network is discussed. More specifically, a general model is presented here in which a set of operational and safety requirements is considered. The model handles the trains overtaking in stations and considers the stations' capacity. The objective function is to minimize the total travel time. Unfortunately, the problem is NP-hard, and real-sized problems cannot be solved in an acceptable amount of time. In order to reduce the processing time, we presented some... 

Guess-and-determine (GD) attacks are general attacks on stream ciphers, which have often been implemented in an ad hoc manner. The authors introduce a heuristic approach to the design of GD attacks, that is a dynamic programming method using a Viterbi-like algorithm which is a well-known decoding algorithm for convolutional codes. The authors also show that with this method, the resulting GD attacks, named heuristic GD (HGD) attacks, on TIPSY, SNOW1 and SNOW2 lead to less computational complexity than the previously known GD attacks. The main advantage of HGD attacks, over ad hoc GD attacks, is that while being powerful, they can be designed algorithmically for classes of stream ciphers,... 

Given an undirected graph with weights associated with its edges, the Steiner tree problem consists of finding a minimum weight subtree spanning a given subset of (terminal) nodes of the original graph. Minimum Spanning Tree Heuristic (MSTH) is a heuristic for solving the Steiner problem in graphs. In this paper we first review existing algorithms for solving the Steiner problem in graphs. We then introduce a new parallel version of MSTH on three dimensional mesh of trees architecture. We describe our algorithm and analyze its time complexity. The time complexity analysis shows that the algorithm's running time is O(lg2 n) which is comparable with other existing parallel solutions. © 2007... 

Current methods of routing are based on network information in the form of routing tables, in which routing protocols determine how to update the tables according to the network changes. Despite the variability of data in routing tables, node addresses are constant. In this paper, we first introduce the new concept of variable addresses, which results in a novel framework to cope with routing problems using heuristic solutions. Then we propose a heuristic routing mechanism based on the application of genes for determination of network addresses in a variable address network and describe how this method flexibly solves different problems and induces new ideas in providing integral solutions... 

This paper presents a priority based dynamic programming approach for long term maintenance scheduling of overhead distribution networks. The proposed approach is based on risk management approach and utilizes the model of decoupled risk factors. Two heuristic factors are defined and utilized in order to establish a maintenance priority list and to curtail the dynamic programming search space. The proposed methodology yields a significant computational saving compare to the previously reported dynamic programming. Risk management approach enables the asset managers to consider the actual condition of electrical equipments and expected consequence of their failures. Furthermore, the decoupled... 

The advent of various real-time multimedia applications in high-speed networks creates a need for quality of service (QoS) based multicast routing. Two important QoS constraints are the bandwidth constraint and the end-to-end delay constraint. The QoS based multicast routing problem is a known NP-complete problem that depends on (1) bounded end-to-end delay and link bandwidth along the paths from the source to each destination, and (2) minimum cost of the multicast tree. In this paper we describe a new representation, called node parent index (NPI) representation, for representing trees and describe harmony operations accord to this representation. The presented algorithm is based on the...