SCIENTIA IRANICA
Year:2006 | Volume:13 | Issue:1|Papers Count:11

This paper addresses minimizing Total Weighted Earliness/Tardiness (TWET) of jobs in a Flexible Job Shop (FJS) problem. The FJS problem is an extension of the classical Job Shop (JS) problem that implies each operation may be assigned to alternative available machines. So, a job may have alternative routing. The FJS problem with a TWET criterion is modeled as a mixed integer programming. The model is proven to be Np-complete. To solve the model, an algorithm, based on a Tabu Search approach (TS), is developed. The proposed algorithm employs TS to find the best routing of each job and a backward procedure to operations scheduling. Two neighboring functions are designed and their effect is investigated on the performance. The numerical experiments show the suggested algorithm efficiently solves the model in a reasonable CPU time.    

A fuzzy queuing coherent hierarchical location-allocation model is developed for congested systems. The parameters of the model are approximately evaluated and stated as fuzzy-numbers. The coverage of demand nodes is also considered in an approximate manner and is stated by the degree of membership. Using the queuing theory and fuzzy conditions, a coherent hierarchical model is developed for the Maximal Covering Location Problem (MCLP). An example problem is solved and presented, along with results. Conclusions and future extensions are also included.    

In this paper, the problem of finding the shortest path between two points in the presence of single-point visibility constraints is studied. In these types of constraint, there should be at least one point on the output path from which a fixed viewpoint is visible. The problem is studied in various domains, including simple polygons, polygonal domains and polyhedral surfaces. The method is based on partitioning the boundary of the visibility region of the viewpoint into a number of intervals. This is done from the combinatorial structure of the shortest paths from the source and destination to the points on the boundary. The result for the case of simple polygons is optimal with O(n) time bound. The running time for the cases of polygonal domains and convex and non-convex polyhedral surfaces are O(n2), O(n2) and O(n3), respectively.    

A unified multiple-access performance analysis and comparison of three multicarrier spread-spectrum multiple-access schemes, namely, MC-CDMA (Multicarrier Code-Division Multiple-Access), MC-FH (Multicarrier Frequency Hopping) and a hybrid of the above systems, called DS-MC-FH (Direct-Sequence MC-FH), in a multirate environment, where each user can have several multirate services, is provided. In MC-CDMA and MC-FH systems, users and their diverse services are differentiated by means of only one kind of signature code. However, in a DS-MCFH scheme, different users and different services of the same user are distinguished through the first and second signature codes, respectively. The performance of the above systems is evaluated and compared, using a unified structure in synchronous and asynchronous nonfading and synchronous correlated Rayleigh fading channels, with a Maximum-Ratio Combining (MRC) receiver. The near-far effect on the systems' performance is also investigated. The (second) signature in the MC-CDMA (DS-MC-FH) scheme is considered to be either a Pseudo-Noise (PN) sequence or a Walsh code. The authors analyses indicate that MC-CDMA systems with Walsh codes outperform the other schemes in different synchronous and asynchronous channels. DS-MC-FH systems with Walsh codes always surpass MC-FH systems. Furthermore, all of the schemes, except synchronous MC-CDMA systems with Walsh codes, are susceptible to a near-far effect with an MRC receiver    

In this paper, two methods to solve multi-response statistical problems are presented. In these methods, desirability function, genetic algorithm and simulation methodology are applied. The desirability function is responsible for modeling the multi-response statistical problem, the genetic algorithm tries to optimize the model and, finally, the simulation approach generates the required input data from a simulated system. The methods differ from each other in controlling the randomness of the problem. In the first method, replications control this randomness, while, in the second method, the randomness is controlled by a statistical test. Furthermore, these methods are compared by designed experiments and the results are reported.  

This paper presents the results on the development of a novel micromachined parallel plate tunable capacitor with a wide tuning range. Different from conventional parallel plate capacitors, this novel tunable capacitor consists of one suspended top plate, suspending with four oblique arms and one bottom fixed plate. These oblique arms increase the length of the cantilevers, in order to cause more deflection and, hence, increase the tuning range of the capacitor for constant bias voltage. Applying a DC voltage between two plates provides electrostatic actuation for capacitance tuning. According to electromagnetic simulation. a tuning range of 146% has been achieved with a 1.7 volt bias voltage. An empirical behavioral model is extracted from simulated y parameters of the structure  

In this paper, the performances of both uncoded and coded multiple access TH-UWB systems,introduced in [1-3] in multipath Rayleigh fading channels, are evaluated. The receiver is a selective diversity combining receiver, known as SRake. Based on a Gaussian distribution assumption for the multiple access interference at the output of the SRake receiver and by using a virtual branch technique, as introduced in [4,5] the bit error rates for uncoded and coded schemes are derived. The performance analysis shows that the effective order of diversity achieved by the coded scheme is the product of the number of branches of the SRake receiver and the Hamming distance of the code applied. Furthermore, the results indicate that the coded scheme significantly outperforms the uncoded scheme without requiring any extra bandwidth, further than what is required by the uncoded system.    

In this paper, the designing of a parallel channel queuing system is considered in which it is desirable to serve the customers immediately upon their arrival. To design such a service facility, the first and the second moments of the number of customers are obtained in an infinite channel server under the assumption that the arrival process has a general probability structure and the service times of customers are independent and identically distributed random variables. With the knowledge about the above two moments, one can obtain the approximate number of channels such that the queue length becomes zero.    

This paper presents an algorithmic approach to solving the problem of excessive travel in C.N.C.machine tools, by introducing an efficient method to compute the shortest path between given sets of points (origin and destination) in an R2(x,y) plane. When a work piece is located (as an obstacle) between sets of points, it is proved that the optimum path between these points would be formed by sequences of connected straight line segments whose intermediate end points are vertices of an appropriate polygonal (closed control barrier). The case of one origin, one destination and a set of barriers is considered. This method is computationally efficient.    

State evaluation of universities is important for administrators to serve as a basis for performance monitoring, policy formulations and distribution of funds. In this paper, a formal methodology was proposed to build a rational basis for evaluation and ranking of Iranian state run universities. In the first step, the entropy method from MCDM is applied and tested using real data from 47 state universities. Poor results of entropy led to the development of a new nonlinear programming formulation. This new model was then reduced to an interactive linear programming formulation in order to solve the problem. Real data is used to test and validate our methodology.    

The inventory system under consideration consists of one central warehouse and an arbitrary number of retailers controlled by a continuous review inventory policy (R, Q). Independent Poisson demands are assumed with constant transportation times for all retailers and a constant lead time for replenishing orders from an external supplier for the warehouse. Unsatisfied demands are assumed to be lost in the retailers and unsatisfied retailer orders are backordered in the warehouse. An approximate cost function is developed to find optimal reorder points for given batch sizes in all installations and the related accuracy is assessed through simulation.