MODARES TECHNICAL AND ENGINEERING
Year:2006 | Volume:- | Issue:22|Papers Count:10

Full wave analysis plays a key role in the accurate modeling of microwave and millimeter-wave structures. Using a suitable full wave analysis with an efficient optimization technique provides enough accuracy to realize microwave structures. This article addresses the design of microwave and millimeter-wave filter and diplexers using full wave analysis with emphasis on an optimization technique. A new design method based on mode-matching and genetic algorithm is presented. The accuracy of the method is its main advantage. A design algorithm based on analytical modeling is developed. The results of software have been used to design a microwave filter in the of 28GHz frequency band. The measurement results confirm the accuracy and efficiency of the proposed method.      

In this paper, we examine the effect of the energy difference between the L- and the D-valleys in compound semiconductor materials, carrier effective mass, and the scattering processes on the electron transport characteristics in MESFETs. To do this, we use the Monte Carlo simulation to demonstrate the superiority of the InGaAs MESFET, made on a semi-insulating InP substrate, over both InP and GaAs MESFETs. Furthermore, we study the effects of device structure on the electron transport characteristics. For the first time we study electron transport characteristics in the channel of a LDD InGaAs MESFET with an InP source. This structure demonstrates to have the highest average electron velocity through out its channel among the other MESFETs.      

In a voice conversion system speech signal of A speaker (i.e. source speaker) is modified so that it sounds as if it had been pronounced by B speaker (i.e. target speaker). This process, sometimes, is called speaker conversion (changing speaker identity). Achieved signal from speaker conversion system is desired to have high quality and very natural. To satisfy this, three major methods are proposed as follows: VQ_based, LMR_based and GMM_based voice conversion methods. DTW is the most popular way to warp corresponded words in two sentences. In this paper, DTW is used to design corresponding transfer function. To decrease the distance between two speakers, DTW warps the couple phonemes of two speakers, instead of two words or couple sentences while a linear temporal transform which depends on phonemes is used for error decreasing. By using other appropriate corrections that are used in learning and designing of the linear transforms, a high quality voice conversion system is achieved.    

In this paper, we used a shape matching algorithm to recognize Farsi digits. For each sampled point on the contour of a shape, we obtain a descriptor showing the distribution of the other points of the contour, with respect to this point. Based on these descriptors, we find the corresponding points of the two contours and take the sum of their distances as a dissimilarity measure between two shapes. Then we define a geometric transformation that maps the sampled points of the one shape to the corresponding points of the other shape. The bending energy of this transform is taken as the second dissimilarity measure between two shapes. We optimized the parameters of the matching algorithm for the recognition of Farsi digits and used the method of minimum distance from the class prototypes for the recognition. In a test on a set of 1288 digits, we obtained a recognition rate of 89.9%. This result was obtained without any post processing.      

In this paper, we show how to obtain suitable differential characteristics for block ciphers with neural networks. We represent the operations of a block cipher, regarding their differential characteristics, through a directed weighted graph. In this way, the problem of finding the best differential characteristic for a block cipher reduces to the problem of finding the minimum-weight multi-path way between two known nodes in the proposed graph. We applied Hopfield network to find the minimum-weight multi-path way. In this technique, the probability of convergence to a local minimum increases when the number of rounds of the cipher increases. We also applied Boltzmann machine to avoid local minima. We applied these techniques to find 3-round, 4-round and 5-round differential characteristics of Serpent block cipher, and repeated the optimization procedures for each characteristics 100 times. With Hopfield network, we obtained suitable results 100, 20 and 1 times for 3-round, 4-round and 5-round of the Serpent respectively. With Boltzmann machine, we obtained suitable results 100, 99 and 30 times for 3-round, 4- round and 5-round of the Serpent respectively. These results show that simulated annealing help avoiding the many local minima of energy function. We compare the probabilities of our obtained differential characteristics for Serpent with the probabilities of eight differential characteristics previously reported in other papers. The comparison shows that our proposed technique obtains better results in 6 cases, and the same results in 2 cases. We also found a 7-round differential characteristic with a probability of 2-125 with Boltzmann machine. Neglecting the reported Bommerang differential characteristics of Serpent, our obtained 7-round differential characteristic is the first report on a differential characteristic for more than 6 rounds of this cipher. The results of experiments indicate the efficiency of neural networks to find suitable differential characteristics of block ciphers.    

Gas sensitive metal oxide layers used in fabrication of resistive gas sensors are prepared by different deposition techniques. The technical data reported on some basic and practically important specifications of these devices, although fabricated based on the same gas sensitive oxide, are anomalously different. The influence of the fabrication technique used for the deposition of the gas sensitive layer on determination of the significant specifications of the transient response of a resistive gas sensor is experimentally investigated for the first time. ZnO and SnO2 layers were prepared by LPCVD, PVD, EPD and powder pressing techniques. Prototype gas sensors based on these layers were fabricated. The transient responses of these devices to a step change in the composition of the surrounding atmosphere were recorded and compared. It was shown that the thickness, porosity and pore micro-structure of the gas sensitive layer are the most effective parameters in determination of the transient response. The relationship between these parameters and the temporal variation of the electrical conductivity of the gas sensitive layer was qualitatively analyzed. Oxide layers of higher porosity resulted in gas sensors of faster response, but response time increased with the thickness of these gas sensitive layers. The sensors produced by EPD technique demonstrated the fastest responses while those produced by CVD were the slowest among the samples investigated.    

Different methods exist for jamming mitigation and we should choose a method based the jammer type and other parameters. One of the jammers is narrow band FM jammer and we use subspace projection techniques for the suppression of this type of jammer. In subspace projection technique, we estimate the IF of signals and construct the subspace vector that is orthogonal to jammer vector by increasing the number of antennas in the array; we enhance the performance of system. The subspace projection method was simulated in MATLAB and the receiver output SINR in several conditions was calculated. In the case of IF estimation error, the SINR of the system is reduced; in this condition the use of an antenna array will increase the performance of the system compared to a single antenna.    

In this paper, the performance of 11 different distances for image retrieval and classification, based on color, shape and texture, is evaluated. The precision-recall measure and the correct classification rate of the k-NN classifier are used to evaluate retrieval and classification performances, respectively. The experimental results for a database of 1000 images from 10 different semantic groups, based on color histogram, directional edge histogram and Gabor features are presented and discussed.