Sharif Digital Repository

The use of Spread-Time CDMA for ultrawideband (UWB) wireless communication is proposed and its capability for supporting many high data rate users is investigated. Furthermore we will study a hybrid technique, namely, Spread-Time/Time-Hopping to increase the capacity of the above mentioned technique and simultaneously minimizing the effect of near-far in a typical CDMA system  

Recently, a group of novel devices based on conducting interfaces has been proposed. These conducting interfaces can be implemented, for example, by using the inversion layer of MOS structures, trapped charges, or depletion layer charges. It has been shown that these structures can support surface electromagnetic waves. In this paper, the coupling of surface electromagnetic waves supported by conducting interfaces is first analysed. Next, by direct solution of Maxwell's equations and applying appropriate boundary conditions; exact analytical formulation of the dispersion equation has also been derived. Results driven by a coupled mode theory approach are justified by solving the exact... 

In this paper we examine demodulation methods for nonuniform Pulse Position Modulation (PPM), which is generated by the crossings of a modulating signal with a sawtooth wave. Here, we are interested in PPM at the Nyquist rate and illustrate the efficiency of the iterative methods. Several iterative methods such as Wiley/Marvasti, Time-varying, adaptive weight, Zero-order-hold, Voronoi, linear interpolation algorithms are discussed, where the PPM demodulation is converted as a nonuniform sampling reconstruction problem. Also another approach called the inverse system approach is introduced and the performance of the two algorithms is assessed. © Springer-Verlag 2004  

In this paper, we propose a novel method for demodulating Pulse Position Modulation (PPM) signals based on Reed-Solomon decoding algorithm. It is shown that the proposed technique outperforms the conventional demodulating method base on Reed-Solomon algorithm in case that the PPM signals are corrupted by Additive White Gaussian Noise (AWGN). © Springer-Verlag 2004  

In several recent papers, layered waveguide structures with conductive interfaces have been discussed, whose propagation properties are controlled by a transverse voltage. These structures have wide range of applications. In this paper, surface wave excitation on one-dimensional photonic crystals at the presence of interface conductivity is discussed. It is shown that excitation of electromagnetic surface waves can be controlled by the interface conductivity. This property can be used in constructing miscellaneous optical devices such as optical modulators, switches and tunable optical filters. Devices based on these waves can be superior to surface plasmon waves since they are not lossy.... 

In this paper, a new integrated programmable diffractive optical element is described, which is composed by stacking of the plasma wave optical modulator/switch structure1. This structure has the possibility of high density integration by means of the state-of-the-art fabrication technologies. The device has the in-design capability to be either programmed electrically or optically. Both the optical and electrical programming require transverse DC biases across the waveguide modulators. Programming time in both approaches could be as low as 10ps. As proposed in the paper, the device has numerous applications in different areas including decision making, pattern recognition, high density... 

In this report, the time-dependent carrier concentration in electrolytic cells, due to some basic periodic shapes, has been studied. Exact analytical solutions for step, sinusoidal and periodic pulse reverse inputs are derived. The effect of frequency, forward and reverse amplitudes, and pulse modulation width are investigated. Analytical solution of the model is obtained as a Fourier series, using Laplace transformations applied to a one-dimensional model with linearized Butler-Volmer electrode kinetics. The results are applied to an acid-copper bath and discussed. It is shown that there exist optimal operation points or optimal current waveforms to obtain efficient and enhanced deposition... 

In this article we have acquired exact solitary wave solutions for the truncated Ω- fractional Lakshamanan–Porsezian–Daniel model with Kerr, parabolic, and anti-cubic nonlinear laws employing extended auxiliary technique. Diverse set of exponential function solutions acquired relying on a map between the considered equation and an auxiliary ODE. Obtained solutions are recast in several hyperbolic and trigonometric forms based on different restrictions between parameters involved in equations and integration constants that appear in the solution. A few significant ones among the reported solutions are pictured to perceive the physical utility and peculiarity of the considered model using... 

Integrated treatment planning for cancer patients has high importance in intensity modulated radiation therapy (IMRT). Direct aperture optimization (DAO) is one of the prominent approaches used in recent years to attain this goal. Considering a set of beam directions, DAO is an integrated approach to optimize the intensity and leaf position of apertures in each direction. In this paper, first, a mixed integer-nonlinear mathematical formulation for the DAO problem in IMRT treatment planning is presented. Regarding the complexity of the problem, two well-known metaheuristic algorithms, particle swarm optimization (PSO) and differential evolution (DE), are utilized to solve the model. The... 

Optical Neural Networks (ONNs) can be promising alternatives for conventional electrical neural networks as they offer ultra-fast data processing with low energy consumption. However, lack of suitable nonlinearity is standing in their road of achieving this goal. While this problem can be circumvented in feed-forward neural networks, the performance of the recurrent neural networks (RNNs) depends heavily on their nonlinearity. In this paper, we first propose and numerically demonstrate a novel reconfigurable optical activation function, named ROA, based on adding or subtracting the outputs of two saturable absorbers (SAs). RAO can provide both bounded and unbounded outputs by facilitating an... 

In this paper, we design a dimming compatible visible light communication (VLC) system with asynchronous and optimum indoor positioning method in a standard office room in combination with asynchronous and optimum indoor positioning method according to illumination standards under channel constraints. We use overlapping pulse position modulation (OPPM) to support dimming control by changing the code weights. The system parameters such as a valid interval for dimming together with an upper bound for bit rate according to the channel delay spread are investigated. Moreover, considering the dispersive VLC channel and using Monte Carlo (MC) simulations, a method is proposed to determine the... 

In this paper, we report our evaluation of the strength of random number generator and RSA key-pair generator of some commercially available constrained hardware modules, i.e., tokens and smart cards. That was motivated after recent related attacks to RSA public keys, which are generated by constrained network devices and smart cards, and turned out to be insecure due to low-quality randomness. Those attacks are mostly computing pair-wise GCD between the moduli in public keys, and resulted in breaking several thousands of these keys. Our results show that most of the tested hardware modules behave well. However, some have abnormal or weak random generators which seem to be unsuitable for... 

The focus of this study is the performance of the relaying network with incremental selective decode-and-forward (ISDF) protocol in non-selective slow Nakagami-m fading channels. To enhance bandwidth efficiency, when the direct transmission is not successful the relay is used to retransmit a clean copy of the source signal. The proposed protocol achieves a significant reduction in the power consumption and an improvement in performance compared to the fixed decode-and-forward (DF). The exact symbol error rate (SER) of M-PSK modulation for the ISDF protocol over general fading channels is derived. However, as the exact SER analysis is very complicated, we provide an approximated SER... 

In an electronic warfare environment, an electronic support system receives signals emitted by different radars. One of the key tasks in an electronic support system is detection of Pulse Repetition Interval Modulation for these signals. In this paper we present a new robust and yet simple method for detection of complex PRI Modulation. Simulation results show that the proposed method is more robust against noise (spurious and missing pulses) and unwanted jitter than previous methods  

In this paper, we consider a broadband secondary transmitter-receiver pair which interferes with N narrowband primary users and study the effect of cognition and cooperation on the maximum stable throughput. In our study we focus on four transmission protocols as well as two channel types, i.e., flat fading and frequency selective fading. In the cooperative protocols, the broadband transmitter relays the packets of the primary users which have not correctly decoded at the primary receiver. The analysis includes random packet arrivals at the transmitters which may impact on the maximum stable throughput. Moreover, sensing errors at the secondary user are considered. In this paper, we derive...