Sharif Digital Repository

We derive some bounds on the Optimum Asymptotic Multiuser Efficiency (OAME) of randomly spread CDMA as extensions of the result by Tse and Verdú. To this end, random Gaussian and random binary antipodal spreading are considered. Furthermore, the input signal is assumed to be Binary Phase Shift Keying (BPSK). It is shown that in a CDMA system with K-user and N chips when K and N → 8 and the loading factor, K over N, grows logarithmically with K, the OAME converges to 1 almost surely under some condition. It is also shown that a Gaussian randomly spread CDMA system has a performance close to the single user system at high Signal to Noise Ratio (SNR) when the loading factor is kept less than... 

In this paper, extended composite right/left-handed (E-CRLH) transmission line (TL) metamaterial structures, with two left-handed (backward) and two right-handed (forward) pass bands, are investigated. Also, design procedures in order to design dual- and quad-band E-CRLH-TLs are presented in detail and the parameters of these structures are extracted by clean formulas, while satisfying arbitrary phase shifts at the operating frequencies. Finally, the dispersion and characteristic impedances of these transmission lines are derived and plotted. The results of this paper can be applied to any type of TL-based dual- and quad-band microwave component  

A novel technique is presented to design efficient full-rate space-time block codes (STBCs) over correlated Rician fading channels. The authors first derive a formula for the achievable information rate of a linear dispersion coded multi-input single-output system as a function of correlation at the transmitter in a Rician fading channel. In addition, the authors obtain the bit-error-rate (BER) equations for this system, when multiple phase shift keying and multiple quadrature amplitude modulation schemes are used at the transmitter. Therefore with regard to derived formulae and using a genetic algorithm, a method is presented to construct STBCs over correlated Rician fading channels. The... 

A body insonified by a constant (time-varying) intensity sound field is known to experience a steady (oscillatory) force that is called the steady-state (dynamic) acoustic radiation force. Using the classical resonance scattering theorem (RST) which suggests the scattered field as a superposition of a resonance field and a background (non-resonance) component, we show that the radiation force acting on a cylindrical shell may be synthesized as a composition of three components: background part, resonance part and their interaction. The background component reveals the pure geometrical reflection effects and illustrates a regular behavior with respect to frequency, while the others... 

In high-voltage applications, due to the use of high-ratio step-up transformers, the value of parasitic capacitance of these transformers is large compared to the low-voltage transformers. Moreover In these applications it is prefered to use single-capacitor filters instead of filters with inductors. In this paper a simple electrical model for two-winding high frequency transformer considering all of the parasitic elements is presented. This model is used in a full-bridge based dc-dc converter with capacitive output filter controlled by phase-shifted method. The operation of this converter in DCM is analysed and its analytical relations in different time intervals is derived and confirmed by... 

Here, an in-depth study of a novel electro-optic phenomenon suggested earlier in optical waveguides, referred to as the conducting interfaces, is carried out. It is shown that it would be possible to achieve a very strong optical modulation effect in slab waveguides fabricated on the standard III-V InAlGaAs compound platform. The electro-optic effect is obtained by controlling the density and population of the electron and hole states in the well layer. The additional phase shift contributed to the reflection phase of the guided electromagnetic wave constitutes an ultrastrong source of optical modulation and phase control. It is estimated that a Mach-Zhender configuration based on this... 

When a transparent plane-parallel plate is illuminated at a boundary region by a parallel monochromatic beam of light, Fresnel diffraction occurs because of the abrupt change in phase imposed by the finite change in refractive index at the plate boundary. The intensity of the diffraction fringes varies periodically with changes in the optical path difference of the light passing through the plate edge. The optical path difference depends on the incident angle of the light, the refractive index of the plate and surrounding medium, and the wavelength of the light. Plotting the intensity of the diffraction fringes versus optical path difference provides a general phase shift method for phase... 

Cobalt-based catalysts were prepared on gamma alumina supports, and their behaviour for different Fischer-Tropsch synthesis (FTS) conditions was assessed. Although Co/γ-Al 2O 3 is a well-known FTS catalyst, its durability ought to be improved to make the industrial process economically feasible. The effect of lanthanum doping on the catalyst lifetime was examined utilising reactor tests and catalyst characterization techniques including TPR, ICP and N 2 porosimetry. Reactor test results revealed that an optimum amount of lanthanum improved catalyst activity and selectivity. Increasing amounts of lanthanum doping up to about 1.1wt% seemed to modify the chemical composition of the support... 

A 6-bit passive phase shifter for S frequency band has been designed in a standard HEMT technology. A new switched-network topology has been proposed for implementing the 5.625 phase shift step which these digital bits are series with each other. We used Advanced Design System (ADS2010) to perform simulations. These digital bits will produce 64 different mode phase shift of -177.15 to +177.15 degree. In all cases simulation obtained less than 1 % error  

In this paper, a reliable low power single phase grid-connected inverter for photovoltaic modules is proposed. The inverter has improved lifetime since large electrolytic capacitor is replaced with small film capacitor due to circuit topology, power processing and proper control scheme. Thus the main limiting component in single phase grid connected inverters is obviated. The proposed inverter consists of two power processing stages. First a dual active bridge (DAB) converter boosts the voltage of PV panel to approximately 440 V and eliminates impact of high dc bus voltage ripples. A proper feed forward control is proposed to regulate the dc bus voltage with high ripple. Simulation results... 

In this paper, we investigate the dynamic analysis of a strongly nonlinear microrobot using a three-term harmonic balance method. The employed locomotion concept, namely "friction drive principle," is based on the superposition of a horizontal vibration at the interface between the robot and work floor and an active variation of friction force, obtained by the vertical vibration of the base at the same interface. The equation of motion for the system reveals a parametrically excited oscillator with discontinuity for which the elastic force term is proportional to a signum function. The obtained periodic solution not only is of high accuracy, but also can predict the contribution of the... 

This paper presents the design of a 6-bit active digital phase shifter in 0.18-μm CMOS technology. The active phase shifter synthesizes the required phase using a phase interpolation process by adding quadrature phased input signals. It uses a new quadrature all-pass filter for quadrature signaling with a wide bandwidth and low phase error. The phase shifter has simulated RMS phase error of <0.85° at 2.4-5 GHz. The average voltage gain ranges from 1.7 dB at 2.4GHz to -0.14 dB at 5 GHz. Input P1 dB is typically 1.3±0.9 dBm at 3.5 GHz for overall phase states  

The exact symbol and bit error probabilities of linearly modulated signals in frequency non-selective Rician fading channels with Lth diversity branches are derived, where coherent detection with maximum ratio combining (MRC) is used at the receiver. For performance evaluation, the multiplicative distortion is combined with the additive Gaussian noise and form an additive noise is formed. This method allows computing the exact symbol and bit error probabilities of M-ary phase shift keying (M-PSK), M-ary quadrature amplitude modulation (M-QAM), M-ary amplitude modulation (M-AM) and M-ary amplitude modulation phase modulation (M-AMPM) for any arbitrary bit mappings. The results contain several... 

We extend the result by Tse and Verdú on the optimum asymptotic multiuser efficiency of randomly spread code division multiple access (CDMA) with binary phase shift keying input. Random Gaussian and random binary antipodal spreading are considered. We obtain the optimum asymptotic multiuser efficiency of a K-user system with spreading gain N when K and N → ∞ and the loading factor, (K/N) , grows logarithmically with K under some conditions. It is shown that the optimum detector in a Gaussian randomly spread CDMA system has a performance close to the single user system at high signal-to-noise ratio when K and N → ∞ and the loading factor, (K/N), is kept less than (log3 K/2). Random binary... 

In this paper, a novel reduced switch cascaded transformer multilevel inverter is proposed. In this structure, a new structure is replaced with conventional H-bridge inverter. It consists of four high frequency switches, two line frequency switches, and two transformers which can be assembled in one core with a specific winding connection. This topology can produce 5 or 7 voltage levels depending on transformers turns ratio. The simplicity and modularity are its advantages compared to the existing structures. Beside these advantages, due to its similarity with H-Bridge converter, it is possible to use a modified technique based on conventional phase shift pulse width modulation to generate... 

Rotating field gradients (RFGs), generated by simultaneously applying sine- and cosine-modulated gradient waveforms along two perpendicular directions, provide an alternative diffusion sensitization mechanism for magnetic resonance imaging and spectroscopy. Two RFGs with a 90-degree phase shift between them are applied around the 180-degree RF pulse in a spin echo sequence to measure the diffusion orientation distribution function (dODF) directly. The technique obviates transforming the data from a space reciprocal to the displacement space. Here, we compare RFG results with those obtained by two pulsed field gradient (PFG) techniques: q-ball imaging (QBI) and its extension to constant solid...