Sharif Digital Repository

Over the past few years , demand for wireless communication services have grown at a formidable rate. As such we need to find new ways to support more and more wireless users in a confined space. this means that the capacity of networks should be significantly increased to overcome formidable rate of subscribers growth. That is why the smart antennas have become very popular in communication systems. Smart antennas are a kind of adaptive antenna arrays with smart signal processing algorithms used to find direction of arrival(DOA) of the signal, and use it to calculate suitable weights to direct the antenna beam on the desired user and nulling interferences. In this thesis, DOA estimation... 

In recent years due to the advances in semiconductor technology and the ability to implement small-scale manufacturing, communication at 60 GHz band receive more attention than ever has been. The use of this frequency band is not only about 7-9 GHz unlicensed broadband access provided, but getting smaller devices make it possible to use multiple antennas at the transmitter and receiver, and thus taking advantage of multiple input – multiple output systems. Antennas used in these systems should have features such as small size , low weight, high gain and bandwidth and also they should keep their high efficiency and gainat high frequency range (about a few GHz ). To achieve these features... 

The main objective of this project is to discuss a DOA estimation system performance in a multipath environment. For this goal, first the basics of the DOA Estimation systems are discussed. Evaluation of different types of algorithms and choosing an appropriate one is an important step in this regard. Moreover the comparison of different array structures is necessary to choose the appropriate one. The major part of the project has been dedicated to discussion of multipath phenomenon as a major source of error in DOA estimation and studying the multipath mitigation algorithms to eliminate the aforementioned error. A model based on the concept of Bistatic Radar Cross Section is proposed to... 

In the new generations of mobile communication systems, smart antennas are used to achieve higher capacities. Since these systems have the ability to form a radiation pattern where the desired signal is received with minimum distortion and inteferers are received with a significant attenuation therefore their application makes it possible to have a large signal to inteference ratio. In addition by setting the main beam direction towards the desired signal, it is conceivable to realize the the communication with a lower power. The optimal methods for angle-of-arrival estimation and beamforming in one and two dimensional structures has been studied in the literature, but the three dimensional... 

Novel two dimensional and three dimensional antenna array geometries for smart antenna application are introduced. MMSE beamforming algorithm, using these arrays, in presence of signal, noise and interferences is implemented. Beamforming approach is used for every type of array assuming uniform and log-normal distributions for the interference amplitudes. Equal volume cylindrical or prism type arrays with circular, hexagonal, triangular, square and star cross sections with equal number of elements are considered for comparison. Novel geometries consisting of rotated cross sections are studied as well. In each case the relative SIR’s are compared in these geometries, showing that the... 

The main objective of this project is to analysis and design of ultra ultra wideband dual polarization vivaldi antenna array for electronic support measure system. In modern satellite communication and radar systems, antenna are required to have the characteristics of both ultra wide band and dual-polarization so as to satisfy various needs such as expanding the information volume, improving the quality of receiving information and increasing the ratio of bandwidth utilization. For this reason we first design a single element of vivaldi antenna for 6-18GHz range and then with design of SMA connector shown that result with and without SMA is same. In the next step we design(under... 

Due to the short wavelengths in millimeter waves and the specific characteristics of wave propagation in this frequency band, the methods used for modeling channel response in lower frequency bands are not applicable without appropriate modifications. Considering the fact that many obstacles in the channel have sizes comparable to the wavelength (eg branches and tree leaves, brick spacing and roughness height of many surfaces), statistical-empirical methods can be an efficient way of modeling propagation in the presence of such components.
Here an attempt has been made to obtain an appropriate model for the propagation channel using propagation models proposed in the literature. Surface... 

The interferometer method, as one of the most accurate schemes for wideband direction finding (DF), is used. The interferometer method has various algorithms which can be implemented depending on the required specifications. The advantages and disadvantages of these algorithms have been evaluated and the appropriate algorithm for a general practical case in view of the ambiguity resolution is proposed. The receivers’ channel phase tracking error is of significant concern in practice, in interferometric DF systems. The induced error due to channels phase tracking error is estimated. Furthermore the use of physically realizable antennas, achievement of high accuracy, minimum number of antennas... 

In this thesis, a simplified method was introduced to model the wave scattering by the metallic objects in a microcell, in order to determine the coverage pattern. First, the equivalent images method was introduced, which models the building walls effect. This method is fast, memory saving and accurate enough to estimate the scattered fields. Two-ray incidence model accuracy was investigated and confirmed in modeling the scattering from the metallic objects. This method also enhances saving in time and memory. The accuracy of single scattering assumption was investigated and confirmed in a scenario of multiple metallic scatterers. Based on this assumption, we can superimpose the scattered... 

Multiple-Input Multiple-Output (MIMO) communications at millimeter-wave (mm-wave) frequencies (e.g., in the 60 GHz band) is a modern technology recently considered for various applications, such as emerging 5G services for multi-user MIMO (MU-MIMO) and high-resolution frequency-modulated continuous wave (FMCW) MIMO radars to support multi-gigabit throughputs in short-range environments via spatial multiplexing-diversity. Nevertheless, the impairments of communication channels in this frequency band, including significant propagation loss and severe blockage effect, are quite challenging to allow an efficient communication link. Hence, beamforming/beam steering can play a crucial role in 60... 

Switched Parasitic Antennas, as a kind of Smart Antennas, have several applications in Telecommunications and Radar systems. In the realm of wireless applications, these antennas provide the system with higher capacity owning to the fact that they are able to direct the main beam toward the desirable user. Moreover, they can be used to degrade the adverse effects of Multi-path. Multi-path reduction not only benefits mobile communication but applies to many applications of radar systems. These antennas, in addition, can be utilized in Direction Finding. In the case of other types of Smart Antennas, the above mentioned benefits can be achieved with high costs due to the fact that there are a... 

In this thesis, some of the well-known DOA algorithms namely Bartlett, Capon, Linear prediction, Maximum Entropy, Pisarenko harmonic decomposition, Minimum norm and MUSIC are analyzed and compared with each other in the identical situations using a uniform linear array. Then based on these comparisons, the appropriate algorithm is selected. In the next step since the linear arrays can't estimate both angles of arrival (Azimuth and Elevation) simultaneously, two dimensional (2D) geometries are introduced and are also compared to each other. Then one of them (circular array) is selected for dipole antenna arrays. The performance of the selected array is evaluated using isotropic elements for... 

Nonlinear transmission line (NLTL) a guiding structure contains nonlinear elements. For discrete NLTLs, this structure is periodically loaded with nonlinear elements. In the structure of this line nonlinear elements such as ceramic capacitors, nonlinear magnetic elements, vector diodes, Mos transistors, etc can be used. With the rapid development of semiconductor devices and microwave band systems, the demand for implementation of NLTLs has increased. These lines provide wide band-width, cut-off frequencies over the hundreds of GHz and picosecond rise time for various systems.Among the applications of NLTL, the generation of high power RF pulses has attracted the most attention. In this... 

Transferred Electron Device is a semiconductor device being capable of generating oscillation in microwave frequencies. Applying a bias voltage to the device and putting it in a resonant circuit, a periodic waveform in the output current is observed. The cause of the generation of oscillation in the device is related to the characteristics of velocity-field in the device. In some part of the characteristics, it is observed that increasing the field inside the device causes the velocity of electrons to be reduced. This causes the generation of the negative conductance and the oscillation of the output current under some conditions. The purpose of this thesis is to represent an equivalent... 

Due to the widespread use of satellite communications including live TV broadcast, DVB-S and satellite Internet, it is necessary to accurately assess the level of signal received and the EIRP of the satellite. Nilesat21(7°W), Eutelsat3B(3°E), Badr(26°E), Express AM6(53°E) and Intelsat20(68.5°E) have been chosen to be meticulously assessed and analyzed.The chosen satellites contain the westernmost for Tehran to the easternmost, which has been listed above from west to east, respectively.In addition, the signal attenuation is closely monitored in the city of Tehran during rainfall for the above-mentioned satellites. In the meantime, the rain rate is also recorded.Another major factor that... 

In this thesis, a modified method based on frequency discriminator technique is presented for measuring phase noise of microwave oscillators. In the proposed method, we use two oscillators with similar phase noise, but their free running frequencies are different in the range of 10MHz~50MHz. Mixing these two oscillators results in the IF frequency, for which the phase noise power at each offset has been doubled.The delay line is used in IF frequency and a phase shifter at microwave or millimeter wave range is used to provide quadrarure condition. Hence the cable loss at IF frequency is low and the sensitivity of system will be very good. Comparison of the measured data at X-band (the... 

Millimeter-wave imaging is used in a vast applications like security, medical and non-destrcutive testings. Frequency scanning antenna is one of the optimum solution in mm-wave imaging systems, because of its accuracy, agility and low cost. In this method, changing the frequency plays the main role and leads the main beam to scan the space. In this thesis, by observing different frequency scanning structures, like microstrip antenna, slotted waveguide antenna array, substrate integrated waveguide leaky-wave antenna, different structures are designed and simulated. The antenna, which results in the best performance for an imaging system is selected for fabrication and measurement. The... 

Nowadays superconducting devices and especially high temperature superconductors (HTSs) have found very wide applications in microwave engineering. This technology is raised in the form of planar microwave structures and in the different shapes of superconducting microstrip transmission lines, strip lines and coplanar waveguide (CPW) lines and different kind of applications like resonators, couplers, filters, antennas, etc. Important advantage of these structures is very low loss, small size and high quality factor. However nonlinear behavior of superconducting microwave devices, due to the dependence of the surface impedance on the applied field, limits the power handling capability of... 

tIncreasing the bandwidth of a receiver is limited to the bandwidth of the system's internal components such as amplifiers, power dividers / combiners, and directional couplers, which are widely used in array antenna systems. Wilkinson splitters are often used for balanced power dividers and hybrid branch line couplers are used for directional couplers because their macrostrip structure makes them lighter and cheaper, and ultimately increases their use. Unfortunately, these elements provide up to 20% fractional bandwidth, which is not enough in our intended use. Also, the design of the power divider at millimeter wave frequencies has challenges such as matching impedance across all bands and... 

A microwave transceiver system requires an array of amplifiers. Low-noise amplifiers (LNAs) are integral parts of receivers while transmitters are based on several stages of power amplifiers (PAs). In modern communication standards, power amplifiers have to achieve high efficiencies over increasingly larger dynamic ranges and banwidths, while maintaining tight linearity requirements. In these systems, PAs must be highly efficient to simplify the thermal management and to reduce dc power requirements. Recently, the use of monolithic microwave integrated circuit (MMIC) for commercial and military applications have been paid more attention to, given the fact that the possibility of building...