Sharif Digital Repository

In this thesis, we investigate the previously introduced Code-Shifted Reference (CSR) time hopping (TH) ultra-wideband (UWB) communication systems as a potential transmitted scheme reference technique. In this system, the orthogonality between the reference and data bearing signals is acquired in code domain. Also, we consider a simple scheme to clean to obtain a clean template to be used in the correlator receiver for the CSR –UWB, this method was used in UWB system before. In this method, which we call as averaged (Ave.) CSR-UWB, the template for each symbol is obtained by taking the average over all the frames of the symbol interval. It is shown that Ave-CSR system achieves higher... 

Ever increasing demand for higher data rate in wireless communication in the face of limited or underutilized spectral resources has motivated the introduction of cognitive radio for dynamic access to spectrum. In dynamic spectrum access a new type of users called secondary users measure the spectrum to see if it is occupied by licensed users (primary users or PU). When channel is empty secondary users can use it to transmit signal. This approach is called spectrum sensing. Hidden PU problem can severely defect detection ability of non-cooperativ spectrum sensing systems. Cooperative spectrum sensing (CSS) uses spatial diversity of spectrum sensors to tackle this problem. There are two kinds... 

In this thesis we examine and study the transmission of Ultra Wide Band (UWB) signals over single mode fiber in a radio over fiber system. We begin by developing mathematical model for the key optical components employed in radio over fiber systems. The authenticity of these models is verified with experimental results. Following the above models as well as experiment results, the transmission of UWB signals over optical fiber is investigated. The impact of limited operating bandwidth and nonlinearity of the electrical to optical transfer function on UWB signals is examined. Two types of UWB signals, namely, multi-band orthogonal frequency division multiplexing (MB-OFDM) and impulse radio... 

At almost all frequencies, we have good methods to generate and detect electromagnetic radiations. One crucial exception is the low terahertz range,where despite intensive research there is a severe lack of proper devices for many applications in this frequency range such as oscillators and detectors that is so-called THz gap and includes 0.3 to 10 THz spectrum. There are different types of THz detectors but bandwidth of dection of most of them is narrow. Among these detectors, superconducting transition edge balometer detector has detectivity in wide frequency range. But generally, detectivity of these detectors strongly depends on their ability to absorb radiations of electromagnetic waves... 

This study presents a comprehensive investigation into the characterization of three distinct homogeneous multilayer self-standing thin films, composed of stacked reduced graphene oxide (rGO) planes, which were synthesized using a modified Hummer’s method. In order to investigate their structural, electrical, and optical properties, the samples were characterized by field emission scanning electron microscopy (FESEM), Raman spectroscopy, four-point probe measurements, Impedance measurements, Fourier-transform infrared spectroscopy (FTIR), and Time domain Terahertz Spectroscopy (TDS). The FESEM images obtained from the samples reveal a smooth sheet-like surface with few wrinkles.... 

In recent years, UWB multiple access communication systems have received a great attention due to its many advantages. The most important advantages of a UWB system can be summarized as its high data rates, low power consumption, fading immunity and high precision routing. One of the important problems in a UWB system implementation is in its receiver part in which in order to use a coherent RAKE receiver, the channel coefficients have to be estimated. Since UWB channel is a dense multipath fading channel with a very high number of resolvable paths, the channel coefficient estimation based on using training sequence is very complicated. To resolve this problem, transmit reference (TR)...