Sharif Digital Repository

Today, phased array antennas play important role in wireless communication systems. Utilizing optimum channel capacity, having better signal to noise ratio and, high data rate are the most important features of these systems.These systems are composed of several thousands of separated transceivers which provide possibility of directional electromagnetic radiation by changing the phase and the amplitude of the signal in each transceiver independently. So, the main components of such a system are amplifiers, phase shifters and, amplitude controllers. Also a high resolution phased array military radar system is another vital application of these systems at frequency band of 33-36 GHz.The... 

In this work a theoretical overview of Parity-Time symmetry and its application in multilevel atomic systems are presented. Because of equivalence between the Schrodinger equation and the paraxial propagation equation, complex index of refraction plays the role of complex potential. On the other side, the possibility to manipulate loss, gain and the index of refraction simultaneously, Optics is a fertile ground to investigate Parity-Time symmetry and one of this symmetry’s applications is reduction of loss in optical systems. Furthermore, In order to realize Parity time symmetry a four level atomic system under electromagnetically induced transparency condition is proposed. Optical gain... 

Graphene, a monolayer carbon-atomic honeycomb lattice crystal, has attracted attention due to its peculiar carrier transport properties owing to the gapless energy spectrum. Due to lack of applicable light source in terahertz (THz) band, further researches to finding materials that are suitable for semiconductor lasers in this band to be continued. In this thesis, the population inversion possibility that is the first provision of laser operation, was analyzed in graphene optical pumping situation. The analysis was done by numerical solving of carrier concentration and energy density relations.As a result of this analysis population inversion is obtained when the stimulation square pulse has... 

Considering energy efficiency ways a key role in designing future wireless networks( 5G mobile networks). Moreover Spectrum efficiency is another critical issues in designing wireless networks. Cognitive radios can improve the spectrum efficiency. On the other hand, radio frequency (RF) energy harvesting has emerged as a promising technique to supply energy for wireless networks and thereby increase their energy efficiency. In this thesis, we propose a new technique for the RF-powered CRNs.To this end,We consider a cognitive radio network comprised of a primary user and a secondary user. The primary user, uses a typical frequency band for transmit data in a time slot basis and both the...