Department of Electrical and Electronics Engineering

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    Effect of molecular adsorption on the electronic structure of single walled carbon nanotubes
    (BITS Pilani, 2015) Talapunur, Vikramaditya
    The study of adsorption of various gaseous molecules on Carbon nanotubes (CNTs) are of great importance because of its vast applications in science and technology. Most of these applications are based on electronic and transport property changes as a result of adsorption. Single walled carbon nanotubes (SWCNT) based sensors have gained significant interest especially due to the modification of their electrical properties with adsorption of simple chemical species on the surface. Due to their high surface to volume ratio, they may be able to act as good sensors and also as gas storage materials. It has also been demonstrated experimentally that the electronic properties of SWNTs are very sensitive to the chemical environment. Conventional sensors which are widely used to detect gases are comprised of thin film metal oxides of tin oxide, zinc oxide or indium oxide. These sensors lack flexibility, suffer with poor response times and fails to operate at lower temperatures. Carbon nanotubes can act as a better alternative for these applications because of their unique morphology. The main principle involved in detecting various gases by CNTs is by monitoring the conductance change associated with the charge transfer between CNTs and the adsorbed gases.
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    Resource Allocation in Downlink OFDMA Systems An Evolutionary Approach
    (BITS Pilani, 2012) Sharma, Nitin
    A downlink wireless system features a centralized basestation communicating to a number of users physically scattered around the basestation. The purpose of resource allocation at the basestation is to intelligently allocate the limited resources, e.g. total transmit power and available frequency bandwidth, among users to meet users' service requirements. Channel-aware adaptive resource allocation has been shown to achieve higher system performance than static resource allocation, and is becoming more critical in current and future wireless communication systems as the user data rate requirements increase. Adaptive resource allocation in a multichannel downlink system is more challenging because of the additional degree of freedom for resources, but offers the potential to provide higher user data rates. Multiple channels can be created in the frequency domain using multiple carrier frequencies, a.k.a. multicarrier modulation (MCM), or in the spatial domain with multiple transmit and receive antennas, also known as multiple-input multiple-output (MIMO) systems. This thesis aims to study the system performance, e.g. total throughput and/or fairness, in multiuser multicarrier and multiuser MIMO systems with adaptive resource allocation, as well as low complexity algorithms that are suitable for cost-effective real-time implementations in practical systems. First contribution of this thesis is the use of Particle Swarm Optimization (PSO), a stochastic optimization technique, for sub-channel allocation in downlink of OFDMA systems followed by power allocation using water filling algorithm. In PSO aided subchannel allocation the search and subchannel allocation is performed simultaneously as compared to traditional methods where the subchannels are first sorted in accordance of their gains and then allocation is performed. This significantly reduces the complexity of PSO aided allocation. This fact makes PSO aided subchannel allocation a suitable choice for practical wireless systems like WiMAX (802.16e) where the convergence rate plays a very important role as the wireless channel changes rapidly. The second contribution to this thesis is a novel genetic algorithm adaptive resource allocation in MIMO OFDM systems. We impose a set of proportional fairness constraints to assure that each user can achieve a required data rate, as in a system with quality of service guarantees. With the proposed algorithm, the sum capacity can iv be distributed fairly and flexibly among users. Since the optimal solution to the constrained fairness problem is extremely computationally complex to obtain, we propose a suboptimal algorithm that separates subchannel allocation and power allocation. In the proposed algorithm, subchannel allocation is first performed using novel Genetic Algorithm, assuming an equal power distribution. An optimal power allocation algorithm then maximizes the sum capacity while maintaining proportional fairness. Finally, we present a joint solution to subchannel, bit and power allocation problem for downlink of MIMO OFDM systems. Using SVD, the MIMO fading channel of each subchannel is transformed into an equivalent bank of parallel Single Input Single Output (SISO) sub-channels. To achieve the capacity bound, one must solve a multiuser subchannel allocation and the optimal bit allocation jointly. We propose the use of Non-dominated Sorting Genetic Algorithm (NSGA) – II, which is a MultiObjective Genetic Algorithm (MOGA), for joint allocation of bits and subchannels, in the downlink of MIMO OFDMA system. NSGA – II is intended for optimization problems involving multiple conflicting objectives. Here the two conflicting objectives are Rate Maximization and Transmit Power Minimization.
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    Design and Analysis of Dielectric Resonator Antenna (DRA) for Wideband Applications
    (BITS Pilani, 2015-08-18) Kumar, Jitendra
    In the last few decades, two classes of novel antennas have been explored for wireless communications that are Microstrip Patch Antenna (MPA) and the Dielectric Resonator Antenna (DRA). Both are highly appropriate for the development of modern wireless communication. DRA do not have any metallic loss, and hence it is highly efficient than its counterpart when operated in microwave and millimeter wave frequencies. Dielectric Resonator Antennas (DRAs) are open resonating structures made out of high permittivity low-loss dielectric materials. DRA has some interesting characteristics like high radiation efficiency, small size, high power handling capacity, low losses, wide operational bandwidth and ease to integrate with existing technologies as compared to the other resonating antennas. In addition to these interesting characteristics, their resonance frequencies, their excited modes and their radiation characteristics are determined by their dielectric constant, their geometry and their coupling mechanism. This great flexibility of the DRAs in terms of their shape and feeding mechanisms in combination with their other advantageous inherent properties make them suitable candidates for many commercial applications. However, still antenna designers are facing problem of widening bandwidth and improving gain in addition to miniaturization and optimization of radiation. A DRA with rectangular cross section is very adaptable as it provides more degrees of freedom than cylindrical and hemispherical shape. The thesis initially discusses and evaluates detailed development and progress in the design and performance of DRAs taken place within the microwave industry on DRA through a concise review of literature. That deals with fundamental concepts, theory and comprehensive review of the radiation characteristics of DRAs of different possible shapes, such as cylindrical, hemispherical, rectangular and hybrid with choices of different feeding techniques. Material selection for selecting the dielectric material for a DRA using Ashby’s approach is carried out successfully and it was observed that Roger TMM10 (ε_r=9.8) material is very promising materials for DRA as DR element. This material is also verified for rectangular DRA and hybrid DRA that provides better bandwidth as compared to other dielectric materials. Thus, this dielectric material is used for the design, simulation and fabrication of proposed DRA mentioned this thesis. Next, comparative study of different rudimentary geometries (rectangular, cylindrical and hemispherical) of DRAs design is carried out for selecting the favorable shape of DRA. These rudimentary geometries of DRA give a better understanding of design parameters of an antenna and their effect on return loss, impedance bandwidth, gain, efficiency and resonant frequency. Then commonly used feeding techniques such as microstrip fed, coaxial probe fed; aperture slot fed and coplanar waveguide fed are analyzed using CST Microwave Studio to choose the suitable feeding technique for proposed DRA. It is found that the rectangular DRA fed by coaxial probe provide better resonance level as compared to other geometries. Then a technique for the bandwidth and gain enhancement using air gap-slots is verified for Gammadion Cross DRAs which guarantees larger impedance bandwidth and higher gain with compact shape. Finally, this slot-technique is utilized to widen the impedance bandwidth and to tune the resonant frequency of the antenna for improving and optimization of design parameters. Following this technique, fabrication and characterization of two novel shapes Asymmetric DRA and Tetraskelion DRA is described. The Rogers copper-clad high-frequency laminates are available in thickness of 0.2 inches (=0.51 cm). Hence, to fabricate the structure, three slabs are joined by applying glue from the edges under high pressure to form 1.5 cm thick slab. The structure of proposed DRA is then fabricated with the help of abrasive jet machine, diamond cutter, diamond filer and diamond drill machine. In the measurements, the reflection coefficients of the DRAs is measured using vector network analyzer (VNA), while the radiation patterns, antenna gains, and efficiencies are measured using a basic antenna measurement setup i.e. Compact Antenna Test Range (CATR) System. These antennas have a simple-interesting structure and relatively reduced volume of 8.24cm3 and 6.48cm3 for Asymmetric DRA and Tetraskelion DRA respectively. The Asymmetric DRA offers an impedance bandwidth of 51% (from 4.1 to 6.7 GHz) at 5.2 GHz resonance while Tetraskelion DRA resonates at 5.25 GHz, and offers an impedance bandwidth of 57.5% (from 3.85 to 6.96 GHz). The measured peak gain of the Asymmetric DRA is 5.3 dBi at the resonant frequency with a high radiation efficiency of 98% while peak gain over the complete bandwidth of operation is 8.2 dBi. However, the peak gain of the Tetraskelion DRA is 4.1 dBi at the resonant frequency with a high radiation efficiency of 95.6%. These types of antennas are very useful for large number of practical applications such as WLAN, WiMax, Vehicular Communication and C-band. Thus concept of a small DRA with wideband operation in the 4-7 GHz frequency band is presented in this thesis. Two new configurations; Asymmetric and Tetraskelion shaped DRA that offer significant enhancements to parameters such as small size, wide operational bandwidth, high gain and high radiation efficiency are fabricated and investigated. In conclusion, this work offers a new, efficient and relatively simple alternative for antennas to be used for multiple requirements in the wireless communication system.
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    MEMS Resonator based Bandpass Filter Design for Wireless Communication Transceiver
    (BITS Pilani, 2015-08-18) Sundaram, G.MEENAKSHI
    The recent advances in microelectronics technology and the tremendous growth of the newlinewireless-communication market have drawn muchinterest into radio-frequency MEMS newlinedevices (RF MEMS) such as filters, oscillators, and switches, which constitute the newlineubiquitous components of radio front systems. This thesis presents material selection, newlinedesign, and characterization of a capacitive micro-machined resonator and filter. The newlineresonator structures such as disk and Lame are studied in detail. Effects of coupling newlinebeams in filter response are analyzed with two different coupling structures such as newlinestraight and V-shaped beams. newlineMaterial selection and design are the two important aspects which govern the performance newlineof resonators and filters like any other MEMS device. In view of the recent advances and newlinetrends in device miniaturization, the fabrication and design compatibility among CMOS newlineand MEMS components is imperative. Material selection for the devices considered has newlinebeen optimized by using Ashby approach.
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    Low Power High Speed and Compact Ternary VLSI Circuit Designs using Carbon Nanotube Field Effect Transistors
    (BITS Pilani, 2015-07-18) Murotiya, Sneh Lata
    Carbon nanotube field effect transistor (CNTFET) shows great promises as extension to Silicon MOSFET for building high performance and low power VLSI circuit. Three-valued (ternary) logic is a promising alternative to traditional binary logic for accomplishing simplicity and energy efficiency in modern digital design. Ternary logic has an elegant association with CNTFET because the best way to design ternary circuit is the multiple-threshold method and desired threshold voltage can be easily achieved by utilizing different diameter of CNT in CNTFET device. newlineThis thesis develops designs of ternary arithmetic and logic unit (TALU) and content addressable memory cell using CNTFETs. First, 2-bit hardware optimized ternary ALU (HO-TALU) is presented. 2-bit HO-TALU gets minimization in required hardware at both architectural as well as at circuit level. At architecture level, HO-TALU has a new adder-subtractor (AS) module which performs both addition and subtraction operations using an adder module only with the help of multiplexers. Thus, it eliminates a subtractor module from the conventional architecture. At circuit level, HO-TALU minimizes ternary function expressions and utilizes binary gates along with ternary gates in realization of functional modules: AS, multiplier, comparator and exclusive-OR. AS module has a minor loss in power-delay product (PDP) but multiplier, comparator and exclusive-OR modules show improved PDP. As a consequence, HO-TALU gets significant reduction in device count with marginally increase in PDP for addition and subtraction operations only in comparison with CNTFET-based ternary designs available in the literature. Design of 2-bit HO-TALU is modified to develop a 2-bit HO-TALU slice which could be easily cascaded to construct N-bit HO-TALU. newlineTernary full adder (TFA) which is a basic sub-block of AS module, is modified using different circuit techniques to improve its efficiency in terms of PDP. Three new designs of TFA are presented.