Modern mobile communication systems will provide enhanced high-speed data, multimedia, and voice services to mobile users. The integration of such heterogeneous traffic types implies that the network must provide differentiated Quality of Service (QoS). Beam forming techniques have been proposed to increase the spectral efficiency of the wireless channel. Using beamforming in the network will lead to intra-cell handoffs within the cell due to user mobility. In a commercially deployed next generation cellular system, it is likely that beam forming and QoS guarantees to the users will co-exist. In this work we propose a resource allocation and management scheme tailored for a network that employs smart antennas in support of a heterogeneous user mix. Resource management in a wireless system should take care of channel impairments and non-ideal antenna patterns. Mobile users moving from one beam to the other give rise to resource reallocation issues. Depending on the scatterer distribution in the cell, the Angle of Arrival (AoA) of the users will also change, affecting the interference pattern in the cell. In a system with data and multimedia users, some of the users are likely to be elastic in their demands for bandwidth. In this work, we propose a resource allocation and management scheme tailored for systems with smart antennas having heterogeneous users. The algorithm works by comparing the received power in the beams. Elasticity of user requirement for data services is exploited to provide adaptive QoS, thereby reducing the call dropping probability due to user mobility. Simulation results showing the channel and Multiple Access Interference (MAI) effects on system performance are presented. The effect of channel coding to provide Bit Error Rate (BER) guarantees is studied. We also show the throughput advantage obtained using the resource management algorithms. It is also seen that the throughput of the system increases for a user population having a higher elasticity. A modified resource allocation algorithm to reduce the blocking probability of the calls is presented and performance verified using simulation. The probability of call dropping in an unmanaged system due to user mobility is shown. Our studies show that using managed system the call drop probability can be minimized. / Master of Science

This thesis investigates optical feed methods for phased array antennas. The technical and practical limitations are analyzed and an optimum design is determined. This optimum optical feed is a two-beam interferometric approach which uses acoustooptic phase control. The theory is derived; a computer model is developed; and the limitations are determined. Design modifications are suggested which reduce limitations and greatly extend the range of applications. / Master of Science

The electromagnetic interaction between antenna elements is referred to as mutual coupling. This phenomenon can cause undesirable effects in an antenna array such as distortion of the radiation pattern and loss of efficiency. This report models mutual coupling in a scattering parameter context and utilizes microwave network theory to establish a set of nonlinear simultaneous equations describing a compensation network. Iterative numerical techniques are then used to solve for the parameters of the compensation network. Three specific networks are investigated. Compensation examples are presented for two and eight element arrays. / M.S.

yes / Size constraints and mutual coupling on the performance of a two-element PIFA assembly are investigated for a design frequency of 2.4 GHz. A benchmark antenna assembly, employing a normal metallic ground plane is compared with an EBG modified ground plane. The height of the antenna elements over the EBG is optimised, and an isolation factor of 9.12 dB is achieved for a gap of 2.5 mm. Prototype structures have been constructed and measured for both cases.

In this thesis a mathematical analysis is made of the input impedance to a cylinder antenna with an axial slot. It is excited by a parallel-wire line connected across the center of the slot. This causes standing waves along the slot. The analysis is then based on an analogy between the slot and a transmission line. Following this analogy equations for the transmission-line parameters are developed for the slotted cylinder. The slot distributed inductance and capacitance are determined by assuming an infinite slot length, while the conductance is obtained for a finite slot. The phase constant, which is contained in the expressions for the line parameters, is a function of the line parameters. Thus, a final answer requires the solution of simultaneous equations. This is done on a digital computer. This analysis applies to the case where the wavelength of excitation is of the same order of magnitude as the diameter of the cylinder. Possible frequencies for which this antenna might be used are in the microwave range. / Master of Science

In 1987 a NASA panel recommended the creation of the Mission to Planet Earth. This mission was intended to apply to remote sensing experience of the space community to earth remote sensing to enhance the understanding of the climatalogical processes of our planet and to determine if, and to what extent, the hydrological cycle of Earth is being affected by human activity. One of the systems required for the mission was a wide scanning, high gain reflector antenna system for use in radiometric remote sensing from geostationary orbit. This work describes research conducted at Virginia Tech into techniques for beam scanning offset Cassegrain reflector antennas by subreflector translation and rotation. Background material relevant to beam scanning antenna systems and offset Cassegrain reflector antenna system is presented. A test case is developed based on the background material. The test case is beam scanned using two geometrical optics methods of determining the optimum subreflector position for the desired scanned beam direction. Physical optics far-field results are given for the beam scanned systems. The test case system is found to be capable of beam scanning over a range of 35 half-power beamwidths while maintaining a 90% beam efficiency or 50 half-power beamwidths while maintaining less than 1 dB of gain loss during scanning. / Master of Science

The increasing demand for mobile communication services without a corresponding increase in RF spectrum allocation motivates the need for new techniques to improve spectrum utilization. The CDMA and adaptive antenna array are two approaches that shows real promise for increasing spectrum efficiency. In this research, we investigate the performance of different blind adaptive array algorithms in the CDMA systems. Two novel algorithms, least-squares despread respread multitarget array (LS-DRMTA) and least-squares despread respread multitarget constant modulus algorithm (LS-DRMTCMA), are developed, and a MATLAB simulation testbed is created to compare the performance of these two novel algorithms with those of the multitarget least-squares constant modulus algorithm (MT-LSCMA) and multitarget steepest-descent decision-directed (MT-SDDD) algorithm. It is shown from the simulation results that these two novel algorithms can outperform the other algorithms in all the test situations (e.g., AWGN channel, timing offset case, frequency offset case, and multipath environment). It is also shown that these two algorithms have less complexity and can converge faster than the other algorithms. / Master of Science

This dissertation is concerned with the dynamics and control of spacecraft consisting of a rigid platform and a given number of retargeting flexible antennas. The mission consists of maneuvering the antennas so as to coincide with preselected lines of sight while stabilizing the platform in an inertial space and suppressing the elastic vibration of the antennas. The dissertation contains the derivation of the equations of motion by a Lagrangian approach using quasi-coordinates, as well as a procedure for designing the feedback controls. Assuming that antennas are flexible, distributed parameter members, the state equations of motion are hybrid. Moreover, they are nonlinear. Following spatial discretization and truncation, these equations yield a system of nonlinear discretized state equations, which are more practical for numerical calculations and controller design. Linearization is carried out based on the assumption that the inertia of the rigid body is large relative to that of flexible body. The equations of motion for a two-dimensional model are also given. The feedback controls are designed in several ways. Disturbance-minimization control plus regulation is considered by using constant gains obtained on the basis of the premaneuver configuration of the otherwise time-varying system. ln the case of unknown constant disturbance, proportional-plus integral (PI) control has proven very effective. Pl control is used to control the perturbed motions of the platform with multi-targeted flexible appendages. A new control law is obtained for the system with small time-varying configuration during a specified time period by applying a perturbation method to the Riccati equation obtained for Pl control. According to the the proposed perturbation method, the control gains consist of zero-order time-invariant gains obtained from the solution of the matrix algebraic Riccati equation (MARE) for the post-maneuver state and first order time-varying gains obtained from the solution of the matrix differential Lyapunov equation (MDLE). The solution of the MDLE has an integral form, which can be approximated by a matrix difference equation. The adiabatic approximation, which freezes the matrix differential Riccati equation or Lyapunov equation is also discussed. Comparisons are made based on system stability by Lyapunov’s second method. A spacecraft consisting of a rigid platform and a single flexible antenna is used to illustrate disturbance-minimization control, and a spacecraft consisting of a rigid platform and two flexible antennas reorienting into different directions is used to demonstrate the effectiveness of the disturbance-accommodating control. A time-varying spring-mass-damper and a two-dimensional model, representing a reduced version of the original spacecraft model, are considered to demonstrate the perturbation and adiabatic approximation methods. To illustrate the effect of nonlinearity on the dynamic response during reorientation, a numerical example of the spacecraft having a membrane-type antenna ls presented. / Ph. D.

The rise of satellites in Low Earth Orbit (LEO) is causing more terrestrial electromagnetic interference in the important L- and X-band frequencies which are crucial for astronomical observations. This thesis introduces reflectarray design which can serve as a basis for an interference mitigation solution for radio telescopes. In the envisioned application, When the reflectarray is placed around the circumference of an existing radio telescope, it can drive a null into the radio telescopes radiation pattern sidelobe distribution. Since the reflectarray only occupies a small potion of the rim of the paraboloidal main reflector, its presence does not significantly effect the main lobe peak gain. Since Iridium and Starlink are the target mega-constellations, the reflectarray must be dual band. To cover the operational bandwidths of these constellations, the target bandwidth in the L-band (Iridium) is 0.7%, and that in the X-band (Starlink) is 17.1%. This makes the design of the reflectarray challenging as the frequencies are widely separated and the bandwidth in the X-band is wide The work of this thesis marks a first step in this effort. It includes a reflectarray design containing a multi-layer stack consisting of: (1) a grounded substrate, (2) an X-band slot loaded unit cell geometry, (3) a dielectric superstrate, and (4) an L-band layer containing crossed dipoles. The dual band reflectarray is dual linearly polarized to maintain symmetric response. The reflectarray is designed and simulated using full-wave solvers. The results show that the reflectarray designs are capable of pattern shaping at both bands and operate across the required bandwidths. This architecture could serve as a basis for future reflectarrays capable of nulling satellite interference from mega-constellations in observatory applications in the future. / Master of Science / The signal clarity issues stemming from the increasing number of satellites in Low Earth Orbit (LEO), particularly in the vital L- and X-band frequencies essential for global communications and radio astronomy, are the motivation of this thesis. The endeavor concentrates on designing a dual-band dual-polarized reflectarray antenna which may ultimately be used to help mitigate interference in these bands in radio telescopes. The work is focused on the frequency ranges utilized by the major satellite networks Iridium and Starlink, which operate within the L-band (1616-1626.5 MHz) and X-band (10.7-12.7 GHz). Recognizing the significance of these frequencies for global communication and also to radio astronomy, the reflectarray is designed to contribute to a an interference mitigation system which would ultimately allow for coexistence between radio telescopes and communications systems satellites. Targeting bandwidth achievements of 0.7% for the L-band and 17.1% for the X-band, the focus is on nulling interference arising across these frequency bands and thereby increasing the sensitivity of the radio telescope operating amongst these mega-constellations. The thesis documents a multilayered reflectarray antenna, containing a wide-band X-band layer of slot antennas on one layer and an L-band superstrate layer containing crossed dipoles at another, both of which utilize dual linear polarization for symmetric operation. The completed reflectarray can operate simultaneously in both bands. It has been shown in the two papers cited by {ellingson2021sidelobe,budhu2024design} that reflectarrays placed along the rim of radio telescopes main reflector can be used to drive nulls in the sidelobe envelope of its radiation pattern thereby nulling incoming interference. The antenna design of this thesis suggests a possible candidate for these interference mitigation systems where both bands are targeted.

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