VTechWorks Repository :: Browsing by Author "Rappaport, Theodore S."

Browsing by Author "Rappaport, Theodore S."

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Active Flow Control For Reduction of Unsteady Stator-Rotor Interaction In a Turbofan Simulator
Feng, Jinwei (Virginia Tech, 2000-10-10)
The research effort presented in this dissertation consists of employing active trailing edge blowing control to reduce the unsteady stator-rotor interaction in a turbofan simulator. Two active flow control systems with different wake sensing approaches are successfully implemented on the engine simulator. The first flow control system utilizes Pitot probes as flow sensors. Use of Pitot probes as sensors is appropriate as a first step toward a more in depth investigation of active trailing edge blowing control. An upper performance limit in terms of wake-filling can be obtained and serves as the baseline in evaluating other control systems with indirect wake sensors. The ability of the system to achieve effective wake filling when subjected to a change in inlet flow conditions demonstrates the feasibility and advantage of active flow control. Significant tonal noise reductions in the far field are also obtained. The second control system involves using microphones as indirect wake sensors. The significance of these acoustic sensing approaches is to provide a practical TEB approach for realistic engines implementations. Microphones are flush mounted on the inlet case to sense the tonal noise at the blade passing frequency. The first sensing approach only uses the tone magnitude while the second novel sensing approach utilizes both the tone magnitude and phase as error information. The convergence rate of the second sensing approach is comparable with that of the Pitot-probe based experiments. The acoustic results obtained from both sensing approaches agree well with those obtained using Pitot probes as sensors. In addition to the experimental part of this research, analytical studies are also conducted on the trailing edge blowing modeling using an aeroacoustic code. An analytical model for trailing edge blowing is first proposed. This model is then introduced into the two-dimensional aeroacoustic code to investigate effect of various trailing edge blowing managements in the tonal sound generation.
An adaptive multistage interference cancellation receiver for CDMA
Kaul, Ashish (Virginia Tech, 1995-03-23)
Most of the previous research on multistage interference cancellation receivers for Code Division Multiple Access (CDMA) systems has relied on the use of simulation techniques for performance evaluation. This thesis formulates a model for an adaptive multistage interference cancellation receiver within a CDMA system to be employed at the cellular radio base station. A closed form expression for the probability of bit error for this adaptive multistage interference cancellation receiver is derived, using a Gaussian approximation for Multiple Access Interference (MAI). The Bit Error Rate (BER) after any stage of interference cancellation can be computed from the signal to noise ratio, number of users and processing gain of the CDMA system. The BER expressions are extended to derive asymptotic limits on the performance of interference cancellation as the number of cancellation stages approaches infinity, demonstrating a fundamental limit on the performance improvement that can be expected from any multistage interference cancellation scheme. Furthermore, the analysis quantifies conditions under which interference cancellation may degrade performance. This thesis also extends a software implementation of the Multistage Rake receiver for a wide range of channel models including Gaussian noise, MAI, multipath propagation and near-far effects. Simulation results demonstrate the robustness of the Multistage Rake receiver to near-far effects and manifold capacity improvement compared to conventional demodulation techniques.
Advanced Site-Specific Propagation Prediction Techniques
Durgin, Gregory David (Virginia Tech, 1998-04-16)
This thesis describes advanced techniques for site-specific propagation prediction. The need for accurate site-specific propagation is discussed in the context of current trends in the wireless industry. The first half of the report is dedicated to measuring and modeling continuous wave (CW) local-area path loss. Specifically, the text uses examples from a 5.85 GHz CW measurement campaign in and around suburban homes. Not only do these measurements demonstrate the validity of the original models and techniques presented in the thesis, but the results themselves may prove particularly useful for developing in-home wireless devices operating in the National Information Infrastructure band. This unlicensed spectrum was allocated in January of 1997 and holds promising applications for public and private telecommunications, home-based wireless internet, wireless local loops, and any number of wideband wireless applications. There is an in-depth development of deterministic propagation prediction techniques in the latter half of the thesis. The use of geometrical optics for terrestrial microwave propagation is discussed as well as an overview of the numerous ray tracing techniques that exist in the literature. Finally, a new 3D ray launching method is presented which improves upon many of the existing ray tracing algorithms. The thesis demonstrates how this algorithm is capable of recovering very detailed channel information from a wideband deterministic propagation prediction.
AMPS co-channel interference rejection techniques and their impact on system capacity
He, Rong (Virginia Tech, 1996-07-05)
With the rapid and ubiquitous deployment of mobile communications in recent years, cochannel interference has become a critical problem because of its impact on system capacity and quality of service. The conventional approach to minimizing interference is through better cell planning and design. Digital Signal Processing COSP) based interference rejection techniques provide an alternative approach to minimize interference and improve system capacity. Single channel adaptive interference rejection techniques have long been used for enhancing digitally modulated signals. However these techniques are not well suited for analog mobile phone system (AMPS) and narrowband AMPS (NAMPS) signals because of the large spectral overlap of the signals of interest with interfering signals and because of the lack of a well defined signal structure that can be used to separate the signals. Our research has created novel interference rejection techniques based on time-dependent filtering which exploit spectral correlation characteristics exhibited by AMPS and NAMPS signals. A mathematical analysis of the cyclostationary features of AMPS and NAMPS signals is presented to help explain and analyze these techniques. Their performance is investigated using both simulated and digitized data. The impact of these new techniques on AMPS system capacity is also studied. The adaptive algorithms and structures are refined to be robust in various channel environments and to be computationally efficient.
Antenna Array Systems: Propagation and Performance
Ertel, Richard Brian (Virginia Tech, 1999-07-28)
Due to the enormous performance gains associated with the use of antenna arrays in wireless networks, it is inevitable that these technologies will become an integral part of future systems. This report focuses on signal propagation modeling for antenna array systems and on its relationship to the performance of these systems. Accurate simulation and analytical models are prerequisite to the characterization of antenna array system performance. Finally, an understanding of the performance of these systems in various environments is needed for effective overall network design. This report begins with an overview of the fundamentals of antenna array systems. A survey of vector channel models is presented. Angle of arrival and time of arrival statistics for the circular and elliptical (Liberti's Model) models are derived. A generalized optimum output SINR analysis is derived for space-time processing structures in frequency selective fading channels. The hardware and software of the MPRG Antenna Array Testbed (MAAT) is described. A literature review of previous antenna array propagation measurements is given. Antenna array measurement results obtained with the MAAT are used to compare the properties of the received signal vector in the various environmental conditions. The influence of channel parameters on the ability of antenna arrays to separate the signals of two users on the reverse link is studied using simulation. Finally, forward link beamforming techniques are reviewed.
Antenna effects on indoor wireless channels and a deterministic wide-band propagation model for in-building personal communication systems
Ho, Chung-Man Peter (Virginia Tech, 1993-04-22)
While the application of antenna diversity in a narrow band communication system is well understood, little research has been done on antenna effects in wide-band channels. Research has shown that circular polarization (CP) is more robust in combating multipath than linear polarization in line-of-sight channels. One objective of this thesis is to study the effects of antenna polarization and antenna pattern on multipath delay spread and path loss in indoor obstructed (OBS) wireless channels. A wide-band experiment was performed in a two-floored modern office building at 2.4SGHz in August 1991. Some preliminary results are as follows. Circular polarization cannot reduce delay spread in OBS channels and CP signals are more vulnerable to depolarization in OBS channels. Our results show that vertically polarized (VP) directional antennas at both the transmitter and the receiver can give better delay spread and path loss results than other antenna combinations. The performance of VP directional antennas are found to be sensitive to the alignment of the antennas, and the performance gain over omnidirectional antennas degrades as shadowing effects increase. In the second half of the thesis, a deterministic wide-band propagation model that can predict channel impulse responses inside buildings is proposed and implemented. The three dimensional image-based propagation model includes effects of antenna pattern, antenna polarization, geometry of the building, and building materials. Comparisons between measured and predicted power delay profiles are given in Chapter Seven. Preliminary results show that the worst case path loss error is IOdB, and the standard deviation of path loss error is 4.6dB. For most cases, predicted rms delay spread values are 20ns within the measured values. Possible prediction errors are due to unmodeled furniture inside the offices and limitations of Geometrical Optics (GO) assumptions. The algorithm is shown to be more efficient than brute force ray tracing algorithm if the number of objects are on the order of a few hundred. Acceleration techniques for the algorithm are also discussed in the thesis.
An Antenna Specific Site Modeling Tool for Interactive Computation of Coverage Regions for Indoor Wireless Communication
Bhat, Nitin (Virginia Tech, 1998-03-02)
A goal of indoor wireless communication is to strategically place RF base stations to obtain optimum signal coverage at the lowest cost and power. Traditionally, transceiver locations have been selected by human experts who rely on experience and heuristics to obtain a near-optimum placement. Current methods depend on involved on-site communication measurements and crude statistical modeling of the obtained data which is time consuming and prohibitive in cost. Given the inherent variability of the indoor environment, such a method often yields poor efficiency. As an example, it is possible that more power than required or extra number of transceivers were used. This thesis describes an interactive software system that can be used to aid transceiver placement. The tool is easy to use and is targeted at users who are not experts in wireless communication system design. Once the transceiver locations are selected by the user within a graphical floor plan, the system uses simple path-loss models to predict coverage regions for each transceiver. The coverage regions are highlighted to indicate expected coverage. Earlier work assumed isotropic transceivers and had limited directional transmitter support. This thesis describes how the tool has been enhanced to support a wide range of 3D antenna patterns as encountered in practical situations. The tool has also been expanded to accommodate more partition types and to report area of coverage. The resulting system is expected to be very useful in the practical deployment of indoor wireless systems.
The applicability of APT towards meeting control needs in discrete parts manufacturing
Bidani, Sandeep (Virginia Tech, 1989-09-05)
For about ten years, Texas Instruments has been developing a software environment of integrated tools for designing, debugging and documenting process control solutions that run on programmable controllers. The product - the Applications Productivity Tool (APT), allows process and control engineers to design and program in a graphical environment that compiles into machine code (relay ladder logic). APT is primarily targeted for the batch manufacturing industry in which engineers combine elements of both discrete and continuous control strategies. The objective of this research was to determine the applicability of APT in discrete parts manufacturing, using two applications of discrete manufacturing. One of these applications was a Fischertechnik model of a manufacturing system, configured to simulate the production of three distinct parts. The other application was the flexible manufacturing system being assembled in the Computer Integrated Manufacturing Laboratory (CIM Laboratory), which is equipped to produce models of a robot and a CNC milling machine.
The application of multiuser detection to cellular CDMA
Buehrer, R. Michael (Virginia Tech, 1996-06-19)
This research investigates the application of multiuser detection to Code Division Multiple Access for cellular communications. This investigation focuses on the use of multiuser receivers at the base station of mobile radio systems. The first two chapters are dedicated to multiuser detection in general. An extensive literature survey is performed on the research concerning multiuser receivers to date. Six major receiver structures are chosen for extensive simulation studies. The bit error rate performance of these receivers is investigated in several system environments. Additionally, practical issues are considered such as computational complexity and robustness to code tracking errors. From this work, one receiver structure is identified for further study, namely multistage interference cancellation. The theoretical performance of this receiver is analyzed using a standard Gaussian Approximation and an Improved Gaussian Approximation for AWGN and fading environments. Additionally, the resistance of the receiver to interference energy levels is explored. Parameter estimation is an important issue for interference cancellation. Simple methods of improving parameter estimation are examined, as is the effect of parameter estimation error on system performance. A baseband hardware implementation is detailed and several design challenges are presented. Results are given for the performance of the implemented receiver and shown to match well with theory and computer simulation. Finally, the implications of this research are discussed.
An application of the hyperbolic navigation radio system for automated position and control
Purdy, Daniel S. (Virginia Tech, 1989-08-05)
As automation in the construction site of the future becomes a reality, position location systems are necessary to provide real-time data to an operator. This thesis addresses problems associated with development of a real time automated position location system using a method similar to hyperbolic navigation methods. The Automated Position and Control (APAC) project is a joint effort between the Civil and Electrical Engineering departments at Virginia Polytechnic and State University and Bechtel Eastern Power Corporation.
Bit error simulation of FSK, BPSK, and pi/4 DQPSK in flat and frequency-selective fading mobile radio channels using two-ray and measurement- based impulse response models
Fung, Victor (Virginia Tech, 1991-08-15)
A software simulation tool has been developed to simulate the performance of digital mobile radio systems in fading channels. The simulation tool generates average bit error rate (BER) and bit-by-bit error patterns with user-specified parameters. The simulation tool can simulate various communication system parameters (i.e. modulation scheme, data rate, signal to noise ratio, and receiver speed, etc.) and different channel environments (i.e. outdoor and indoor fading channels). Additive white Gaussian noise and co-channel interference effects for outdoor channels are also simulated. Using the simulation tool, we studied average BER results for FSK, BPSK, and Ï /4 DQPSK with Nyquist pulse shaping in indoor and outdoor, flat and frequency-selective fading channels. The BER performance of Ï / 4 DQPSK using the data rate and the pulse shaping requirement in the U.S. Digital Cellular Standard are studied. BER results for high data rate (> 450 kbps) transmissions in indoor channels generated by a measurement-based channel model, SIRCIM, are compared with results in channels generated by the classic two-ray Rayleigh fading model. Simulation results show that irreducible BER is not only a function of rms delay spread, but is also a function of the temporal and spatial distribution of multipath components. The simulation results are also applied into a real-time bit-by-bit error simulation using a hardware simulator between a data source and a data sink. Simulation results of the transmission of a video image in mobile radio fading channels are shown. This simulation methodology allows subjective evaluation of link quality between a source and sink in a laboratory in real-time without the need of building radio frequency hardware.
BSML: A Binding Schema Markup Language for Data Interchange in Problem Solving Environments
Verstak, Alex; Ramakrishnan, Naren; Watson, Layne T.; He, Jian; Shaffer, Clifford A.; Bae, Kyung Kyoon; Jiang, Jing; Tranter, William H.; Rappaport, Theodore S. (Hindawi, 2003-01-01)
We describe a binding schema markup language (BSML) for describing data interchange between scientific codes. Such a facility is an important constituent of scientific problem solving environments (PSEs). BSML is designed to integrate with a PSE or application composition system that views model specification and execution as a problem of managing semistructured data. The data interchange problem is addressed by three techniques for processing semistructured data: validation, binding, and conversion. We present BSML and describe its application to a PSE for wireless communications system design.
A comparison of two novel channel coding techniques for CDMA
Kim, Young M. (Virginia Tech, 1995-05-05)
In Code Division Multiple Access (CDMA) systems forward error correction (FEC) is an important factor in improving system performance. Because of the multiple access interference observed in asynchronous CDMA, FEC is required to improve system capacity. Several alternative coding techniques are available, including "trellis codes" specifically designed for CDMA, very low rate convolutional codes and conventional convolutional codes. An analytic approach is developed to compare the performance of different coding techniques for CDMA on the same basis by extending several improved analysis techniques which were developed for uncoded CDMA. Analytic results are presented for additive white Gaussian noise (AWGN) channels and frequency non-selective Rician Fading (FNRF) channels. The performance of coded CDMA systems in frequency selective Rayleigh fading channels is also analyzed using simulation techniques. Both analytic and simulation results show that by using trellis codes or very low rate convolutional codes significant performance improvement is achieved over conventional convolutional codes. Trellis codes outperform very low rate convolutional codes in AWGN and FNRF channels while both codes show similar performance in frequency selective Rayleigh fading channels.
A Comprehensive In-Building and Microcellular Wireless Communication System Design Tool
Skidmore, Roger R. (Virginia Tech, 1997-06-09)
Indoor wireless communication systems are becoming increasingly prevalent in work environments. The need to quickly and efficiently provide in-building and microcellular coverage without sacrificing quality is critical to cellular and emerging personal communication system (PCS) operators. Traditionally, indoor wireless communication system design has been carried out by human experts relying on experience to determine a satisfactory system configuration. This thesis describes the algorithms and technical considerations implemented in a comprehensive propagation planning tool, SMT Plus, which has been designed to predict the coverage regions of both in-building and microcellular wireless communication systems. The goal of SMT Plus is to provide both wireless service providers and equipment manufacturers with an efficient, easy-to-use coverage prediction tool for use in the design of any indoor or campus-wide wireless system. Using site-specific building information combined with on-site signal strength measurements, the tool provides system planners with a highly accurate model of the propagation environment among a group of buildings. SMT Plus provides a comprehensive solution to the planning and installation of wireless communication systems in and around buildings.
A Comprehensive Method and System for the Design and Deployment of Wireless Data Networks
Skidmore, Roger Ray (Virginia Tech, 2003-04-25)
Increasingly, wireless subscribers are demanding reliable data capabilities from wireless networks. The ability of wireless network engineers and Information Technology (IT) professionals to rapidly design, deploy, and maintain wireless communication systems that can provide strong, reliable data service is severely hampered by a lack of adequate systems and methods for simulating the performance of such networks a priori. Unlike older generation wireless systems that could be readily deployed on the basis of strong received signal strength and simple circuit-switched channel allocation protocols, modern and emerging wireless data networks are more noise and interference limited and rely on packet-based protocols. A heavier emphasis is needed on predicting and simulating throughput, bit error rate, frame error rate, user priority classes, and overall network capacity. This research provides wireless network engineers and IT professionals with a comprehensive system and method for the simulation and design of wireless communication systems that combines site-specific databases, equipment-specific distribution system modeling, and advanced ray tracing propagation analysis to directly predict throughput, frame error rate (FER), and other critical performance parameters for emerging wireless data networks.
Computer-based bit error simulation for digital wireless communications
(United States Patent and Trademark Office, 1993-08-03)
A communications stimulation system allows a user to perform a quantitative or subjective test of digital baseband devices over wireless channels using actual measured or modeled propagation data. The digital wireless communication simulation system is capable of simulating the transient nature of channels and radio hardware so that loss of synchronization can be included in the simulation. The simulator is a combination of computer software and hardware that computes a convolution, in the time domain, of a sequence of binary digits or data symbols (i.e., the data stream) with a computer model of a radio transmitter, a propagation channel or channels and a receiver. The transmitter typically comprises a coder, a pulse shaper, a modulator, and a spreader. The propagation channel or channels may include impulsive and average noise levels, co-channel interference and adjacent interference levels, fading and multipath propagation events, and non-linear channel and radio system effects. The receiver system typically comprises at least a filter bank, a demodulator, a despreader, a synchronizer, a detector, and a decoder. The data stream may either be random or applied by the user. The software computes the bit-by-bit sequence for replay at a later time. Once stored, the bit-by-bit error sequence can be clocked through a hardware data port and compared with an applied data stream in real time. The output of the hardware data port is a real time sequence of bits that has errors due to the bit-by-bit simulation computed earlier by the software.
Design and analysis of shipboard electrical distribution
Russell, Kevin Joseph (Virginia Tech, 1990)
This research is a preliminary design and feasibility analysis of a new type of shipboard electrical distribution system for Naval vessels. The design combines three traditionally separate disciplines: damage control, marine engineering, and electric power engineering to produce a hybrid system well suited for present and future surface warship environments. The design structure is a combination of shore-based power utility and shipboard distribution. The primary section is an interconnected bus feeder ring which resembles a shipboard vertical loop firemain. The bus feeder ring emulates the firemain’s network structure because it is well suited for both normal and emergency operating conditions. The distribution ring is used to transfer power between fire zones to load centers which radially feed loads within each zone. The electrical feasibility of the system was established through standard power system load flow contingency analysis, use of Navy design specifications, and direct comparison with an icebreaker radial electrical system. The new system could best be applied to small ships where the effective use of zone distribution is difficult, or where automation is needed to implement reduced manning. For large ships, this system would provide additional design alternatives which could help to reduce intersystem design interference where the requirements for one system impinge on those of another. As a final point, this system provides a viable network for facilitating the application of shore-based automatic switching technology to Naval vessels.
Design and development of a frequency hopper based on the DECT system for the 902-928 MHz ISM band
Dominique, Francis (Virginia Tech, 1995-12-15)
The Digital European Cordless Telecommunications (DECT) standard defines a high-performance digital communications system for two-way transfer of speech and data information. This thesis is concerned with the development of a frequency hopping Spread spectrum transceiver based on the DECT radio frequency (RF) front end, and fully compliant with FCC Part 15 rules for transmissions in the 902-928 MHz ISM band. A theoretical design of a slow frequency hopper (SFH) based on an existing DECT RF front end and a custom-designed system controller was developed. An innovative FH synchronization technique that eliminates the need for a separate FH tracking loop and can be implemented completely at baseband was designed. The practicality of the design was verified through extensive simulations. The SFH design was then implemented in prototype hardware using a digital signal processor acting as the system controller and using the DECT RF front end. Results show that the existing DECT physical layer can be successfully modified for spread spectrum transmissions in the ISM band fully compliant with FCC regulations.
Design and Implementation of a Pilot Signal Scanning Receiver for CDMA Personal Communication Services Systems
Blankenship, T. Keith III (Virginia Tech, 1998-04-16)
In cellular and personal communications services (PCS) systems based on code division multiple access (CDMA), a pilot signal is used on the forward link for synchronization, coherent detection, soft handoff, maintaining orthogonality between base stations, and, in the future, position location. It is critical that the percentage of power allocated to the pilot signal transmitted by each base station be fixed properly to ensure the ability of the CDMA network to support subscriber demand. This thesis reports on the design and implementation of a prototype receiver for measuring pilot signals in CDMA PCS systems. Since the pseudonoise (PN) signal of the pilot channel is a priori information, the receiver searches for pilot signals by digitally correlating the received signal with this known, locally generated pilot signal. By systematically changing the phase of this locally generated pilot signal, the receiver scans the received signal to identify all possible signs of pilot signal activity. Large values of correlation indicate the presence of a pilot signal at the particular phase of the locally generated pilot signal. The receiver can also detect multipath components of the pilot signal transmitted from a given base station. One issue associated with this receiver is its ability to keep the signal power within the dynamic range of the analog-to-digital (A/D) converter at its input. This necessitated the design of an automatic gain control (AGC) mechanism, which is digitally implemented in this receiver. Simulation studies were undertaken to assist in the design and implementation of the pilot signal scanning receiver. These simulations were used to quantify how various non-idealities related to the radio frequency (RF) front-end and A/D converter adversely affect the ability of the digital signal processing algorithms to detect and measure pilot signals. Because the period of the pilot signal is relatively long, methods were developed to keep the receiver's update period as small as possible without compromising its detection ability. Furthermore, the high sampling rate required strains the ability of the digital logic to produce outputs at a rate commensurate with real-time operation. This thesis presents techniques that allow the pilot signal scanning receiver to achieve real-time operation. These techniques involve the judicious use of partial correlations and windowing the received signal to decrease the transfer rate from the A/D converter to the digital signal processor. This thesis provides a comprehensive discussion of these and other issues associated with the actual hardware implementation of the pilot signal scanning receiver.
Design and Implementation of a Practical FLEX Paging Decoder
McCulley, Scott L. (Virginia Tech, 1997-10-24)
The Motorola Inc. paging protocol FLEX is discussed. The design and construction of a FLEX paging protocol decoder is discussed in detail. It proposes a decoding solution that includes a radio frequency (RF) receiver and a decoder board. The RF receiver will be briefly discussed. The decoder design is the main focus of this thesis as it transforms the RF frequency modulated (FM) data from the receiver and converts it to FLEX data words. The decoder is designed to handle bit sampling, bit clock synchronization, FLEX packet detection, and FLEX data word collection. The FLEX data words are then sent by the decoder to an external computer through a serial link for bit processing and storage. A FLEX transmitter will send randomly generated data so that a bit error rate (BER) calculation can be made at a PC. Each receiver'9s noise power and noise bandwidth will be measured so that noise spectral density may be calculated. A complete measurement set-up will be shown on how these noise measurements are made. The BER at a known power level is recorded. This enables Eb/No curves to be generated so that results of the decoding algorithm may be compared. This is performed on two different receivers.

Browsing by Author "Rappaport, Theodore S."

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