Browsing by Author "Woerner, Brian D."
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- 2GHz W-CDMA Radio TransceiverCheung, Tze Chiu (Virginia Tech, 1998-12-15)A radio transceiver has been designed and built for a W-CDMA experimental system operating at 2GHz. The radio is an RF front-end for mobile terminals. The radio provides the functions of modulation and demodulation to enable transmitting and receiving digital information through the air link. The radio comprises one transmitter in conjunction with transmit power control (TPC), two independent receivers in conjunction with automatic gain control (AGC) and automatic frequency control (AFC), and one common synthesizer. Data exchange between the radio and the baseband processor takes place in an 8-bit digital format. Digital-to-analog converters (DAC) at the transmitter and analog- to-digital converters (ADC) at the receivers provide the interface between the radio and the baseband processor. DACs are also used to convert the 7-bit command codes from the processor to analog signals for the TPC, AGC and AFC. The radio transceiver is designed to meet the stringent requirements imposed by the W-CDMA system. The 70dB TPC enables the adaptive power control for combating the near-far problem. The high linearity of the transmitter provides the use of linear modulation with less than 40dBc adjacent channel power suppression. The 0.03125ppm tuning resolution of the AFC maximizes the receiver sensitivity. The 80dB AGC produces a constant demodulated signal level to the ADC regardless of the RF signal level. This thesis documents the design methodology for this radio transceiver.
- Adaptive Antenna Arrays Applied to Position LocationBreslin, Donald F. (Virginia Tech, 1997-08-04)Wireless communication has enjoyed explosive growth over the past decade. As demands for increased capacity and quality grow, improved methods for harnessing the multipath wireless channel must be developed. The use of adaptive antenna arrays is one area that shows promise for improving capacity of wireless systems and providing improved safety through position location capabilities. These arrays can be used for interference rejection through spatial filtering, position location through direction finding measurements, and developing improved channel models through angle of arrival channel sounding measurements. This thesis provides an overview of the technical challenges involved in position location of wireless users and details the hardware development of a multi-sensor testbed at the Mobile and Portable Radio Research Group at Virginia Tech. This testbed is to be used for position location experiments as well as a host of other adaptive signal processing applications.
- Adaptive Equalization for Indoor ChannelsMorton, John M. (Virginia Tech, 1998-08-03)This thesis describes the use of adaptive equalization techniques to compensate for the intersymbol interference (ISI) that results when digital data is transmitted over a multipath radio channel. The equalization structures covered in this work are the linear transversal equalizer (LTE), the fractionally spaced equalizer (FSE), the decision-feedback equalizer (DFE), and the maximum-likelihood sequence estimation (MLSE) equalizer. This work also covers adaptive algorithms for equalization including both the least mean squares (LMS) and the recursive least squares (RLS) algorithm. All these equalizer structures and algorithms will be modeled using various simulation modules. Equalization for both stationary and mobile radio channels is considered. Stationary channels are modeled with a simple exponentially decaying profile. The mobile radio channel is represented using a two-ray Rayleigh fading model for an outdoor environment. The SIRCIM channel modeling tool is used to create channel profiles for an indoor mobile radio channel. Adaptive arrays and their similarities to linear equalizers are also studied in this thesis. The properties and performance of simple adaptive array systems using the LMS and RLS algorithms are examined through simulation. This thesis concludes with an in-depth study of the use of adaptive equalization for high-speed data systems operating in an indoor environment. Both stationary and slowly varying radio channels are examined. Simulations of DFE and MLSE equalizers operating in such a system show that both equalizer structures provide better BER performance over a system with no equalization. These simulation results also show that the MLSE equalizer provides better performance than the DFE in almost all cases, but requires a great deal more computations.
- An adaptive multistage interference cancellation receiver for CDMAKaul, 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.
- AMPS co-channel interference rejection techniques and their impact on system capacityHe, 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.
- Analysis and Characterization of Fiber Nonlinearities with Deterministic and Stochastic Signal SourcesLee, Jong-Hyung (Virginia Tech, 2000-02-10)In this dissertation, various analytical models to characterize fiber nonlinearities have been applied, and the ranges of validity of the models are determined by comparing with numerical results. First, the perturbation approach is used to solve the nonlinear Schrödinger equation, and its range of validity is determined by comparing to the split-step Fourier method. In addition, it is shown mathematically that the perturbation approach is equivalent to the Volterra series approach. Secondly, root-mean-square (RMS) widths both in the time domain and in the frequency domain are modeled. It is shown that there exists an optimal input pulse width to minimize output pulse width based on the derived RMS models, and the functional form of the minimum output pulse width is derived. The response of a fiber to a sinusoidally modulated input which models an alternating bit sequence is studied to see its utility in measuring system performance in the presence of the fiber nonlinearities. In a single channel system, the sinusoidal response shows a strong correlation with eye-opening penalty in the normal dispersion region over a wide range of parameters, but over a more limited range in the anomalous dispersion region. The cross-phase modulation (CPM) penalty in a multi-channel system is also studied using the sinusoidally modulated input signal. The derived expression shows good agreement with numerical results in conventional fiber systems over a wide range of channel spacing, ∆f, and in dispersion-shifted fiber systems when ∆f > 100GHz. It is also shown that the effect of fiber nonlinearities may be characterized with stochastic input signals using noise-loading analysis. In a dense wavelength division multiplexed (DWDM) system where channels are spaced very closely, the broadened spectrum due to various nonlinear effects like SPM (self-phase modulation), CPM, and FWM (four-wave mixing) is in practice indistinguishable. In such a system, the noise-loading analysis could be useful in assessing the effects of broadened spectrum due to fiber nonlinearities on system performance. Finally, it is shown numerically how fiber nonlinearities can be utilized to improve system performance of a spectrum-sliced WDM system. The major limiting factors of utilizing fiber nonlinearities are also discussed.
- Analysis and Dynamic Range Enhancement of the Analog-to-Digital Interface in Multimode Radio ReceiversFox, Brian L. (Virginia Tech, 1997-02-01)The rapidly developing wireless market has spawned a multitude of different standards for cellular, PCS, and wireless data. To allow users the ability to access services conforming to disparate standards, multimode handsets capable of software reconfiguration are needed. These "software radios" are distinguished from their traditional counterparts by their strong reliance on digital channel filtering and demodulation which may be reprogrammed to receive different standards. In these radios, higher dynamic range is required from the analog portion, most notably, the analog-to-digital converter (ADC). This research examines through analysis and simulation the performance requirements of analog-to-digital converters for use in radios which are conformant to the AMPS, IS-54, GSM, and IS-95 cellular standards. Simulations reveal the degradation in performance under conditions of off-channel interference, fading, and converter nonlinearities. Included in this analysis is the design of automatic gain control (AGC) for narrowband and IS-95 spread spectrum systems to optimize quantization noise and distortion due to A/D overload. Lastly, methods for improving the dynamic range of the analog-to-digital interface such as nonuniform quantization, companding, and dither are presented. The development of a novel A/D using a direct-sequence pseudo-noise (DSPN) technique in conjunction with an asymmetrical quantizer is presented and compared with standard dither techniques. Advantages of this technique compared to ordinary ADC's include an almost one bit improvement in resolution, quantization noise whitening, elimination of A/D offsets, and the ability to simultaneously digitize multiple analog signals with a single quantizer. The technique requires no synchronization and is easily implemented.
- Analysis and simulation of the Kerr effect in long haul in-line fiber amplifier transmission systemsMa, Xiaobing (Virginia Tech, 1994-12-05)The next generation of transoceanic submarine cable systems will use in-line fiber amplifiers to replace electrical regenerators. This new approach requires a better understanding of the fiber waveguide, especially the nonlinear characteristics. It has been demonstrated, both by numerical simulation and experiments, that the Kerr effect has the most significant degradation effects on these systems with a single or a few channels. In this dissertation, the numerical simulation is the principal approach but this is supplemented with some analytical studies. There are two phenomena that are directly associated with the Kerr effect: spectrum broadening and four wave mixing (FWM). The broadened signal spectrum enhances the dispersion effect and consequently increases the inter symbol interference. This distortion is significant when the dispersion is relatively large. U sing erbium doped fiber amplifiers, amplified spontaneous emission (ASE) noise is added to the signal The FWM effect between the noise and signal causes a loss in the signal power. In the close vicinity of the zero dispersion wavelength, the FWM effect is maximized due to the loose phase match condition. In order to reduce these degradation effects, dispersion compensation has recently been proposed. Using this approach, it is possible to optimize the system configuration to achieve the best performance. In this dissertation, the dispersion compensation distance, dispersion coefficient, channel bandwidth, bit rate and the degree of imperfect compensation are all subject to the optimization. The evaluation is obtained by the numerical simulation using the mean squared error (MSE), which can be derived as the difference between the wavefronts received by a back-to-back receiver and a receiver at the end of the channel. Although the MSE can't be related to the bit error directly, this research provides the insight into how dispersion and noise behave in the presence of the Kerr effect and points the direction for future experimental research.?
- Analysis, Design and Performance Evaluation of Optical Fiber Spectrum-Sliced WDM SystemsArya, Vivek (Virginia Tech, 1997-06-03)This dissertation investigates the design and performance issues of a recently demonstrated technique, termed as spectrum-slicing, for implementing wavelength-division-multiplexing (WDM) in optical fiber systems. Conventional WDM systems employ laser diodes operating at discrete wavelengths as carriers for the different data channels that are to be multiplexed. Spectrum-slicing provides an attractive low-cost alternative to the use of multiple coherent lasers for such WDM applications by utilizing spectral slices of a broadband noise source for the different data channels. The principal broadband noise source considered is the amplified spontaneous emission (ASE) noise from an optical amplifier. Each slice of the spectrum is actually a burst of noise that is modulated individually for a high capacity WDM system. The stochastic nature of the broadband source gives rise to excess intensity noise which results in a power penalty at the receiver. One way to minimize this penalty, as proposed and analyzed for the first time in this work, is to use an optical preamplifier receiver. It is shown that when an optical preamplifier receiver is used, there exists an optimum filter bandwidth which optimizes the detection sensitivity (minimizes the average number of photons/bit) for a given error probability. Moreover the evaluated detection sensitivity represents an order of magnitude ( > 10 dB) improvement over conventional PIN receiver-based detection techniques for such spectrum-sliced communication systems. The optimum is a consequence of signal energy fluctuations dominating at low values of the signal time bandwidth product (m), and the preamplifier ASE noise dominating at high values of m. Operation at the optimum bandwidth renders the channel error probability to be a strong function of the optical bandwidth, thus providing motivation for the use of forward error correction coding (FEC). System capacity (for BER = ) is shown to be 23 Gb/s without coding, and 75 Gb/s with a (255,239) Reed Solomon code. The effect of non-rectangular spectra on receiver sensitivity is investigated for both OOK and FSK transmission, assuming the system (de)multiplexer filters to be N'th order Butterworth bandpass. Although narrower filters are recommended for improving power budget, it is shown that system penalty due to filter shape may be kept < 1 dB by employing filters with N > 2. Moreover spectrum-sliced FSK systems using optical preamplifier receivers are shown, for the first time, to perform better in a peak optical power limited environment. Performance-optimized spectrum-sliced WDM systems have potential use in both local loop and long-distance fiber communication systems which require low-cost WDM equipment for high data rate applications.
- Antenna Array Systems: Propagation and PerformanceErtel, 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 systemsHo, 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.
- The Applicability of the Tap-Delay Line Channel Model to Ultra WidebandYang, Liu (Virginia Tech, 2004-09-15)Ultra-wideband (UWB) communication systems are highly promising because of their capabilities for high data rate information transmission with low power consumption and low interference and their immunity to multipath fading. More importantly, they have the potential to relieve the "spectrum drought" caused by the explosion of wireless systems in the past decade by operating in the same bands as existing narrowband systems. With the extremely large bandwidth of UWB signals, we need to revisit UWB channel modeling. Specifically we need to verify whether or not the traditional tap-line delay channel model is still applicable to UWB. One essential task involved in channel modeling is deconvolving the channel impulse response from the measurement data. Both frequency domain and time domain techniques were studied in this work. After a comparison, we examined a time domain technique known as the CLEAN algorithm for our channel modeling analysis. A detailed analysis of the CLEAN algorithm is given, as it is found that it is sufficient for our application. The impact of per-path pulse distortion due to various mechanisms on the tap-delay line channel model is discussed. It is shown that with cautious interpretation of the channel impulse response, the tap-line delay channel model is still applicable to UWB.
- The application of multiuser detection to cellular CDMABuehrer, 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.
- Applications of Multiwavelets to Image CompressionMartin, Michael B. (Virginia Tech, 1999-06-08)Methods for digital image compression have been the subject of much study over the past decade. Advances in wavelet transforms and quantization methods have produced algorithms capable of surpassing the existing image compression standards like the Joint Photographic Experts Group (JPEG) algorithm. For best performance in image compression, wavelet transforms require filters that combine a number of desirable properties, such as orthogonality and symmetry. However, the design possibilities for wavelets are limited because they cannot simultaneously possess all of these desirable properties. The relatively new field of multiwavelets shows promise in removing some of the limitations of wavelets. Multiwavelets offer more design options and hence can combine all desirable transform features. The few previously published results of multiwavelet-based image compression have mostly fallen short of the performance enjoyed by the current wavelet algorithms. This thesis presents new multiwavelet transform methods and measurements that verify the potential benefits of multiwavelets. Using a zerotree quantization scheme modified to better match the unique decomposition properties of multiwavelets, it is shown that the latest multiwavelet filters can give performance equal to, or in many cases superior to, the current wavelet filters. The performance of multiwavelet packets is also explored for the first time and is shown to be competitive to that of wavelet packets in some cases. The wavelet and multiwavelet filter banks are tested on a much wider range of images than in the usual literature, providing a better analysis of the benefits and drawbacks of each. NOTE: (03/2007) An updated copy of this ETD was added after there were patron reports of problems with the file.
- Beamforming for MC-CDMAVenkatasubramanian, Ramasamy (Virginia Tech, 2003-01-31)Orthogonal Frequency Division Multiplexing (OFDM) has recently gained a lot of attention and is a potential candidate for Fourth Generation (4G) wireless systems because it promises data rates up to 10Mbps. A variation of OFDM is Multi-Carrier CDMA (MC-CDMA) which is an OFDM technique where the individual data symbols are spread using a spreading code in the frequency domain. The spreading code associated with MC-CDMA provides multiple access technique as well as interference suppression. Often times in cellular and military environments the desired signal can be buried below interference. In such conditions, the processing gain associated with the spreading cannot provide the needed interference suppression. This research work investigates multi-antenna receivers for OFDM and MC-CDMA systems; specifically this works investigates adaptive antenna algorithms for MC-CDMA for very different channel conditions. Frequency domain beamforming is studied in this research predominantly through simulation. As an alternative a time domain beamforming is also studied. Time variations in the channel can disrupt the orthogonality between subcarriers. Minimum Mean Square Error (MMSE) detection coupled with MMSE beamforming is proposed for time varying channels. Semi-analytic results are derived to study the Bit Error Rate (BER) performance. These results show significant performance improvement in the presence of interference. Joint MMSE weights in space and frequency is also investigated and semi-analytic results are derived to study their BER performance.
- Channel Estimation Strategies for Coded MIMO SystemsTrepkowski, Rose E. (Virginia Tech, 2004-08-06)High transmission data rate, spectral efficiency, and reliability are necessary for future wireless communications systems. In a multipath-rich wireless channel, deploying multiple antennas at both the transmitter and receiver achieves high data rate, without increasing the total transmission power or bandwidth. When perfect knowledge of the wireless channel conditions is available at the receiver, the capacity has been shown to grow linearly with the number of antennas. However, the channel conditions must be estimated since perfect channel knowledge is never known a priori. In practice, the channel estimation procedure can be aided by transmitting pilot symbols that are known at the receiver. System performance depends on the quality of channel estimate, and the number of pilot symbols. It is desirable to limit the number of transmitted pilot symbols because pilot symbols reduce spectral efficiency. This thesis analyzes the system performance of coded multiple-input multiple-output (MIMO) systems for the quasi-static fading channel. The assumption that perfect channel knowledge is available at the receiver must be removed, in order to more accurately examine the system performance. Emphasis is placed on developing channel estimation strategies for an iterative Vertical Bell-Labs Layered Space Time (V-BLAST) architecture. The channel estimate can be sequentially improved between successive iterations of the iterative V-BLAST algorithm. For both the coded and uncoded systems, at high signal to noise ratio only a minimum number of pilot symbols per transmit antenna are required to achieve perfect channel knowledge performance.
- Characterization of Ultra Wideband Communication ChannelsMuqaibel, Ali Hussein (Virginia Tech, 2003-03-05)Ultra-wideband (UWB) communication has been the subject of extensive research in recent years due to its unique capabilities and potential applications, particularly in short-range multiple access wireless communications. However, many important aspects of UWB-based communication systems have not yet been thoroughly investigated. The propagation of UWB signals in indoor environments is the single most important issue with significant impacts on the future direction, scope, and generally the extent of the success of UWB technology. The objective of this dissertation is to obtain a more thorough and comprehensive understanding of the potentials of UWB technology by characterizing the UWB communication channels. Channel characterization refers to extracting the channel parameters from measured data. The extracted parameters are used to quantify the effect of the channel on communication UWB systems using this channel as signal transmission medium. Data are measured in different ways using a variety of time-domain and frequency-domain techniques. The experimental setups used in channel characterization effort also include pulse generators and antennas as integral parts of the channel, since the pulse shape and antenna characteristics have significant impact on channel parameters. At a fundamental level, the propagation of UWB signals, as any electromagnetic wave, is governed, among other things, by the properties of materials in the propagation medium. One of the objectives of this research is to examine propagation through walls made of typical building materials and thereby acquire ultra-wideband characterization of these materials. The loss and the dielectric constant of each material are measured over a frequency range of 1 to 15 GHz. Ten commonly used building materials are chosen for this investigation. These include, dry wall, wallboard, structure wood, glass sheet, bricks, concrete blocks, reinforced concrete (as pillar), cloth office partition, wooden door, and styrofoam slab. The work on ultra-wideband characterization of building materials resulted in an additional interesting contribution. A new formulation for evaluating the complex dielectric constant of low-loss materials, which involves solving real equations and thus requiring only one-dimensional root searching techniques, was found. The results derived from the exact complex equation and from the new formulation are in excellent agreement. Following the characterization of building materials, an indoor UWB measurement campaign is undertaken. Typical indoor scenarios, including line-of-sight (LOS), non-line-of-sight (NLOS), room-to-room, within-the-room, and hallways, are considered. Results for indoor propagation measurements are presented for local power delay profiles (local-PDP) and small-scale averaged power delay profiles (SSA-PDP). Site-specific trends and general observations are discussed. The results for pathloss exponent and time dispersion parameters are presented. The analyses results indicate the immunity of UWB signals to multipath fading. The results also clearly show that UWB signals, unlike narrowband signals, do not suffer from small scale fading, unless the receiver is too close to walls. Multipath components are further studies by employing a deconvolution technique. The application of deconvolution results in resolving multipath components with waveforms different from those of the sounding pulse. Resolving more components can improve the design of the rake receiver. The final part of this research elaborates on the nature of multiple access interference and illustrates the application of multi-user detection to improve the performance of impulse radio systems. Measured dispersion parameters and their effects on the multiple access parameters are discussed.
- Coding performance on satellite channels using AX.25 protocolJesser, William Augustus (Virginia Tech, 1993-05-05)A form of data transmission which is increasing in popularity is satellite communication. In order to insure that data is received correctly, certain error control strategies are employed. In packetized transmission, automatic-repeat-request (ARQ) schemes and error correcting codes have been employed. Currently a combination of both is not employed by the AX.25 protocol. The purpose of this project is to determine which error correcting code should be combined with ARQ to provide the optimum performance. This project investigates the performance of the (7,4) Hamming code, (23,12) Golay code, Reed-Solomon, RS , codes over the Galois fields of 16, GF(16), and 256, GF(256), elements, and common rate convolutional codes of various constraint lengths. The codes are evaluated on three primary criteria, which include: throughput vs. input bit error rate, coding complexity, and burst error performance. The class of Reed-Solomon codes over GF(256) was chosen to be superior due to the throughput and burst error performance. The encoding complexity is small, but the decoding is more complicated. The conclusion is that error correcting codes should be employed in ARQ satellite systems. However, the error correcting strength of the code must be determined by observing the channel characteristics.
- Combined Space-Time Diversity and Interference Cancellation for MIMO Wireless SystemsTsai, Jiann-An (Virginia Tech, 2002-05-02)There is increasing interest in the exploitation of multiple-input and multiple-output (MIMO) channels to enhance the capacity of wireless systems. In this study, we develop and evaluate a channel model, evaluate the corresponding channel capacity, and design and analyze a simple orthogonal transmit waveform for MIMO channels in mobile radio environments. We also evaluate the system performance of various interference cancellation techniques when employing multiple-receive antenna in interference-limited systems. The first part of this dissertation presents two major contributions to MIMO systems. The analytical expression for space-time MIMO channel correlation is derived for a Rayleigh fading channel. The information-theoretic channel capacity based on this correlation is also evaluated for a wide variety of mobile radio channels. The second part of this dissertation presents two major contributions to the area of orthogonal waveform design. We analyze the bit-error-rate (BER) performance of a proposed space-time orthogonal waveform for MIMO mobile radio communications. The application of the proposed space-time orthogonal waveform to a conventional cellular system is also evaluated and briefly discussed. Finally, this dissertation investigates a number of interference cancellation techniques for multiple-receive antenna systems. Both adaptive beamforming and multiuser detection are evaluated for various signal waveforms over a variety of mobile radio channels.
- A comparison of two novel channel coding techniques for CDMAKim, 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.