A Geographic Information System (GIS) viewshed is the result of a function that determines, given a terrain model, which areas on a map can be seen from a given point(s), line or area. In the communications industry, this function has been used to model radio wave coverages and to site transceiver towers for cellular phones. However, there are errors involved with this function and, without the requisite data, it cannot account for building heights that may affect visibility in urban areas. This paper examines the ability to accurately show line-of-sight (LOS) radio wave coverages in order to establish the viability of replacing existing field methods with GIS viewshed analysis. An origin point capable of supporting a line-of-sight radio wave transmitter was chosen from within the Virginia Tech campus study area. A viewshed analysis was performed with ESRI's ArcView GIS, using this site as the observation point and a 30-meter resolution Digital Elevation Model (DEM) from the US Geological Survey. To check the accuracy of the viewshed, we transmitted at 27.5 GHz, a LOS frequency that has properties common in the wireless telecommunications industry. We also transmitted at 900 MHz from the same point to provide a comparison of the 27.5 GHz frequency to a non-line-of-sight radio wave.The data was recorded and then entered into the GIS where the visibility attribute of each point was compared to the viewshed's prediction of visibility. After this comparison, footprints and heights of campus buildings were included in the model, by adding them to the ground heights of the DEM. Another viewshed analysis was performed using the same origin site and the new building height DEM as the elevation grid. This second viewshed was compared with the recorded visibility attribute and signal strength data. The use of more complete surface data was shown to have a more positive correlation with the recorded data than the previous model. The comparison of these two viewsheds demonstrated how well the viewshed function can represent real-world visibility and showed that the accuracy of the viewshed function is dependent on the accuracy of the elevation model. The signal strength attribute enabled us to create a communications viewshed, or 'commshed'. The commshed consists of all data points that received a signal strong enough to carry data. The building viewshed and commshed were then compared to see to what degree the 27.5 GHz frequency deviates from true line-of-sight paths. There were slightly fewer points in the commshed than points 'in view' in the viewshed model due to the frequency's small wavelength and the lack of vegetation data in the elevation grid. However, when a power margin is added to the transmitter strength in order to overcome unpredictable conditions, the commshed's predictive accuracy increases as well. This means that for LOS radio systems running under normal operating conditions, the viewshed is an excellent predictor of receiving areas. A 900 MHz commshed was also created so that we could study the relationship between a line-of-sight radio wave and a wave that is not blocked by structures. This comparison showed that, as expected, a viewshed is a better predictor of LOS radio waves than non-LOS waves. The data sets and analysis presented here should help communications companies ascertain the best way to incorporate GIS and the viewshed function into their wave coverage mapping and tower siting processes. / Master of Science

This thesis proposes a system architecture for a high-data rate mobile Local Multipoint Distribution Service (LMDS) Network. Its goal is to provide a workable "strawman" design that can serve as a basis for further research. The architecture is designed to offer broadband services to train commuters via LMDS. The thesis provides a broad overview of system aspects, such as Doppler shift, modulation selection, and error correction coding. These parameters and others are important in providing a robust design. This thesis discusses a set of criteria that seek the best approach in terms of economical feasibility, throughput capabilities, design complexity, data routing, and robustness in serving multiple mobile units simultaneously. These criteria are examined through link budget analysis, layout designs, and throughput efficiency. System throughput specifications are calculated for services, such as web browsing, email, ftp, and voice services to 100 train commuters. The information rate was 134.4 Mbps. The information rate plus overhead, which includes routing, bit and framing synchronization, and error correction coding, was approximately 201.51 Mbps. Using Carson's rule, the total required bandwidth for downstream transmissions was approximately 263 MHz. This throughput requirement was a criterion in selecting the appropriate system architecture. Three approaches were evaluated: LMDS Infostations, Tower Sites, and Infostations/Tower Sites. Infostations are low-powered wireless cells designed to offer individual pockets of high bandwidth connectivity for broadband services. Tower Sites use switched antenna beams to offer continuous services to train commuters. The hybrid solution, Infostations/Tower Sites, offers continuous services with increased power requirements and increased base stations separation when compared to the Infostation approach. Link budgets were examined for the Infostations and Tower Site approach. The initial required power for the Infostation was 1 mW. A 42.4 dB Eb/No link margin was computed using the Friis equation. The initial required power for the Tower Site approach was 500 mW. A 10.2 dB Eb/No link margin was computed with this approach. Tradeoffs with the non-fixed parameters were made to vary the link margins. An economically feasible number of required units were also determined. Approximately 3,000 low-powered Infostations would be needed to offer continuous service. Only 93 Tower Sites would be required and 4*93 Tower Site/Infostation units would be needed to supply continuous, seamless services over a 230-miles coverage area. The LMDS Tower Site was chosen to be the most suitable approach because of its robustness in meeting the pre-defined criteria. / Master of Science

The recent allocation and auctioning of the largest spectrum currently available has positioned Local Multipoint Distribution Service (LMDS) technology as a major contender in the race to provide broadband connectivity to customers. With the focus on solving the "Last Mile" problem, LMDS provides a significant reduction in both the time it takes for system deployment as well as the cost of the implementation. However, as with all emerging technologies, the lack of standardization and the risks involved in being the pioneer of an emerging market have resulted in a slower commercialization of such systems. The issues mentioned above served as the catalyst for this paper, to examine the factors that affect deployment strategies for an LMDS system in a rural commercial environment. In order to provide a better understanding of this technology, a review on the background issues in the areas of terrain, coverage/capacity, and networking will be presented. By evaluating these criteria, a strategy that can be employed to facilitate the implementation of LMDS systems is generated. Finally, the application of these methodologies will be demonstrated on a case study performed for a commercial environment in the county of South Boston, Virginia. Through evaluating the possible designs and the feasibility of the business case, it was determined that a point-to-multipoint system will best match the needs of the proposed location. / Master of Science

This thesis presents the results of a 28 GHz continuous-wave (CW) diffraction measurement campaign in the Washington, DC area. It describes the measurement approach including information on equipment and testing methods. Also described are the various parameters that affected the diffraction loss. Observed diffraction losses showed little dependence on polarization and building material. For diffraction angles greater than 5 degrees, a simple linear equation was fit to the data and accurately describes the diffraction loss. A logarithmic equation describes the dependence at smaller angles. The model developed shows very good agreement with theory and other measurements. Also included are an overview of the fixed wireless industry, a discussion of system design issues, and a review of the historical and mathematical development of diffraction theory. / Master of Science

Providers of high-speed, wireless data services need to know where in their service area to place transmitters to reach potential customers. Viewshed analysis, a technique found in Geographic Information Systems (GIS) software, can be used to model propagation of the wireless signals from different locations to find the best transmitter site. To carry out viewshed analysis, digital data are required for all obstructions the signal may encounter along its path. One such obstruction, terrain, can be represented in the GIS by easily available Digital Elevation Models (DEMs). Another obstruction is buildings, which are common in populated areas, and therefore of particular concern to wireless providers. Unfortunately, digital data for buildings in U.S. cities and towns are often non-existent, difficult to obtain, or very costly. In light of the difficulties surrounding acquisition of building data for wireless propagation studies, this study used Multiple Regression analysis to construct models to predict building count and dimensions. U.S. Census Housing and Demographic data, aggregated at the Census Block level, served as the predictor variables in the regression equations. The models were built from sample data collected from four U.S. cities. For each variable to be predicted (Y), the top models were compared to find the optimum one. The model chosen for Building Count (per Block) showed quite good results, and future research in the prediction of this variable shows promise. Results for the models of Average Building Height and Average Building Footprint Area (both per Block) were not nearly as encouraging, but additional work modeling these variables may still yield insights. / Master of Science

Digital Elevation Models (DEMs) are computer-generated representations of the earth's surface. These surfaces can be used to predicted Line-of-Sight (LOS) radio propagation. DEM resolution can affect the results of this prediction. This study examines the effect of DEM resolution on accuracy by comparing varied resolution terrain data for a portion of Blacksburg, Virginia using the prediction of ESRI's ArcView® viewshed algorithm. Results show that resolutions between one-meter and thirty-meters have little effect on the aggregate accuracy of the viewshed. / Master of Science

This thesis focuses on the interaction between digital video quality and channel coding in a wireless communication system. Digital video is a high-bandwidth, computationally intensive application. The recent allocation of large tracks of spectrum by the FCC has made possible the design and implementation of personal wireless digital video devices for several applications, from personal communications to surveillance. Wireless video research has centered on contextual coding mechanisms; different sections of a video data stream are coded differently based on the perceived importance of the coded bits. Besides the complexity introduced into a system by this type of coding, no metric based solely on physical layer parameters can be used to predict video quality across different system architectures. A tool was built to explore the video/channel coding relationship. This tool simulates a packet-based digital wireless transmission in various noise and interference environments. The basic communications system models the DAVIC (Digital Audio-Visual Council) layout for the LMDS (Local Multipoint Distribution Service) system and includes several error control algorithms and a packetizing algorithm that is MPEG-compliant. This work introduces a statistical approach to monitoring the video quality. The error-event-mean-arrival-rate, lambda_distance, is a metric derived from the physical layer that can be used to predict the expected video quality across systems with different channel coding algorithms. This metric proved to be uniformly consistent in predicting video quality for sequences corrupted by Gaussian and non-Gaussian noise and protected by a variety of error correction techniques. / Master of Science

[pt] Este trabalho apresenta o desenvolvimento de novo método de previsão da distribuição cumulativa de probabilidades da atenuação por chuvas para climas tropicais que em conta o percentual estimado da contribuição dos sistemas de chuva convectiva e estratiforme na região. O método proposto mostrou desempenho muito superior ao obtido com o método atualmente recomendado pelo ITU-R quando comparado com medidas experimentais realizadas, nas freqüências de 15 e 18 GHz, em cinco enlaces em um clima tropical. O método de previsão desenvolvido foi aplicado ao cálculo de enlaces de sistemas LMDS, cujas características e metodologia de planejamento foram também descritas no trabalho. Nestes sistemas a atenuação por chuvas é o principal fator limitante da cobertura, em função da operação nas faixas de 24 a 30 GHz. / [en] This work presents the development of a new method for the prediction of rain attenuation cumulative distributions in tropical climates. The method takes into account the estimated percent contribution of convective and stratiform rain systems. The proposed method shows much better accuracy than the method currently recommended by the International Telecommunications Union (ITU-R), when tested against experimental data from five links operating at 15 and 18 GHz. The prediction method was applied in link budgets for Local Multipoint Distribution Systems (LMDS) that operate in the 24 to 30 GHz frequency band where rain attenuation is the main propagation impairment. A design methodology for these systems is also presented.

[pt] Um aspecto muito importante a ser considerado no dimensionamento e planejamento de sistemas de comunicações rádio celulares móveis e de rádio acesso, é a determinação da cobertura, para o que tem sido propostos, na literatura técnica, vários métodos de cálculo. O cálculo de cobertura dos sistemas wireless, e sua otimização, requerem o desenvolvimento de programas de computação, que se constituem numa poderosa ferramenta para o planejamentos de sistemas. No presente trabalho, são analisados diversos fatores que devem ser considerados nesse dimensionamento e planejamento de sistemas wireless, com ênfase na análise e desenvolvimento de algoritmos apropriados para a determinação da distância de cobertura dos sistemas de rádio acesso WLL (wireless Local Loop) e do sistema LMDS. Os resultados são apresentados através da análise dos diversos fatores que influenciam na área de cobertura destes sistemas. Os algoritmos foram implementados, com auxílio de programadores, no programa NETDimension, que foi utilizado para gerar alguns exemplos de planejamento. / [en] A very important aspect that must be considered in the dimensioning and planning of a cellular mobile and acess radio communication systems is the coverage prediction. The technical literaute several methods of calculations have been proposed. The coverage calculation of wireless systems, and its optimization, require the development of computer programs, that are powerful tools for systems planning. In this work, a wide range of factors that must be considered in the planning of wireless systems are analyzed, with emphasis in the development of algorithms for coverage prediction in WLL (Wireless Local Loop) and LMDS systems. The results are presented and several factors, which have influence in the coverage area of these systems, are analyzed. The algorithms were implemented with help of programmers, in the NETDimension software, and examples of design of radio acess systems developed usign the software are presented.

Channel impulse responses collected on the Virginia Tech campus show combinations of specular multipath and diffuse scattering at LMDS frequencies. An algorithm is presented that estimates link performance based on the channel impulse response. Presented and analyzed are representative impulse responses (one is primarily specular in nature and one shows significant diffuse scattering) to show that the proposed algorithm is appropriate for analyzing channels exhibiting either of these characteristics. Monte Carlo simulations logged the sequence number of each bit error to gain an understanding of the distribution of errors over time. The results show that for these static channels the errors occur randomly rather than in bursts, leading to the conclusion that average bit-error rate statistics are appropriate for channel characterization. Zero-Forcing (Z-F) and Minimum Mean Square Error (MMSE) equalizers employed on these channels had a significant impact on the link quality. In many cases, the performance of the MMSE equalizer performed only slightly better than the Z-F equalizer. However, when deep nulls were present in the channel response, the MMSE equalizer performed significantly better. Algorithms for determining the number of taps necessary to approach an optimum equalization are presented for both types of equalizers and a '98%' rule of thumb is presented. The algorithm's role in adaptive and cognitive radio systems is discussed and two applications are presented to illustrate its utility. / Ph. D.