In this study, the properties of the reflected paths and scattering phenomena were investigated in the LMDS band (28 GHz). We used the newly developed sampling swept time delay short pulse (SSTDSP) sounder to collect field data in certain locations on the Virginia Tech campus. The sounder collected the channel impulse response analog waveform, sampled, digitized and reconstructed it. The stored data were used to produce the power delay profile and other channel parameters. In particular, we collected scattered and reflected data regarding the channel response with different incident angle and distance set-ups from brick and limestone walls. We used the reflected pulse width and maximum excess delay derived from each power delay profile to analyze the rough surface scattering phenomena. We found that limestone and brick walls exhibited some diffuse scattering. The reflected pulse of a limestone wall had more maximum excess delay spread than did a brick wall at -15dB power threshold. The mean maximum excess delay for the reflected pulse of the limestone wall measurement set-ups was more than two times that of the brick wall. With equal transmitter and receiver distances to the wall, we found that as the incident angle increased, the maximum excess delay decreased but the perpendicular reflection coefficient increased. It is recommended that for future study, a second generation SSTDSP sounder will replicate the measurement with larger distance and angle set-ups as well as in non-line-of-sight areas. / Master of Science

The growth of broadband Internet access has paved the way for the development of many newer existing technologies. As the costs of implementing broadband access soar, the best alternative will be to use wireless technologies. At a carrier frequency of about 28 GHz, the potential benefits of Local Multipoint Distribution Service (LMDS) will eventually outweigh those of its current competitors in the wired market. Since the rural communities will reap the most benefits from this technology due to its low costs, studies on the channel behavior and terrain relationships must be done. This thesis aims to provide a preliminary study on the effects of propagating an LMDS signal over a lake surface. Currently, there is not enough information to prove the feasibility of deploying an LMDS system for this terrain. Some background on the technology and rough surface theory is provided for the reader to better understand the environment that is being investigated. Simulation results are presented as a guide to assist future researchers to conduct a field measurement campaign. A block diagram of a potential measurement system is also provided to aid in the development of the tools necessary for the measurement of an actual channel. / Master of Science

This thesis takes the reader through an overview of issues pertinent to Local Multi-point Distribution Services (LMDS). The reader will first learn what LMDS is and then review the system architectures that are made available for LMDS technologies. After summarizing the basics of LMDS, we will compare it with some competing technologies. The reader will then be guided through the aspects of millimeter (mm) wave radio link design. This should be a good lead into the experiment section, since it is suspected that the reader would want to be aware of what design techniques are involved in mm-wave radio link design and what issues may pose potential problems and how they may be mitigated. Of the potential problems posed in the mm-wave radio link design section, one will be further investigated experimentally. This is the investigation of the effect of vegetation on the magnitude, phase and error vector magnitude (EVM) performance of an LMDS channel. The motivation for this experiment came from the review earlier work, which showed an unexpected relationship between carrier to noise ratio (C/N) and bit error rate BER. / Master of Science

Millimeter wave (mm-Wave) systems have become very attractive recently as lower frequency spectrums used for mobile device communications have been experiencing a “spectral crunch” due to the dissemination of smartphones. Channel characterization of the outdoor urban environment, where networks for mobile devices require the highest data capacity, has been quite scarce and even non-existent for cellular (rooftop to ground) setting measurements. Our project characterizes the urban environment at 38 GHz in a cellular setting and 38 and 60 GHz in a peer-to-peer setting. A sliding correlator channel sounder with an 800 MHz RF bandwidth at 38 GHz and 1.5 GHz RF bandwidth at 60 GHz was constructed to measure the channel using a bandwidth that is larger than the expected bandwidths of future mm-Wave channels. Directional antennas were utilized during the measurements to imitate mm-Wave systems using beam steering antenna arrays, which also allowed for AOA characterization. Path loss and RMS delay spread statistics are provided. Finally, an outage study was performed to test the outage likelihood in an urban environment with many multi-story buildings. / text

Over the years, the Internet has become increasingly popular and people's dependence on it has increased dramatically. Whether it be to communicate to someone across the world, find blueprints, or check sports scores, the Internet has become a necessary resource for everyone. In emergency situations, this need increases further. After the terrorist attacks on the Pentagon, it took several days to restore communications. This is not an acceptable time frame when people's lives are at stake. Virginia Tech's Center for Wireless Telecommunication has developed a prototype of a rapidly deployable high bandwidth wireless communication system at 28 GHz (Local Multipoint Distribution Service frequency). This system provides a large bandwidth radio link to a disaster zone up to 5 km away and puts Ethernet speeds and 802.11 accesses to users within hours. Because of the possible variability in locations that the system can be deployed, it is necessary to find the most useable channel at the site as quickly as possible. In addition to 28GHz radio links, the system also has a built-in channel sounder that measures and captures the channel impulse response of the current channel. Until now, there has been limited research on the relationship between the channel impulse response and the usability of the channel quantified using bit error rate. This thesis examines several different channels captured by CWT's channel sounder and simulates the BER using Cadence's SPW with time-domain models of the channels. This thesis goes on further to show that BER greatly depends on the channel impulse response and the symbol rate. / Master of Science

This thesis presents a model to predict signal strength for frequencies between 902 MHz and 28 GHz. The model approximates diffraction using the knife-edge concept and equations proposed by Lee (1985). LOS pathways are calculated using the Bresenham algorithm and the corresponding elevations are obtained from a 30m DEM base map. The base map was generated by the procedure outlined in Rose (2001) and includes building elevations. The effect of Fresnel zones on prediction accuracy is considered. The effect of interpolating elevations along the Bresenham line is also considered. An Inverse Distance Weighting algorithm was used to interpolate the elevations. The accuracy of the model was evaluated using received signal strength data compiled from studies conducted at 902 MHz, 24.12 GHz and 27.525 GHz. In addition to the compiled data, data was also collected for this study at 2.4 GHz. 257 receiver locations were evaluated; 70 samples were Line-of-Sight. The study area incorporates the Virginia Polytechnic Institute and State University campus. Incorporating Fresnel zones, Interpolating elevations and calculating double blockages do not have an effect on the program's overall ability to predict signal strength. However, for obstructed pathways, it is not adequate to simply use path loss as an estimate of signal strength. Accurate estimates of diffraction gain are crucial for obstructed pathways. In addition, examination of the standard deviation for the data sets indicates that the model is independent of frequency. The average error across the frequencies is positively correlated with frequency, indicating that the model predicts signal strength better for higher frequencies. The smaller wavelengths associated with the higher frequencies require a more directional antenna and are therefore less sensitive to multipath interference. In addition, the smaller wavelengths are less able to diffract around buildings and terrain features. / Master of Science

This thesis describes an architecture for policy-based quality of service (QoS) for fixed broadband wireless systems. An implementation of the proposed architecture for the Local Multipoint Distribution Service (LMDS) wireless network in Blacksburg, Virginia is described in detail. The focus of the research work was to enable simpler management of the LMDS system and to design and enable network QoS. The thesis examines various means to provide QoS in the network. It highlights issues related to enabling QoS in the VT-LMDS network, like prioritized access, resource management, service differentiation, and lack of predictability in network performance. Quality of service assumes a definition based on the context and application of interest. This research focuses on enabling service differentiation and intelligent resource management based on network conditions and link utilization. A software application that serves as a model of the described architecture was developed using the C++ programming language. The tool uses the Simple Network Management Protocol (SNMP) for the network management operations. The design, implementation issues and the advantages and shortcomings of the tool are outlined and a short primer on the use of the tool is provided. Finally, possibilities for future work in this area especially towards enabling the tool to work with other vendor-specific LMDS systems and non-LMDS fixed broadband wireless systems are examined and the issues in implementing one such system are described. / Master of Science

This thesis proposes the design of the UAV-LMDS communication system for military and commercial use. The UAV-LMDS system is a digital, wireless communication system that provides service using unmanned aerial vehicles (UAVs) flying at 60,000 ft. acting as communication hubs. This thesis provides background information on UAV-LMDS system elements, a financial analysis, theory, link budgets, system component design and implementation issues. To begin the design, we develop link budgets that are used to characterize system parameters. We present detailed antenna designs for the antennas aboard the UAV. We also present communication equipment block diagrams. Included are technical details on military and commercial geostationary satellites used to link transmissions in the system. Implementation issues in the military system are discussed. Mobility and the effects of vegetation in the propagation path are investigated and a co-channel interference study is done. This thesis shows that by using UAVs and LMDS, a viable, broadband, wireless communications system can be created for military and commercial use. / Master of Science

This thesis describes a possible integrated terrestrial-satellite network system for disaster recovery and response. The motivation of this thesis was based on the adjacent spectrum allocations between the Virginia Tech terrestrial Local Multiple Distribution Service (LMDS) system and a Ka-band satellite system, and potentially being able to provide as an additional Ka-band satellite network backbone to the Virginia Tech terrestrial LMDS system for better and faster communications deployments. The Spaceway satellite system's design parameters were adopted typically for a Ka-band satellite system. The LMDS system was assumed to use IEEE 802.16 standard protocols although it currently uses its own proprietary protocols. Four possible topologies integrating both terrestrial and satellite network were investigated. The study showed that the task was more problematic and complicated than anticipated due to incompatible network protocols, limitations of available hardware components, the high path loss at Ka-band, and the high cost of the equipment, although the adjacent frequency bands do suggest a possible integrated network. In this thesis, the final selected topology was proposed and designed. The technical characteristics of the earth station used for coupling both terrestrial and satellite networks were determined by a link budget analysis and a consideration of network implementations. The reflector antenna used by the earth station was designed. In addition, other system design concerns and engineering tradeoffs, including adjacent satellite interference, rain attenuation, antenna pointing error, noise temperature, and modulation and multiple access selection, were addressed. / Master of Science

Virginia Tech researchers have developed a broadband channel sounder that can measure channel quality while a wireless network is in operation. Channel measurements from the broadband sounder hold the promise of improving TCP/IP performance by trigging configuration changes in an adaptive data link layer protocol. We present an adaptive data link layer protocol that can use different levels of forward error correction (FEC) codes and link layer automatic retransmission request (ARQ) to improve network and transport layer performance. Using a simulation model developed in OPNET, we determine the effects of different data link layer protocol configurations on TCP/IP throughput and end-to-end delay using a Rayleigh fading channel model. Switching to higher levels of FEC encoding improves TCP/IP throughput for high bit error rates, but increases end-to-end delay of TCP/IP segments. Overall TCP/IP connections with link layer ARQ showed approximately 150 Kbps greater throughput than without ARQ, but lead to the highest end-to-end delay for high bit error rate channels. Based on the simulation results, we propose algorithms to maximize TCP/IP throughput and minimize end-to-end delay using the current bit error rate of the channel. We propose a metric, carrier-to-interference ratio (CIR) that is calculated from data retrieved from the broadband channel sounder. We propose algorithms using the carrier-to-interference ratio to control TCP/IP throughput and end-to-end delay. The thesis also describes a monitor program to use in the broadband wireless system. The monitor program displays data collected from the broadband sounder and controls the settings for the data link layer protocol and broadband sounder while the network is in operation. / Master of Science