An extensive radio wave propagation measurement campaign was conducted at 1.3 GHz and 4.0 GHz inside four buildings, including a sports arena, a modern closed-plan office building, and two dissimilar, open-plan factories. Measurements were recorded at 57 locations using base station antenna heights of 1.7 meters and 4.0 meters. Results were obtained for mean and maximum excess delay, rms delay spread, time delay jitter, differential delay jitter, and path loss through analyses of impulse response estimates, which were obtained via repetitive 5 ns probing pulses. The effects of frequency, antenna height, topography (line-of-sight or obstructed direct path), and building environment on delay spread and path loss are quantified. Results indicate that, on average, the frequencies and antenna heights used in this study have minimal impact on rms delay spread and path loss. However, topography and building environment significantly affect these parameters. RMS delay spread values as high as 230 ns were observed in open plan factories. Computed path loss power law exponents are 1.84 and 2.35 for line-of-sight and obstructed topographies, respectively. A second campaign was conducted to determine the effects of antenna directivity and polarization on propagation parameters. On average, line-of-sight indoor channels offer 8 dB of cross-polarization discrimination, whereas obstructed environments offer less than 3 dB. Directional antennas provide a significant reduction in rms delay spread over omni-directional antennas. In line-of-sight environments, circular polarization provides an additional delay spread reduction. / Master of Science

Designers of low-power radio systems for use in urban areas would benefit from the capability for accurate computer-based predictions of signal loss due to shadowing. This thesis is intended to fill a need for prediction methods that exploit a building database and consider the three-dimensional profile of the radio path. Models are presented that allow the application of Fresnel-Kirchoff diffraction theory to arbitrarily oriented buildings of simple shapes. Building location information used by the diffraction models is in a form compatible with a geographic information systems (GIS) database. Diffraction screens are constructed at all building edges, including those of both horizontal and vertical orientations, in order to consider all possible diffractions and to compute field contributions often ignored. Multiple buildings and edges of the same building that introduce multiple successive diffractions are considered with a rigorous, recursive application of the diffraction theory that requires sampling the field distribution in each aperture. Robust and computationally efficient numerical methods are applied to solve the diffraction integrals. The software implementation of these methods is tested with example runs and comparisons with 914 MHz continuous-wave measurements taken on the Virginia Tech campus. / Master of Science

M.S.

Two-dimensional spatially selective radiofrequency (2DRF) pulse is a type of excitation radiofrequency (RF) pulse in magnetic resonance imaging (MRI) which selects and excites spins in two dimensions simultaneously. Due to its extra selectivity on the second dimension compared with conventional one-dimensional RF pulse, 2DRF pulse could realize reduced field of view (rFOV) imaging. 2DRF-enabled rFOV imaging can accelerate imaging speed, and improve image quality significantly. Advances of 2DRF pulse have been achieved in recent years. However, its wide application in routine clinical practice is hindered by rigid design of 2DRF pulse, the long pulse duration, the inflexibility in excitation pattern, and the user-unfriendliness for non-specialists. The objective of this thesis work is to develop a high-efficient and user-friendly 2DRF pulse to improve its usability and efficiency in routine clinical practice, especially for non-specialists. Finally, the feasibility and applicability of the developed 2DRF pulse was verified in human spine T₁ₚ MR imaging and chemical exchange saturation transfer (CEST) imaging. / A high-low echo planar (EP) 2DRF pulse was first developed to realize rFOV imaging. Developed 2DRF pulse was evaluated in both phantom and human brain. The advantages of being robust to moderate gradient system delay and higher image signal to noise ratio (SNR) of the proposed 2DRF pulse were characterized by the better excitation profiles compared with conventional EP 2DRF pulse sequence. Additionally, the flexibility of the developed 2DRF pulse was verified by the constant image qualities and excitation profiles at different on-line setups. These results demonstrated the feasibility and user-friendliness of the high-low EP 2DRF pulse and revealed its potential application in routine clinical practice. / The high-low EP 2DRF pulse was then incorporated into T₁ₚ sequence to realize rFOV spine T₁ₚ imaging. Quantitative T₁ₚ imaging is time consuming in particular for high spatial resolution images obtained with high spin-lock frequency (FSL) though fast acquisition techniques could be employed. In addition, spine T₁ₚ imaging is vulnerable to the artifact induced by respiration and fat-water chemical shift. Reduced field of view in phase encoding (PE) direction helps to shorten the acquisition time, while preserving the SNR and resolution. The respiration artifact can be eliminated as no spin in chest or abdomen is excited. The 2DRF-enabled rFOV T₁ₚ imaging shortened scan time to the half and minimized respiration artifacts. rFOV CEST imaging was realized by the high-low EP 2DRF pulse. The feasibility of 2DRF pulse enabled rFOV CEST imaging was demonstrated. / 二維空間選擇射頻脈衝（2DRF）是一種磁共振成像中的激發脈衝。它能夠在兩個維度上同時地選擇性激發自旋。和傳統的一維選擇射頻脈衝相比，2DRF 在另一維度上額外的選擇能力使之能夠實現小視域成像。2DRF小視域成像能夠大幅加快成像速度以及提高圖像質量。近年來，2DRF的理論和實驗成果日益豐碩，但2DRF的廣泛臨床應用受到了其相對複雜的設計，較長的激發時間，不夠靈活的激發模式，對組織磁敏感性的過於靈敏以及對於非專業人士的難以操控性等諸多因素的阻礙。這篇碩士論文所述工作的目的是開發一種高效以及易操控的2DRF,以提高它在臨床應用中的實用性和易用性。而後，在人體脊柱T1ₚ磁共振成像和化學轉移飽和交換（CEST）成像技術中，驗證了所提出的2DRF的可行性和適用性。 / 一種高-低回波平面2DRF被首次提出并用於實現小視域成像。這種2DRF在水模和人體腦部成像中得以檢驗。相較於一般的平面回波2DRF，其具有對梯度線圈系統延遲的魯棒性，以及較高的信噪比。這些優勢體現于較好的激發圖樣中。此外，此種2DRF的靈活性在多種設定中取得的穩定的圖像質量和激發圖樣中得到體現。 / 所提出的高-低回波平面2DRF隨後與T1ₚ磁共振成像序列相結合實現了小視域脊柱T1ₚ成像。量化T1ₚ成像比較費時，特別是在使用較高自旋鎖定頻率或者較高的成像分辨率時。此外，脊柱T1ₚ成像容易受到呼吸運動以及水脂化學位移導致的偽影的影像。小視域成像在相位編碼方向上減少了成像範圍，從而在保證圖像信噪比和分辨率的同時減少了圖像採集時間。由於沒有腹部的自選電子被激發，呼吸運動偽影也得以消除。小視域脊柱T1ₚ成像大幅減少成像時間至常規時間的一半以下且最大程度地消除了呼吸運動偽影。通過應用這種2DRF,小視域化學轉移飽和交換轉移成像隨後也得以實現。在研究中，這種成像技術的可行性得到了證明。 / Zhang, Qinwei. / Thesis M.Phil. Chinese University of Hong Kong 2014. / Includes bibliographical references (leaves 69-79). / Abstracts also in Chinese. / Title from PDF title page (viewed on 11, October, 2016). / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only.

Title from PDF of title page (University of Missouri--Columbia, viewed on Feb 19, 2010). The entire thesis text is included in the research.pdf file; the official abstract appears in the short.pdf file; a non-technical public abstract appears in the public.pdf file. Thesis advisor: Dr. C. Mark Cowell. Includes bibliographical references.

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Modem technology has revived interest in the once popular area of meteor scatter communications. Meteor scatter systems offer reliable communications in the 500 to 2000 km range all day, every day. Recent advances in microprocessor technology have made meteor scatter communications a viable and cost effective method of providing modest data rate communications. A return to the basic fundamentals has revealed characteristics of meteor scatter propagation that can be used to optimize the protocols for a meteor scatter link. The duration of an underdense trail is bounded when its initial amplitude is known. The upper bound of the duration is determined by maximizing the classical underdense model. The lower bound is determined by considering the volume of sky utilized. The duration distribution between these bounds is computed and compared to measured values. The duration distribution is then used to specify a fixed data rate, frame adaptive protocol which more efficaciously utilizes underdense trails, in the half duplex environment, than a non-adaptive protocol. The performance of these protocols is verified by modeling. / Thesis (Ph.D.)-University of Natal, Durban, 1987.

There is need to improve prediction results in rain attenuation in order to achieve reliable wireless communication systems. Existing models require improvements or we need fresh approaches. This dissertation presents a model of rain attenuation prediction for terrestrial and satellite radio links based on a novel approach. This approach postulates that the difference in rain attenuation for various locations is attributed to the dissimilar rain drop sizes and rain cell diameter sizes and that cell sizes derived from local measurements would depict the true nature of rain cells better than the cells derived from long term rain data gathered from different climates. Therefore all other link parameters used in the attenuation equation are presented by the use of mathematical analysis; but the rain cell size is derived from local rain rate measurements. The physical link aspects considered in the mathematical attenuation model are: the Fresnel ellipsoid of the link path, the effect of elevation angle, the rain cell diameter size and the shape of growth of rain rates in the cell. The effect of the elevation angle of the link on the scale of attenuation is accounted for through the proposed coefficient of elevation equation. The coefficient of elevation is considered to modify the size of the rain cell diameter in proportion to the elevation angle of the link and the rain rate growth is taken to be of the truncated-Gaussian form. On the other hand, the rain cell diameter is derived from rain rate measurements as a power law model and substituted in the attenuation expression. The rain cell size model evaluated in this dissertation is based on point rain rate measurement data from the disdrometer located at the University of KwaZulu-Natal, South Africa. The “Synthetic Storm” technique is applied to develop the rain cell diameter distributions and the rain cell diameter model. In addition, the impact of the rain cell diameter size model in site diversity and cellular network-area planning for the region is discussed. To validate the model for terrestrial links, attenuation data collected from Durban, South Africa is used while that for satellite links, attenuation data from 15 links which are located in tropical climatic zones are used. In each case, the new model is tested against some well-known global rain attenuation prediction models including the standard ITU-R models. The performance of the proposed models for the sampled radio links based on error estimations shows that improvements have been achieved and may be regarded as a universal tropical model especially for satellite links. / Thesis (M.Sc.Eng.)-University of KwaZulu-Natal, Durban, 2011.

The advantages offered by Super High Frequency (SHF) and Extremely High Frequency (EHF) bands such as large bandwidth, small antenna size, and easy installation or deployment have motivated the interest of researchers to study those factors that prevent optimum utilization of these bands. Under precipitation conditions, factors such as clouds, hail, fog, snow, ice crystals and rain degrade link performance. Rain fade, however, remains the dominant factor in the signal loss or signal fading over satellite and terrestrial links especially in the tropical and sub-tropical regions within which South Africa falls. At millimetre-wave frequencies the signal wavelength approaches the size of the raindrops, adversely impacting on radio links through signal scattering and absorption. In this work factors that may hinder the effective use of the super high frequency and extremely high frequency bands in the Southern African region are investigated. Rainfall constitutes the most serious impairment to short wavelength signal propagation in the region under study. In order to quantify the degree of impairment that may arise as a result of signal propagation through rain, the raindrops scattering amplitude functions were calculated by assuming the falling raindrops to be oblate spheroidal in shape. A comparison is made between the performance of the models that assume raindrops to be oblate spheroidal and those that assume them to be spherical. Raindrops sizes are measured using the Joss-Waldvogel RD-80 Distrometer. The study then proposes various expressions for models of raindrops size distributions for four types of rainfall in the Southern Africa region. Rainfall rates in the provinces in South Africa are measured and the result of the cumulative distribution of the rainfall rates is presented. Using the information obtained from the above, an extensive calculation of specific attenuation and phase shift in the region of Southern Africa is carried out. The results obtained are compared with the ITU-R and those obtained from earlier campaigns in the West African sub region. Finally, this work also attempts to determine and characterize the scattering process and micro-physical properties of raindrops for sub-tropical regions like South Africa. Data collected through a raindrop size measurement campaign in Durban is used to compare and validate the developed models. / Thesis (Ph.D.)-University of KwaZulu-Natal, Durban, 2014.

The inability of service providers to constantly meet the design target of 99.99 % availability of the line-of-sight (LOS) microwave links has caused concern among both the operators and consumers. The non-availability of the links is predominantly due to propagation impairments along the propagation link. These propagation effects include cloud, snow, fog, gas attenuation, rain and atmospheric scintillation. Various studies have shown high vulnerability of radio communication systems operating at microwave (3-30 GHz) and millimeter wave (30-300 GHz) to rainfall attenuation especially in the tropical regions characterized by heavy rainfall and relatively large rain drops when compared to the temperate regions. In order to understand the effects of attenuation due to rain on communication systems in any locality (region), a good knowledge of the raindrop size distribution (DSD) and the rainfall rate estimates is necessary for accurate prediction and estimation of the rainfall attenuation. For this study, experimental raindrop size measurements gathered over a period of three years, using the Joss-Waldvogel RD-80 disdrometer installed at the roof top of the Electrical, Electronic and Computer Engineering building, University of Kwa-Zulu Natal, Durban, a subtropical location in South Africa, is analysed. Disdrometer measurements, sampled at one-minute rate over a period of nine months from Butare, an equatorial site in Rwanda, is also analysed for the purpose of comparison. The estimated R0.01 values for Durban and Butare are employed for the purpose of analysis. Based on the statistical analysis of the measured data samples, DSD parameters are proposed from the negative exponential, modified gamma, Weibull and the lognormal models. The DSD models are compared to models from other countries within and outside the region. The Mie scattering approximation at temperature of 20oC for spherical raindrop shape is adopted for the estimation of the scattering functions. The study further investigates the influence of critical raindrop diameters on the specific rain attenuation for the annual, seasonal and various rainfall regimes in southern Africa. This is achieved analytically by integrating the total rainfall attenuation over all the raindrop sizes and observing the differential change in the attenuation over a given range of drop size diameters. The peak diameter at which the specific rainfall attenuation is maxima is determined for different rainfall regimes. Finally, the cross-polarisation discrimination (XPD) due to rain over Durban is computed at two elevation angles. The results of this study will be helpful for the proper design and allocation of adequate fade margins to achieve the expected quality of service (QoS) in a radio communication system operating in the Southern Africa region. / Thesis (Ph.D.)-University of KwaZulu-Natal, Durban, 2014.