Thesis (M.S.)--Boston University

Thesis (M.S.)--Boston University

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This thesis addresses itself to the solution of a number of problems which arise in the development of Radio Frequency filters, particularly those involving coupled transmission lines as resonant elements. The text is divided into a number of sections dealing with individual topics. After a brief description of Darlington filter design principles, there is an account of the pivotal role of quarter wave sections and their vital part in the realisation of high frequency filters. This is followed by the development of new material relating to equivalent circuits of physical lines in terms of quarter wave sections, and its application to improvements in the design of wide-band filters. There follows an account of a new procedure for calculating the dimensions of comb-line and inter-digital filters. This section includes a new proposal for the inversion of Getsinger's procedure to permit the calculation of rectangular rod dimensions and spacings from given electrical data. There is also an algorithm for use with round rods which circumvents the tedious manual interpolation procedure devised by Cristal. There follows an investigation of and a proposed solution to a long-known but unexplained discrepancy existing between the calculated and measured pass-band width of the class of comb-line filters. With the new procedure the discrepancy reported previously to be of the order of 10% is eliminated. Finally, experimental evidence is adduced to verify the algorithms outlined in the preceding chapters.

The following various multiple access schemes based on CDMA are investigated: Hybrid OCA-FD/SC-CDMA: Whereas conventional FD/SC-CDMA schemes do not permit adjacent carrier spectra to overlap, this scheme overlaps adjacent carrier spectra intentionally. Even though interference arises from adjacent carrier, higher chance of multipath diversity and spectrally efficiency is achieved in comparison with conventional FD/SC-CDMA. It will be shown that OCA compensates for capacity loss incurred by subdivision of available spectrum for frequency division multiplexing, and also achieves even higher capacity for chip waveforms with smooth spectral shape at no extra system complexity while merits of FD/CDMA such as lower complexity and higher diversity gain for noncoherent reception are still fully exploited. In terms of capacity, for flat fading channel and rectangular pulse cut off at mainlobe-null in frequency domain, FD/SC-CDMA combined with OCA schemes gives roughly 56% gain in comparison with SC-CDMA and conventional FD/SC-CDMA. For frequency selective channel and noncoherent reception, even higher gain is achievable. Hybrid slow frequency hopping (SFH)/SC-CDMA: Overlapped carrier allocation (OCA) schemes can be applied to hybrid SFH/SC-CDMA. In power controlled systems, SFH/SC-CDMA is known to be much worse than pure DS-CDMA in terms of capacity. Introduction of OCA to SFH/SC-CDMA improves capacity significantly, and consequently it becomes comparable to pure DS-CDMA whilst merits of frequency hopping such as strong immunity to near/far effect is preserved. FD/MC-CDMA: In this scheme, available spectrum is subdivided into multiple discrete subspectra, and they are interleaved. Then diversity gain becomes equal to that of ordinary MC-CDMA frequency diversity. As a result of less subcarriers than ordinary MC-CDMA, equaliser becomes less complicated. Guard interval imposed to overcome timing synchronisation error and intersymbol interference helps to suppress inter-subcarrier interference. Successive subcarriers are apart by multiple of the chip rate, and so inter-subcarrier interference is reduced or nearly rejected. In FD/MC-CDMA, longer guard interval instead of windowing is more effective. SFH/MC-CDMA: This scheme replaces hardware implementation of frequency hopping with simple coding technique. Hence frequency hopping gives no extra hardware complexity unlike SFH/SC-CDMA. Even fast frequency hopping can be simply implemented. Likewise in FD/MC-CDMA, frequency diversity is fully exploited. In the absence of nonlinear distortion, FD/MC-CDMA outperforms other multiple access schemes under consideration in terms of capacity, hardware complexity, and flexibility of resource management in single rate and multi-rate applications. In practice, power-limited mobile terminals can not afford to impose sufficient output backoff on power amplifier, and consequently nonlinearity generates intermodulation products (IMP). IMP's degrade signal-to-noise ratio and make synchronisation even more difficult. Unlike narrow-band OFDM, intermodulation products become noise-like after despreading at the receiver, and so cross-talk does not happen. Flexibility in pulse shape, carrier frequency, and the width of spectrum makes performance analysis more troublesome.

The growing usage and demand for microwave communications has led to the increase in system density, particularly in urban areas, and consequently to the increase in the probability of mutual interference between systems sharing the same frequency band. Satellite and point to point microwave communication systems might have to rely on site shielding to reduce the level of this co-channel interference. In urban areas, there is a great possibility of a building obstructing the interference path, thus providing protection. However, there could be more than one interference path. Scattered interference from other buildings in the vicinity of the site is one which can be found to be almost as harmful as the direct interference. Although site shielding has long been identified as an interference reduction technique, most studies have been devoted to the protection obtained from the obstruction of interference paths, namely diffraction. There is little information available regarding the effects of building scatter in site shielding. The work reported in this thesis was set to carry out theoretical and experimental investigations and characterisation of building scatter. The studies are aimed at the effects of building scatter on site shielding at microwave frequencies. Building scatter prediction models are developed based on Fresnel-Kirchhoff diffraction theory and verified against results obtained from measurement campaigns conducted at a frequency of 11.2 GHz using firstly a perfectly conducting reflector and later a number of buildings in urban environments as the scattering obstacles. Scattering prediction models are developed as extending to that originally applicable to the far field for use in the near and very near field regions of the scattering surface. The very near field model is found to be particularly useful in site shielding applications in urban environments. Furthermore, it has been possible to establish the scattering characteristics in terms of angular and distance dependence of the scattering coefficient using the expansions of the Fresnel integral with appropriate approximations. The effects of building features and surface variations, e.g. protruding and recessed features, windows and surface deviations, are analysed and characterised. These effects are particularly significant in the interpretation of measurement results obtained from buildings in typical urban environments. The thesis provides a prediction procedure which radio system planners and design engineers can use for determining the effects of building scatter on the site shielding factor for specified radio path geometries. The procedure is expected, through UK study group 3, to add considerably to a revised ITU-R (CCIR) procedure and recommendations for building scatter effects in radiowave propagation. The work has also contributed regularly to the technical output of European COST project 235.

The overall growth in cellular, fixed and satellite communications markets, has exceeded many expectations and there is a widespread anticipation that the demand for wireless telecommunication systems will continue to expand in the foreseeable future. Such systems rely in their planning, design and implementation on the availability of radiowave propagation models. In the particular case of land mobile radio systems and wireless fixed access systems, obstacles in the form of vegetation volumes, e.g formations of trees, are likely to influence radio propagation, giving rise to absorption and scattering of radio signals. In this context, this thesis investigates suitable techniques to characterise and model the effects of inhomogeneous volumes of vegetation on the propagation modes of radiowaves. The thesis proposes an enhanced model based on the Radiative Energy Transfer theory (RET) which was discretised to accommodate forests formed by different vegetation species with their distinct propagation characteristics. The discretised model computational structure, comprises several element cells, whose characteristic propagation parameters may be assigned independently. The discretised RET (dRET), is therefore capable of gathering the interactive responses between the element cells comprising the computational structure, leading to the determination of the received signal inside or around a given illuminated vegetation medium. The performance of the proposed model, was assessed utilising results from an extended range of measurements, carried out in different environments. Such measurements comprised those necessary for the model input parameters extraction. Others enabled the model assessment through comparison between the model predictions and the actual directional profile of the measured received signal results. An initial assessment of the model was carried out in the laboratory, using an idealised test forest formation placed inside an anechoic chamber, whereas the final model assessment was performed in an outdoor tree groupings formed by several different full size trees. Both indoor and outdoor measurements, confirmed good overall model performance and predictions of both absorption and scattering propagation modes caused by the presence of vegetation in the radio path. This was demonstrated at micro- and millimetre wave frequency bands, centered at 11.2, 20, 40 and 62.4 GHz frequencies. The thesis provides a valid tested method to evaluate the dRET propagation parameters for various isolated volumes of vegetation. Such parameters, may subsequently be utilized into the proposed propagation model, which is shown to be capable of dealing with typical and non homogeneous forests thereby effectively predicting the received signal directional profile at several locations inside and around the inhomogeneous forest. The thesis has many novel features. These include the development and extension of the basic dRET model removing many limitations. The parameter extraction including the effects of the receive antenna radiation pattern is another novel contribution. Further novelty lies in the application of the dRET model to mixed, finite and inhomogeneous vegetation formations. As a result of these refinements and extensions, the dRET propagation model has been shown to yield predicted results which agree well with measurements.

The ever growing application of wireless communication systems requires accurate models for characterising radiowave propagation when affected by the presence of a variety of obstacles. In particular if the obstacles take the shape of vegetation volumes, like single trees or groups of trees and are present in the radio path, they give rise to absorption and scattering of radio signals. This thesis presents a literature review of common models for radiowave propagation through vegetation, the theory of Radiative Energy Transfer (RET) is one of these models and provides an accurate analysis of radiowave propagation through a vegetation media. Extensive measurements have been designed and conducted in a controlled indoor environment to provide valuable measurement data for later development of deconvolution approaches. It can be shown that the measured directional spectra are convolution products of the phase function pattern and the receiver antenna radiation patterns, which impacts determination of the RET input parameters. Consequently, in order to achieve more accurate determination of the RET input parameters, the adverse influence caused by receiver antenna radiation patterns have to be removed from measured directional spectra by implementing a process of deconvolution. This thesis provides successful implementation of two iterative based deconvolution techniques on the measurement directional spectra. To the author's knowledge, this is its first kind of application to eliminate distortion caused by the receiver antenna radiation pattern during measurements. This thesis reports a number of novel approaches. These include the further development and extension of deconvolution techniques such as combining the Bennia-Riad criterion and an error function to determine optimal parameters, as well as using pre-filtering techniques to improve the deconvolution results. Development of clearly defined criteria based on the knowledge of the central-limit theorem and discussion of loss of information avoidance during convolution is another novel contribution. Further novelty lies in the modification of the two methods to suit implementation on the measurement data from radiowaves impacting on vegetation volumes. As a result of these refinements, extracted RET input parameters from the restored patterns after applying the deconvolution processes show evident improvements compared to those extracted from directly measured patterns. Early stage results of this project are published in the IEEE Proceedings on Next Generation Applications, Services and Technologies.

In the face of a shifting political climate in Latin America, movements for indigenous rights and autonomy are leveraging community media in new ways transcending the state-market binary. Through ethnographic research with Zapotec media producers in Oaxaca and the supportive organizations forming points of connection between radios and activists, I argue that the strength of the indigenous community media movement in Oaxaca, and its potential to build a movement to resist destructive state and market forces, is best explained by Gilles Deleuze and Felix Guattari’s concept of the rhizome, which portrays Oaxacan indigenous media as a map of heterogeneous interconnections defying structural hierarchies and binaries. With this picture of a rhizomatic media movement, I demonstrate how radios have paved the way for innovations, revealing creative ways that indigenous groups are connecting with each other and the outside world, while asserting agency in their interactions with the market and the state.

In the 1980s the future of public service broadcasting in Britain was called into doubt. Technological developments in cable, satellite and digital technologies were, it was argued, poised to end the condition known as 'spectrum scarcity'; while the emergence of a neo-liberal Conservative government, pledged to rolling back the frontiers of the state', was of the opinion that the current system of public service broadcasting provision was no longer necessary given the number of broadcasting channels now available; broadcasting, in its view, would increasingly be able to mirror the publishing industry in its structure and future regulation. Critics however, were loathe to accept the argument that technological considerations alone ought to drive broadcasting policy; and two key questions emerged. Firstly, how was public service broadcasting to be defended in a climate increasingly hostile to public service ideals and institutions in general; and secondly, and as a result of the first question, how was public service broadcasting to be understood? This thesis seeks to answer both these questions and argues that in the process of clarifying the nature of public service broadcasting in the past, that solutions for its defence in the future will be found. Public service broadcasting, was not, it will be argued, simply about institutions like the BBC, but evidence of a much broader and widely shared (across the political divides) understanding of the proper role of broadcasting in a democratic society (at least until the 1980s). In short, public service broadcasting in the past was never simply a response to a set of technological conditions; instead it was forged from a set of political, economic, Administrative and cultural ideas about the nature of society and broadcasting's role in it; and hence its ability to respond to the new conditions of the 1990s and beyond.