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Thesis (Ph.D.) OGI School of Science & Engineering at OHSU, March 2008. / Includes bibliographical references (leaves 116 - 124).

Thesis (Ph. D. in Computer Science)--Naval Postgraduate School, September 2002. / Dissertation supervisor: Michael Zyda. Includes bibliographical references (p. 159-164). Also available online.

Thesis (MScIng)--University of Stellenbosch, 2007. / Bibliography. Also available via the Internet.

Spread-spectrum multiple access (SSMA) is conceptually attractive for interactive data communications over broadcast channel shared by a large number of potential users. The present thesis Involves a study of asynchronous direct sequence SSMA interactive data communication systems. The study includes a unified analysis and assessment of system delay-throughput performance. Two important message delay components which are affected by the design of the transmitter/receiver pair result from code acquisition and bit-errors. A new code synchronizer design featuring a number of parallel correlators is developed, and an analysis of synchronizer performance as it relates to SSMA applications is provided. It is shown that average acquisition delay decreases in proportion to Increase In the number of correlators when this number is small. Receiver bit-error probability for any given channel occupancy is derived. Three protocols suitable for SSMA transmissions under different operating conditions are proposed. Using one of these protocols, the expected number of transmissions before a message is received error-free is estimated by averaging bit-error probabilities over a postulated channel occupancy probability distribution. This distribution is verified using data obtained from channel simulations. Delay-throughput characteristics of the SSMA system are thus obtained by evaluating the above delays and other exogenous delays at various traffic levels. Results relevant to the given system and traffic models, transmission protocol, and transmitter/receiver structure are obtained assuming that users' codes are uncorrelated. Subsequently, it is shown that the results are easily modified to account for code cross-correlations. Assessment of SSMA delay-throughput performance Is accomplished by comparisons with pure ALOHA, slotted ALOHA and queueing channels. These comparisons necessitate extension of existing analysis of slotted ALOHA channels to include the effects of Gaussian channel noise, as well as development of an analysis procedure for noisy pure ALOHA channels. It is shown that in power-limited situations, the capacities of ALOHA and queueing channels can be maximized with respect to the transmission bit rate. Delay-throughput comparisons show that at throughput levels much lower than the capacities of these channels, average delays for SSMA are higher than those of the other channels. However, capacities of SSMA channels are generally higher than those of the other channels, which occupy only a fraction of the available bandwidth at power levels favourable to SSMA. In such cases, the throughput which results for a given delay clearly favours SSMA. Comparisons are also performed with respect to m-parallel ALOHA or queueing channels, where ra is the number of channels accommodated by the available bandwidth. In this case the capacities of SSMA channels are generally less than those of m-parallel slotted ALOHA or queueing channels, but approximately equal to those of m-parallel pure ALOHA channels. Therefore, SSMA presents a viable alternative to m-parallel pure ALOHA multiple access for interactive data communications. SSMA is especially favourable for transmissions of long messages In a wide-band broadcast channel with limited power. / Applied Science, Faculty of / Electrical and Computer Engineering, Department of / Graduate

In this thesis, the author pursued two objectives. The first objective was to present a working example of a convenient, "idiot-proof", interactive computer program (designed with the user - not the programmer - in mind). The second objective was to investigate how various types of users interact with the computer, with the intention of reaching some conclusions about which program interfaces were most appropriate and convenient for various user types. In addition, some theories about the effects of various behavioural variables were investigated. The experimental tool used for this research was a simple interactive computer game in which the participants searched for the optimum profit in a three-dimensional space, given a fixed time limit. , Frequent periodic measurements were automatically collected on user performance, attitude, requests for reports, utilization of special features, and other variables; also, the solution protocol of each participant was recorded. The users were cateqorized by coqnitive style (heuristic/analytic), risk attitude, and previous computer experience as determined by a battery of pre-tests and questionnaires. In analyzinq the results, it was found that experience level was the dcminatinq factor on all dimensions: novices were slower, finished less frequently, and were siqnificantly less confident than experienced players. A hiqhly structured proqram interface was found to be more appropriate for these new users., Experience was also the dominating factor in the use of reports, although novices did show a marked learning effect over time - as did all users on most dimensions of performance and behaviour. As previously hypothesized, analytic-types and risk-takers played significantly faster and were more confident than heuristic-types and risk-averters, respectively. Concerning utilization of special program features, it was found that input response defaults influenced users in unfamiliar situations (ones which were new or did not have clear-cut responses), and didn't affect them at all in familiar circumstances. Analytic-types made least use of defaults. Bisk-averters were least likely to abbreviate commands. Also, the extent to which commands were abbreviated depended much upon their length, Finally, in the area of solution protocols, it was indeed found that heuristic-types were much less structured in their approach to solving the problem than analytic-types. . / Business, Sauder School of / Graduate

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Typescript (photocopy). / Digitized by Kansas Correctional Industries / Department: Computer Science.

Typescript (photocopy). / Digitized by Kansas Correctional Industries