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UBC Theses and Dissertations

Spread-spectrum multiple access for interactive data communications Leung, Victor Chung Ming


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.

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