This thesis reports on an investigation of various system architectures and receiver structures for cellular communications systems which discriminate users by direct sequence code division multiple access (DS-CDMA). Attention is focussed on the downlink of such a spread spectrum system and the influence of a number of design parameters is considered. The objective of the thesis is to investigate signal processing techniques which may be employed either at the receiver, or throughout the system to improve the overall capacity. The principles of spread spectrum communication are first outlined, including a discussion of the relative merits of spreading sequence sets, and a description of various signal processing techniques which are to be applied to the multi-user environment. The measure of system performance is introduced, and the conventional DS-CDMA system is analysed theoretically and through simulation to provide a reference performance level. Adaptive algorithms, which iteratively approximate the minimum mean square error (Wiener) receiver filter, are then investigated, both in stationary additive white Gaussian noise (AWGN) and in a more realistic radio channel. The inter-dependence of the system chip-rate, maximum Doppler offset induced by the motion of the receiver and the tuning parameters of the adaptive algorithm are demonstrated. Aspects of forward error correction (FEC) coding are then investigated, with convolutional coding on the data used both as an alternative to and as a supplement of direct sequence spreading. The most efficient use of the available expansion in bandwidth is shown to be dependent on a balance between FEC coding power and the capacity of the spreading sequence set chosen. Methods of combining multiple access interference cancellation techniques with convolutional coding and Viterbi decoding are considered. New structures are proposed which incorporate FEC decoding at the intermediate stage of the canceller, and the performance of these receivers is analysed theoretically. Simulations confirm that significant capacity improvements may be achieved, at tolerable increases in computational complexity and processing delay.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:641284 |
| Date | January 1998 |
| Creators | Band, Ian W. |
| Publisher | University of Edinburgh |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://hdl.handle.net/1842/10722 |
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