The origins of spread spectrum are in navigation and military systems. Techniques originally developed to reduce the effects of the intentional jamming also proved suitable for communications through dispersive channels in cellular applications. In 1949 the first time hoping spread spectrum multiple access system has been introduced by John Pierce. A direct-sequence spread spectrum system has been proposed by De Rosa-Rogoff later in 1950. He has also introduced the concept of processing gain. However the commercialisation of cellular direct-sequence code division multiple access (DS-CDMA) systems became possible only during the 1980's and 1990's. In 2000 and beyond we will be witness to the standardisation and commercialisation of wide band CDMA system with a bandwidth of 5 MHz or more. While CDMA presents a number of advantages for cellular mobile communications, it has its own drawbacks. Good air interface designs provide efficient solutions for the terrestrial cellular system. We first take a look at the problem of narrow-band and partial band interference suppression in a CDMA system. By using Poor's model for partial-band interference, some interference suppression techniques are analysed. Then the concept of hybrid genetic prediction is introduced which outperforms the nonlinear techniques in terms of SNR improvements. The performance of a CDMA system can be degraded by Multiple Access Interference (MAI) due to the presence of many users in the same bandwidth. After analysing the structure and performance of existing multi user detection techniques, a low complexity bit level detector is proposed to reduce MAI in synchronous CDMA (S-CDMA) system for an AWGN channel. It is shown how it is possible to reach almost the single user bound by combining detection theory and the fundamentals laws of evolution found in nature. Nonlinear mappings are added to the proposed detector to increase detector's performance by reducing the misleading effect of noise on the detection process. Then for Asynchronous CDMA (A-CDMA), a novel packet level genetic detector is proposed. The near-far resistance feature of the proposed detector is studied. The effect of different parameters of genetic engine i.e. chromosome length, gene's value or mutation and crossover probabilities on its performance are characterised. Different initialisation methods are introduced. For high bit rate CDMA system in multimedia applications in a dispersive CDMA channel, Inter Symbol Interference (ISI) becomes another highly important factor that degrades system performance. It is shown how it is possible to use a signal sub-space based detector as a core detector for a hybrid genetic Multi user detector.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:326201 |
| Date | January 2000 |
| Creators | Abedi, Saied |
| Publisher | University of Surrey |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://epubs.surrey.ac.uk/843016/ |
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