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.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:388886 |
| Date | January 1997 |
| Creators | Lee, Joohee |
| Publisher | University of Surrey |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://epubs.surrey.ac.uk/844189/ |
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