Since recent times the importance of wireless communications has experienced a phenomenal growth, thus incurring a considerable increase in usage of the frequency spectrum available for communications and hence creating the need for efficient methods for its utilisation. Multiple-access is one efficient method, which enables several users to share a common frequency bandwidth. A multiple-access technique known as Collaborative Coding Multiple Access (CCMA) has been theoretically investigated and has been shown to give an increase in channel capacity and bandwidth efficiency. This technique uses a special type of codes known as collaborative codes to enable multiple users to communicate simultaneously over a common frequency bandwidth, thus eliminating the use of time, frequency and orthogonal code divisions. However to implement this technique practically, requires a multiple access channel (MAC) that coherently combines the signals originating from different users to give a composite signal. The combining of signals to implement a CCMA scheme can be easily achieved in baseband channels, where signals can be represented as voltages or currents which can add, to give uniquely identifiable code combinations. However when extended to bandpass carrier channels, the combining of signals over radio channels involves the addition of signals of differing, and, in the mobile situation particularly, varying relative phases and amplitudes. This can give rise to a major detection problem due to the resulting destructive signal combining and thus making CCMA non- practical over radio channels. This thesis describes a novel multiple-access scheme known as Complex-Valued Collaborative Coding Multiple Access (CV-CCMA), to provide a practical system over radio channels that achieves the theoretical promises of CCMA. CV_CCMA overcomes the problem associated with the non-coherent combining of multiple signals over radio channels by employing a complex-valued collaborative code and a combined detection and channel estimation process to estimate the data of each user as well as their associated amplitude and phase variations induced by the channel. The research is initiated with the principles and the theoretical foundations of the CV_CCMA technique. A combined signal detection and channel estimation process is presented. The choice of a suitable collaborative coding scheme is investigated. The conditions for the selection of valid codesets are established for a given number of users, modulation levels and code length. In addition, the effect of channel fading is also investigated. AT-user M-PSK CV_CCMA system operating in Rayleigh and Rician fading channels, is modelled and simulated to establish the error-performance. The results from this investigation are obtained for different number of users and modulation levels. The effect of varying signal power levels is also investigated. Observing the obtained results an additional code selection condition is established for a Rician channel, with an improved error performance. The channel capacity of the CV_CCMA scheme operating in a flat Rayleigh fading channel is obtained. In a given T-user M-PSK CV_CCMA scheme the channel outputs result in multi-level signals. Therefore the channel capacity of CV_CCMA is compared with equivalent multi-level single access schemes. It is shown that the channel capacity of CV_CCMA is greater than the corresponding multi-level schemes. The theoretical channel capacity limits of CV_CCMA is also compared with the limits of TDMA, FDMA and CDMA operating in a flat Rayleigh fading channel. The enhancement of the error performance of CV_CCMA in fading channels is investigated by using space diversity reception and error-control coding. It is found that by using these techniques the error performance of CV_CCMA can be greatly enhanced. For example with a dual-diversity scheme in a flat Rayleigh fading channel, the achieved gain is in the order of 20 dB. Using error-control coding, a coding gain is achieved depending on the type of code used. It is also found that using both of these techniques contributes towards further performance gains depending on the number of diversity paths and the coding scheme. The thesis is finally concluded with an overview of the contributions of this thesis, the achieved results and their implications and suggestions for future work in this subject.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:299028 |
| Date | January 1999 |
| Creators | Soysa, Gamaethige Francis Robert Sulak |
| Publisher | University of Sussex |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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