Year

2008

Degree Name

Doctor of Philosophy

Department

School of Electrical, Computer and Telecommunication Engineering - Faculty of Engineering

Abstract

Various kinds of wireless communication devices and systems provide a number of different functions and services to meet different demands for people. Some of these devices and systems coexist in the same area and share the common frequency bands according to some coexistence mechanisms such as cooperative and non-cooperative mechanisms. These mechanisms including power control, frequency hopping and time division multiplexing technique can handle electromagnetic interference between coexistence devices to some extent, but for the coexistence systems the interference problems between these systems are still very serious issues which affect coexistence system performance. In this thesis we consider the system coexistence interference problems in the spectrum shared environments. Rather than applying the techniques of power control, frequency control, time control and spatial control to avoid interference, we attempt to address the fundamental nature of system transmission. The general philosophy is to combine the orthogonal frequency division multiplexing (OFDM) technique with a spectrum spread method to generate an interleaved spectrum spread OFDM (ISS-OFDM) multiple subband signal, so that the system transmission subbands are selected adaptively and system coexistence interference is avoided and suppressed. This approach reveals the potential ability of system coexistence. Simulated results on system performance such as peak to average power ratio (PAR), signal frequency diversity and time diversity, and system bit error rate (BER) are presented to verify that system transmission bandwidth can be adaptively selected to avoid interference of coexistence systems and improve system performance. We then consider the implications of choosing or dropping the subbands with different levels of interference from the multiple subbands of the ISS-OFDM signal, and show that (i) it is possible to implement the information transmission without information loss by selecting some of the subbands with an interference level below the threshold, and dropping the subbands with an interference level over the threshold, and (ii) it is possible to derive the interference thresholds, based on which the adaptive selection subband transmission is implemented. We also show that it is possible to replace the interference thresholds over multipath fading channels by the interference thresholds over the Gaussian channels, so that the derivation process of interference thresholds over the multipath fading channels is greatly simplified. Through the theoretical analysis and investigations, we show that the ISS-OFDM technique can be applied to the coexisting systems sharing the frequency bands in the industrial, scientific and medical (ISM) band. Coupled with a technique for cognitive radios, the ISS-OFDM can be applied to a wide class of problems covering the interference suppression and spectrum efficiency improvement.

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