Electrical and Electronic Engineering - Theses
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ItemTechnologies for millimetre-wave fibre-radio systems( 1999)The recent advances in mobile wireless technology have increased the need for more transmission bandwidth to accommodate for future telecommunication services. The millimetre-wave (mm-wave) hybrid fibre-radio system is one of the potential technologies for provision of these broadband services. With the advent of low-loss and high bandwidth optical fibre in telecommunication networks, it is an ideal medium for distributing the broadband mm-wave information. In addition to the provision of broadband services and high capacity, the hybrid fibre-radio system also enables customer mobility. This thesis investigates the performance of different architectures for distributing radio signals over fibre in mm-wave fibre-radio applications. In particular, networks incorporating radio signal distribution as mm-wave frequency and as baseband data over fibre are investigated. There have been many different downlink implementations incorporating mm-wave radio signal distribution over fibre for mm-wave fibre-radio system, however there has been little focus on the uplink path. For the first time a simple mm-wave uplink path in a mm-wave fibre-radio system is presented in this thesis which incorporates direct modulation of a multi-section mode-locked distributed-Bragg reflector (DBR) laser at the base station. The performance of the multi-section laser as a mm-wave optical transmitter is presented, and a detailed characterisation of the multi-section laser stabilised via hybrid mode-locking at fundamental and subharmonic frequencies is carried out. The uplink path implementation using the multi-section laser is further extended to incorporate multi-channel transmission. Efficient multi-channel transmission incorporating the multi-section laser requires the separation of the stabilisation and modulation functions of the drive signal applied to the laser. Two different stabilisation techniques namely fundamental hybrid and subharmonic synchronous mode-locking are considered for multichannel operation, and their performance investigated. A detailed theoretical analysis is then presented that quantifies the impact of fibre chromatic dispersion-induced rf power penalties when using the multi-section laser to transmit mm-wave frequencies over fibre. The model is also used to study the performance of the laser as a function of its operating conditions. The thesis also presents a detailed investigation of the implementation and performance of a mm-wave hybrid fibre-radio system with baseband data delivery over optical fibre, a technique which has not been previously reported. The first realisation of a mm-wave fibre-radio system with baseband data delivery over the optical fibre network is presented. This system incorporates a novel modulation scheme for simultaneous transmission of baseband digital data and remote local oscillator (LO) signal delivery. The modulation scheme exhibits several unique features including the use of a single dual-electrode Mach-Zehnder modulator and the delivery of the LO signal is such a way that it is not affected by fibre dispersion. A full-duplex mm-wave fibre-radio system comprising broadband baseband data transmission is implemented with custom-designed mm-wave diplexers and antennas. A theoretical model is also developed to quantify the performance of the baseband data modulation scheme. The analysis is used to determine the sensitivity of the modulator input parameters on the system performance and good agreement is obtained between the model and measurements of the fibre-radio link incorporating baseband data modulation.
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ItemFeedforward compensation of directly modulated semiconductor lasers for analogue fibre communications systems( 1991)Optical fibre communication has become very attractive in the field of telecommunication. Recently, subcarrier multiplexing has emerged as a method for providing a way for each customer to directly access voice, data, and video transmissions. Although digital transmission is becoming very popular in telecommunications, analogue transmission still has its attractive features due to its low cost and compatibility to recent customer electronics. Directly modulated semiconductor lasers are important optical sources in optical communication systems. In analogue fibre communications, nonlinear distortion and intensity noise in these directly modulated semiconductor laser are main obstacles. The thesis first presents experimental characterization of semiconductor lasers, including light-current characteristics, static longitudinal mode spectrum, and small-signal frequency response. Since the linearity of semiconductor lasers plays an important role in analogue fibre communications, distortion characterization of semiconductor lasers has been investigated in considerable detail. This thesis gives details of distortion characteristics, particularly in experimental work. The main work presented in this thesis is an optical feedforward compensation scheme. This feedforward system was successfully built for experimental investigation. Since nonlinear distortion in semiconductor lasers is a severe problem in analogue fibre-communications, the feedforward compensation has been employed to compensate the distortion. Reduction of harmonic distortion and intermodulation distortion was achieved by feedforward compensation. The thesis describes the optical feedforward system in considerable detail. A simultaneous reduction of intensity noise and distortion was achieved using the feedforward technique. This provided a great improvement in signal-to-noise ratio in analogue subcarrier multiplexed systems. The experimental results are presented in this thesis. Also, the theoretical calculation of noise limitations in the feedforward system is presented in this thesis.