Electrical and Electronic Engineering - Theses
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ItemLinearization of analogue optical transmitter by feedforward compensation( 1993)In recent years, analogue optical systems have received a lot of attention. Although conventional optical systems mostly employ digital format because of the low power budget requirement and good immunity to noise and distortion, analogue systems prove to have significant advantage over digital systems in applications like video distribution and satellite communication systems. However, analogue transmission systems require transmitters of low noise and low distortion. A number of linearization schemes have been proposed to reduce the distortion introduced by the analogue transmitter. One of the most widely used linearization scheme for analogue transmitter is feedforward compensation. Previous research has shown that feedforward compensation can reduce laser intensity noise as well as distortion. The work described in this thesis is to investigate design optimization of feedforward compensation and to develop a prototype of this optimized system. Experimental testing shows that the feedforward prototype is capable of reducing distortion products by 15 dB over 2.7 GHz. This work also involves modelling the feedforward system to investigate the factors limiting the operation bandwidth of the system. The model developed is a useful design tool for feedforward systems because the distortion reduction performance of the feedforward system can be predicted by characterising the individual components that made up the linearization scheme. Theoretical analysis of an alternative implementation of the feedforward system is also performed. Unlike conventional feedforward systems this new implementation uses only one laser source. Although the distortion reduction performance is degraded, the advantage is that problems associated with the use of two optical sources of different wavelengths is eliminated.
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ItemTraffic modelling and analysis for cellular mobile networks( 1993)This thesis is concerned with the modelling and analysis of call control policies in cellular mobile networks. It addresses the important problem of finding policies which give sufficient priority to handover attempts between cells over new call attempts so that network congestion will not lead to handover failures and subsequent call dropouts. The major contribution of the thesis is the analysis of a class of priority queuing systems and a methodology for the modelling of cellular networks with non-uniform offered traffics. Three related priority queuing systems are considered for application to cellular mobile networks: a non-preemptive priority queue; a cutoff priority queue with a hysteresis mechanism and a non-preemptive priority queue with channel reservation and hysteresis. A matrix-geometric solution of the same form is shown to be common to all of these systems with a matrix-exponential form found for the delay distributions. It is also shown that these systems can be represented by an equivalent M/G/1 queue with multiple vacations and this reveals some insight into their behaviour. A new result is derived for the M/G/1 queue with multiple vacations and impatient customers and this allows for the priority systems to be extended so that new call arrivals are subject to a fixed timeout in queue. In some situations, this provides a more realistic model of the behaviour of new call attempts. Handover delay performance is treated at length. It is found that queuing of handover requests is highly desirable and that handover delay performance can be further improved by classifying handover requests and giving higher priority to the more urgent handover requests. Microcellular networks, for which handover delay requirements are quite stringent, are also considered. A micro cellular traffic model is proposed and a call repacking policy, which is particularly well-suited to microcells, is analysed. The effectiveness of a class of state-dependent call acceptance policies in improving handover delay performance is also considered. The performance of cellular networks is much dependent on the distribution of offered traffic. An approximation technique is developed which enables a wide class of call control policies to be investigated. It is based on decomposing a network into a number of subnetworks which are then assumed to be stochastically independent. This technique turns out to be fairly accurate under reasonable traffic assumptions.