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Mobile Satellite Communication Systems are the best, and probably the only solution to a full 'global communication coverage'. Despite the intense scrutiny and inherent difficulties in gaining acceptance by the telecommunications market and the recent switch back towards GEO satellites, mobile satellite systems in non-geostationary LEO (500-2000km) and MEO (10000-12000km) constellations still remain an attractive solution in an integrated satellite/terrestrial scenario as they offer lower delays and lower power requirements than GEO satellites (35800km). Since Quality of Service (QoS) and service availability are major subscriber concerns, the investigation of factors which influence them are of central importance to the design of such systems. In non-GEO mobile satellite systems both the QoS and the service availability are very much dependant on the changing dynamics of the constellation and on the time varying nature of the propagation environment. Motivated by the above issues, we present, analyse and evaluate the coverage and availability of first generation constellation proposals in terms of LOS and bit-error-rate (QoS) requirements. Handover management is also identified as an important issue affecting the QoS and therefore handover strategies and mechanisms for various satellite constellations are presented and analysed. Based on the statistical but predefined nature of the constellation dynamics as well as the influence of the propagation channel and its dependence on the constellation design we propose two new channel adaptive handover algorithms in an effort to reduce the handover signalling whilst maximising at the same time the QoS as perceived by the user in terms of reduced call dropping rate for a typical circuit mode telephone call. Finally, as the current trend of the telecommunication services is towards the provision of packet oriented services, we focus the final part of this study on the performance investigation, in terms of throughput versus delay characteristics, on the provision of GPRS-like services over mobile satellite systems and compare GEO and non-GEO delivery. It is concluded, based on the MAC protocol proposed in the last chapter, that depending on the type of traffic and on the network load, the LEO approach doesn't always give superior performance in terms of delay characteristics to that of a MEO. The results and findings presented in this thesis can be used as a reference for optimising and designing future mobile satellite systems. Key words: Availability, QoS, handover, MAC protocols, satellite channel.

Recent years have witnessed a tremendous growth of research and development to provide mobile users with "seamless" communication through wireless media. Service provision in mobile networks is closely intertwined with network mobility management and therefore efficient management of subscriber mobility remains a challenging and an important area of research. This thesis examines management of signalling traffic in mobile networks and packet network design approaches, with specific contributions relating to i) reduction of network database signalling costs in GSM; ii) minimization of mobility signalling over the air-interface based a new technique for subscriber location management; iii) an efficient network design technique for packet communication networks with application to GPRS, based on a novel hybrid GA-Heuristic approach. A novel location management technique is proposed to reduce signalling costs in the core network, by reducing the rate of HLR-related transactions. By intelligent distribution of the HLR related signalling over a number of VLRs, the HLR is prevented from being a potential bottleneck and a single point of failure in the signalling network. Based on the analytical model developed, it is demonstrated that within the call-to-mobility range considered, the total network signalling cost as well as location management costs can be significantly reduced, compared to current strategy adopted in GSM, To address location management signalling in the access network, a number of dynamic schemes are considered and compared with the proposed adaptive multilayer technique. Through detailed simulations under various scenarios, the superiority of the proposed scheme, in terms of significant savings in the total signalling traffic (i.e. location update and paging) and ease of implementation, compared to other techniques is demonstrated . The location management scheme of GSM is used as baseline for comparisons. Finally, efficient design of packet communication networks, with application to GPRS backbone architecture, is addressed. To meet the cost and traffic requirements whilst ensuring that the delay and reliability constraints are also satisfied, a hybrid GA-heuristic approach is developed as an alternative to purely GA based and heuristic-only approaches. Under a common set of parameters, the performance of various techniques are compared and it is shown that the hybrid approach is capable of producing solutions that are superior to other typical network design methods.

The World's ever increasing demand for communication capacity has been the catalyst for the development of a range of next generation satellite reflectors. This new generation are significantly larger than those currently in orbit. Their dimensions prohibit transportation into space in their operational configuration. This thesis investigates the use of deployable tetrahedral trusses for the reflecting surface support structures of a 50 m diameter Land Mobile Communication System. A deployable structural system was selected ahead of other possible forms, as it satisfied the majority of the criteria of low part count, quick assembly times and an economical packaging volume thereby minimizing transportation and on-orbit fabrication costs. The composite material examined is formed from a polyethersulphone thermoplastic matrix reinforced with high strength, low modulus carbon fibres, C-PES.The modal characteristics of a series of scaled sub-units of the proposed structure were examined. These units, manufactured from the two types of material considered, C-PES and Perspex, were subjected to a range of excitation functions. The C-PES units were thermally cycled under high vacuum to simulate the space environment. The accelerated thermal cycling of the composite structural units revealed that a shift in resonant frequency occurred, together with some surface cracking which could affect the long term stability of the material. The effects of surface pigmentation on the thermal response were also considered. An algorithm has been developed which allows low cost materials to be used to predict the behaviour of geometrically similar units manufactured from the composite material. The interaction between the predicted dynamic structural behaviour and the electrical performance of the satellite is also addressed and suggests that some form of active control system will be required if the maximum defocus parameter is not to be violated.

The last decade has witnessed rapid growth and development within the telecommunication industry. This, in particular for speech coding, has been primarily driven by the enormous expansion of digital mobile communication. Whilst the available bandwidth in wired based terrestrial network is a relatively cheap and expandable resource, it becomes unavoidably limited in satellite or mobile communication systems. At the same time, a high quality speech communication system is preferred. This, on the other hand, requires high data rates. Therefore, key factors to design speech coder are both the optimisation of the bandwidth usage and provision of high quality of service. The research carried out in this thesis has mainly focused on the design and development of low to medium bit rate narrowband and wideband speech coding algorithms which are based on Analysis-by-Synthesis Linear Prediction Coding (AbS-LPC). In order to reduce the computational complexity as well as to improve the perceptual quality, the secondary excitation used in the developed coder is based on sparsely populated pulse vectors instead of a Gaussian codebook, used in the original famous Code Excited Linear Prediction (CELP) coder. The algorithms subsequently developed also incorporate a multi-rate operation i.e. speech and channel coding bit allocation can be varied for different level of error protection according to channel conditions. Moreover, the developed coder is designed to operate between 8 and 14 kb/s, at a gross bit rate of 22.8 kb/s in order to provide a reliable service in the existing mobile communication network. In addition to the multi-rate speech coders, a new approach to variable rate speech coder is presented. In this course of research, longer frame lengths are used when possible i.e. when the speech segment contains similar- characteristics. This is mainly to reduce the parameter updating rate, and hence the bit rate. Speech segmentation and voice classification are obtained via speech recognition. Different types of excitation are used for different voice classes in order to improve the speech quality and to further reduce the bit rate. However, some applications require very high quality speech which narrowband speech cannot offer. The work in this research also includes the development of wideband CELP coder that operates at much lower bit rate than the current standards whilst maintaining toll or near toll quality of speech.

Satellite resource management efficiency has been identified as one of the key factors in the commercial success of mobile satellite systems, since optimisation of all link budget elements is crucially important in order to make the most out of the satellite limited resources (bandwidth, power) which in turn have a direct impact on the cost of the system. The compensation techniques used in order to overcome the fading effects experienced in the link are generally applied by considering the worst-case channel conditions, resulting in inefficient utilisation of the transmission power as well as frequency spectrum. There is no dynamic control adaption used in current mobile satellite systems except simple power control. In addition, a single scheme is neither capable of providing an optimum solution for fade mitigation nor closing the link budget at all times. Therefore, it is necessary to use more than one technique at a time which offers the best solution in terms of spectrum and power efficiency. The efficiency of conventional systems, therefore, can be improved if it has the ability to match the effective user bit rate to the channel conditions by using a hybrid scheme. Hence, the aim of our work is to develop a physical layer by using a hybrid scheme, which results in higher throughput under favourable channel conditions. This technique also introduces a reduction of the data rate during bad channel conditions without the need to increase the transmitted power significantly. The novelty of this research work is centred on the switching mechanism used for such adaptations. For the first time, we propose an adaptive system based on the Rice factor variation. The suitability of this parameter for the proposed adaptive system is investigated first by considering the real time variation in the environment. We are proposing a feed back type of system in which the receiver estimates the Rice factor and sends it to the transmitter. Upon receiving this information, the transmitter selects the optimum modulation and coding scheme for the transmission in order to improve spectral and power efficiency of the system. In order to develop an adaptive physical layer, the main issues related to mobile satellite systems should be identified. Therefore, the key differences between the terrestrial and satellite mobile communication systems are presented at the beginning. A brief description about T/S-UMTS and the air interfaces proposed for standardisation of S-UMTS are presented and compared in the following chapter. Subsequently, a suitable baseline model was chosen and the simulation aspects are presented. The following chapter presents some of the parameters that have to be estimated in order to develop an adaptive physical layer. The performance evaluation of adaptive modulation and coding is presented in the last chapter. Key words: S-UMTS, SW-CDMA, Adaptive modulation and coding. Rate matching, SNR estimation, modulation detection. Rice factor estimation. Histogram comparison. Re-encoding.

This thesis describes new techniques of detection and fast start-up channel estimation for transmitting digital data over a fast 'frequency-selective' fading channel of the type encountered in fading mobile communications. The model of the channel involves a large number of reflected signals delayed in time by different amounts and reaching the receiver via different paths, each of which is represented by an independent Rayleigh fading channel. Such a model gives rise to frequency-selective fading and an appreciable degree of intersymbol interference. Base bend representation of the quadrature amplitude modulated data transmission system is first studied.

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