In this thesis timing synchronisation and resource allocation issues in future mobile satellite systems have been examined. Specifically, in the third chapter the timing synchronisation problem generated by the integration of a GSM-like system and a satellite one was addressed. In the fourth chapter the RLC/MAC layer of the GPRS protocol, which provides for resource allocation amongst packet mobile users, was examined in a MEO satellite environment. In chapter five, resource allocation algorithms were examined under different perspectives and were addressed through a combination of DCA techniques and heuristic algorithms. In the sixth chapter the process introduced in chapter five was adjusted to fit a time-variant system.
7.1
M ain C ontributions
The main contributions of this thesis are as follows: In the third chapter:
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• A novel approach, based on the division of the satellite footprint into de termined patterns, was introduced in order to find solutions to the timing synchronisation problems for an integrated GSM and satellite environment. This is considered to be an im portant contribution from a theoretical point of view, since it addresses and aids the research in one of the most im portant problems of such a future integrated system. The research work treated the problem as a theoretical approach th a t could form the basis of a feasibility study, and some implications such as the ones affecting the satellite architec ture and system capacity were also discussed. Detailed study of the impact of these implications necessitates further work, and therefore they are discussed further in the section ‘Suggestions for further work’.
• In addition to the application of the above approach to a ‘fixed-satellite cell’ satellite system, the approach was applied to ‘fixed-earth cell’ satellite systems, emphasising the futuristic nature of such a system. For the latter case we concluded th a t the system is unfeasible, and showed th at the former approach is more practical, while highlighting some of the main design considerations.
In the fourth chapter:
• A detailed simulation platform was developed in order to evaluate the perfor mance of the GPRS RLG/MAC protocol over a MEO satellite system. GPRS is expected to address the mobile world’s increased demand for the support of packet-switched services in a mobile environment. The integration of such a protocol with the satellite world could satisfy this demand in a seamless way.
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The specific research evaluated the performance of this system and identified areas in which further enhancements could be introduced.
In the fifth chapter:
• In a limited spectrum mobile world, an efficient distribution of channel re sources amongst the users is of high importance. In this chapter, algorithms th at enable an optimised ‘packing’ of channels were identified. Additionally, the algorithms were used in conjunction with a novel approach according to which the channels were distributed to the users and not to cells. As a result, significant performance enhancements were achieved.
• Further investigation, included the identification of the most efficient heuristic technique to be used in such environments. Therefore, in this chapter specific optimisation techniques were tested for their applicability to the problem at hand, as well as different algorithms for their efficiency. These methods and algorithms were applied in the next chapter for evaluation in time variant systems.
In the sixth chapter:
• The techniques th a t were used in the fifth chapter were applied to a dynamic system, in order to achieve a real-time optimised system performance. The simulated annealing technique was incorporated into the algorithms used in our dynamic model. Although simulated annealing is conventionally used in ‘static’ problems in order to approach their global optimum, we applied it
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to a dynamic problem by using a constant tem perature for all the ‘micro states’ of our system. The results achieved after the introduction of simulated annealing were significantly improved. The use of our algorithms together with constant tem perature annealing can therefore offer major system capac ity benefits over existing algorithms. We expect th a t our model of constant tem perature annealing, in particular, would offer m ajor benefits also when ap plied to cell-based DCA systems. We expect th a t it would improve the system blocking probability while allowing the use of low complexity algorithms for the DCA part. Therefore, this option is further discussed in the ‘Suggestions for further work’ section. Finally, this chapter also discussed some practical implications of such an approach on satellite and system architecture, as well as the complexity of the controlling hardware and software.
7.2
Suggestions for further work
• In the third chapter we are proposing a new footprint cell-division method in order to m itigate GSM timing synchronisation problems over a satellite system. The cell sizes th at are proposed could be small. Therefore there is a trade-off between cell sizes and TDMA timeslot duration. The decrease in cell size will result in an increase in signalling messages required, for location area updates, routing area updates as well as for handovers. The increase in signalling with respect to the cell size should be explored further. Additionally, since the application of this technique to different channel environments impacts on the
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satellite architecture and system design, a feasibility study would be suggested. Such a study would compare the application of this theoretical approach to different architectures and investigate cost and practical trade-offs. Finally, cell structures other than the ‘traditional’ hexagonal cell patterns, could be looked into, in order to attain better suitability of the model. Some suggestions for such patterns are included in the thesis. Complexity issues when looking at such new cell patterns could include appropriate frequency re-use patterns.
• In the fourth chapter the same research could be applied to different altitude constellation systems, such as LEO, and the impact of the altitude on the GPRS system loads th a t can be supported in this new environment should be evaluated.
• In the fifth and sixth chapters, different optimisation techniques, such as ex haustive search, th at could produce more optimal results, but could also intro duce significant increase in the execution time and processing capacity required can be applied. Their efficiency could be compared with th a t of the heuristics used in this work, and then a comparison on the basis of the two previously mentioned limiting factors should take place. Additionally, Genetic Algo rithms (GA) techniques are becoming increasingly popular and have proved to offer very efficient alternatives to other optimisation techniques. A fur ther work item could be to evaluate the applicability of GA algorithms to the problem presented in chapters 5 and 6.
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• When simulated annealing is used in the time variant system, different sim ulation sets were executed in order to achieve a balance between improved performance and number of signalling messages sent. A more detailed study in this direction could follow, resulting in a system design which would take into account detailed signalling messages, and the estim ation of extra capacity required.
• The system in chapter 6 requires a very complex beam-forming network due to the requirements of control for the switched beams, the dynamic frequency- to-beam assignment and the number of beams themselves, while also requiring advanced packaging technologies to simultaneously achieve light weight, low volume and low cost satellite systems. These practical considerations will have to be outweighed against the advantages th a t such a system would introduce, in order to assess whether such a system would be cost effective for a particular network design.
• The idea of channel reshuffling between resource allocation events in a DCA system using our proposed, in chapter 6, constant tem perature simulated an nealing algorithm, could be applicable to both cell-based as well as non cell- based systems (as is the case with our model). When applied to a cell-based system, the distance parameters refer to distances between cells. We would expect the application of this algorithm to a DCA cell-based system to offer
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similar levels of improvement as observed in chapter 6. Therefore, further investigation of the above option is suggested as further work.
In summary, this thesis has addressed a range of issues of importance to satellite communication systems and has presented and evaluated a number of innovative solutions.