Application aux Futurs Systèmes Multimédia par Satellite en Bande Ka'> Abstract

Abstract

The Present thesis deals with the uplink in future satellite communication systems for multimedia applications in Ka band. More precisely we are concerned by the design of an adaptive waveform to cope with the stringent propagation conditions in Ka band. Such waveform will provide higher system capacity by adopting spectral efficient waveform at clear sky propagation. It will also improve system availability by selecting power efficient waveform during fading events. CPM based waveform is adopted as a solution for the waveform design. Motivations are the followings: First, constant envelope signals allow us to operate user terminal amplifier near saturation which provide higher power efficiency. Secondly, high power performances are obtained when CPM is serially coded with convolutional code with iterative decoding. Finally, waveform parameters diversity (modulation order, modulation index, pulse shape...) is very helpful to design an adaptive waveform.

To start, the usefulness of Fade Mitigation Techniques for satellite communications systems operating in Ka band is emphasized and the state of the art of different FMT is given. Then, we show that, in our context, adaptive waveform is the most attractive solution to cope efficiently with propagation conditions.

Simulations results of several CPM schemes over AWGN (Additive White Gaussian Noise) channel show that iterative decoding provide high power performance when performed with serially coded CPM. The EXIT chart is a very practical way to analyse waveform performance and BER behaviour. The selection of the adopted waveform parameters is a very crucial task and many considerations have to be taken into account, among them:

Power and spectral performance
Transmitter and receiver complexity
Decoding delay
Out-of-band transmitted power

The proposed adaptive waveform is mainly based on the modulation index variation while keeping the same denominator. This solution leads to the same receiver structure and complexity, it require, however, a specific filter banc associated to each modulation index. Three CPM schemes with different modulation indexes are considered: the binary 3RC, the quaternary 2RC and the octal 2RC. Simulation results show that binary scheme is more efficient at low . However quaternary and octal CPM offer higher spectral efficiency during favorable propagation conditions. Therefore, those schemes seem to be more suitable for adaptive waveform. The octal 2RC is particularly an attractive scheme, it requires, however, higher receiver complexity.

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