Etude et optimisation des techniques MC-CDMA pour les futures générations de systèmes de communications hertziennes
Stéphane Nobilet · HAL (Le Centre pour la Communication Scientifique Directe) · 2003
Future radiocommunication systems will have to accomodate high data rate while allowing a great mobility to the users. In order to achieve this goal, new signal processing techniques must be investigated. One of the techniques currently under significant research is the MC-CDMA technique. This combines orthogonal frequency division multiplex modulation with spread spectrum technique. This thesis deals with the study and the optimization of MC-CDMA communication systems. After a general presentation of different combinations of OFDM and spread spectrum techniques, MC-CDMA systems performances are evaluated for multipath Rayleigh fading channel and BRAN channels for the case of single and multi-user detections. The influence of the spreading sequences is then studied on the dynamic range of the transmitted multicarrier signal envelope and on the multiple access interference. For that purpose, the envelope variation of MC-CDMA signals is considered through the study of the crest factor and the global crest factor. These quantities make it possible to estimate the fluctuations of the transmitted signals in uplink and downlink respectively. In order to minimize the multiple access interference, an optimized spreading sequence allocation procedure is presented and different code selection criteria are proposed. Finally, the focus is moved to the optimization of uplink MC-CDMA systems. In order to avoid the insertion of Nu groups of pilot subcarriers used to estimate the Nu uplink channels, a method based on the channel reciprocity property is proposed. This solution consists in using the downlink channel estimation to predistort the signal at the mobile station before transmission. Two alternative techniques are considered. The first is based on a time division duplex of the uplink and downlink whereas the second relies on a frequency division duplex of the uplink and downlink.