TH`ESE - Library of Ph.D. Theses | EURASIP

AbstractThis work deals with the use of an antenna array at the base station of a mobilecommunication system for transmission. In reception, solutions that exploit theantenna array are now well established. In transmission, however, the problem remainsopen. Two approaches are possible : exploit the array by using beamformingtechniques or by using diversity techniques. These two approaches are based on oppositeassumptions about the channels correlation, which implies a greater or smallerdistance between antennas, depending on the environment. In practice, these assumptionsare not verified. Here, we aim to deal with the problem as a whole for betterexploiting the antenna array. This work treats the mono-user case, as well as themulti-user scenario.In the mono-user case, we propose a transmission scheme composed of a classicaltransmit diversity technique applied to virtual antennas, which are directive andideally independent. These virtual antennas are produced by means of purely spatialfilters applied to the real antennas. This set of filters is called precoder, and actsas the interface between the transmit diversity technique and the transmit antennaarray. The precoder function is twofold; to beamform the transmit signal, and tocreate diversity by means of ideally independent virtual antennas, thus combiningtransmit diversity and transmit beamforming. We propose two approaches to obtainthis precoder, one based on the minimization of the variance of the received powerat the mobile, and the other directly based on the minimization of the error rate.Compared to more classical approaches, the performance of the new methods revealstheir advantage.Then, we extend this scheme to a multi-user context, for which transmit diversitytechniques have not yet been developed. We propose to jointly adapt the precoders byminimizing the total transmit power while satisfying the users’ target BER. Similarlyto the mono-user case, the precoders allow to combine beamforming and transmit diversity.Compared to classical multi-user beamforming techniques, the new approachadds diversity to the link and obtains in return better performances.vii