Study and optimization of multi-antenna systems associated with multicarrier modulations

Since several years, multi-antenna systems are foreseen as a potential solution for increasing the throughput of future wireless communication systems. The aim of this thesis is to study and to improve the transmitter and receiver’s techniques of these MIMO (Multiple Input Multiple Output) systems in the context of a multi-carrier transmission. On the one hand, the OFDM (Orthogonal Frequency Division Multiplex) modulation, which transform a frequency selective channel into multiple non frequency selective channels, is particularly well adapted to the conception of MIMO receivers with low complexity. On the other hand, two techniques allowing to improve the exploitation of frequential and/or temporal diversities are associated with OFDM, namely linear precoding (LP-OFDM) and CDMA in a MC-CDMA (Multicarrier Code division Multiplex Access) scheme. We have associated LP-OFDM and MC-CDMA with two MIMO techniques which require no channel state information at the transmitter, that is STBC (Space-Time Block Coding) and spatial multiplexing. It is shown that for these two systems MIMO MC-CDMA and MIMO LP-OFDM, spatial, temporal and frequential diversities are efficiently exploited. The advantage of the association with orthogonal STBC is that low complexity receivers can be implemented. In fact, for a STBC LP-OFDM system, a simple linear MMSE equalizer leads to performance close to those obtained with a Maximum Likelihood (ML) receiver allowing the use of large linear precoding matrices. For STBC MC-CDMA, like SISO MC-CDMA, single-user detectors leads to the best tradeoff performance/complexity. When LP-OFDM or MC-CDMA are associated with spatial multiplexing, the surplus of complexity for the receivers is inherent to the multi-antenna interference. We have also demonstrated that the implementation of a simple MMSE receiver requiring the inversion of the MIMO channel matrix leads to very good performance. A similar process can be applied for non orthogonal STBC. Combined with an iterative process when channel coding is inserted at the transmitter, all these systems can reach the matched filter bound performance close to the gaussian curve performance using simple operations. The gain in spectral efficiency and performance is to the advantage of spatial multiplexing techniques because they benefit from the linear augmentation of the capacity with the minimum number of transmit and receive antennas. The trade-off between throughput and complexity will indicate the choice of the MIMO system to associate with LP-OFDM and MC-CDMA. Theoretical channels as well as realistic channels are simulated with or without correlation between antennas.