Study On Optimal Transmission And Multiuser Detection For STBC Systems

With the increasing demandings for the high-speed and wide-band wirless access, it is now a serious challenge to effectively make use of the valuable frequency resources. As the key components of the air interface of the future wireless communications system, MIMO and space-time coding have attracted more and more attention of researchers. Among various space-time coding technologies, space-time block coding (STBC) becomes the current focus, due to its simple implementation and superior diversity performance. Therefore, this thesis makes a primary study on the STBC systems, with the emphasizing on the optimal transmission design and multiuser detection.First of all, the multi-dimensional time-domain beamforming for the space-time-orthogonal frequency division multiplexing (ST-OFDM) system is proposed. For the traditional optimal transmission designed for the single-carrier STBC system, it's difficult to keep a good performance when the system is working in the frequency-selective channel. The proposed scheme performances well in the frequency-selective fading channel, avoiding the drawbacks of the optimal transmission designed for the single-carrier system. We make a quantitative analysis of the complexity of the proposed scheme, along with a comprehensive performance compare with other existing schemes. The proposed scheme is heuristic when considering applying other single-carrier optimal transmission designs into the multi-carrier systems. Specially, we clarify the relationship between the space-diversity degree and the dimensions of the beamforing, which is an instructive supplement to the research results of Zhou and Giannakis.Thereafter, this thesis proposes a beamformer for the cancellation of co-channel interference (CCI) in the space-frequency-OFDM (SF-OFDM) system. The performance of the beamformer based on null-steering has been investigated. Simulation results show that the proposed method has a good CCI cancellation ability. Meanwhile, the system performance can be enhanced by using the time delays of the signals from different direction of arrivals.Next, a particle swarm optimization (PSO) based multiuser detector for the receive-diversity aided STBC systems is proposed. Due to the receive-diversity, the signals received at the different antennas are faded independently, resulting in an independent log-likelihood function for each antenna. To resolve the multi-objective dilemma when choosing one signal estimation for multiple recevie antenna-branches, each particle updates its velocity in a Pareto optimal way. Simulations and comparisons with the genetic algorithm and the maximum likelihood detection verified the effectiveness of the proposed scheme.Thereafter, an improved multiuser detector based on genetic algorithm for the spatial diversity reception assisted STBC systems is proposed. To more effectively resolve the multi-objective dilemma caused by the spatial diversity, the genetic algorithm only selects the best optimal individuals in terms of each goal of the multi-objective optimization for further exploiting. Simulations show that the genetic algorithm employing this strategy achieves a lower bit-error rate when compared with the strategy of selecting all the Pareto Front. The proposed strategy also avoids the complex procedure of non-dominated judging.In the end, we propose a multiuser detection scheme using a radial basis function (RBF) network for the STBC systems with multiple receive-antennas. The neuron centers of the RBFs are jointly carried out in space and time domains, which lead to a powerful technique to combat the interference resulting from different sources. Simulations compare this detector with traditional receiver structures such as the minimum mean square error (MMSE) receiver and maximum likelihood detection. It shows that this RBF-based space-time multiuser detection can be flexibly trade off between the bit error rate performance, the center initialization-rate and the number of the receive-antennas.