Research On Space-Time Signal Processing In CDMA System

With the fast development of mobile communications and emerging service, the design of mobile communication systems faces more challenges, for example, larger system capacity, higher signal quality and multimedia communications. In order to satisfy these requirements, we have to solve the problem by using advanced multiple access methods and digital signal processing. In early 90's, smart antenna technique has been applied to commercial mobile communications; it is an important breakthrough in mobile communication techniques. Nowadays, smart antenna technique which is regarded as the core technique in 3G system,has been generalized to space-time signal processing. This thesis treats space-time processing in CDMA system. In this thesis, space-time 2D-RAKE receiver version, beamforming algorithms in transmitting mode, space-time multi-user detection and space-time coding are studied. The main contents are as follows: Introduces the characteristics and advantages of CDMA, and an overview of space-time processing in CDMA system. Based on uniform linear array (ULA) model, space-time channel and space-time signal are studied; a kind of model of space-time vector channel is given. Three kinds of new beamforming algorithms are proposed: firstly, A robust adaptive beamformer in the presence of array imperfections is presented by improving the block matrix of the generalized side lobe canceller (GSC), it is convenient to use its partial adaptive processing to reduce the computation load. Secondly, although sample matrix inversion algorithm (SMI) algorithm is sounded, the main problems is identified, the distortion of beam patterns occurred as a result of being hypersensitive to the perturbation during computing the covariance matrix inversion of received signal. In order to solve this problem, we propose an adaptive dialog-loading algorithm (ADL-SMI). The simulation results reveal that these two algorithms are robust to noise, and have good performance in beamforming. At last we develop another robust beamforming algorithm. The weight vector is only related with the steering vectors, and is independent of the received signal, so it can completely avoid to be influenced by the matrix perturbation when computing the covariance matrix inversion of received signal. Simulation results reveal that this new beamforming algorithm can form good beam pattern with low side lobe, and is robust to noise. In the thesis, two novel robust 2D-RAKE receivers are proposed. First 2D-RAKE receiver has two parts, one part is a new beamforming algorithm, which has been proposed in chart 3, and the other part is RAKE receiver. The simulation results show that this novel 2D-RAKE receiver can completely eliminate the multi-access interference (MAI) and resist the multipath interference. Secondly, We propose a space-time 2D-RAKE receiver with GSC, and give a adaptive version, The simulation results reveal that the new receiver precedes the conventional one in eliminating MAI and resisting multipath interference, and it can converge rapidly. Combined with smart antenna, space-time interference cancellation multi-user detection with multipaths channel is proposed. At the same time, hard decision is replaced by soft decision, and partial parallel interference canceller is given. Computer simulations illustrated that the performance of the algorithms proposed are much better than of traditional detection. Most modellings on space-time transmit diversity are based on the assumption that there is single-path arriving at the receive antenna from a transmit antenna. However, there are multipaths to arrive at the receive antenna from a transmit antenna in practice. A novel space-time transmit diversity scheme where orthogonal spread codes are used respectively at the transmit antennas is presented to solve this problem. Simulation results reveal that the space-time transmit diversity scheme presented in this paper have much better performance than conventional transmit diversity in multipath channel. We also find that this space-time transmit di...