Research On Channel Estimation And MIMO Transmission Techniques In TDD Systems

With the integration of Internet and multimedia applications in next generation wireless communications, the demand for wide-band high data rate communication services is growing. In order to satisfy the requirements of the next generation mobile communication systems with higher data rate and wider coverage, OFDM and MIMO techniques have been the key technologies in the physical layer. As an important branch of MIMO technology, MIMO precoding technology has drawn much attention because of its excellent performance and low complexity. On the other hand, the channel reciprocity in TDD systems can be utilized to make the transmitter acquire the channel state information without the overhead of feedback, which contributes to the realization of MIMO precoding technology.This thesis conducts the research on precoding technologies in TDD system. Firstly, it introduces the basic principles of MIMO technology, OFDM technology and TDD mode. And then it does the in-depth research on channel estimation and MIMO transmission, which are the two key technologies for utilizing channel reciprocity of TDD systems effectively. Following list summarizes major contributions of these research works.According to the virtual subcarriers in practical OFDM systems, this thesis makes theoretical quantitative analysis of the effects brought by virtual subcarriers on channel estimation. And based on the ideas of iteration in time domain and channel prediction in frequency domain respectively, two channel estimation algorithms are proposed. These two channel estimation algorithms improve the channel energy leakage in time domain, which is introduced by virtual subcarriers, and enhance the performance of channel estimation correspondingly.For MIMO detection algorithms, based on the widely used K-Best sphere decoder, it is proposed a breadth-first low complexity sphere decoding algorithm. This algorithm significantly reduces the number of the constellation points being searched, through dividing the process of keeping the retained paths in each layer detection into two steps.According to the fact that channel time variations result in non-ideal channel reciprocity, it is analyzed the effects of time-varying channel on MIMO transmission. And with the consideration of channel time variations, two MIMO transmission algorithms with joint designing of both transmitter and receiver are proposed. Theses algorithms are based on the ideas of seeking for the precoder and decoder, which are matched with each other, and interference cancellation among subchannels respectively. Moreover, corresponding theoretical analysis is presented to demonstrate the improved performance and reduced complexity of the proposed MIMO transmission algorithms.