Synchronization, Channel Estimation And Diversity Technologies For Cooperative Communication Systems

Multiple-Input-Multiple-Output (MIMO) is an effective technology to increase transmission rate and improve communication reliability of wireless communication systems. However, MIMO system is based on antenna arrays, and the utilization of multiple antennas in MIMO systems is often impractical because of the limitation of the size, cost and complexity. In order to overcome this practical problem, relay-assisted cooperative communication systems come into being. In relay assisted cooperative communication system, a single antenna mobile terminal can use antennas of other mobile terminals that locate in different areas to assist its own communication, hence the traditional MIMO system is simulated and the diversity gain is achieved, while the complexity of the terminal is kept from increasing. Relay-assisted cooperative communication combines MIMO and relay technologies, so that it can achieve diversity gain easily and effectively. As a result, it has become the focus of the research in wireless communications.In this thesis, several key transmission technologies in cooperative communi-cations will be researched into, including synchronization, channel estimation and diversity schemes of cooperative communication systems.(1) In the study of synchronization scheme, the thesis focuses on synchronization for distributed multi-antenna systems. First, the traditional Unequal-Period Synchronization-Pattern (UPSP) based timing algorithm is improved to overcome the adverse effect caused by large carrier frequency offset (CFO). After that, a joint time-frequency domain synchronization is proposed, which works quite well in amplify-and-forward cooperative systems with a large number of relays and multiple CFOs.(2) The study of channel estimation is based on amplify-and-forward cooperative systems. First, the channel estimation algorithms based on frequency domain pilots are introduced. After that, a time-domain channel estimation algorithm based on cyclic orthogonal training sequence is proposed, which has high accuracy and low complexity. Finally, we design a new time-domain channel estimation algorithm to fit time-varying channels, according to which discrete pilot is multiplexed by a certain Chu sequence. And the proposed algorithm has great potential in practical application.(3) The thesis aims to design full-rate, full-diversity and low-complexity space-time codes for amplify-and-forward cooperative systems using OFDM. The extended orthogonal space-time code and quasi-orthogonal space-time code are involved in the frequency-selective fading cooperative systems. By means of limited feedback, the diversity performance is proved effectively.Finally, based on the summary of theoretical analysis and simulation results of this thesis, the author lists several possible improvement ideas for the proposed cooperative system.