| The world of wireless communications is rapidly evolving. New wireless technologies which improve the transmission rate, system capacity, spectrum efficiency of the wireless system and which are more adapted to channel conditions are continuing to emerge. Distributed system attracts more and more research interests because it can achieve the significant benefit of diversity as in traditional MIMO system through cooperative diversity when there is only one antenna at the mobile terminal. This thesis involves space-time coding into distributed system with cooperative diversity. The distributed space-time coded system is mainly investigated in this thesis.The thesis first proposes and investigates the problem of synchronization of signal in physical layer in distributed space-time coded system. In distributed space-time coded system, the problem of synchronization is unique. Because there exists multiple nodes or transmitters in the distributed space-time coded system, there may exist multiple timing offsets and frequency offsets. Thus, it is impossible for the receiver to synchronize to multiple transmitters at the same time.The thesis combines the distributed space-time coding with orthogonal frequency-division multiplexing to construct the DSTBC-OFDM system. The DSTBC-OFDM system can mitigate the influence of the multiple timing offsets, because OFDM is not sensitive to timing offsets.How to mitigate the serious interference caused by the multiple carrier frequency offsets in DSTBC-OFDM system is the key problem in the thesis. Finally the thesis proposes a simple frequency domain equalization technology to mitigate the interference caused by the multiple carrier frequency offsets. The simulation results show that the equalization technique is quite effective when the carrier frequency difference between the distributed transmitters is small, and moreover, by setting the receiver's frequency to the mean value of the transmitters'frequencies, a better performance can be obtained. |
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