Research On Carrier Frequency Offset Of Asynchronous Cooperative OFDM Communication

Cooperative communication is a kind of virtual MIMO technology, which combines the advantages of many other transmission technologies, and has been widely used in the new generation wireless communication. Although cooperative communication technology makes MIMO technology widely used, different from the traditional MIMO technology, cooperative nodes is distributed in space, which causes the problem of asynchronization. In cooperative communication system, asynchronous problem includes clock, time delay and frequency. This paper mainly studies the problem of carrier frequency offsets in cooperative OFDM system.Firstly, this paper studies the main technologies in cooperative communication system. It introduces the concept of cooperative communication and the very important cooperative forwarding protocols, also compares the performance of different protocols. Because OFDM technology as an important role in resisting to multipath delay, so this paper discusses the theory of OFDM in detail, and studies the MIMO-OFDM system and cooperative OFDM system. In addition, space time block code and distributed space time block code are also been studied.Then, the paper studies cooperative STBC-OFDM transmissions when the cooperative transmitters are not perfectly synchronized in time and carrier frequency. Based on an approximate channel model, this paper analyses the performance of STBC-OFDM cooperative transmissions, and propose a joint ICI mitigation and STBC decoding algorithm, it can effectively eliminate the impact of asynchronization.Finally, this paper goes further research for mitigation method of carrier frequency offset in cooperative OFDM system, and develops an efficient method to mitigate CFO, by exploiting the cyclic prefix(CP) of the OFDM transmissions. This method can mitigate or even remove completely the CFO. Another special feature of this algorithm is that it is implemented as a pre-processing procedure independently from cooperative-encoding and cooperative-decoding details, which makes the CFO problem transparent to the cooperative OFDM transmission designs, so it having general applications in cooperative OFDM transmissions.