Study On Synchronization Technology Of OFDM-Based Cooperative Relay System

Expanded from distributed multi-antenna systems, by using multiuser collaboration, the cooperative communication system solves the problem of traditional MIMO systems requiring multiple transmit antennas, thus the cooperative communication system has been widely applied in wireless communication. Introduction of Orthogonal Frequency Division Multiplexing(OFDM) into cooperative communication system can provide higher data rates and spectral efficiency, and enhance the capability of anti-multipath fading. Accurate synchronization of the system is the basis for cooperating with OFDM communications system to make correct data transmissions; the introduction of OFDM enables cooperative communication system to be more sensitive to synchronization error. The thesis focuses on precise timing synchronization and carrier frequency synchronization of the cooperative OFDM communication system in multipath fading channel environment.Main works of this thesis are:(1) influences from timing offset and carrier frequency offset in multipath fading channel on OFDM system are theoretically derived and demonstrated by simulation; comparison between synchronizations of the cooperative OFDM system and the OFDM system are discussed; and the necessities of the first-path precise detection of cooperative OFDM system and the accurate estimation of carrier frequency offset are explored;(2) considering jointly the timing synchronization and carrier synchronization of cooperative OFDM system, the low-redundancy preamble frame structure is designed, and the first-path precise detection and carrier frequency synchronization are completed at the same time; compared to conventional synchronization, the separate part for timing synchronization is not a must for the preamble frame;(3) there exists such a problem: the algorithm for precise timing synchronization of traditional cooperative OFDM takes usually the energy of metric function as the decision condition for synchronous position, thus the algorithm synchronous position will be the most energetic path instead of the first path expected. In regard to this problem, this thesis presents an algorithm(based on weighted CAZAC sequence) for first-path precise detection; according to noise power, this algorithm can achieve first-path precise detection by adaptive threshold setting. Simulation results show that, in multipath fading channels, the proposed algorithm can achieve the first-path precise detection;(4) there exists another problem: in a multipath fading channel, frequency synchronization of traditional cooperative OFDM system can be interfered by scattering components, leading to insufficient accuracy of frequency offset estimation, and that the training symbols of frequency offset estimation would be mostly different from those of timing synchronization. With respect to this problem, based on the designed frame structure, this thesis proposes a two-step algorithm for frequency offset estimation, which joints together the time domain with the frequency domain. Firstly, in the time domain, the timing metric function was used to separate the estimated frequency offsets of all paths' impulse response; then, according to principle of maximum ratio, estimated frequency offsets of all paths were consolidated and taken as the crude value of frequency offset estimation; finally, the time domain signals compensated by the crude value of frequency offset estimation were switched to the frequency domain, and training symbols of frequency domain were extracted for estimating the residual frequency offset. Simulation results show that the mean square error performance of frequency offset estimation of the frequency synchronization scheme proposed in this thesis is better than that of the traditional algorithms.