Symbol Timing And Carrier Frequency Synchronization Methods For OFDM In Non-cooperative Communication

Non-cooperative communication, as a means of communication unauthorized access, is a communication mode who can access to cooperative communication system under the condition of having no interference in both sides of normal cooperation communication. In non-cooperative communication, because of the unknown originator information and the complexity of interference, it is difficult to meet the performance requirements by the existing synchronous methods to achieve symbol timing and carrier frequency synchronization for Orthogonal frequency division multiplexing(OFDM) system, who is also the key technology for OFDM demodulation in non-cooperative communication environment. Thus the OFDM synchronization technologies in the paper is analyzed and researched in the non-cooperative communication, it includes the carrier frequency synchronization for OFDM system based on the cyclic prefix(CP), and the symbol timing and carrier frequency synchronization for OFDM system who is based on the offset quadrature amplitude modulation(OQAM). Main solutions obtained are shown as follows:1. In non-cooperative communication system, the carrier frequency synchronization for OFDM system based on CP is studied because the synchronization performance of carrier frequency synchronization is not well in low signal-to-noise. A method of carrier frequency synchronization for OFDM system is proposed in multipath channel by analyzing the relationship between subcarrier signals’ amplitude after Fourier transform in the receiver. The original cost function of amplitude difference firstly is based on the relationship between subcarrier signals. And then cost function by the simplified is based on the product of the subcarriers’ amplitude. Finally through the properties’ analysis of the cost function, it can be equivalent to a sine function, and then get the carrier frequency offset by polynomial interpolation. Simulation experiments show that the method has good estimation performance under low SNR and multipath channel, and lower computational complexity.2. The joint synchronization of the symbol timing and carrier frequency for OFDM system based on OQAM modulation is studied in non-cooperative communication environment. The joint synchronization method is proposed under multipath channels through the cyclic characteristics of the OFDM/OQAM signal. Firstly, The cyclo-stationarity is derivated. Secondly, the additive noise on the non-zero cyclic frequency is disturbed, through the analysis of the characteristics of the signal second-order cycle characteristics. Thirdly, through the two pairs of the cycle frequencies who are even times of sub-carrier spacing, the estimation expression of symbol timing offset and carrier frequency offset can be got by the cyclic characteristics. Finally, the required Estimation values are obtained through estimating the second order cyclic cumulants of the received signal etc. Simulation results show that symbol timing offset and carrier frequency offset estimation does not be affected by the additive noise and would be independent of each other in conformity with the theoretical analysis, and also show that the estimation performance of the algorithm is good in the multipath channels.