Research On OFDM Synchronization Technologies In Mobile Satellite Systems

As continuous development and maturity of 3G and Beyond 3G services, the demand for global high-speed data transmission and multimedia services is increasing. Mobile satellite communication is suitable for high-speed transmission due to its large coverage area, broadcast and multicast working mode and other intrinsic properties. Whereas, the frequency selective fading channel characteristics and deficient spectrum resources become constraints crying for new efficient solutions. OFDM(Orthogonal Frequency Division Multiplexing) technology has characteristics of strong anti-interference ability and high spectrum efficiency and thus it is suitable for mobile satellite communication system to complete high-speed transmission. However, the superiority of OFDM is built on orthogonality among subcarriers and it is vulnerable to synchronization errors. The multipath effect and Doppler frequency shift of satellite channel will lead to synchronization error, so in this paper, we will study on synchronization technology for OFDM in a mobile satellite communication system.Characteristics of mobile satellite communication channel are introduced first and multipath effect and Doppler frequency shift are analyzed, then we choose the wideband satellite communication channel model in suburb as simulation environment. After that, we give the implementation process of an OFDM system and focus on the causes of synchronization errors, doing theoretical analysis and simulation on the effects of the timing offset and frequency offset to the whole OFDM system.Synchronization algorithms based on training sequences are adoptted in this paper. Symbol timing synchronization is carried out at first. We study on the Schmidl method and H.Minn method, and simulate their performance under the suburban satellite channel. A new timing estimation method with conjugate symmetric structure in training sequences is proposed in this paper. Its timing metric has sharp peak so timing performance is more accurate and superior to the previous two methods.Frequency synchronization is divided into two processes: fractional and integer frequency offset estimation.Generally, fractional frequency offset is estimated and corrected to eliminate the ICI(Inter-Carrier Interference), then the integer frequency offset estimation is conducted. We do intensive study on Schmidl frequency synchronization method, and propose an improved scheme which adopts the average of several times estimation as the final estimation value, so the results are more accurate. Afterwards, Kim algorithm is researched on. Given the premise that it reduces the cost of the training sequence, Kim method is better than that Schmidl method. Finally, we give a frequency synchronization scheme based on the the training sequence used in timing estimation, the conjugate relations among which is used in fractional frequency offset estimation. Then the interger frequency offset is estimated with a new method. BER(Bit Error Rate) of signals at the receiver is computed and simulation shows that the proposed scheme performs basically close to the Kim algorithm on frequency synchronization while has better performance on timing synchronization.