Research On Timing/Frequency Synchronization In OFDM Systems

Orthogonal Frequency Division Multiplex(OFDM) has drawn much attention from researchers all over the world, due to its good ability in combating multipath fading and high spectral efficiency. It has been already widely applied in Digital Audio Broadcasting(DAB),Digital Video Broadcasting(DVB), and Asymmetric Digital Subscriber Line(ADSL), and so on. As one of multicarrier modulation techniques, OFDM is very sensitive to timing and carrier frequency offset error, so timing and frequency synchronization for OFDM systems is very important.Recently, many training symbol-based carrier frequency synchronization schemes have been proposed for burst OFDM systems, In burst OFDM systems, synchronization should be fast and accurate, and therefore the number of training symbols employed should be less, and high frequency offset estimation accuracy and large estimation range are required. Many proposed schemes have shortcomings. For example, some schemes employ more training symbols, resulting in lower data transmission rate. Frequency offset estimation accuracy of some schemes is high, but the estimation range is limited or the computational complexity is high. Other schemes enlarge the estimation range, but frequency offset estimation is not very accurate. In this paper, we focus on the carrier frequency synchronization in burst OFDM systems and then an improved training symbol-based method for frequency offset estimation was proposed. In our scheme, both the training symbol and the cyclic prefix were used to obtain the fine frequency offset estimation in time domain, and the coarse frequency offset estimation was accomplished in frequency domain by peak detection. Simulation results show that, in multipath fading channel, the frequency offset estimation accuracy is improved significantly while the estimation range is enlarged to the overall signal bandwidth. Meanwhile, the residual frequency offset can be restricted within 2% of the subcarrier spacing, if SNR reaches 12dB. Moreover, many frequency synchronization schemes only consider frequency synchronization in the receiver (assuming good timing synchronization). However, in OFDM systems, timing synchronization and frequency synchronization always influence each other, and therefore joint timing and frequency synchronization is needed. To address this requirement, people have presented a lot of joint synchronization methods. But the structure of the training symbol always affects the accuracy of timing or frequency synchronization. To solve this issue, an improved timing and frequency synchronization scheme based on a new training symbol was proposed. In this method, a plateau was firstly obtained by using the same repeated halves in the training symbol, and then the timing synchronization was accomplished by using the symmetric conjugate of the training symbol in the plateau. Meanwhile, frequency synchronization was finished by utilizing phase rotation of the same repeated halves in the training symbol. Simulation results show that, in multipath channel, proposed method achieves an accurate estimation both in symbol timing and carrier frequency offset for OFDM systems.