Research On Frequency Synchronization For The OFDM And OFDMA Systems

Orthogonal Frequency Division Multiple Access (OFDMA) is a multiplexing/multiple access technique based on OFDM technology. It has all the advantages of OFDM technology such as high spectral efficiency and robustness property against frequency selective fading channels. Recently, OFDMA has been adopted or proposed for a variety of wireless applications. Similar to OFDM, the performance of OFDMA is also sensitive to the frequency synchronization errors. Carrier Frequency Offset. (CFO) between the transmitter and the receiver results in the loss of orthogonality among subcarriers and ICI, which further introduces MAI in OFDMA and degrades the system performance. In OFDMA downlink, synchronization is a single-parameter estimation problem .Many CFO estimation and compensation algorithms developed for OFDM can apply to the OFDMA downlink. The real challenge lies in the uplink of OFDMA. In OFDMA uplink, each user has its own CFO, so the synchronization has become a multiple-parameter estimation problem, and the compensation of CFO has also became more difficult.First in the .thesis, basic principle about OFDM and some synchronization algorithm to estimate CFO is introduced. We also present a new frequency synchronization scheme based on time oversampling. From the simulation results, it is shown that the accuracy of the proposed algorithm, as well as the resistance to frequency selective fading and noise, is much better than that of the algorithm based on cyclic prefix.Then we introduce OFDMA systematically. The multiple access technique in OFDMA is discussed along with the comparison of different subcarrier assignment schemes. An estimation algorithm based on the signal structure is described for estimation the CFOs of all users using only one OFDMA block. Simulation results illustrate the high accuracy and efficiency of this algorithm. With the estimation results, we must compensate the synchronization errors. The traditional method is feeding back the estimation results to the transmitter with downlink channel, but the performance is not good enough over time-varying radio channels, so we should implement it at the receiver. For the interleaved subcarrier-assignment scheme, minimum mean square error criteria are used to construct the orthogonal spectral signals from one OFDMA block contaminated with interference caused by CFOs of multiple users, and a low-complexity implementation of the proposed algorithms is developed. The simulation results illustrate that the algorithms significantly improve the system performance.