On Synchronization In Orthogonal Frequency Division Multiplexing Systems

OFDM is a promising technology for its high spectral efficiency and robustness to selective fading and narrow band interference. As MIMO technology can be adopted to increase system capacity, data rate, and make the transmission more reliable, it has now become a key technology in wireless communications. OFDM and MIMO are widely believed to be the candidate technologies for next generation wireless communications (Beyond 3G or 4G) systems. A MIMO-OFDM system is a seamless merger of MIMO and OFDM, which can potentially provide larger coverage range, more reliable transmission, higher date rate and spectrum efficiency.However OFDM and MIMO OFDM systems are very sensitive to time and frequency synchronization errors, especially the frequency synchronization error. This dissertation investigates the topic of time and frequency synchronization for OFDM and MIMO-OFDM systems. The content of the dissertation is shown as following.It is well known that PN sequence can be used for OFDM synchronization, while the security is an essential requirement for modern communication systems. Unfortunately, m sequences and Gold sequences are not able to meet the security requirement in many cases. Cascaded GMW sequences and enhanced phase control sequences have large linear span, good autocorrelation and cross correlation properties, therefore can be used for synchronization in SISO-OFDM and MIMO-OFDM systems separately. In this dissertation, a simple but efficient method, which can be used to construct cascaded GMW sequence, is proposed. Another method to generate enhanced phase control sequence is proposed as well.The effect of time and frequency synchronization error on the performance of OFDM system is systemically summarized in this dissertation. An adaptive threshold and new synchronization strategy are proposed for the synchronization using PN sequences. Moreover, the performance of the proposed synchronization method is compared with that of conventional method. Furthermore, a new method capturing the first path of the multi path channel is also proposed for OFDM systems, which has been shown to have better performance. Along with this new method, a new capturing algorithm is proposed.Frequency synchronization is very important for OFDM systems too. In this dissertation, I present some new methods such as a new iterative frequency synchronization method, a new frequency synchronization method using repeated sequences and cyclic prefix, a high performance frequency synchronization with large estimation range by correlating the received data with local training sequences to eliminate the influence of training sequences, and a new method using CAZAC sequences. The criterion on how to determine the period of repeated sequences in multi-path channel is also analyzed. These methods enrich the already existing frequency synchronization methods. The estimation variance of frequency synchronization methods using cyclic prefix proposed by van de Beek and a more precise estimation variance than that of Schmidl's are got.Novel synchronization methods for MIMO-OFDM systems are proposed, which can be considered as the extension of Tufvesson's method for SISO-OFDM systems.