Synchronization Technique For Ofdm Systems

One of the requirements of next generation of wireless mobile communication system is to provide the transmission of the multimedia services with higher speed and wider bandwidth. In the wireless environment, the frequency resources, power and wireless fading channel etc. limit higher data transmission. In theses years, OFDM (Orthogonal Frequency Division Multiplexing) technique has been paid more and more attention because it has strong transmission ability in wireless channels, which comes from its own parallel transmission mechanism, inherited robustness to Inter Symbol Interference (ISI) and the diversity with the multi-path channel. Meanwhile, OFDM system has very high channel utilization ratio because of the orthogonal properties among its sub carriers. While the OFDM technique also has some shortcomings such as being sensitive to the time synchronization error and the frequency offset. In detail, at first, in the wireless environment, the receiver has no knowledge about the time delay between the transmitter and itself in general situation. In the OFDM system, the OFDM data is transmitted as one symbol, so in order to do the correct demodulation, the correct start of symbol should be got from the received signals to reduce the effect of ISI. Secondly, the high frequency utilization and the robust transmission of OFDM are founded on the orthogonal properties of the sub carriers, so if the frequency offset is generated and the orthogonal properties among sub carriers is lost, that will cause the Inter Carrier Interference (ICI) and high BER of system. As a result, in the OFDM system, how to synchronize in a quick and accurate manner is quite an important issue, which affects the performance of the whole system. So the corresponding time and frequency synchronization algorithms should be proposed and realized in the OFDM receiver design. This paper studies the synchronization technique in the OFDM system, which is to do efficient estimation and compensation of time and frequency offset to enhance the system performance. Because more and more communication system has adopted the OFDM technique as its core techniques and they have different application background, system parameters and data structures, our paper only focuses on the synchronization issue in the IEEE 802.11a WLAN system. In this system, the data are transmitted in burst frames and there exists a preamble, which is composed of ten repeated short training sequences and tow repeated long training sequences in front of each frame. This redundancy provides potential to do time and frequency synchronization in IEEE 802.11a OFDM system. This thesis focuses on this special frame structure, utilize time synchronization, fractional-frequency offset coarse estimation algorithm in the time domain and integer-frequency offset estimation in the frequency domain to meet with the high requirement of synchronization in the OFDM system.This thesis is organized as follows:Chapter 1 introduces this thesis's background and contents. Chapter 2 gives a brief introduction of OFDM technique. Then it discusses the synchronization issues and the existing synchronization algorithms in the OFDM system. In Chapter 3, the physical layer of IEEE 802.11a WLAN system, which is based on OFDM technique and its realization by MATLAB are explained. Chapter 4 proposes the improved synchronization algorithms of frame and coarse frequency offset estimation. The simulation results illustrate that the proposed algorithm has better synchronization performance especially in the multi-path fading channel with low SNR Chapter 5 discusses the integer frequency offset estimation algorithm and it is proved that this algorithm has good estimation performance by simulation. Chapter 6 gives the conclusion of the whole thesis.