| WLAN (Wireless Local Area Networks), a kind of LAN using wireless media, is a production of the combination between wireless communication technology and computer networking technology. WLAN specified by IEEE802.11a, whose PHY adopts OFDM modulation, can achieve a high transmission rate up to 54Mbit/s. There has been a great interest in researching and applying OFDM technology into high-speed wireless communications in recent years. OFDM technology based on digital communication systems is robust in multi-path environment, and it can achieve high bandwidth efficiency and can be implemented with a simple structure. These advantages make OFDM technology particularly suitable for high-rate data transmission in wireless environments. This thesis mainly focuses on the research of synchronization techniques of IEEE802.11a's OFDM systems, and puts forward a synchronization scheme after considering the simulation result.Following the description of the OFDM's basic principle and necessary elements for setting up an OFDM system, this thesis discusses some non-ideal effects on the performance of OFDM digital communications, which include carrier frequency offset, symbol timing offset and sampling clock deviation, and so on. In this thesis, we also discuss some typical synchronization schemes, such as, uses the specific training symbols, the redundant information contained in the cyclic prefix and the pilot signal to achieve synchronization. New designed synchronization schemes often adopt ideas from these schemes, and compare the performance and practicability to them.In this thesis, we put forward a synchronization scheme according to IEEE802.11a, suitable to use in the burst mode communications system. In the time domain this schreme utilizes the training symbols to achieve packet header detection; frequency offset estimation and symbol timing. During the frequency domain, it makes use of the pilot signal to fulfill the tasks of carrier frequency tracking and sampling clock tracking. This scheme has such advantages as simple computation and easy to realize. It satisfies the demand for rapid synchronization in the bust-mode communications environment. The system's BER comparison result, between the ideal synchronization and the synchronization algorithms, shows the synchronization scheme's good performance in slow time-variant channel.This thesis is arranged as following: the developing process of WLAN and OFDM technology is introduced in Chapter 1. Chapter 2 discusses the basic principle and structure of OFDM system. In the mean time, some non-ideal effects on the performance of OFDM system are carefully analyzed. Then in Chapter 3, some typical synchronization schemes are also introduced. In Chapter 4 we introduce the concrete configuration of the OFDM system used in IEEE802.11a based on WLAN's PHY. In Chapter 5, we put forward an IEEE802.11a based synchronization scheme, and analyze the performance of every algorithm in this scheme, as well as their optimization. Then the BER is calculated by computer simulation to show the performance. In Chapter 6 IEEE802.11a prototype's design including hardware and software is introduced, and the HDL implement details of coarse CFO estimation and correction are given. In the end, Chapter 7 discusses the system improvement and some key techniques in OFDM system that needs further research work. |
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