| With the capability of supporting high-rate transmission in multipath channels, orthogonal frequency division multiplexing (OFDM) technique has been introduced into more and more wireless applications. The growing popularity stems from its advantages of simple equalizer, pulse-noise immunity and spectral efficiency. It is very promising to be a core technology of the fourth generation mobile communications.While the OFDM technique also has some shortcomings such as being sensitive to the time synchronization error and the frequency offset. If the time synchronization error generated, the orthogonal properties among the sub carriers will lost, it will course the very serious Inter Symbol Interference (ISI) and Inter Carrier Interference (ICI), and then, the demodulation performance will be greatly interfered. Therefore, the accurate time synchronization is extremely important for OFDM system. Furthermore, a complete communication system absolutely needs a proper channel encoding and decoding module. In IEEE 802.11a WLAN system, the data are transmitted in burst frames and there exists a preamble, which is composed of ten repeated short training sequences and two repeated long training sequences in front of each frame to realize the time synchronization. For channel encoding and decoding, (2, 1, 7) convolutional code and Viterbi decoding algorithm is used.In the thesis, firstly, it discussed the basic theory of OFDM. Secondly, it introduced parameters of OFDM in 802.11a physical layer and studied the OFDM encoding process. After that it analyzed various kinds of important function blocks in 802.11a protocol. Then, it focused on the implementation of two important function blocks: they are synchronization block and convolutional encoding/ Viterbi decoding block with FPGA (Field Programmable Gate Array). Finally, the method was proposed and implemented in VHDL language, and the waveform simulations verify its correctness by comparison with the Matlab simulation results. |
PDF Full Text Request