| Orthogonal frequency division multiplexing (OFDM) is a multicarrier transmission technique which has become a popular method of communication, due to its high spectrum efficiency, strong immunity to severe multi-path fading and high-data-rate transmission. OFDM has been used in several systems such as digital television, digital broadcasting and wireless local area networks, and it has become a most promising candidate for 4G mobile communications.In the application of OFDM in the wireless communication system, carrier frequency offset (CFO) can have a negative impact. Small CFOs can cause severe performance degradation in OFDM due to the loss of orthogonality among the subcarriers. This dissertation focuses on the synchronization effects which influence the OFDM systems significantly and studies the key synchronization technologies of OFDM in wireless mobile communications.The content includes:1.The influence of carrier frequency offset on OFDM systems has been analyzed.2. The advantages and disadvantages of the existing CFO estimation algorithms have been summarized, and a new idea has been given for designing high spectrum efficiency and high data-rate transmission CFO estimation algorithms.3.Novel CFO estimation algorithm has been developed, based on the Expectation-Maximization principle for OFDM systems in AWGN channel.In this method, the length of pilot samples could be shorted for no loss of the BER performance. When SNR is large, it is shown that its performance is close to the idea case even using short length pilot sequences.4. Joint CFO and channel estimation method is proposed based on the Expectation-Maximization in multi-path channel.In this method, just partial pilot samples are required in the preamble symbol. The new algorithm has high transmission efficiency and low complexity. According to the following theoretical analysis and simulation result, it is shown that its performance is still good when some subcarriers are interfered. |
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