Study On Precise Synchronization Algorithms For Burst OFDM Systems

Orthogonal Frequency Division Multiplexing(OFDM) provides higher data transmission rate and spectral efficiency and the ability to overcome multipath fading effect, which makes it a vital technology of next generation wireless communication systems. Multipath propagation results in inter-symbol interference(ISI) that severely degrades the system performance. The ISI can be avoided by precise first-path detection. Burst transmission requires a fast and precise synchronization scheme, but the random arrival time of the received signal makes it difficult to detect the first path precisely. Hence, the research of precise timing synchronization algorithm with first-path detection is important for the application of burst transmission systems.This thesis studied the precise synchronization algorithms for burst transmission OFDM systems in an actual engineering scenario and focused on the first-path detection algorithms. The main contribution of this thesis is listed as follows.â‘  The impact of timing and frequency errors on OFDM systems in multipath fading channels was theoretically derived and demonstrated by computer simulations. The connections between first-path detection and timing synchronization were discussed to elaborate the necessity of first-path detection.â‘¡ Most of the conventional algorithms use energy as the timing metrics which results in the dominant path is regard as the synchronization point rather than the first path. This thesis proposed a first-path detection algorithm based on constant amplitude zero auto-correlation(CAZAC) sequences. The probability density function of timing metric was derived, in the light of which a first-path search method was designed, and a constant false alarm rate(CFAR) adaptive threshold was set according to the Neyman-Pearson criterion for accurately first-path validation. Simulation results show that the proposed algorithm can realized fast and precise detection of the first path in multipath fading channels.â‘¢ The timing and frequency synchronization for OFDM systems are mutually depended and constrained. Conventional algorithms have to switch for multiple times between time and frequency domain, which causes high implementation complexity and delay and thus limit their applications in burst scenario. And the synchronization precision cannot be considered jointly. Therefore, a frequency synchronization algorithm which is robust to timing error was proposed to form a joint synchronization scheme with the aforementioned first-path detection algorithm. The adaptive threshold for fine timing was adjusted to reduces the performance loss due to residual frequency offset. The proposed joint scheme can be executed in time domain for burst transmission systems to achieve fast and precise synchronization. And the frequency synchronization algorithm has quite a wide range of frequency offset estimation. Simulation results on the first-path detecting probability, timing and frequency estimation mean square errors(MSE) show that the proposed joint synchronization scheme is significantly superior to the conventional ones to satisfy the requirement of the first-path detection and frequency synchronization for burst transmission OFDM systems.