Carrier Synchronization Technology, Ofdm-based Leo Satellite Broadband Communication System

The aims of the future B3G/4G wide-band telecommunication systems are seamless roaming of mobile communication and multimedia communication. The satellites are important parts of B3G/4G systems. The synchronization of carrier frequency should be completed perfectly when OFDM-one of the key technologies of B3G/4G is applied in the LEO satellite systems with large Doppler shift.High-speed data streams are transmitted in several orthogonal sub-channel in OFDM system. It slows down the data rate in each sub-channel and attenuates the Inter-Symbol Interference caused by multipath fading channels. Carrier frequency offset destroy the orthogonality of sub-channels and the Inter-Channel Interference will lead to worse results than the single carrier communication systems . The estimation algorithms of frequency in OFDM systems can be sorted in three kinds: data-aided algorithm; algorithm based on structure of OFDM symbol; decision-directed algorithm. They also can be sorted by estimated object: algorithms for fractional frequency offset and algorithms for integer frequency offset. This thesis introduces all kinds of algorithms and presented new algorithms.The channel of LEO satellite is different from wireless channels on the ground. Low SNR and large Doppler shift are the two characteristic of LEO satellite channel. The estimation of carrier frequency and design of loop parameters are analyzed in this thesis when the two characteristics are considered. The Cramer-Rao bound of maximum likelihood estimation of carrier frequency in OFDM is presented. Then how to choose the Phase Discriminator in the carrier recovery loop and how to design the loop filter are discussed. It is pointed that the estimation algorithm with linear error function has the best performance when SNR is low and the design of loop filter is compromise of acquisition time and performance.The performance of OFDM system can be improved when frequency offset exists. Three method are presented: ICI self-cancellation; Rectification of pulse shaping function; Orthogonal coding.Finally this thesis design an OFDM system on LEO satellite based on the parameters of typical satellite orbit. The results of simulation show that the synchronization of carrier frequency can meet the system requirements.