Research And Implementation On Synchronization Of OFDM Based Communication Systems

Multi-input multi-output orthogonal frequency division multiplexing (MIMO-OFDM) is regarded as one of the most highlighted wireless transmission schemes for its insensitivity to multi-path fading and high spectral efficiency which increases its capacity linearly with the number of antennas. Synchronization is a major research topic in MIMO-OFDM system due to its sensitivity to symbol timing offset (STO) and carrier frequency offset (CFO). Moreover, Orthogonal frequency division multiplexing (OFDM) based cooperative communication systems have obtained major attention in the wireless communications community in recent years due to its various potentials regarding the enhancement of diversity, achievable rates and coverage. However, the distributed and independent nature of the cooperative nodes result in the multiple symbol timing offsets (MSTO) and multiple carrier frequency offsets (MCFO), which makes the synchronization problem more complex and challenging compared with the traditional centralized MIMO systems. In this dissertation, the synchronization problems of OFDM based MIMO and cooperative communication systems are focused on. The main contribution of this dissertation is as follows:Firstly, the principles of MIMO and OFDM are explained in detail. Another emphasis is devoted to analyze how the synchronization error affects the MIMO-OFDM system performance.Secondly, to combat the long capture time and great computation of the long training sequence in 4G MIMO-OFDM systems, a novel timing synchronization algorithm using repeated shift-orthogonal constant amplitude zero auto correlation (CAZAC) sequences constructing the synchronization training preamble is proposed. At the receivers, the proposed algorithm utilizes segment fast Fourier transform (FFT) and inverse fast Fourier transform (IFFT) instead of the traditional methods based on slide correlation to detect the time offsets of each antenna element.Following that, key physical layer transmission technologies and synchronization problem particularity of localized DFT-S-OFDM uplink cooperative communication systems are described and explained.Finally, a new MCFO compensation algorithm for localized DFT-S-OFDM uplink cooperative communication systems is introduced by fully exploiting the diversity gain of space frequency block coded (SFBC) and the characteristic of inter-carrier interference (ICI). This algorithm can achieve nearly the ideal performance with large ranges of MCFO, while no matrix inversion is needed. Furthermore, the effects of MCFO on the bit error ratio (BER) performance for DFT-S-OFDM uplink cooperative communication systems are analyzed and the BER lower bound in multi-path Rayleigh fading channels is given.