Research On Synchronization Techniques For MIMO-OFDM System Based On Training Sequence

Orthogonal Frequency Division Multiplexing (OFDM) technology has attracted popular interest for its huge advantages over the conventional single carrier technology, such as robustness in combating multi-path fading, improving spectral efficiency and so on in recent years. Multiple Input Multiple Output (MIMO) technology takes multiple antennas at the transmitters and receivers to gain the higher channel capacity and spatial diversity. As far as the crucial technologies in B3G/4G systems, MIMO-OFDM combined the above technology has been widely implemented in IEEE802.11a,802.16e,3GPP Long Term Evolution (LTE) systems and so on. However, for MIMO-OFDM system which takes OFDM as the core technology is very sensitive to time offset and frequency offset, and the primary problems to be solved must be the problems of timing and frequency synchronization in the system. For the centralized system, the antennas at the transmitters and receivers are placed closely, therefore there are only one time offset and one frequency offset. But in the distributed system, the transmitters and receivers are sparsely placed, and ITD and CFOs have to be taken into account. The synchronization algorithms in each of the system would be researched in the paper, and the improving synchronization methods are shown, discussed and emulated.Synchronization algorithms pre-existed of two types of MIMO-OFDM system models are deeply researched, discussed and emulated in the paper firstly. The MODY method, WY method in the centralized MIMO-OFDM system model, and the UPSP method, WYS method in the distributed MIMO-OFDM system model are researched and simulated in detail, and the advantages and drawbacks of these approaches are pointed out through discussing the simulation results.As to the centralized MIMO-OFDM system, an advanced synchronization approach with the conjugate structure based on ZC sequence is presented. Depended on the great correlation characteristic of ZC sequence in time domain and frequency domain, the proposed approach combines the two steps of timing synchronization to one step, which reduces the computation load, and adopts the method of processing the multi-path signals to increase the accurate possibility. The simulation results demonstrate that the improved approach has a better synchronization performance in AWGN channel and multi-path channel than MODY method and WY method.Aiming at the distributed MIMO-OFDM system, the paper shows a synchronization approach with training sequence of each transmit antenna staggered in frequency domain, which has different period in time domain. The sequences in the frequency domain have the same length as one OFDM symbol in the novel approach which has lower overhead over the system, and form different periods of symmetric conjugate structure in time domain. The simulation results show that the given synchronization algorithm has high estimate precision, low algorithm complexity, lower overhead over the spectrum resources of synchronization training sequence, and furthermore, with the proposed approach, the estimation range of the carrier frequency offset (CFO) can be flexibly extended to cope with larger CFOs.