The Research On Synchronization For MIMO-OFDM Systems

As one of the key technology in wireless communication, orthogonal frequency divisionmultiplexing (OFDM) can greatly improve spectrum utility rate. By using serial/parallelconversion to split a high-rate data stream into a number of lower rate orthogonal sub-carriersa well-performed sustainability to interference and multi-path loss is acquired. Multiple-inputmultiple-output (MIMO) is a technology that takes advantages of spatial multiplexing tofurther improve spectrum utility rate. MIMO-OFDM, a brand new technology that combinesMIMO and OFDM together, is more superior to both two of them and is preferable for thenext generation communications. Similar to the SISO-OFDM technology, MIMO-OFDM isnot sustainable to error during synchronization process. For example, unsynchronized timingcannot determine the start time of FFT window and will bring down inter-symbol interference(ISI) to the network; unsynchronized frequency will undermine orthogonality among eachsub-carrier that result in inter-symbol interference (ISI). Hence, an accurate and efficientsynchronization technology is the key research of MIMO-OFDM.In this paper, we studied some typical timing and frequency synchronization algorithms.In order to overcome the high BER of timing synchronization and the complex of frequencysynchronization algorithm, a specific threshold evaluation program was proposed. Thisalgorithm adopted the training sequence which was known in the transmitter, it couldaccurately calculate channel impulse response and determine the threshold function.Meanwhile, we designed the training sequence with repeated conjugate staggered structure.The sequence could make the training sequences of each transmitter keep cross-correlation bythe phase rotation. In the process of timing synchronization, on the basis of producing nosidelobe, it could achieve effective frequency offset.The main work in this paper is as follows:1. By analyzing the characteristics of MIMO-OFDM system and some synchronizationalgorithm, and then compared two timing synchronization threshold algorithms, we found outsome existing problems and the insufficiency in these two algorithms, some goals wereobtained for designing our threshold function.2. A specific threshold evaluation program was proposed for different MIMO-OFDMsystems. Firstly, we determined the performance comparison between the threshold parameterand shift of maximum peak. Then, the relationship of threshold parameter and MSE of timingsynchronization was utilized to determine threshold parameters with lowest MSE. Finally, wegot the threshold function and put it into different Q×P MIMO-OFDM systems to discussour scheme. 3. Based on three training sequence existing problems and insufficiency, this paperconstructed a new training sequence and proposed a frequency synchronization algorithm.The proposed preamble with an alternative central symmetric structure and the last pattern isthe complex conjugates of the first. Using the training sequence cross-correlation propertiesand crossing symmetry mechanism, we could simplify the carrier frequency offset algorithm.4. We simulated the proposed threshold method and frequency offset estimationalgorithm. The results showed that the proposed threshold function could improve theaccuracy of timing synchronization and frequency synchronization algorithm could achieveeffective frequency offset.