| MIMO technology has promoted the development of wireless communicationgreatly, which also has been widely used in TD-LTE-Advanced system. As many asnine multi-antenna transmission modes are defined in TD-LTE-Advanced, and up toeight layers and four layers are supported in downlink and uplink transmission,separately. In addition to MIMO transmission, the adaptive technology is anothercritical technology in TD-LTE-Advanced system. According to the CSI informationUEs fed back, eNodeB chooses appropriate configurations for UEs to adapt thecomplex variation of the channel in mobile communications better, such asmodulation and coding mode, transmission mode and pre-coding matrix. However,whether or not these two key technologies can play their roles well depends on thevalidity and reliability of the CSI feedback mechanism. Therefore, in this paper, thefeedback mechanism of MIMO transmission modes in TD-LTE-Advanced isthoroughly researched.In this paper, the pre-coding technology and MIMO detection algorithm ofTD-LTE-Advanced are first researched. For pre-coding technology, the codebookbased pre-coding and beamforming technology are studied successively, and somecommonly-used multi-user beamforming algorithms are simulated and analyzed infocus. By comparison, the SLNR based beamforming algorithm, which can reducethe interference between users effectively, has better performance than others. Then,the research on MIMO detection algorithm is carried on, and the usual algorithmsare also simulated and analyzed. Through a comprehensive comparison of detectionperformance and complexity, the MMSE algorithm is selected as the detectionalgorithm of this paper. Based on the analysis above, the SINR calculation formulaof MIMO transmission in multi-cell is provided.Afterwards, nine feedback modes defined in TD-LTE-Advanced standard areconcluded, summarized and compared in this paper. And based on the system-levelsimulation platform of TD-LTE-Advanced built in this paper, various feedbackmodes related to MIMO transmissions are simulated and evaluated, and theapplicabilities of these modes are also analyzed. On this basis, a switch algorithm offeedback modes to improve the performance of cell-edge users is proposed. Thesimulation result shows that, by employing this switch algorithm, the throughput ofcell-edge users are improved effectively, while the feedback overhead increasing isvery little.Finally, taking advantage of the channel reciprocity between uplink and downlink in TD-LTE-Advanced system, the CQI correction scheme in frequencydomain for slowly fading channels is proposed in this paper. And combined withchannel prediction, the CQI correction scheme in both time and frequency domainfor fast fading channels is also proposed. Simulation results indicate that by utilizingthe proposed CQI correction scheme, the initial retransmission rate can be reducedeffectively and the system throughput can be improved obviously. In addition, bythe proposed CQI correction, the performance of wideband CQI feedback mode isclose to or even better than the performance of sub-band CQI feedback mode, whichwill reduce the feedback overhead enormously. |
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