MIMO技术作为LTE系统的关键技术之一，能够满足LTE系统在高系统容量和高数据传输速率上的需求。LTE物理下行PDSCH信道支持MIMO技术中的发射分集、空间复用和波束赋形等技术，从而能够使系统具备高吞吐量和高频谱效率的能力。LTE下行MIMO支持7种不同的传输模式，在不同的环境或场景下应用不同的下行传输模式能够使系统性能达到相应环境或者场景下的最佳效果。为了使系统达到最佳的传输效果，本文主要针对LTE下行MIMO模式的自适应切换技术展开研究。 通过对LTE系统的特点和关键技术、MIMO技术的基本理论和LTE下行MIMO 7种模式基本理论以及各模式所适用场景的分析搭建了TDD LTE下行PDSCH信道仿真平台。通过仿真平台对发射分集、空间复用和波束赋形的信道容量以及系统吞吐量性能的仿真得出了模式内和模式间切换的理论依据。 根据UE所反馈的CQI信息所对应的MCS值，依据在不同场景下使系统频谱效率达到最大化的原则，得出模式3内部和模式7内部以及模式3与模式7之间自适应切换的条件。提出了试探机制下的模式3内部和模式7内部自适应切换算法，以及模式3与模式7之间的自适应切换算法。 利用所搭建的仿真平台分别在不同信道相关性下对试探机制下的模式3内部和模式7内部自适应切换算法进行了系统吞吐量性能仿真，得出了采用试探机制的模式内自适应切换能够使系统达到该模式下的最佳性能，有效地提高系统吞吐量性能。通过在不同信道相关性下对模式3与模式7模式间自适应切换算法的仿真，得出在不同场景下自适应的切换到相应场景所适合的发射模式同样可以有效地提高系统的吞吐量性能。

As one of the key technology of LTE system, MIMO could meet the demand of high system capacity and high data rate in LTE system. The transmit diversity, spatial multiplexing and beamforming that are include by MIMO technology which supported by the downlink channel of PDSCH in LTE system, through which the system could achieve high throughput and high frequency spectrum efficiency ability. Seven different transmission modes which are supported by the LTE downlink MIMO system, through application different transmission mode, the system could achieve the best transmission effect in corresponding environment or situations. In order to make the system to achieve the best effect of the transmission, the LTE downlink MIMO mode adaptive switch technology is researched in this paper. The characteristics and the key technology in LTE system，the basic theory of MIMO technology，seven different transmission mode in LTE downlink MIMO system and the scene of different mode applied are analyzed, then the TDD LTE downlink PDSCH channel simulation platform is built based what has been analyzed. Through the platform the channel capacity and system throughput about transmission diversity, spatial multiplexing and beamforming are simulated. Through which we got the theory of adaptive switch about internal mode and between modes. According to the CQI information corresponding MCS value that the UE feedback, base on the theory of making the system achieving the best spectrum efficiency in different situations, we got the conditions of mode 3 internal adaptive switch, mode 7 internal adaptive and the adaptive switch between mode 3 and mode 7. Then the LTE downlink MIMO internal mode adaptive switching algorithm based on the test mechanism about mode 3 and mode 7 were put forward. At the same time the adaptive switching algorithm about between mode 3 and mode 7 is put forwrad also. Through the simulation platform that built in the paper, the system performance of throughput about mode 3 internal and mode 7 internal adaptive algorithms that base on the test mechanism are separated simulated in different channel correlation. Through analysis the simulation result we got using the internal mode adaptive switch that base on the test mechanism could achieve the best performance at the transmission mode and the system throughput performance could be effectively enhanced. Through the simulation result about the adaptive switch between mode 3 and mode 7, the UE in the different scene adaptive switch to the appropriate mode could effectively improve the system throughput performance also.