Research On Diversity And Multiplexing Tradeoff In MIMO Systems

Multiple-input multiple-output (MIMO) technique can significantly increase the channel capacity, spectral efficiency and transmission quality without additional transmit power and frequency bandwidth. It has become one of the key techniques to improve the performance of wireless communication systems. Existing research shows that MIMO systems can provide diversity gain and multiplexing gain at the same time to improve the transmission reliability and efficiency. Both gains can be obtained simultaneously, but there is a fundamental tradeoff between them. It is the performance limit achieved by coding schemes and it shows the assignment of the spatial resource of MIMO systems.In this paper, the channel model of MIMO systems is first introduced. The channel capacity is analyzed from the viewpoint of information theory and the performance advantages are fully exhibited. Then the diversity gain and multiplexing gain which provided in MIMO systems are introduced and the tradeoff between them is discussed in the independent and identically distributed (i.i.d) channel. This tradeoff can not only show the assignment of the spatial resource but also be used as the evaluation criteria for the existing space-time coding schemes. At the same time, it provides the theoretic guidance for the designs of space-time coding and transmission schemes in practical systems.The diversity gain and multiplexing gain are realized through space-time coding schemes. Based on different design targets, the transmission schemes are different. In this paper, three coding schemes are chosen, which are the Alamouti Space-time Block Code (STBC) with the maximum diversity gain, the Layered Space-time (LST) scheme with the maximum multiplexing gain and the Perfect STBC with the full diversity and full rate. The fundamental principles of them are introduced and the performance from the viewpoint of diversity and multiplexing are discussed.On the basis of understanding the limitations of the existing space-time coding schemes, an adaptive hybrid space-time coding scheme is proposed in this paper. A part of transmit antennas are used to transmit the Alamouti STBC and the others to transmit the Perfect STBC. It combines the transmit diversity and the spatial multiplexing to obtain the diversity gain and multiplexing gain simultaneously. Meanwhile, according to the feedback from the receiver, this scheme adaptively adjusts the allocation of transmit antennas to adapt to the change of the channel. Simulation results show that this scheme obtain the tradeoff between diversity and multiplexing and improve the integral performance of the system.