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Multi-antenna technologies such as beamforming and Multiple-Input, Multiple-Output (MIMO) are anticipated to play a key role in next generation mobile communications system, which are expected to be deployed in the year 2020 and beyond. The MIMO (Multiple-Input Multiple-Output) technologies have directionality, improve spectral efficiency and high network capacity. In this paper, we discuss MIMO architectures where the types of transmit MIMO, processing and applications. We will focus on the principles and applications for MIMO. Multiple-input multiple-output (MIMO) technology uses multiple antennas at the both link ends and does not need to increase additional transmit power and spectrum, leading to promising link capacity gains of several-fold increase in spectrum efficiency. MIMO technology has been adopted in IEEE 802.16 standard and also has been applied in IEEE 802.11. It is normally though that the spatial correlations reduce the capacity of MIMO system, but in fact, with correlation matrices available, a transmitter can apply the stochastic power allocation and the antenna selection. Stochastic power allocation is applied on these selected antennas instead to all to get maximum capacity and accelerate the antenna selection process. MIMO technology will play an important role in the 4G wireless system.
The early studies of MIMO system are mostly based on the ideal channel model that the elements of channel matrix are independently and identically distributed complex Gaussian random variables, but the real MIMO channels may have spatial correlation. Moreover, using MIMO systems will lead to the increase of system cost and complexity, which constrains the development of MIMO technology, however, the fast antenna selection scheme based on correlation matrix can efficiently reduce the complexity of MIMO system.
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