MIMO stands for “Multiple Input Multiple Output”. A MIMO system consists of multiple antennas at both the transmitter and the receiver.
A scattering channel can be problematic because it causes interference and hence sub-optimal reception. Many methods have been introduced to counter the effect of interference, such as interference cancellation, phased array antennas and many of the diversity techniques mentioned in this document. However, it is possible to exploit the spatial diversity aspects of the channel, especially when it is highly dispersive and causes a reasonable degree of scattering. The MIMO concept is designed to take advantage of the channel dispersive nature instead of counteracting it.
There are various techniques that work with a MIMO system. Spatial multiplexing for example works by splitting the data stream to multiple streams (equal to number of transmitting antennas). If the channels are totally independent and uncorrelated it is possible to increase the overall data rate by multiple folds.
Another method is to employ space-time coding where the transmitted data is sent over multiple antennas at different times. With a MIMO system, the combined effect is a reduced power requirement (signal to noise ratio), or an increased throughput if traded off with power.
The scope of MIMO techniques varies from one implementation to another, and a combination of: spatial multiplexing, beamforming, space time coding, array antennas … etc. can all be considered part of a MIMO system implementation.
Theoretically, a NxN MIMO system can increase the throughput in the system by N folds compared to a simple “single input single output” system.