In MIMO- Multiple independent data streams are transmitted simultaneously and as a result it can achieve much higher data throughput in comparison to SISO (Single Input Single Output) case.
To send same data across the different propagation (spatial) paths. This is called spatial diversity .
To improving the data rate of the system, we may choose to place different portions of the data on different propagation paths called spatial-multiplexing.
MIMO use spatial diversity technique to improving the reliability & to improve the throughput spatial-multiplexing is used.