The purpose is to derive the maximum capacity possible by using MIMO systems. For example, if we have Nt transmit and Nr receive antennas, and the channel matrix is full rank, then we have min(Nt,Nr) parallel channels that are interference-free. From information theory, the total capacity of such a system is the sum of the capacities of the individual channels. That is, the total capacity of a MIMO system will be min(Nt,Nr) times the capacity of a SISO system.