In a multipath environment, the data rate increase due to MIMO is equal to the number of MIMO antennas.
Rather than to transmit the same bit of more than two antennas, the Spatial Multiplexing method sends a data bit from the first antenna, and another bit of the second antenna at the same time, the symbol. As long as the receiver has multiple antennas and the signal is of sufficient quality, the receiver may separate the signals. This Spatial Multiplexing method involves the added complexity and expense, both the transmitter and receiver. However, two transmit antennas and two receive antennas, data can be sent twice as fast as compared with systems using space-time codes with a single antenna receiver.
Distributing data over co-channels on spatially displaced antennas, in a multi-path environment provides a temporally displaced set of sub-streams to be seen by the receiver.
Maximum Ratio Combining Diversity – MRC in Receiver Gains
In Maximum Ratio combining each signal branch is multiplied by a weight factor that is proportional to the signal amplitude in Receiver Gains
That is, branches with strong signal are further amplified, while weak signals are attenuated.
With MRC, the attenuation/amplification factor is proportional to the signal amplitude ai = ri for each channel i.