In the past, we had utilized two types of resource for wireless communication: time and frequency. However, the amount of time and frequency resource is highly limited, and especially so in the cellular systems serving massive users.
It has been known that the space resource can be utilized to scale the spectral efficiency by simply having more physical antennas at the transmitter and receiver. As shown in the left figure above, multiple signals are transmitted, which are captured at the receiver antennas. At the receiver side, the challenge in the MIMO system is to successfully distinguish and detect all individual symbols transmitted by the transmitter.
At the transmitter side, two different strategies can be used as shown in the right figure above. In “Spatial Multiplexing,” the transmitter transmits all independent symbols through different transmit antennas, thereby increasing data rate significantly. On the other hand, in the “Diversity” mode, the transmitter transmits the same symbol for all the transmit antennas such that the reliability of the communication is improved in random fading channels.