We write MIMO as a rectangular block array with .
Example 1. A washing machine is a MIMO system. The inputs are cold water, soap powder, and electricity; whereas the outputs are hot soapy water, cold rinsing water and hot air. The state of the washing machine can be complicated, and relate to the rotation of the drum, temperature of various components, washing cycles and so on.
Example 2. A domestic central heating system is a MIMO system with inputs gas, cold air, electricity and cold water; whereas the outputs are hot air and hot water.
Example 3. Mobile telephone networks are MIMO systems. There are multiple transmitting antennas, and multiple receiving antennas. Massive MIMO is used to model 5G wireless transmission.