In RF engineering, when discussing antennas, SISO and MIMO now apply to the number of antenna "inputs" and antenna "outputs" that a communication channel between two or more devices may have. In this case, the SISO RF system will have only a single antenna from one device and a single antenna from another device. Unlike control engineering, the number of "inputs" and "outputs" does not refer to the inputs and outputs of the communication system, but rather to the antennas in the spatial multiplex scheme. Using the MIMO RF antenna, the number of antennas sent from one device and the number of antennas received from another device determine the order of the MIMO channels (TX antenna x RX antenna）

Using the SISO antenna system, signals sent from a device will interact with the environment and will be absorbed or reflected according to these environmental variables. Eventually, the signal energy from the SISO antenna system can reach the receiving antenna from one or more spatial paths. In this case, the path with the highest signal energy is ideal because it is usually the least attenuated or distorted. Signals from other paths can actually be interfering because they can degrade or delay due to interaction with the environment.  

For MIMO systems, these multi-space paths from one device to another are used to enhance communication between two devices, either by increasing the number of active streams used or by increasing the reliability of communication channels. Both methods can improve throughput by increasing channels or improving channel quality, reducing errors, and using channel capacity more efficiently.