Although the terms single input single output (SISO) and multiple input multiple output (MIMO) are derived from control engineering, SISO and MIMO are often talked about when referring to RF antenna. In control engineering, SISO system has only one input and one output. For example, for DUT with only two ports, dual port vector network analyzer (VNA) can be used to measure dut to obtain 2% of S parameter × 2 matrix (S11, S12, S21, S22). MIMO in control engineering refers to a system with any number or combination of multiple inputs and multiple outputs. MIMO systems with "n" inputs and "m" outputs will eventually have a channel matrix in the order of nxm.

In RF engineering design, SISO and MIMO are applicable to the number of possible antenna "inputs" and "outputs" of the communication channel between two or more devices when it comes to antennas. In this case, in SISO RF system, only a single antenna of one device communicates with a single antenna of another device. Unlike control engineering, "input" and "output" quantities do not refer to the input and output of the communication system, but to the antennas included in the spatial multiplexing scheme. With the help of MIMO RF antenna, the number of transmitting antennas of one device and the number of receiving antennas of another device determine the order of MIMO channels (TX antenna) × RX antenna).

With the help of SISO antenna system, the signal transmitted by a device will interact with the environment and be absorbed or reflected, depending on such environmental variables. Finally, the signal energy transmitted from SISO antenna system can reach the receiving antenna from one or more spatial paths. In this case, the ideal path is the path with the highest signal energy, because the attenuation or distortion of this path is usually very small. Signals from other paths may actually cause interference, which may be degraded or delayed due to interaction with the environment.

With the help of MIMO system, such multiple spatial paths from one device to another are used to enhance the communication between two devices, or increase the number of effective streams in use, or improve the reliability of communication channels. Both methods can increase the number of channels, or improve channel quality, reduce errors and make more effective use of channel capacity, so as to improve throughput.

One method is to use space-time transmit diversity (STTD) to send multiple signal copies with different codes to the receiver through multiple spatial channels to enhance the communication link. This method can improve the signal-to-noise ratio (SNR) of the signal, which means less bit errors and higher throughput to the limit of the communication link. Usually, this method will reach the diminishing point of return as the number of antennas used increases, resulting in the actual limit.

Another method is to use each spatial diversity path (i.e. spatial division multiplexing) to send different signals to another device at the same time, so as to generate additional communication flow. At the receiving end, such parallel streams can be divided into multiple channels, which can effectively improve the throughput under favorable conditions.

MIMO systems can be designed to use either method, depending on the best result of the communication link. Therefore, MIMO system needs a method to determine the quality of air interface and spatial diversity options and determine how to optimize the link.

If beamforming / beam control technology is combined with MIMO technology, the system can be enhanced. With beamforming / beam control, the antenna pattern can be modified to better match the spatial path from one device to another, so as to improve the gain and ensure the best results of MIMO links. Through these methods, MIMO link may be better than SISO link in throughput (even signal reliability) under specific conditions. However, MIMO system requires additional RF hardware, signal path, antenna and analog / digital signal processing hardware. Under certain conditions, MIMO systems are usually designed with antennas that are more suitable for matrix manufacturing (such as patch antennas), and may be inferior to SISO antennas under the same shape coefficient. Therefore, considering the antenna performance, the performance of a single optimized antenna may be better than that of MIMO systems.

However, one of the main use cases of MIMO technology is to support cellular or IOT communication from a single base station or router to multiple user equipment. It can serve hundreds of user equipment through very complex MIMO base stations at the same time, while SISO system can hardly meet the requirements of high throughput and reliability.