Though the terms single-input single-output (SISO) and multi-input multi-output (MIMO) originate with controls engineering, SISO and MIMO are now very commonly discussed in reference to RF antennas.
In controls engineering, a SISO system has only one input and one output, such as a DUT (device under test) with only two ports that you can measure with a 2-port vector network analyser (VNA) to acquire the 2 x 2 matrix of S-parameters (S11, S12, S21, S22). MIMO in controls engineering refers to a system with multiple inputs and multiple outputs of any number or combination. MIMO systems with n inputs and m outputs will ultimately have a channel matrix of the order n x m.
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, a SISO RF system will only have a singular antenna from one device communicating with a singular antenna from another device. Unlike in control engineering, the number of inputs and outputs isn’t a reference to the inputs and outputs of the communication system, but instead discusses the antennas that are part of a spatial multiplexing scheme. With MIMO RF antennas, the number of transmitting antennas from one device and receiving antennas from another device dictate the order of the MIMO channel (TX antennas x RX antennas).
With a SISO antenna system, the signals transmitted from one device will interact with the environment and will be absorbed or reflected depending on these environmental variables. Ultimately, the signal energy transmitted from the SISO antenna system may arrive at the receiving antenna from one or more spatial paths. In this case, the path with the highest signal energy is most desirable as it is usually the least attenuated or distorted. Signals from other paths may actually act as interference, as they may have been degraded or delayed by their interaction with the environment.
With MIMO systems, these multiple spatial paths from one device to another are used to enhance communication between the two devices, either increasing the number of effective streams being used or to enhance the reliability of the communication channel. Either method may result in increased throughput, either by adding channels or by enhancing the channel quality, reducing errors and resulting in more efficient use of the channel capacity.
A communications link can be enhanced by using space time transmit diversity (STTD) to send multiple signal copies with different encoding through the multiple spatial channels to the receiver. This method enhances the signal-to-noise ratio (SNR) of the signal, which means fewer bit-errors and a higher throughput to the limit of the communication link. Generally, this approach will reach a point of diminishing returns with the number of antennas used, which leaves a practical limit.
The other method is to use each of the spatially diverse paths (spatial division multiplexing) to simultaneously send a different signal to another device, which results in additional communication streams. On the receive side, these parallel streams can be separated into multiple channels, which effectively multiplies the throughput under favourable conditions.
It is possible for a MIMO system to be designed to take advantage of either method, depending on the best outcome for the communication link. For this to be possible, the MIMO system needs a method of determining the qualities of the air interface and spatial diversity options and deciding how best to optimise the link.
This system can be augmented if beamforming/beamsteering technology is used in conjunction with MIMO technology. With beamforming/beamsteering, the antenna pattern can be modified to better match the spatial paths from device to device, enhancing the gain and ensuring the best outcome for the MIMO link. In these ways a MIMO link may outperform a SISO link in terms of throughput and even signal reliability under certain conditions.
However, MIMO systems require additional RF hardware, signal paths, antennas and analog/digital signal processing hardware. Under certain conditions, a single optimised antenna may be higher-performing than a MIMO system due to antenna performance, as MIMO systems are often designed with antennas that are better suited to being manufactured in a matrix, such as patch antennas and may be inferior to a SISO antenna in the same form factor.
However, one of the main use cases for MIMO technology is to support cellular or IoT communication from a single base station or router to multiple user devices. With a sufficiently complex MIMO base station, hundreds of user devices may be served simultaneously, which would otherwise be a complex or impossible task for a SISO system if a high enough throughput and reliability are desired.
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