MIMO

What is MIMO?

MIMO is a multiple-input, multiple-output technology used in wireless communications. It uses multiple antennas at both the transmitter and receiver to transmit and receive signals, allowing signals to be transmitted through various paths. When the receiving antenna sends different versions of the same signal, the data can reach the receiving antenna stably without being affected by attenuation, which can improve the signal-to-noise ratio, reduce errors, greatly optimize speed, and increase radio transmission capacity. MIMO is the core wireless technology of products on the market that support the IEEE 802.11n (Wi-FI 4) standard, enabling IEEE 802.11n standardized products to have higher connection speeds.

How does MIMO Work?

MIMO technology uses multiple smart transmit and receive antennas. Information arrives at the receiver multiple times in different forms and paths. The receiving antenna can combine data streams from different paths and times. Spatial diversity and spatial multiplexing are two space-time coding techniques implemented by MIMO.

Spatial Diversity

Spatial diversity is a method of improving the reliability of data transmission by sending different versions of the same data stream on multiple antennas. These versions can be the original data stream or new data streams generated after some mathematical processing. At the receiving end, the signal is received through a spatial equalizer, then demodulated and decoded, and finally, the different signal versions of the same data stream are combined to restore the original data. This method can make data transmission more stable and reliable and is suitable for long-distance transmission scenarios where the transmission rate is not high.

Spatial Multiplexing

Spatial multiplexing increases the data transmission rate by sending multiple data streams of the same data on multiple antennas. At the transmitting end, each transmitting antenna has an independent transmission channel and does not interfere with each other; at the receiving end, the receiving antenna receives these data streams separately through a spatial equalizer, then demodulates and decodes, and finally merges each data stream to restore it to the original signal. This method can improve the transmission rate and is suitable for transmission scenarios with high transmission rate requirements but short transmission distances.

Before MIMO: SISO vs SIMO vs MISO

Before the introduction of MIMO, the most traditional wireless devices used single-input, single-output (SISO) technology. Both the transmitter and the receiver had only one antenna, so they could only send and receive signals from one spatial stream. This technology limited the wireless transmission rate and had extremely low reliability.

Subsequently, single-input multiple-output (SIMO) technology was introduced to achieve a more reliable connection. Under this technology, multiple signals containing the same data are sent from a single transmitting antenna, and the receiving antenna can capture the complete data even if some of the signals are lost, thus improving transmission reliability.

What about multiple-input single-output (MISO)? Multiple signals containing the same data are sent from different transmitting antennas. If some transmitting antennas fail or the signal is lost, the receiving antenna can still capture the complete data, similar to the SIMO mentioned above.

SU-MIMO vs MU-MIMO

The implementation of MIMO technology was first used for multiple-input and multiple-output (SU-MIMO) data transmission for a single user. Under this technology, the AP can only transmit one data packet to the client simultaneously. 

With the development of high-density scene requirements, in the Wi-Fi 5 (IEEE 802.11ac Wave 2) standard, MIMO has derived multi-user multiple input multiple output technology (MU-MIMO), which allows a single AP to transmit multiple data packets to multiple clients at the same time, significantly improving transmission efficiency and performance. 

There are two subdivision types of MU-MIMO: cooperative MIMO and massive MIMO. Cooperative MIMO's characteristic is that multiple access points are virtualized into a multi-antenna system, allowing adjacent transmitters to transmit data to multiple users at the same time. Massive MIMO is one of the key technologies for 5G mobile communications. Its antenna capacity has been expanded from traditional MIMO to 64, 128, or even 256 antennas.