MIMO

What Is MIMO?

MIMO, short for Multiple-Input Multiple-Output, is a wireless communication technology that utilizes multiple antennas for transmitting and receiving signals. Primarily employed in Wi-Fi and mobile communications, MIMO technology offers significant enhancements in system capacity, coverage range, and signal-to-noise ratio (SNR). In an MxN MIMO system, M represents the number of transmit (Tx) antennas, and N represents the number of receive (Rx) antennas.

From SISO to MIMO

• SISO

Definition: Single-Input Single-Output (SISO) involves a single path connecting the transmitter (Tx) and receiver (Rx) antennas.

Transmission: Transmits a single signal over one spatial stream.

• SIMO

Purpose: Introduced to enhance reliability.

Configuration: Adds an additional antenna at the receiver.

Functionality: Allows for the simultaneous reception of two signals.

Benefit: Improves reliability through receive diversity, ensuring complete data retrieval even if one signal is partially lost.

Capacity: Remains the same since only one path is utilized.

• MISO

Concept: Uses two Tx antennas while maintaining a single Rx antenna.

Operation: Signals from the two Tx antennas carry the same data and are combined into one signal at the receiver.

Effect: Achieves similar reliability improvements as SIMO by utilizing multiple antennas at the transmitter.

• MIMO

Analysis: Transmission capacity depends on the number of Tx and Rx antennas.

Configuration: Using two antennas each for both the transmitter and receiver.

Advantage: Doubles the rates by transmitting and receiving two signals separately.

Definition: Known as Multiple-Input Multiple-Output (MIMO), utilizing multiple antennas on both ends.

How Many Types of MIMO?

• Single-User MIMO (SU-MIMO): SU-MIMO is the original type of MIMO technology used for single-user data transmission. It utilizes multiple antennas at the transmitter and receiver to improve the performance and reliability of the communication link.

• Multi-User MIMO (MU-MIMO): MU-MIMO allows the transmitter to simultaneously transmit data to multiple users. It enables concurrent transmission to multiple users, increasing the overall system capacity and efficiency. MU-MIMO is supported in Wi-Fi 5 and Wi-Fi 6, with Wi-Fi 6 supporting even higher numbers of users compared to Wi-Fi 5.

• Cooperative MIMO (CO-MIMO): CO-MIMO combines multiple wireless devices or transmitters into a virtual multi-antenna system. This configuration enables neighboring transmitters to transmit data concurrently to multiple users, enhancing the overall system performance.

• Massive MIMO: Traditional MIMO systems typically employ two to eight antennas, while massive MIMO supports a significantly higher number, such as 64, 128, or 256 antennas. This technology brings substantial improvements in system capacity and transmission efficiency.

How Does MIMO Work in Wi-Fi?

MIMO technology plays a crucial role in Wi-Fi networks. Here is a structured explanation of how MIMO works in Wi-Fi:

Space Diversity: In Wi-Fi, MIMO is implemented using space diversity, which involves transmitting different versions of the same data stream on multiple antennas. The data stream may be the original raw data or a transformed version. Each antenna encodes and modulates the data stream before transmission. At the receiver, a spatial equalizer separates the signals received by different antennas and combines them to reconstruct the original data stream. Space diversity improves data transmission reliability, especially in long-distance scenarios that prioritize reliability over high rates. Beamforming, a technology introduced in Wi-Fi 4, can also be considered a form of space diversity. It detects the channel conditions and optimizes the transmitted signals on each antenna, enhancing the signal strength and improving receiver performance.

Spatial Multiplexing: Wi-Fi MIMO also employs spatial multiplexing, which involves dividing the data to be transmitted into multiple data streams. Each data stream is encoded, modulated, and transmitted on a separate antenna. Spatial multiplexing takes advantage of the independence of antennas, treating each antenna as an individual channel. At the receiver, a spatial equalizer separates the received signals from different antennas and combines them to recover the original data streams. Spatial multiplexing significantly improves transmission rates, making it suitable for short-distance scenarios with high data rate requirements.

What Is MxN MIMO?

MxN MIMO is a representation of a WLAN device's MIMO capabilities, with M denoting the number of Tx antennas and N representing Rx antennas. For example, 4x3 MIMO indicates four Tx antennas and three Rx antennas. The number of spatial streams available in a MIMO system depends on the lower value between the number of Tx and Rx antennas.

While access points (APs) typically possess a significant number of antennas, ranging from 4 to 16, terminals like mobile phones usually have only one or two antennas. This difference in antenna capabilities results in underutilization of channel resources during transmission between APs and terminals, wasting the AP's antenna resources. To address this issue, multi-user types of MIMO technology have emerged, with MU-MIMO being a notable example. MU-MIMO enables an AP to simultaneously transmit signals to multiple terminals, fully leveraging the AP's antenna capacity by matching it with the total number of antennas across multiple terminals.