An Introduction to MIMO Radio technology

In radio technology, multiple-input and multiple-output, or MIMO, is a method for multiplying the capacity of a radio link using multiple transmit and receive antennas to exploit multipath propagation.

MIMO has become an essential element of wireless communication standards including IEEE 802.11n, ac and ax (WiFi), WiMAX (4G), 3GPP 4G LTE and 5G NR.

Earlier usage of the term “MIMO” referred to the use of multiple antennas at both the transmitter and the receiver. In modern usage, “MIMO” specifically refers to a practical technique for sending and receiving more than one data signal on the same radio channel at the same time via multipath propagation. MIMO is fundamentally different from smart antenna techniques developed to enhance the performance of a single data signal, such as beamforming and diversity.
MIMO contains three main categories: precoding, spatial multiplexing (SM), and diversity coding.

Products using MIMO technology

CableFree products that use MIMO include:

• CableFree IHPR-MIMO
• CableFree HPR-MIMO
• CableFree Amber Crystal

Functions of MIMO technology

Precoding refers to spatial signal processing at the transmitter, particularly in multi-antenna systems. In its simplest form, it acts as multi-stream beamforming. Traditional (single-stream) beamforming sends the same signal from each antenna with specific phase and gain adjustments to enhance signal strength at the receiver and reduce multipath fading. In line-of-sight scenarios, this creates a well-defined directional signal. However, in environments with significant multipath, like cellular networks, beamforming alone is less effective, especially when the receiver has multiple antennas. In such cases, multi-stream precoding becomes useful, which requires accurate channel state information (CSI) at both the transmitter and the receiver.

Spatial multiplexing, enabled by MIMO antenna systems, splits a high-rate signal into several lower-rate streams, transmitting them from different antennas in the same frequency channel. If the receiver can distinguish these spatially unique signals and has accurate CSI, it can separate them into (almost) parallel channels, significantly increasing data capacity at high signal-to-noise ratios (SNR). This technique works even without CSI at the transmitter but performs better when combined with precoding. Spatial multiplexing can also serve multiple users simultaneously (multi-user MIMO or space-division multiple access (SDMA)), requiring transmitter-side CSI to separate users based on spatial characteristics.

Forms of MIMO

Multi-antenna MIMO (or Single User MIMO) technology is incorporated in standards such as 802.11n for enhanced wireless performance.

Special Cases of MIMO:

• MISO (Multiple-Input Single-Output): Occurs when the receiver uses a single antenna.
• SIMO (Single-Input Multiple-Output): Occurs when the transmitter uses a single antenna.
• SISO (Single-Input Single-Output): A traditional radio system with a single antenna at both the transmitter and receiver.

Single-User MIMO Techniques:

• Bell Laboratories Layered Space-Time (BLAST): Gerard J. Foschini (1996)
• Per Antenna Rate Control (PARC), Varanasi, Guess (1998), Chung, Huang, Lozano (2001)
• Selective Per Antenna Rate Control (SPARC): Ericsson (2004)

Limitations:

MIMO requires large antenna spacing at the base station, typically multiple wavelengths, to ensure effective performance. However, at the receiver, especially in handsets, space constraints limit antenna separation, though advanced antenna designs and algorithms are being explored to address this challenge.

For Further Information

Please Contact Us for more information on our exciting range of solutions using MIMO radio technology.