MIMO Review: What Is MIMO (Multiple-Input and Multiple-Output)?

What Is MIMO?

In radio, MIMO (multiple-input and multiple-output) is a method for multiplying the capacity of a radio link using multiple transmission and receiving antennas to exploit multipath propagation. MIMO has become an essential element of wireless communication standards including IEEE 802.11ac (Wi-Fi 5), IEEE 802.11n (Wi-Fi 4), HSPA+ (3G), WiMax, as well as LTE (long term evolution). Also, MIMO has been applied to power-line communication for three-wire installation as part of the ITU G.hn standard and the HomePlug AV2 specification.

In wireless, at one time, the term MIMO was used to indicate the use of multiple antennas at the transmitter and the receiver. Nowadays, MIMO specifically refers to a practical technique for sending and receiving more than one data signal simultaneously over the same radio channel through exploiting multipath propagation.

Although the “multipath” phenomenon may be interesting, it is the usage of orthogonal frequency division multiplexing to encode the channels that is responsible for the increase in data capacity. MIMO is fundamentally different from smart antenna techniques developed to enhance the performance of a single data signal like diversity and beamforming.

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MIMO Forms

In general, there are two forms of MIMO.

Multi-user Types

Full multi-user MIMO (MU-MIMO) or network MIMO can have a higher potential, the research on (partial) multi user MIMO, or multi-user and multi-antenna MIMO technology is more active.

#1 Multi-user MIMO

MU MIMO is a set of multiple-in and multiple-out technologies for multipath wireless communication, in which multiple users or terminals, each radioing over one or more antennas, communicate with one another.

#2 Cooperative MIMO (CO-MIMO)

CO MIMO is a technology that can effectively exploit the spatial domain of mobile fading channels to bring significant performance improvements to wireless communication systems. it is also named distributed MIMO, network MIMO, virtual MIMO, as well as virtual antenna arrays.

#3 Macrodiversity MIMO

Macrodiversity is a type of space diversity scheme taking advantage of several receivers or transmitter antennas for transferring the same signal. The distance between the transmitters is much longer than the wavelength, as opposed to microdiversity where the distance is in the order of or shorter than the wavelength. Macrodiversity MIMO schemes pose unprecedented theoretical and practical challenges.

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#4 MIMO Routing

MIMO routing routes a cluster by a cluster in each hop where the number of nodes in each cluster is larger or equal to one. It is different from conventional (SISO) routing since the latter’s protocols route node-by-node in each hop.

#5 Massive MIMO

Massive MIMO is a technology where the number of terminals is much less than the number of base station (mobile station) antennas. In a rich scattering environment, the full advantages of the massive MIMO system can be exploited using simple beamforming strategies like MRT (maximum radio transmission), MRC (maximum ratio-combining), and ZF (zero forcing).

Also read: What Is Dual-Band WiFi & Differences to Single-Band WiFi?

Multi-antenna Types

Multi-antenna MIMO or single-user MIMO technology has been developed and applied in some standards such as 802.11n products. The following are some special cases of MIMO.

• Multiple-input single-output (MISO): It is a special case when the receiver has a single antenna.
• Single-input multiple-output (SIMO): It is a special case when the transmitter has a single antenna.
• Single-input single-output (SISO): It is a conventional radio system where neither transmitter nor receiver has multiple antennas.