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5 things to know about MU-MIMO technology in Wi-Fi networks

'Multiuser multiple input, multiple output' might not roll off the tongue, but MU-MIMO technology is increasingly important in modern wireless networks. Here's what you should know.

Like a metropolitan freeway system during a population boom, the legacy wireless LAN has long struggled to accommodate exponential increases in network traffic, as the number of connected devices continues to surge. Essentially, multiuser multiple input, multiple output is a Wi-Fi feature that increases network capacity by widening the "roads" between wireless access points and clients.

MU-MIMO first appeared in Wave 2 of the 802.11ac, or Wi-Fi 5, standard, representing a significant step toward high-density Wi-Fi. MU-MIMO technology works by dividing an AP's available bandwidth into separate spatial streams, thus enabling a single AP to multitask and communicate with more than one node at a time. Wi-Fi 5 Wave 2 includes 4x4 MU-MIMO support, for example, which means an AP with four transmit antennas can broadcast to up to four clients simultaneously.

MU-MIMO technology continues to improve and impress as part of the Wi-Fi 6 standard, with unprecedented support for up to eight spatial streams. And, while MU-MIMO receives less buzz than its counterpart, orthogonal frequency-division multiple access, or OFDMA, it nonetheless plays an important and evolving role in high-efficiency Wi-Fi.

Here are five things about MU-MIMO technology you should know.

1. What is SISO vs. MIMO and SU-MIMO vs. MU-MIMO?

Early wireless stations were designed to broadcast to just one device at a time using a single antenna, in a configuration known as (SU-SISO). Later, single-user MIMO (SU-MIMO) -- first rolled out as an 802.11n, or Wi-Fi 4, feature -- enabled additional spatial streams via two or more antennas. SU-MIMO -- sometimes just called MIMO -- supports stronger, clearer and more efficient data transmissions between AP and endpoint.

Essentially, MU-MIMO is a Wi-Fi feature that increases network capacity by widening the

MU-MIMO technology further changed the rules of wireless engagement, enabling an AP or router to use its separate spatial streams to talk to multiple endpoints or users concurrently. By dynamically divvying up available bandwidth among a group of clients, a wireless station can achieve more efficient connectivity, resulting in greater network capacity.

2. What are SISO, SIMO, MISO, MIMO, etc.?

SISO, SIMO (single input, multiple output), MISO (multiple input, single output) and MIMO might sound like alphabet soup, but each acronym essentially just describes a variation on the same theme of how many streams a transmitting device can send and a receiving client can receive.

  • SISO. The transmitting device and the receiving client each have one antenna for a single stream.
  • SIMO. The transmitting device has one antenna, but the receiving client has multiple antennas to receive multiple streams.
  • MISO. The transmitting device can send multiple streams using multiple antennas, but the receiving client has only one antenna.
  • MIMO. Both the transmitting device and the receiving client(s) can send and receive multiple streams using multiple antennas. Alternatively, the transmitting device has multiple antennas that it uses to simultaneously transmit to multiple clients, some or all of which have only one receiving antenna each.
MU-MIMO vs. SU-MIMO chart
MU-MIMO Wi-Fi technology supplants SU-MIMO functionality.

Engineers at Aruba Networks once described SIMO and MISO as "degenerate forms of MIMO with a single antenna chain at one or the other end of the connection," calling the additional acronyms superfluous and unnecessarily confusing. SISO and MIMO are certainly the more relevant and frequently used terms.

3. What do 1x1, 2x2, 3x3, 4x4, etc., mean in MU-MIMO technology?

These figures indicate the number of antennas in a wireless AP, router or endpoint and thus the number of simultaneous spatial streams it can support. A Wi-Fi 5 Wave 2 AP might bear a 4x4 label, for example, indicating it has a four-antenna configuration -- with four transmitters and four receivers -- and can support four concurrent spatial streams. A 2x2 smartphone has two antennas, while a 1x1 device has just one. A Wi-Fi 6 AP has up to eight transmitting and receiving antennas (8x8), enabling it to simultaneously transmit to eight 1x1 clients, four 2x2 clients, two 4x4 clients, etc.

4. What is downlink MU-MIMO vs. uplink MU-MIMO?

In Wi-Fi downstream traffic, the AP is the transmitter, and the client node is the receiver. In upstream traffic -- when an end user uploads video to the cloud, for example -- the endpoint is the transmitter, and the AP is the receiver. When Wi-Fi 5 Wave 2 introduced MU-MIMO technology, it included only downstream connectivity -- from AP to clients -- which is known as downlink MU-MIMO. Upstream connections, however -- from clients to AP -- remained single-user, with only one node able to broadcast to an AP at a time. Imagine an interstate highway with four southbound lanes and just one single-file northbound lane. (Note: Uplink SU-MIMO enables a client with more than one transmitting antenna to simultaneously send multiple spatial streams to the AP but not at the same time as other endpoints.)

According to Cisco, IEEE considered including uplink MU-MIMO in Wi-Fi 5 Wave 2 but decided against it. Uplink MU-MIMO technology is highly complex and ultimately wouldn't be ready for prime time for many years.

5. When will Wi-Fi technology include uplink MU-MIMO capabilities?

The first wave of the Wi-Fi 6 wireless standard makes downlink MU-MIMO functionality mandatory for APs that support four spatial streams or more. It does not yet include uplink MU-MIMO capabilities.

While not written in stone, networking analysts expect uplink MU-MIMO technology will make its big debut as part of Wi-Fi 6 Wave 2. It's important to note that, in order for an endpoint to take advantage of uplink MU-MIMO capabilities, it would need at least two antennas. Otherwise, upstream traffic would presumably revert to a single-user connection, via backward compatibility.

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