MIMO & Beamforming

MIMO, Spatial Multiplexing, MU-Mimo & Beamforming

When I was writing my recent article about how 802.11ax (WiFi 6) works, I mentioned MIMO and beamforming, I recalled all of the times I'd had questions about these, the confusion between them and a misunderstanding about what they actually do.

I assumed there'd be some good web explanations out there, and there are, but none which cover both features without going into some horrible, mind-melting technical detail, so here goes...

MIMO (Multiple Input Multiple Output)

MIMO was first introduced in 802.11n ("Wi-Fi 4") and it means that a Wi-Fi device has multiple antennae and radios (transceivers), as opposed to just one. It can be used for 'Spatial Multiplexing' or 'Beamforming' or both. These two, or three terms are often confused and how they work is commonly misunderstood. In particular, you may see someone speak about their router supporting "MIMO and Beamforming". Well, beamforming is a type of MIMO, and if they support beamforming, they probably support spatial streaming too. Let's try to clear this up...

Spatial Multiplexing

Spatial Multiplexing is a method for multiplying the capacity of a radio link and requires a MIMO setup of 2 or more antennae. The number of spatial streams your device supports is normally denoted as 2x2, 3x3 or 4x4 for two, three of four antennae/radios. If a device supports dual-band (2.4Ghz & 5Ghz) it may have another set of antenna which is why some routers might have 8 antennae. That said, a single antennae can transmit or receive more than one frequency at once so it's not actually necessary to have 8 antennae for 4x4 dual band (but it does look fancy!).

With Spatial Multiplexing, each antennae/radio uses the same Wi-Fi channel but its transmitting different data. In a 2x2 scenario, that will be two halves of the data being transmitted. The receiving device can then reassemble back into one data stream. This is the equivalent of sending loads of trucks down two lanes of a highway instead of sending them all down one lane - the receiver gets his goods at twice the speed.  In a 3x3 setup, you have 3 lanes and 4x4 you...well, you get it!

But wait a minute; all of these streams are coming from the same base, on the same channel. There isn't a separate lane. They're all going to crash into each other!   Well, as you might have guessed, no, they don't, thanks to "Multipath Propagation".

Multipath Propagation is a phenomenon whereby a single radio signal from a single antenna may reach a receiver by two or more paths as a result of reflection, refraction or atmospheric ducting. The, now, out of phase multiple signals can then cause interference with each other, causing a loss of signal. This is not good; it's what causes weird effects on your FM radio when you go into a tunnel or 'ghosting' on your analogue TV when you use an indoor antenna (if you're old enough to remember that).  Multipath propagation is roughly similar to it being difficult to understand someone in a very echoey hall - you hear multiple, delayed voices, each interfering with each other.

Spatial Multiplexing exploits this phenomenon. I explained earlier how we use multiple antennae on the base, all transmitting  on the same frequency, however, because they will each reach the receiving antennae via a slightly different path (including all reflected signals) the receiving equipment can distinguish the multiple signals by comparison to a reference signal. The reflections can be off walls, ceilings, furniture, people or anything. The receiving client must also have 2 or more antennae to support this.

Maximum Wi-Fi Speeds

Many wireless routers and access points will advertise their maximum data rate, or even include it in the name (for example "Acme AC1900"). These rates are generally a function of adding up or multiplying every possible stream and technology and getting the highest number.  With an 802.11ax device, the specification allows up to 8x8 spatial streams and channel widths of up to 160Mhz. Each 40Mhz stream gives a PHY* rate of 300Mb/s but multiply that by 8 (spatial streams) and then 4 (160Mhz channel width) and you get 9.6Gb/s speed - that's your headline speed.  Add in any other bands supported and the number goes even higher (Marketing people! Am I right?!). 

Admittedly, you have to have 'some' number on your specification, so as long as everyone understands what it means, and everyone uses the same system, it can be acceptable but it should be clear what the number represents.   Firstly, if you wanted to reach 9.6Gb/s you'd need to have an 8x8 client, a perfect environment with no interference, maximum channel width, minimum guard interval, highest QAM level, no other nearby WLANs or Wi-fi devices interfering and a PC/wireless interface fast enough to keep up with that speed. Then, of course, that 9.6Gb/s is the PHY speed, before you add in wireless protocols (physical and multi-layer software), error correction, other signalling and that rate is for both directions (RX/TX) across all clients. That headline 9.6Gb/s keeps on dropping and ends up as a small fraction.

To be clear, 802.11ax is still a welcome improvement over previous Wi-Fi standards and does provide considerably increased performance in most scenarios.

*PHY means the speed of the PHYsical transport layer - the total capacity of the system if you ignore all overheads. Those overheads are significant at each layer - data has to be packeted, labelled, error corrected and resent, sent in both directions. 

Beam Forming

Beam Forming (or 'Transmission Beamforming') is a method for increasing the signal strength to a receiver in a specific location."Beam forming" can be a confusing term, because there isn't an actual 'beam' in one direction and there are no adjustable or directional antenna making a 'beam'.  It might more correctly be called "adjusting multiple transmissions of the same signal to move hot and cold spots to best suit the position of devices" but that's not very catchy - "Beam forming" sounds cool!

Beam forming, like spatial streaming, uses multiple antenna (2 or more). Unlike spatial multiplexing, which transmits different data on each of the streams/antennae, with beam forming, the same data is transmitted on all streams.  Due to the difference in positions of the antennae at each end, there will be areas where the waves coincide, thus the overall signal is stronger, and areas where they do not, and therefore the overall signal is weaker. By varying the phase of each signal - i.e. adjusting each waveform position relative to the other streams, you can adjust the location of these stronger positions and make them coincide with where your actual receiver is.

During the initial setup and reassessment phases, both ends of the link determine the optimum settings. With the 'implicit' method, the transmitter assumes characteristics of the receiver from the received signal. In the 'explicit' method, the receiver sends specific telemetry about the receive characteristics so that the transmitter can adjust specifically for the receiver's profile.  The explicit method is therefore preferable and can provide 2-3dBm of gain.

You can operate beam forming and spatial streaming at the same time but you need at least four antennae (minimum two for each).

MU-MIMO (Multi User MIMO)

802.11n (Wi-Fi 4) supported multiple spatial streams but only for a single client at a time, servicing each one sequentially - every station waiting its turn.

With 802.11ac (Wave 2), we saw the introduction of MU-MIMO (Multi-User MIMO).  MU-MIMO means that the base can transmit to multiple users concurrently (simultaneously) with each one getting its own spatial stream. It can also send multiple streams to a single device, or a mixture of the two. 

In all cases, for a client device to receive more than one stream, it must have at least two antennae (2x2).  This probably isn't obvious from looking at the device as antennae are usually not visible. Even the presence of more than one antenna doesn't mean it supports MIMO as multiple antennae can also be used for diversity or dual-band operation. To check for MIMO support, you need to check the device's specification.

802.11ac supports MU-MIMO but only for the downstream data (transfer from base to client).  On 802.11ax, MU-MIMO is supported on both TX and RX.

MU-MIMO and ODFMA

Elsewhere I've written about OFDMA, which is another method of enabling simultaneous transmission to multiple clients. OFDMA works by allocating sub-groups of subcarriers (bins) to different client devices, splitting the total bandwidth but it doesn't increase overall capacity. MU-MIMO, on the other hand, does increase total capacity.  OFDMA supports larger numbers of users (OFDMA can support up to 74 users in 802.11ax) whereas MU-MIMO is limited to 8 (on the largest 8x8 bases). OFDMA also has lower latency and doesn't require continuous sounding (repeated re-assessment of the link which takes 1ms every time).  MU-MIMO is also not effective at longer ranges whereas OFDMA is. Further, OFDMA can increase range anyway because it can make dynamic changes to switch off poorly performing bins.  This is covered in greater depth in my How does 802.11ax (Wi-Fi 6) work article.

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