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OFDMA and MU-MIMO: what? Let's talk about beamforming

3.6.2020

Translation: Patrik Stainbrook

We wouldn't have 5G without beamforming. These «shaped radio beams» also allow Wi-Fi routers to provide improved signal strength or range.

Beamforming isn't just exclusive to radio. This technology can theoretically be applied wherever wave fields exist. In acoustics, beamforming algorithms can be used with directional microphones to locate sound sources. Or you could use it to ensure that sound travels better between your surround speakers and yourself. Literally, beam shaping is about determining the source of wave fields in order to irradiate or sonicate it in a targeted manner.

A Wi-Fi router without beamforming technology will, with a bit of luck, transmit in all directions equally, as long as there's no interfering object in the way. A model with beamforming will specifically target a terminal device. However, this requires that the router knows the location of this device, which is why both points of contact need this technology installed.

By the way, beamforming originated during the Second World War. In the 1940s, it was used to improve submarine sonars. Complex mathematics weren't the only essential field in its development, however. The advancement also required the sophisticated use of converters, preamplifiers, digitizers and hardware for beamforming calculations.

Beamforming requirement: one antenna isn't enough

In Wi-Fi, beamforming ensures that the router focuses its signal on the receiving device. It does this by sending out several overlapping signals – more on this later. The prerequisite for this is a router with several antennas or one with MIMO technology. Wi-Fi routers with «Multiple-Input, Multiple-Output» functionality are able to use different antennas. They can also transmit four downlink data streams in parallel. The first MIMO routers were released towards the end of 2009, in combination with the Wi-Fi 4 standard 802.11n. But 802.11n doesn't specify how beamforming should be implemented in routers. As a result, there were few proprietary 802.11n products with beamforming technology.

Beamforming would only reach the masses with Wi-Fi-5 or 802.11ac respectively. Starting in 2014, new routers wouldn't only transmit up to eight different downlink data streams simultaneously using different antennas. Thanks to MU-MIMO, you could also supply different terminals with the data. However, the new standard didn't just provide «Multi-user Multiple-Input, Multiple-Output» abilities, more data throughput and many other improvements; it also finally included some specified beamforming techniques. And although an 802.11ac router isn't required for beamforming, virtually all manufacturers implement it in their products since then.

Beam me up Scotty: MU-MIMO uplink and OFDMA

For new technology to work, it must be supported not only by the Wi-Fi router, but also by the respective terminal device. If your old PS3 still doesn't get up to speed after a router upgrade, it's not because your new machine is defective, but because of the outdated 802.11g chip in the console.

Starting with Wi-Fi-6, which has been certified since September 2019, MU-MIMO technology will be upgraded. However, we're still waiting for this MU-MIMO uplink. Current Wi-Fi 6 routers don't yet have this. This technology will only be implemented starting with future Wave-2-802.11ax products. The uplink isn't completely new, but it was previously withheld from a few Wave-2-802.11ac products.

Furthermore, MU-MIMO is getting a good partner with Wi-Fi-6. The same partner it has in 5G devices: OFDMA.

MU-MIMO as well as OFDMA are multi-user technologies – they'll both be able to exchange varied data with different devices simultaneously with downlink and uplink. They both ensure that latency times are reduced and that Wi-Fi efficiency is improved. But the «Orthogonal frequency-division multiple access» technology takes a different approach by additionally dividing a radio channel into smaller sub-channels. For example, a 20 MHz channel can be split into up to nine smaller channels. Following that, a time interval is assigned to the respective sub-channel. The router now alternately supplies the sub-channels or different end devices with data, whereby all this happens at extremely short intervals. Tiny windows of time ensure that everything runs seemingly seamlessly.

At first glance, OFDMA and MU-MIMO might seem like. However, appearances are deceptive, as these two techniques complement each other perfectly. If an application requires high bandwidth, MU-MIMO is on hand to increase link efficiency better than OFDMA could. OFDMA performs better with low bandwidth or small data packets. If low and high bandwidths are mixed, OFDMA is also a good choice. An AX router decides on the basis of predefined or adapted algorithms whether it transmits data via multi-user MIMO, OFDMA or even single-user MIMO.

The bottom line is that OFDMA increases efficiency, reduces latency more effectively and can serve significantly more devices with small packets at the same time than MU-MIMO can. In return, MU-MIMO increases the capacity, brings a higher speed per user and is best suited for when you want to pull a big movie from the net.

Now what is beamforming really?

For a Wi-Fi router to be able to handle different antennas, you'll need either MU-MIMO or OFDMA. But that's far from everything a modern router can do. With Wi-Fi-6, more than 50 features are planned, some of which are already available in current products or will be here soon. And there are also other aspects that can significantly affect performance. Quadrature amplitude modulation, which is also used for 5G like both MU technologies, can't remain unmentioned at this point.

But back to beamforming, which adds additional benefit: if an antenna sends its signal circularly in all directions, the signal becomes weaker at the edge of the reception area. You can negate this by beaming a cone-shaped pillar of frequency towards your desired endpoint instead of a circular shape. No antennas need to be physically aligned for this. A targeted strong beam is achieved by phase shifting and multipath propagation of the signal. Different antennas, which are located close to each other, transmit the same signal at extremely short intervals. The different signal waves are layered and lead to interference. Depending on the location, the superposition of the waves can result in destructive or constructive interference. The waves either cancel each other out or amplify each other and act in bundles.

Beamforming must therefore be carried out correctly in order to get a strong and focused signal. A complicated story, but one that is essential for 5G. As the new standard operates in higher frequency bands and higher frequencies have a shorter range than lower ones, technology is needed to compensate for this.

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Martin Jud

Senior Editor

martin.jud@digitecgalaxus.ch

I find my muse in everything. When I don’t, I draw inspiration from daydreaming. After all, if you dream, you don’t sleep through life.

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OFDMA and MU-MIMO: what? Let's talk about beamforming

Beamforming requirement: one antenna isn't enough

Beam me up Scotty: MU-MIMO uplink and OFDMA

Now what is beamforming really?

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