MU-MIMO vs SU-MIMO Wi-Fi

Introduction

The airwaves are a much different medium for Wi-Fi than cables are for Ethernet. One of the first differences is that Ethernet can be full-duplex, transmissions can be sent and received simultaneously, whereas Wi-Fi can only be half duplex. A radio can only transmit or receive at once, not both at the same time.

Furthermore, in the past Wi-Fi radios could only send transmissions to one other radio at a time. However, with multi user multiple-input and multiple-output (MU-MIMO) that changes.

Understanding single user MIMO

To understand the new multi user MIMO technology, you first need to have at least a basic understanding of the single user technology. It came along as an optional technology with the 802.11n wireless standard in 2007. It enabled multiple streams of data to be simultaneously transmitted or received between two Wi-Fi devices using multiple antennas and beamforming technology. It helps increase the speed at which data passes between those two Wi-Fi devices.

Issues with single user MIMO

The most obvious down side to single user MIMO is that the multiple streams of data must be sent or received between just one device at time. However, there are more cons as well. For instance, single user MIMO requires both the transmitting and receiving Wi-Fi radios support the MIMO technology, along with having multiple antennas. The multiple antennas add cost, weight, and size to the Wi-Fi devices and the processing of the MIMO signals requires more resources as well. These are especially evident with the smaller devices, like smartphones and tablets.

Understanding multi user MIMO

Multi user MIMO is being released in the second wave of the 802.11ac wireless standard. It enhances the MIMO technology by enabling Wi-Fi to simultaneously transmit those multiple streams to different Wi-Fi devices, instead of just one single device as with the older version.

So for instance, say an access point is capable of sending four data streams simultaneously, it could send all four to a device that can accept four. Alternatively, it could send two streams to one device and the other two streams to two different devices. In all, three different devices would be receiving streams simultaneously.

Benefits of multi user MIMO

The most obvious benefit of multi user MIMO is that those multiple streams of data can be sent or received to or by different devices, increasing the possible throughput of the network. However, there are many more benefits as well.

For instance with multi user MIMO, Wi-Fi devices receiving one of the MIMO data streams doesn’t have to have multiple antennas. The receiving Wi-Fi devices must support the multi user MIMO technology, but if there’s only one antenna then it could still receive one of the multiple data streams from the wireless router or access point. Furthermore, the wireless router or access point is the device that does the heavy processing of the MIMO signals, thus it’s less taxing on the processors of the Wi-Fi devices. This all means saved cost, weight, and size for the devices supporting the multi user MIMO technology. In turn, we should see more devices with this technology than we did with single user MIMO.

Positive side effects of multi user MIMO

Multi user MIMO helps increase the speeds and throughput for the entire network more than the previous technology did. Even non-MIMO devices will see a benefit since there is potentially more airtime for them, increasing their throughput as well.

In addition to increasing speeds, multi user MIMO also has the potential to increase the capacity of wireless networks. Since Wi-Fi devices can be served quicker, the more devices there can be. A noticeable difference could be especially realized on public Wi-Fi hotspot networks with a dense amount of users in certain area.

One very interesting side effect of the signal processing of multi user MIMO is that the data is scrambled while traveling from the wireless router or access points through the airwaves to the Wi-Fi devices. This basically means unsecure or unencrypted connections, such as when on public Wi-Fi hotspots, can see an increase in security. Any eavesdroppers nearby capturing the Wi-Fi traffic won’t be able to make use of any of the data that’s transmitted as multi user MIMO. This is also useful on private networks as well that are using the personal (PSK) mode of Wi-Fi security. Normally, any eavesdropper nearby that knows the Wi-Fi password can decrypt the wireless traffic from the other users, but that won’t be the case for multi user MIMO traffic.

The rules and regulations for single MIMO and beamforming that were designed with the 802.11n wireless standard weren’t as standardized as they are with the multi user MIMO and the 802.11ac standard. Having a more standardized technology means that products from different manufacturers will all support the same technology and methods. More products with the technology, means the better the chance the technology will make a difference on your network.

Issues with multi user MIMO

The major stipulation of multi user MIMO is that it currently only works on the downlink connection of Wi-Fi: the transmissions from the wireless router or access point to the Wi-Fi devices users have connected. Right now Wi-Fi devices can only transmit a single stream of data to the wireless router or access point with the multi user MIMO technology no matter how many antennas there are on either device. Though single user MIMO actually worked on both the uplink and downlink connection, multi user MIMO is typically seen as a better option overall when you weigh all the pros and cons of each technology.

Another important fact about multi user MIMO is that it only works on the 5GHz Wi-Fi band. Though 802.11ac devices also support the older standards, like 802.11n or even 802.11b/g, this newer MIMO technology only works for connections utilizing the actual 802.11ac standard, which is a 5GHz-only technology. Thus other than the positive side effects like I discussed, increased total throughput, the multi user MIMO technology doesn’t directly affect devices using 802.11n or earlier.

Apart of the MIMO technology is beamforming, where the Wi-Fi signals are directed towards the intended recipient rather than always being sent equally in all areas. This raises a potential issue for rapidly moving Wi-Fi devices. The movement can complicate and slow the MU-MIMO performance. However, the wireless router or access point should detect this when it arises and will likely revert to single user MIMO, which isn’t so sensitive to movement, for any problem devices when needed, while continuing multi user MIMO with any other devices. It’s safe to say multi MIMO won’t be much of a benefit to networks with a majority of roaming devices.

Deploying multi user MIMO

Though the multi user MIMO technology has been included in the 802.11ac standards from the IEEE, and product vendors are including it in what they call wave 2 of 802.11ac, the Wi-Fi Alliance likely won’t include it in their certification criteria until 2016. There aren’t many Wi-Fi products that support it just yet anyways, and those first to come are typically in the consumer market, like for wireless routers. We likely won’t see many enterprise products support it until 2016 and after.

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