The significance of the ‘MU’ in 802.11ac MU-MIMO

Firstly, what is the ‘MU’ feature in 802.11ac MU-MIMO? Put simply it allows multiple Wi-Fi client devices (e.g. mobile phones, tablets, and laptops) to exchange data with an access point radio, in parallel. Previously only one Wi-Fi client device at a time could exchange data with an access point radio. An important consequence of this is that the aggregate throughput of access points can spend longer at higher levels and so make more efficient use of network resources. Another consequence is that traffic analysis will be more difficult when there are multiple simultaneous talkers.

The number of Wi-Fi client devices that can exchange data simultaneously with an access point radio is limited by the number of spatial streams that each supports. The 802.11ac amendment to the 802.11 standard allows for radios with up to eight spatial streams, although only recently have four stream MU-MIMO processors become available. Each spatial stream is a distinct stream of data that requires an antenna of its own linked to one radio. A connection between an access point and a Wi-Fi client device will use one or more streams. In practical terms this means a four stream 802.11ac processor with MU-MIMO in an access point can communicate in parallel with four single stream client devices, or two single stream client devices and one two stream client device, or two client devices each using two streams, or of course one four stream client device.

At this time a typical 802.11ac setup may use an 80 MHz channel width and an 800 ns guard interval, with connections perhaps achieving MCS 7. If that setup were fully MU-MIMO enabled it would then have a theoretical aggregate throughput of 4*292.5 Mbps i.e. 1.17 Gbps. Out of interest I performed a test as I wrote this in very good RF conditions using a Sony Xperia Z Ultra and Samsung Galaxy NotePRO 12.2 connected to D-Link DAP-2695. I used them for no other reason than they happen to be sitting on the next desk and are all are very current. All of these are 802.11ac devices, but not MU-MIMO. The Sony device achieved a link speed of 325 Mbps with RSSI at -42 dBm; it delivered 205.7 Mbps up and 207.95 Mbps down. The Samsung device achieved a link speed of 866 Mbps with RSSI also at -42 dBm; it delivered 208.87 Mbps up and 413.89 Mbps down. These were the best figures from among a handful of tests on each client device. Some test results achieved only half of these rates or less, but most were similar. These links are clearly 80 MHz, 400 ns, MCS 7 and MCS 9 for the Sony and Samsung respectively, with one and two streams respectively. Anyway, if these devices were MU-MIMO then my best aggregate download throughput for two Xperia and one NotePRO (for example) would be 2*207.95 + 413.89 = 829.79 Mbps. Add a client on a 600 Mbps 2.4 GHz radio and we can see it is possible for an access point to make full use a GbE link. The theoretical throughput of GbE is 118660598 data bytes per second (about 949 Mbps) using a 1460 data bytes Maximum Segment Size in a normal Ethernet frame of 1518 bytes containing a Maximum Transmission Unit (MTU) of 1500 bytes. Using a 9K MTU improves this to about 123916800 data bytes per second i.e. about 991 Mbps. In practice of course these theoretical GbE maximums cannot be achieved, and Wi-Fi transfer rates are likely to be about half of the link speed.

Let us consider how multiple SSIDs relate to this ‘MU’ feature. An access point radio operates on one logical channel at a time. In fact that logical channel may be composed of multiple contiguous channels or discontinuous ‘bonded’ channels that behave as one large channel. These techniques increase the amount of spectrum used by a radio for a logical channel and so its bandwidth. They do not provide distinct parallel streams of data. As SSIDs are configured to a band and thence a radio, so they will all share the same logical channel of their radio. Consequently all SSID traffic has to take a turn on their radio’s configured logical channel, unless that radio is MU-MIMO enabled. In which case SSID traffic might travel over one or more spatial streams, depending on Wi-Fi client device MU-MIMO capability, and so could travel in parallel with other SSID traffic. So, SSIDs provide no innate transmission parallelism; that can only come from MU-MIMO enabled 802.11ac radios.

How to design a Wi-Fi system – coverage

There are three main concerns when designing a Wi-Fi system: coverage, throughput, and features. In this post we consider coverage.

First a note on coverage as a distinct concern: It is not possible to completely separate issues of coverage from throughput. Better coverage will very often lead to better throughput. We will find out more about this in the post on throughput. Also some features are dependent on coverage. Again we will find out why in that post.

Terminology: A devices that provides Wi-Fi coverage is called an access point (AP). In commercial deployments APs are usually attached to ceilings or are fixed high up on walls, but they can be placed on surfaces. Most home and small business ‘routers’ now contain an AP, along with several other items of network technology. Any Wi-Fi enabled device that connects to an AP is called a station (STA). They includes all modern laptops, tablets, mobile phones, most e-readers, many games consoles, and increasingly TVs and other consumer electronics. In fact an AP is a special kind of STA which provides access to a distribution network for the STAs connected to it. A Wi-Fi system is an example of a wireless local area network (WLAN). Attenuation of a signal is weakening of it.

Coverage is about having a Wi-Fi signal where it is required. This has three main considerations: the number of APs that serve a location, the band of radio frequencies used, and obstacles to coverage.

The first consideration is the number of APs that serve a location. Even though most Wi-Fi systems try to ensure each location is covered by an AP, inevitably AP ranges overlap. So many locations will be covered by more than one AP. Indeed, in some cases we deliberately ensure that this is the case. This can be a kind of insurance policy against problems with APs. If your Wi-Fi coverage is critical or very important in some locations then you may wish to ensure they are covered by more than one AP. However, for Wi-Fi to work well APs with overlapping ranges should work on different radio frequencies. For a number of technical reasons this makes Wi-Fi more efficient, but for now it is enough to say we are trying to avoid interference. The most commonly used frequencies are in what is referred to as the 2.4 GHz band, which is available for use without a radio operator’s license. Consequently it is popular with many kinds of transmitting devices, which can be a problem; more on this later. The 2.4 GHz band is divided into 13 channels in the UK and Europe. Unfortunately each Wi-Fi channel is wider than one of these 13 channels, but four Wi-Fi channels can be well enough separated for tolerable interference levels by centring them on channels 1,5,9,13. In the US the situation is worse as they only have 11 channels in the 2.4 GHz band, so they can use only three Wi-Fi channels centred on channels 1,6,11. Much equipment is designed in America and they set defaults to suit their own market you will often see their pattern of channels 1,6,11 used in the UK too. Anyway, try to arrange things so that the APs that are closest together are not centred on the same channel. As your neighbours will also likely have Wi-Fi you may have to take this into account as well. So it may be best to start planning your channels by looking at what you are receiving from your neighbours at your boundary if coverage there is important.

The second consideration is the band of radio frequencies to use. In fact Wi-Fi can work in two radio frequency bands. The less commonly used higher frequency band is referred to as the 5 GHz band, which is also available for use without a radio operator’s license. In the UK it has 19 channels, although for arcane reasons only 16 are typically available. The 5 GHz band channels are a little complicated so we won’t discuss them here, but importantly 5 GHz Wi-Fi channels fit into them without interfering with their neighbours. These 5 GHz channels have two properties that are important to us that are due to the laws of physics for radio waves at higher frequencies. Firstly they don’t travel as far as 2.4 GHz band radio waves. Secondly it is easier to get them to transmit more information in an amount of time. For many the reduced range is the more important property. You will need to fit more APs working at 5 GHz for the same coverage so the cost of the system is higher. If however higher throughput and/or stability is important, then consider using the 5 GHz band. There is another important point to make about the 5 GHz band. Currently it is much less used so it is easier to get a signal free from interference. If your 2.4 GHz band is a mess with all sorts of transmissions you should consider using the 5 GHz band. The last thing we will say about the 5 GHz band is that the latest and greatest version of Wi-Fi – so called 5G Wi-Fi (802.11ac) – only works in this band. So if you are planning to upgrade from a 2.4 GHz system to 5G Wi-Fi, unless your existing system was designed for 5 GHz you will have more APs requiring extra cabling, almost certainly requiring existing cables to be repositioned, and probably requiring replacement of switches they connect to or extra switches.

The third consideration is that everything that Wi-Fi signals pass through attenuates them. Two things are particularly problematic: water and metal. Water absorbs Wi-Fi signals. As people are about 60% water they are a significant problem. That is why we prefer to place access points high up, so their signals pass more through air and less through people. This is particularly important where many people are close together, such as in locations that host events like conferences and social gatherings. Metal reflects Wi-Fi signals. So access points are generally best positioned away from metal. Unfortunately most buildings contain many metal parts. For example supporting columns contain steel, concrete is usually reinforced with steel, doorways may have a steel support above them, and separate rooms that have been made into one by removing a wall will have steel supporting the span. Some buildings use steel reinforced concrete for walls and floors. Stairs in non-domestic buildings are usually made of steel. Electrical, plumbing, and air conditioning infrastructure is mostly metal. In larger buildings you may need to consider where water and metal infrastructure is as they become significant sized obstructions. As a very rough guide, in the UK a 2.4 GHz signal will be usable in the next room, but weak in the room after that, so corridors can be good locations for access points to cover multiple rooms. Distance also weakens Wi-Fi signals, so downgrade your expectations with larger than average size rooms. Also downgrade coverage expectations significantly for 5 GHz. If it is important to get coverage right or you have doubts get one access point and test it in possible locations. There are many test tools, from free mobile phone apps through to very expensive professional equipment. Lastly, if your job or reputation will be damaged by getting it wrong you should probably call us. The small amount we charge for installing it is partly offset by the better prices we get for equipment, the more appropriate selection of equipment we will make, our more accurate and efficient installation and configuration, and your time saved.