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.

The balance of wired and wireless

Recently I installed a 4G LTE router in a site where there is a poor wired Internet service with no plans for improvement, but a choice of proximate 4G LTE base stations. The resulting wireless throughput is better, the service more reliable, and the prospect of further improvements immanent – partly because of the increasing competition between the wireless Internet service providers (WISPs) offering 4G. Apparently the wired infrastructure is not economically viable to upgrade according to its ISP. This is surprising statement given that the area is very densely populated with consumers and wired infrastructure. Perhaps what they mean is not enough disgruntled customers are leaving for 4G to justify spend on upgrading their service yet. This is not the first area I have come across with that attitude by an ISP. The first time I was told this was also in a build-up area, but it had fewer consumers and more businesses that are probably paying for leased lines anyway, so it was easier to see why there. Anyway, this attitude made me wonder where it is economically viable to put in at least fibre to the cabinet. Obviously the WISP base stations that serve this recent site need to aggregate a lot of data, and at least one of them has no wireless carrier antennas, so I suspect it is using fibre for backhaul. I think this is a case where wired infrastructure can more easily make money. It has the throughput advantage (at the moment) that can justify the cost of digging in a heavily developed area with strong property laws. I expect ISPs to continue to cede customers to WISPs and wired infrastructure to further retrench and focus on highly aggregated throughput.

Now suppose that some clever researcher finds some scrap of information intrinsic in electromagnetic radiation that allows distinct transceivers to be identified, or even just groups of them. This would make a dramatic difference to wireless communication because spectrum becomes less contended. In fact something like that has already been announced in the shape of pCells. I hope for and expect more innovations of this kind. When they arrive they will have a profound effect on wireless communication and wires will retrench further.

Backhaul

I have been disappointed for some time by the lack of progress from 1 GbE to 10 GbE. Links to APs do not serve one client device. APs working in two RF bands are common and extra bands are being tested to provide even more RF PHY. Better use is made of the 5 GHz band by 11ac. Directional antennas and 11ac MU-MIMO further increase capacity of RF PHY. These factors combined with climbing throughput demands means 1 GbE backhaul is already too little for some. Xirrus XR6000 series APs have 4x 1 GbE ports and 1x SFP+ 10 GbE port to handle backhaul from up to 16x 3 stream 11n or 11ac modules. 10 GbE needs to get a lot cheaper soon.