MU-MIMO soon and trends

In April Qualcomm announced their forthcoming 802.11ac MU-MIMO chipsets. These include the QCA 9990 and QCA 9992 chipsets for business grade access points with 4 and 3 stream radios respectively. Their client device chipsets provide 1 and 2 streams. All these MU-MIMO chipsets provide up to 80 MHz channel width, not 160 MHz. Their highest link speed is then 1.73 Gbps on 4 stream access point and ‘home router’ chipsets, while their client device chipsets with 2 streams have a highest link speed of 867 Mbps. So, for an all Qualcomm setup the upper limits for access points and ‘home routers’ are more usefully considered as aggregate capacity limits, e.g. two 2 stream clients could in theory transfer at 1.73 Gbps. In practice of course it is more likely to be about half of that or less. As these chipsets were “expected to sample in the second quarter of 2014” we can expect them in the products in the second half of 2014, along with some of their competitors – Broadcom and Quantenna have made similar announcements.

With MU-MIMO access points can service multiple stations simultaneously, so the available streams can be more fully utilised. The most important effect of this is to effectively increase the capacity of the spectrum. Obviously this is good news for WLAN owners and managers who have spectrum operating around capacity. Although MU-MIMO does not make a connection faster than before, it does provide more uncontended air time to clients, so they should also feel the benefit as better transfer times.

As MU-MIMO is compute expensive we are going to see more PoE+ equipment. As more channels are available in the 5 GHz band, and they are being added to, it makes sense for access points with two or more radios with omnidirectional antennas to be deployed where spectrum is highly utilised. This will add further to power requirements so we may see a growing market for mid-span PoE+ injectors.

802.11ac and MU-MIMO is coming at a good time as expectations and use of WiFi are soaring; a trend that will continue as the Internet of Things and wearable devices gain traction. If rumours are correct, the ever growing bandwidth needs of static and moving images will soon be added to by the demands of holographic displays. Obviously with all this data aggregating over WiFi to Ethernet we need 10 GbE at a sensible price soon.

8x8x8 MU-MIMO WiFi in 2015

Quantenna says they plan to release 8x8x8 MU-MIMO chipsets in 2015
This will be a very important development for anyone owning WiFi networks and of course WLAN/LAN professionals.
8 stream MU-MIMO can provide very high aggregate throughput to the LAN, making more efficient use of the WiFi infrastructure but requiring a 10 GbE LAN to make full use of it.

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.