802.11ac has brought with it MIMO alphabet soup … spatial streams, space-time streams, explicit beam forming, CSD, MU-MIMO. Alphabet soup triggers questions to which curious mind seeks answers. This post is an attempt to explore some questions surrounding explicit beam forming (E-BF) that is available in Wave-1 of 802.11ac. E-BF is a mechanism to manipulate transmissions on multiple antennas to facilitate SNR boosting at the target client.
How is E-BF related to spatial streams?
E-BF is a technique different from spatial streams. E-BF can be used whenever there are multiple antennas on the transmitter, irrespective of the number of spatial streams used for transmission.
In earlier blog posts on 802.11ac practical considerations, we reviewed 80 MHz channels, 256 QAM and 5 GHz migration. Continuing the 802.11ac insights series, in this post we will look at some practical aspects of MU-MIMO, which is the star attraction of the impending Wave-2 of 802.11ac.
MU-MIMO mechanics and 802.11ac standard
Illustration of 802.11ac MU-MIMO
At a high level, MU-MIMO allows AP with multiple antennas to concurrently transmit frames to multiple clients, when each of the multiple clients has lesser antennas than AP. For example, AP with 4 antennas can use 2-stream transmission to a client which has 2 antennas and 1-stream transmission to a client which has 1 antenna, simultaneously. Implicit requirement to attain such concurrent transmission is beamforming, which has to ensure that bits of the first client coherently combine at its location, while bits of the second client do the same at the second client location. It is also important to ensure that bits of the first client form null beam at the location of the second client and vice versa.
Currently, market is inundated with announcements from vendors on 3-stream MIMO APs. Sure enough AirTight has also launched one being at the forefront of Wi-Fi technology. But what sticks out of some of those announcements is lopsided mention of high speed wireless connectivity, even to the extent of misleading claim of 900 Mbps for the dual radio 3-stream APs albeit with a sneaky word “upto” before the number. While connectivity speed is important consideration (actually now a commodity available out of 3-stream Wi-Fi chipsets), that consideration alone does not help to come up with a good game plan for deploying 3-stream Wi-Fi. A more holistic thinking taking into account real world performance, security, and next generation Wi-Fi architecture is required when selecting 3-stream MIMO APs. Read more…
Early 802.11b APs used to have 1 antenna on them, which later became 2 in the 802.11g/a era, which now have become 3 or 6 in the current 802.11n era. So why do number of antennas keep changing as WLAN technology advances to every next generation.