Here we go again. Another post on the benefits of the newly ratified 802.11AC standard. In my first post I explained modulation and how it applies to the .11AC user. In this post I want to focus on MU-MIMO, Multi User – Multiple Input Multiple Output, and what that brings to the table. In order to do so, we need to first address the older 802.11N’s use of MIMO, sans MU.
So, what is MIMO and why should we care? MIMO, Multiple Input, Multiple Output is best thought of as an enhancement to legacy systems that allows for the use of multiple transmitters and receivers in the access point (AP) and/or client device. In simple terms, the AP or client will utilize multiple radios and antennas. Typically seen in 802.11N are combos such as 2×2 (two input chains and two output chains) or 3×3 (yeah, you guessed it, three input and three output chains). The biggest benefit to these multiple chains is the means by which the throughput is increased by taking advantage of multipath signals.
What in the world is multipath, you ask? Multipath is, in a way, interference. When a signal is broadcast, lets say from a laptop to an AP, it is not sent in a single direction to the AP. Instead, it’s usually broadcast in an omnipolar manner. It travels out in (sort of) all directions.* As it travels towards it’s intended recipient, the signal encounters walls, windows, ceilings, pizza boxes, and more. This signal then reflects away from these obstructions and continues to travel at a new angle. Ultimately, the AP “sees” multiple instances of these reflected signals ,but at ever so slightly different times. In pre .11N systems which were, by definition, SISO or Single Input Single Output, these multiple instances could not be interpreted individually and it adversely affected the receiver’s ability to decipher the true signal from the noise. 802.11N introduced the ability to combine these spatial streams with the use of multiple chains (antenna/radio combos). Combining the signals results in a greater SNR or Signal to Noise Ratio. It was a radical improvement to the 802.11 standard that, in combination with other advancements, increased the theoretical maximum throughput rate in 802.11G of 54Mbps to 802.11N’s potential of 600mbps.
Along comes moo….or is it MU? MU, the Multi User component of .11AC’s MU-MIMO is a continuation of the standard. What MU-MIMO brings to the table, in simple terms, is the ability to utilize multiple antennas for multiple clients. For example, if you have a 3×3 AP and multiple clients attempting to transmit or receive data at the same time, the AP can simultaneously transmit/receive with, say, two of the antenna chains to one client and dedicate it’s third to another client. By using the spatial diversity of the antennas themselves, we can push data without making other client devices wait their turn, increasing overall throughput. Not only do you get the benefits of MIMO and it’s ability to combine diverse spatial streams but, add to that the ability to do so for more than one connection at a time.
All of this does, of course, come at the cost of processing power but, Moore’s Law applies and we see higher and higher quality chip sets in both APs and stations. Another cost to consider is the relative loss in RSSI (Received Signal Strength Indicator) when individual clients, with multiple antennas/chains, don’t get to use the full 3×3 antenna array because another client is using some. This loss in RSSI can and does have a direct effect on the MCS rate available (see my first post on modulation), potentially reducing throughput because smaller bits of traffic can be sent at any given time.
All in all, MU-MIMO is but one more advancement in the AC standard that can allow for higher data rates than now legacy 802.11N. It’s part of the reason why 802.11AC is not the future…it’s here, now and it kicks ass.
*Omnipolar isn’t entirely correct. Most antennas are designed to radiate in somewhat of a donut shape, rather than sending the signal out in all directions like a sphere. Many manufacturers can provide the radiation pattern of the APs they offer.