Multi-User MIMO – Marketing Hype vs Reality

by | Jun 5, 2015 | Blog

During the presentation at #WFD7 from Matt Silverman, Technical Leader from Cisco on an Overview from MIMO technology, I started to think about the net effects this might cause our Wireless LANs in the future as 802.11ac Wave 2 in on the horizon for next year.

Though I, like many of you, are intrigued and impressed by the technical complexity and geek-coolness of MU-MIMO… I started to think through some of the implications and how reality is going to trump marketing hype.

Those marketing Wave2 as the “Panacea” of Wi-Fi congestion will be proven wrong. (I added the word Panacea just for my friend Sam Clements)

Here are some of the issues I foresee that will counteract all the marketing machine hype surrounding Wave 2.

802.11ac-WIFI

Not Enough Radio Chains

We were supposed to have 4×4:4 in 802.11n – and at this time it isn’t even on the horizon for a wide variety of reasons. I’m predicting 802.11ac’s promise of an 8×8:8 are just a theoretical limit as well. We’ll be hard pressed to get much more than 4X4:4 out of our production Enterprise-class Wi-Fi gear. Thus many of the benefits of MU-MIMO will be somewhat truncated by minimizing possible radio-chains.

Distance Limitations

We are already sorely limited in our 256-QAM distance limitations based on the extraordinarily high SNR requirements to maintain a clean enough RF SINR differential to keep 256-QAM signaling without inducing errors. In today’s testing most vendors are touting sub-10m distances already for their MCS rates requiring 256-QAM. (my real-world testing places this down below 8m)

When you lower the Tx power by splitting off some of the spatial streams to support multiple clients, you are effectively lowering the SINR to individual client devices. For example, lets say you had a 4×4:4 capable 802.11ac Wave 2 Access Point, and you were servicing two clients. Each client would now receive the RSSI energy of only two of the spatial streams, thus effectively giving the client a -3dB lower RSSI. (When compared to receiving all 4 spatial streams and using built-in MRC to garner the highest possible SNR from the Transmitted radios)

This -3dB loss in RSSI would lower the effective range of 256-QAM even further.

Drop in Data Rates

So given a specific area where Wave 1 – SU-MIMO was in place, turning on Wave 2 MU-MIMO could potentially drop the client’s associated data rates by a huge margin by dropping out of 256-QAM and lowering to a 64-QAM MCS rate instead. This in turn would lower the available time in the frequency – each client now takes longer to transmit – thus lowering the capacity of the frequency and lowering the number of clients that can be serviced by any single AP.

Just this -3dB loss in SINR alone would indicate we should be very cautious indeed in believing all the marketing hype associated with Wave 2 802.11ac. Read the marketing literature with a bit of caution and a skeptical eye to the huge increases in Wireless LAN throughput.

Reality trumps marketing every time!

For another take on 802.11ac Wave 2 MU-MIMO – check out Tom Hollingsworth’s post