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1 Stream vs. 2 Stream

When we were still testing MU-MIMO performance, we used Veriwave test gear that emulated multiple single-stream (1x1) MU-MIMO devices (STAs). But newer phones are using two-stream radios, which some routers may not handle optimally.

For example, Qualcomm's older 4x4 QCA9980 radio can provide peak total throughput for three 1 stream or one two stream and one 1 stream devices. If presented with two two-stream MU-MIMO devices, it's likely the second device gets treated as SU, with no resulting MU-MIMO throughput gain.

According to the slide below taken from a June 2015 Qualcomm presentation, the QCA9984, used in newer routers like NETGEAR's R7800 and Synology's RT2600ac, can support two, two-stream devices.

Older MU-MIMO devices support only three devices per frame

Older MU-MIMO devices support only three devices per frame
(Image credit: Sogi.com.tw)

So let's compare MU-MIMO throughput gain using two 1x1 and two 2x2 devices. I used two octoScope Pal-5 partner devices, which use the QCA9984 and allow setting the number of MIMO streams.

I ran tests with two Pal-5's connected to the NETGEAR R7800, which also uses the QCA9984 in its 5 GHz radio and then the ASUS RT-AC88U, which uses a Broadcom BCM4366. I unfortunately did not note the revision level of the device when I opened it up, so don't know if it's using the C0 rev allegedly required for proper MU-MIMO support. As the data will show, I suspect it's not.

Each router being tested was placed in the center of the octoScope 38 chamber. One Pal-5 connected to antennas on the right side of the chamber; the other connected to antennas on the left side. Programmable attenuators were used to adjust signal levels reported by the Pals to be approximately equal. The same traffic used for the previous phone testing was used, i.e. multiperf3 running four simultaneous connections with a 2048 kB TCP/IP window size.

The bar charts below compare total throughput from the two STAs, since that's what MU-MIMO is supposed to increase. The pairing of Qualcomm-based STA and router using the same QCA9984 chipset should give MU-MIMO its best shot. The results show a 62% gain in total throughput using the pair of 1x1 STAs, but only 17% gain using 2x2's.

1 vs. 2 stream MU-MIMO Gain - NETGEAR R7800

1 vs. 2 stream MU-MIMO Gain - NETGEAR R7800

As unimpressive as that may be, pairing a Broadcom-based router and Qualcomm-based STA is even worse. The pair of single-stream STAs manage to get a 42% total throughput gain. But the pair of 2x2 STAs results in a 58% total throughput loss!.

1 vs. 2 stream MU-MIMO Gain - ASUS RT-AC88U

1 vs. 2 stream MU-MIMO Gain - ASUS RT-AC88U

Here's what the ASUS throughput plots looked like, with MU-MIMO off...

ASUS RT-AC88U - 2 stream Pal STA - MU-MIMO off

ASUS RT-AC88U - 2 stream Pal STA - MU-MIMO off

...and with MU-MIMO enabled.

ASUS RT-AC88U - 2 stream Pal STA - MU-MIMO on

ASUS RT-AC88U - 2 stream Pal STA - MU-MIMO on

Useless MU-MIMO

So it's bad enough that a buggy feature gets prominently hawked as a product advantage when it has a chance of providing benefit. But when it has no chance of doing anything useful, it's way past time to call it out.

Such is the case with MU-MIMO in two-stream products. I keep seeing MU-MIMO listed as in the specs of Wi-Fi extenders and multi-node Wi-Fi systems like NETGEAR's Orbi that have 2x2 client-facing radios.

If we use the old number of streams -1 rule of thumb for the number of single-stream devices that provide peak MU-MIMO gain, we get 1, which makes no sense. So let's say a two-stream MU-MIMO AP/router will provide peak throughput gain with two single-stream devices.

To test this, I put an Orbi RBK30 into the test chamber, fired up the two octoScope Pal-5's again and got the results below. Like all Wi-Fi Systems to date, Orbi is based on Qualcomm silicon. So we know it should be capable of supporting proper MU-MIMO operation. Although the original Orbi has a dedicated 5 GHz 4x4 backhaul radio, the client-facing radios in all models are only 2x2.

The plot below shows clearly that MU-MIMO isn't contributing anything useful here. But at least it's not decreasing throughput like the Broadcom-based RT-AC88U.

1 vs. 2 stream MU-MIMO Gain - NETGEAR RBK30 Orbi

1 vs. 2 stream MU-MIMO Gain - NETGEAR RBK30 Orbi

Closing Thoughts

So there you have it. This is why I don't waste time testing MU-MIMO anymore. For most cases, it provides no significant benefit. And in other cases, it can result in degraded Wi-Fi throughput.

If you're one of the few who have a MU-MIMO router and devices that are boosting total throughput in your network, congratulations! But most of us are not that person. At least, like 3D in TVs, MU-MIMO isn't costing us extra anymore. But it's certainly not a reason to prefer one product over another.

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