Simultaneous beamforming and SU-MIMO?

[maxbraketorque]

I'm thinking about my next wifi router and trying to decide on a 3x3 or 4x4. MIMO and beamforming both require multiple antennas, but from what I've read, they use the antennas differently. As I understand it, beamforming transmits the same signal over multiple antennas with phase differences to produce constructive interference at the client. However, MIMO transmits data in parallel using multiple antennas. So if I want 2x2 SU-MIMO and beamforming, it seems to me that the wifi router needs four antennas. Is this correct, and if yes, do routers actually do this?

 

[sfx2000]

Beamforming is already there for SU-MIMO, it's part of the spec for 802.11ac...

 

[maxbraketorque]

Yep, I know its in the spec, but its not clear to me that both beamforming and SU-MIMO can occur simultaneously for any given number of router antennas. Can a 2x2 router, e.g. RT-AC56U, send out a beamformed SU-MIMO signal? My understanding right now is that it cannot because beamforming requires sending the same data from the two transmit antennas while MIMO requires sending different data from each antenna all on the same channel. Multiplexing wouldn't fix the situation, but broadcasting from two different channels would allow a 2x2 to send out a beamformed MIMO signal, however, from what I've read MIMO sends out all data on the same channel. If both can occur simultaneously on a 2x2 router, what am I missing in the concepts?

 

[System Error Message]

actually a lot of companies mislead in beamforming, what they're talking about is 1 antenna vs multiple. with more antennas location can be done and the router can do its own encoding and tx adjustment better for the client, including better reception (more antennas to compare the data received at the same time).

2x2 means using a larger frequency range, this does not need to relate to the number of antennas, only that the encoding scheme spans over more frequences.

 

[sfx2000]

Need a minimum of two RF chains/streams to do TxBF on the AP side - more radios are better...

The extra radios to offer some additional diversity gain when num_streams on the AP > num_streams on the client, plus a bit of coding gain to boot as the 'unused' stream is still used as part of the coding matrix.

Keep in mind that SM is in the digital domain at the baseband, and beamforming is in the analog domain with the phase delays...

The big difference in MU vs. SU beamforming - in MU, the VHT management frame to initiate client sounding is broadcast to all - MU clients respond individually, in SU beamforming, it's a directed management frame to a specific client.

 

[sfx2000]

actually a lot of companies mislead in beamforming, what they're talking about is 1 antenna vs multiple. with more antennas location can be done and the router can do its own encoding and tx adjustment better for the client, including better reception (more antennas to compare the data received at the same time).

2x2 means using a larger frequency range, this does not need to relate to the number of antennas, only that the encoding scheme spans over more frequences.

I think you might be confusing MIMO configs with Broadcom's SmartConnect technology...

2*2:2 has nothing to do with frequency span - has to do with Tx/Rx chains and spatial streams.

Where things can get interesting - Marvell does 4*4:3 - spatial streams are less than or equal to the number of Tx/Rx radios...

Then one sees some oddities like RealTek - 3R/2T - the third receiver is a sounder...

Anyways - on a 2 stream or more AP - one can do MIMO and Beamforming, even with a single stream client, as the single stream client still does the beamforming reports back to the AP (as the AP does the beamforming), and the AP sends the same bits on all streams, and the client recovers what it needs... (even using Space Time Block Coding).

 

[sfx2000]

actually a lot of companies mislead in beamforming

One more thing... TxBF is handled differently with 11n vs. 11ac

11n - TxBF is optional, and if using TxBF in 11n, all three antennas Tx the same stream - e.g. no SM.

The other challenge with 11n and TxBF, is that there are multiple ways in the 11n spec to accomplish things, and they're mutually incompatible - and that goes for both explicit and implicit TxBF.

11ac cleans all that up - TxBF is mandatory, and SM is mandatory on the AP side, even with TxBF.

IN the real world - TxBF can help in mid-range - usually allowing at least one MCS step higher - in close range, there's no benefit to TxBF, and the AP has some latitude - it will usually still send the VHT NDP announcement, and this is to sound the channel for feedback from the associated clients, but it may choose not to do beam forming for some/all of the clients.

 

[maxbraketorque]

sfx2000, what does SM stand for?

 

[System Error Message]

sfx2000, what does SM stand for?

S&M

 

[sfx2000]

sfx2000, what does SM stand for?

Spatial Multiplexing.