Wireless Water Cooler

Reply
Aruba Employee
Posts: 4
Registered: ‎08-31-2015

Transmit beam forming vs spatial multiplexing

[ Edited ]

Team

 

Can Spatial streams and tranmsit beamforming co-exist ? As per my understadning of 802.11n during spatial mutiplexixing diffrent frames are sent through antenna elements which give better throughput but in tranmsit beamforming two frames will be sent in diffrent phases and if both reaches in client in constructive phase client will get signal with better RSSI . Long story short . Tx beamforming will give tranmsit power to clients with limited throughput while spatila multiplexing will give better throughput to clients if they are nearby .. Does this behaviour changed in 802.11ac ? Can 802.11ac APs beamform individual streams ?

 

George Isaac

newdaywireless.wordpress.com 

Chief Airhead
Posts: 1,009
Registered: ‎07-13-2010

Re: Transmit beam forming vs spatial multiplexing

@Ejohnson can probably help with this question.

Sean Rynearson | Chief Airhead
Aruba, a Hewlett Packard Enterprise Company
Moderator
Posts: 57
Registered: ‎09-28-2011

Re: Transmit beam forming vs spatial multiplexing

Spatial multiplexing is the process where the same informaiton is placed across 2 or more of the available antennas in an AP or Client device.

 

Using this simple definition almost all of the operating modes on a device use spatial multiplexing.

 

Good diagrams found here:

http://www.arubanetworks.com/pdf/technology/whitepapers/WP_80211acInDepth.pdf

 

When you have a multistream capable AP talking to a single stream device it can use CDD (Cyclic Delay Diversity aka Cyclic shift diversity) or STBC (Space time block coding)

 

For CDD sending to a single client the AP transmits a time delayed copy of the single stream on all antennas with time delays on the order of the OFDM symbol period (3.2 microseconds). the time delays are defined in the standard. The time delays decorrelate the copies of the signal being sent so this means that there is no (or very little) beamforming occuring. Beamforming in this case would be detrmental since the AP has no information on where the user is.

 

For STBC one stream is sent over 2 antennas or two streams over 4 antennas. there is no time delay across the signals but the data on the two antennas is decorrelated mathematically using something called an Alamouti code

 

https://en.wikipedia.org/wiki/Space%E2%80%93time_block_code#Alamouti.27s_code

 

again due to the decorrelation of the signals there is no beamforming and the element pattern of the AP dictates the antenna characteristic.

 

For Tx BF and MU MIMO

In 11ac there is a sounding process the sounding process gives the AP a mathematical recipe for ampitude and phase on each antenna for each stream so that the signal peaks directly over the target antenna(s) on the client. In this situation the signal are correlated and the signals from the antennas are combined to form beams and to maximise the ability of the system to support more than one stream.

 

Tx BF and MU MIMO are very closely related technologies. The underlying math for both is the same.

The sounding process allws the AP to create orthognal antenna patterns for each stream that is being sent. In appropriate enviornments (like an indoor space with walls ceiling and floor) the system works to deliver near ideal Multi-Stream/Multi User support.

 

So to summarize

in the simplest sense spatial multiplexing is transmitting signal on multiple antennas that are tied to the same baseband.

CDD/CSD and STBC do this without beamforming

    Signal are de correlated either with large time delays or by mathematical encoding

TxBF and MU MIMO do this with beamforming

     Signals are correlated and orthogonal beams are formed

 

Further reading

 

3 articles starting with this one

http://community.arubanetworks.com/t5/Technology-Blog/Wave-2-Antennas-in-the-spotlight/ba-p/228089

 

and an Airheads deep dive on 11ac

https://www.youtube.com/watch?v=HFo67dEGfcY

 

Aruba Employee
Posts: 4
Registered: ‎08-31-2015

Re: Transmit beam forming vs spatial multiplexing

Spatial multiplexing is the process where the same informaiton is placed across 2 or more of the available antennas in an AP or Client device.

 

Query : One spatial stream in 802.11ac can give upto 433 Mbps theoratical throughput . If we need to achieve 1.3Gbps we need clients which support 3 SS . Here as per my understanding all three spatial streams will carry diffrent information . Is this right understanding ? If all carrying same data payload / information then total throghupt will be only 433 Mbps . Right ? 

 

WIki Definition of beamforming : Beamforming or spatial filtering is a signal processing technique used in sensor arraysfor directional signal transmission or reception.[1] This is achieved by combining elements in a phased array in such a way that signals at particular angles experience constructiveinterference while others experience destructive interference

 

TxBF and MU MIMO do this with beamforming

Signals are correlated and orthogonal beams are formed

 

Query : How do you form orthogonal beams / achieve constructive signal correlation ? Can you do it with 1x1:1 AP ? I dont think we can do without MIMO antenna . If we are using MIMO antenna then are we sending same data payload across multiple antenna elements ?  

 

[I am still not able to go through all links you have shared .]

 

Occasional Contributor I
Posts: 9
Registered: ‎08-07-2013

Re: Transmit beam forming vs spatial multiplexing

Awesome, thank you Ejohnson.

That puts things nicely into perspective.

 

Search Airheads
Showing results for 
Search instead for 
Did you mean: