MIMO - 802.11ac

Reference

https://www.youtube.com/watch?v=0fpyfPUyfic

Multipath:

 A signal from the transmitter may travel through different paths due to reflections from obstacles along the path. These signals arrive at different times and different angles  at the receiver and  interfere with signal from direct line of sight. This causes distortion of signal at the receiver. Prior to 802.11n this multi-path had a adverse effect.

But from 802.11n , the multi path effect has been used constructively, 

1.  Using spatial multiplexing, different data streams are transmitted from different antennas 

2.  Transmit diversity, where the same data is sent through different transmitter radio chain and processed at the receiver

3. Multiple  identical data streams  received at the the receiver by different radio chains can be processed using MRC algorithms

Multi path is a requirement for sending multiple streams. If multipath effect is not there , multiple spatial streams cannot be sent. For example in the outdoor environment obstructions may not be there.

In OFDM, known sequences  of  data are transmitted in the  pilot sub carriers .When the receiver gets the signal , it checks the pilot symbols. It knows what it has to receive. It compares that with what is has received. The difference is noise.

Because of multi path effect, the pilot sub carrier of stream 1 may be better than that of stream 2 at radio chain 1

SU MIMO

When 2 streams are transmitted, the processing unit at the receiver may assign 1 stream to the radio chain 1 and the second stream to radio chain 2 depending on how the pilot sub carriers are received by the  radio chains. Due to multi path, there will be difference in how the  2 signals are received at the radio chains.

When radio chain 1 receives stream 1 and 2 , it checks the pilot sub carriers of stream with what is should be, and cancels the rest, that is stream 2 is noise for radio chain 1.

MU MIMO

Transmitter/AP should be able to send one stream to 1 station and stream 2 to station 2.  Logic is the same as SU MIMO, station 1 should be able to keep the stream sent for it and  remove the stream sent for station 2.

Each stream sent from the AP is sent in a way such that each station has different view of the stream, say station 1 sees stream 1 as a strong signal and stream 2 as a weak signal. Stream 2 can be discarded as noise.

For this AP needs the channel state information.  For this AP sends NDP announcement and the a NDP.  Each station replies with VHT compressed beam forming frame that has the Channel state information, ie how the station heard the AP on different sub carriers. This process is called sounding

Based on the CSI, the AP groups compatible stations for which data can be set simultaneously,

For only stations in a group , data can be transmitted simultaneously.

AP indicates to the station to which group it belongs to and  within the group which index it is assigned to.

When the AP transmits the streams, it indicates the group id and  the spatial stream  assigned to each station. When the station receives the streams,

Assume station 1 assigned to group 1 with 2 spatial streams. Station 1 knows that it has to process stream 1 on radio chain 1 and stream 2 on radio chain 2. It compares the pilot sub carriers of stream 1 and stream 2 and removes everything else.

The MU MIMO frame is very different in 802.11ac

The  L-LTF, L-STF, L-SIG ad VHT SigA  fields are common to all users

The  other part - VHT-STF, VHT-LTF, VHT-SigB , data --- Is specific to each user.

The VHT SigA field has information regarding which sub stream a station has to pick among the diff sub streams

VHT-stf, vht-LTF VHT-SigB , Data --- > user specific

vht stf and vht ltf  - user/station specific information that lets the station know it has to read a particular sub stream meant for it properly 

Channel Estimation or sounding process has to be done frequently