1. GroupID Concept for Downlink MU-MIMO Transmission
January 2010
doc.: IEEE /0073r0
January 2010
GroupID Concept for Downlink MU-MIMO Transmission
Date:
Authors:
Joonsuk Kim, Broadcom Corp.
Joonsuk Kim

2. Background Non-resolvable LTFs Resolvable LTFs
January 2010
Background
Non-resolvable LTFs
The number of LTFs should be sufficient for each STA to estimate the channel for its own spatial streams
Each user cannot estimate HiQi reliably unless ||HiQj||=0
We cannot guarantee this assumption in general.
Advantage
Shorter preamble
Disadvantage
Incompatible with high-performance MU downlink algorithms
Incompatible with certain antenna configurations
Incompatible with interference suppression
Resolvable LTFs
The number of LTFs >= total number of spatial streams
Each user can measure both its signal channel and all interference channels
We can enhance the MU-MIMO performance that cannot be obtained with unresolvable LTFs at the cost of longer preamble.
For resolvable LTFs, we need a mechanism to identify which streams are for which user.
Joonsuk Kim, Broadcom Corp.

3. January 2010
doc.: IEEE /0073r0
January 2010
Group ID Definition
A group ID (y bits) is used to specify a group of K MU-MIMO users in a specified order
Group ID is defined by AP and is communicated to STAs
Group ID may be transmitted within the body of a management frame
Management frame may be a sounding frame or a non-sounding frame
Management frame without sounding may be used to indicate AID list for a Group ID without requesting channel measurement feedback frame (when the management frame transmitter already has CSI information)
A group ID is modified or created by transmitting group-definition-field
See next slide
Group ID may be overloaded. AP may assign same Group ID to mutually exclusive sets of users if necessary
A station can be member of up to 2^y groups
Joonsuk Kim, Broadcom Corp.
Joonsuk Kim

4. Group-definition-field
January 2010
Group-definition-field
Group-definition-field is defined as following
K number of MU-MIMO users per group (K=4 for example)
Several group-definition-fields may be transmitted in one management or sounding frame
Another field indicates how many group-definition-fields are included in the management or sounding frame
Upon reception of Group-definition-field, each STA knows its own STA number (one to four in this example) associated with group ID.
Group ID is managed by AP.
AID2
AID3
AID4
Group ID
AID1
AID stands for Associated Identifier
AID may be replaced by some other field that identifies STAs
Joonsuk Kim, Broadcom Corp.

5. An Example of Sounding Mechanism
January 2010
An Example of Sounding Mechanism
Sounding AP
In sounding packet, four AIDs are included in MAC header or DATA.
Each AID represents MAC address to sound for MU-MIMO
From this order, each destination STA knows the assigned number (1 to 4)
STA1
STA2
STA3
STA4
MU-MIMO Packet
time
Group-definition-field in DATA or MAC header
Group-ID-field in VHT-SIG A
Feedback Frames
- Details are TBD -
Sounding packet includes the MAC addresses of recipients to sound (possible candidates for MU-MIMO packet recipients)
The order of AIDs (Associated ID) in Group-definition-field in the PHY payload indicates the station number (1 to 4).
AP will choose the participating STAs for MU-MIMO packet based on available downlink traffic and current channel state information for all the STAs
All of STAs that were included in Group-definition-field are not required to be recipients of MU-MIMO packet.
Joonsuk Kim, Broadcom Corp.

6. Stream Number Indication in VHT-SIG A [1]
January 2010
Stream Number Indication in VHT-SIG A [1]
VHT-SIG A tells the number of columns of Qk (steering matrix for user k) in the order of assigned user number in the group-definition-field.
All STAs can listen to VHT-SIG A, so each participating user knows when to start to detect its own stream.
VHT-SIG A may include following bits (called MU-MIMO bits)
Some of these bits may be reused for SU-MIMO packet
MU indication
1 bit
# of columns of Qk (Nsts_k) for k=1,2,3,4
x bits
Group ID
y bits
Qk is the steering matrix
for user k
x bits indicate the number of columns of Nsts for user 1,2,3 and 4. Details are TBD
It is possible Nsts_k = 0 for certain k if STA k is not a recipient of MU-MIMO packet.
Joonsuk Kim, Broadcom Corp.

7. January 2010
Summary
MU-MIMO with resolvable LTFs requires a mechanism to indicate which streams are for which user.
Group ID and MU-MIMO bits in VHT-SIG field are introduced for this information
In MU-MIMO bits,
For SU, MU-MIMO indication bit is set to 0
Some of MU-MIMO bits may be reused for other purpose
For MU, MU-MIMO indication bit is set to 1.
Joonsuk Kim, Broadcom Corp.

8. Reference [1] “11-10-0070-00-00ac-802-11ac-preamble.ppt" January 2010
Joonsuk Kim, Broadcom Corp.