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GroupID Concept for Downlink MU-MIMO Transmission
January 2010 doc.: IEEE /0073r0 March 2010 GroupID Concept for Downlink MU-MIMO Transmission Date: Authors: Joonsuk Kim, Broadcom Corp. Joonsuk Kim
Abstract Changes from r1 Remove SU/MU bit from the GroupID Field
January 2010 doc.: IEEE /0073r0 March 2010 Abstract Changes from r1 Remove SU/MU bit from the GroupID Field Modify the summary Add a strawpoll at the end Joonsuk Kim, Broadcom Corp. Joonsuk Kim
Background Non-resolvable LTFs Resolvable LTFs
March 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.
January 2010 doc.: IEEE /0073r0 March 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
March 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.
An Example of Sounding Mechanism
March 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.
Stream Number Indication in VHT-SIG A 
March 2010 Stream Number Indication in VHT-SIG A  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 field (called GroupID field) Some of these bits may be reused for SU-MIMO packet y bits x bits Group ID # of columns of Qk (Nsts_k) for k=1,2,3,4 Qk is the steering matrix for user k x>0, y >0 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.
March 2010 Summary MU-MIMO with resolvable LTFs requires a mechanism to indicate which streams are for which user. Group ID field in VHT-SIGA is introduced for this information Group ID can be announced when a sounding frame is sent, so it does not require significant extra network overhead. PHY adhoc group already approved to adopt this concept last January meeting . Joonsuk Kim, Broadcom Corp.
Reference  “11-10-0070-00-00ac-802-11ac-preamble.ppt"
March 2010 Reference  “ ac ac-preamble.ppt"  “ ac-GrpID-for-DL-MU-Transmission.ppt"  “ ac-TGac-PHY-report-Jan-2010.ppt” Joonsuk Kim, Broadcom Corp.
January 2010 doc.: IEEE /0073r0 March 2010 Straw Poll Do you support adding the GroupID field to VHT-SIGA for 11ac preamble as in 11-10/0073r2, and editing the spec framework document, , accordingly? Y N Abstain Slide 10 Joonsuk Kim, Broadcom Corp. Joonsuk Kim
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