1. HT Features in Mesh Network
Feb, 2008
doc.: IEEE /xxxx
September 2008
HT Features in Mesh Network
Date:
Authors:
L. Chu et al
Liwen Chu, STMicroelectronics

2. HT Features Being Included in 11S Easily
September 2008
HT Features Being Included in 11S Easily
The following features can be used in a mesh network without any change after HT capability element being added to Beacon frame, Peer Link action frames
A-MSDU
A-MPDU
Enhanced BlockAck
RD (reverse direction)
Beamforming
ASEL (antenna selection)
L. Chu et al

3. Unclear HT Features in 11S
September 2008
Unclear HT Features in 11S
It is also not clear if and how the following features are used in a mesh network:
PSMP (power save multi-poll)
HT protection
STBC operation
Transition between 20/40MHz channel and 20MHz channel in 20/40MHz operation
PCO (phased coexistence operation)
L. Chu et al

4. September 2008
PSMP Mechanism
PSMP sequence allows the AP to create effective service periods for S-APSD
Multiple-TID blockAck allows for single frame to respond to (implicit) BAR for multiple TID
2-hop MP (mesh point) may create more collision if EDCA method is used for PSMP in a mesh network
No RTS/CTS protection for the downlink burst can be used
PSMP-
DTT1
(STA1)
PSMP-
DTT2
(STA2)
PSMP-
DTT3
(STA3)
PSMP-
DTT4
(STA4)
PSMP AP
STA
PSMP-
UTT1
(STA1)
PSMP-
UTT2
(STA2)
PSMP-
UTT3
(STA3)
PSMP-
DTT4
(STA4)
PSMP Sequence
L. Chu et al

5. PSMP Mechanism (Cont’d)
September 2008
PSMP Mechanism (Cont’d)
What happens if we combine MDA and PSMP
This combination may increase link efficiency
But this combination is different from the original MDA in that
An MP will use PIFS or the EDCA mechanism of the highest AC of the data waiting to be transmitted
Multiple AC’s frames will be transmitted in a MDA txop
An MP will send unicast frames to more than one destination MP in a MDA OP.
PSMP MDA OP should be reserved in the same way as multicast MDA OP
So 11s should disallow the PSMP mechanism.
PSMP-
DTT1
(STA1)
PSMP-
DTT2
(STA2)
PSMP-
DTT3
(STA3)
PSMP-
DTT4
(STA4)
PSMP AP
STA
PSMP-
UTT1
(STA1)
PSMP-
UTT2
(STA2)
PSMP-
UTT3
(STA3)
PSMP-
DTT4
(STA4)
PSMP Sequence
L. Chu et al

6. HT Protection Mechanism
September 2008
HT Protection Mechanism
HT transmission will be protected based on the following fields:
HT Protection
Non-green-field HT STAs Present
OBSS Non-HT Present
L-SIG TXOP Protection Full Support
In a infrastructure HT BSS, HT protection field may be set to one of the following modes
HT no protection mode
HT non-member protection mode
HT 20MHz protection mode
HT non-HT mixed mode
L. Chu et al

7. HT Protection Mechanism (Cont’d)
September 2008
HT Protection Mechanism (Cont’d)
In a IBSS, the HT Protection field is reserved, but an HT STA shall protect HT transmissions as though the following fields have the settings indicated:
The HT Protection field set to non-HT mixed mode
The RIFS mode field of the HT Information element set to 1
The Non-Greenfield HT STAs present field set to 1
The L-SIG TXOP Full Support field set to 0
The OBSS Non-HT STAs present field set to 1
The PCO Active field set to 0
In a non-HT BSS, An HT STA operating a direct link with another HT STA shall operate as though the following fields have the settings indicated:
The Basic MCS Set field set to all zeros
L. Chu et al

8. HT Protection Mechanism (Cont’d)
September 2008
HT Protection Mechanism (Cont’d)
It is not clear how to protect HT transmission in a mesh network
Solution 1: Mesh network uses whatever methods defined for IBSS HT protection
It is easy to be defined
It is inefficient especially when there are no legacy MP in the future
Solution 2: Mesh network defines new HT protection methods
An HT MP sets its protection mode according to the capabilities of its neighbor MPs and STAs in the primary channel or the secondary channel in 20/40 MHz mesh network
When selecting the protection mode, its neighbor MPs in the same mesh network have the same roles as the STAs in the same BSS.
L. Chu et al

9. HT Protection Mechanism (Cont’d)
September 2008
HT Protection Mechanism (Cont’d)
Solution 2: Mesh network defines new HT protection methods (Cont’d)
When two peer MPs report the same protection mode in HT protection field, the protection methods of the related mode will be used
When two peer MPs report different protection modes,
If one MP reports non-HT mixed mode, the protection methods of non-HT mixed mode will be used for the transmission between these two MPs
If one MP reports non-member protection mode and non-HT mixed mode is not reported by any MP, the protection methods of non-member protection mode will be used for the transmission between these two MPs
If one MP report 20 MHz protection mode and neither non-HT mixed mode nor non-HT mixed mode is reported by any MP, the protection methods of 20 MHz protection mode will be used for the transmission between these two MPs
L. Chu et al

10. HT Protection Solution 2: Example
September 2008
HT Protection Solution 2: Example
Non-member protection mode
No protection mode
MP1
MP2
MP3
MP4
MP5
No protection mode
No Non-HT mixed mode
STA1
STA1 and MP3 interfere with each other
AP1
Non-HT MP or non-HTSTA
20MHz HT MP in Non-HT mixed mode
20MHz HT MP in Non-member protection mode
40MHz HT MP in 20MHz protection mode
20MHz HT MP in no protection mode
L. Chu et al

11. September 2008
STBC Operation
STBC achieves significant error rate improvements over single-antenna systems
STBC related operations includes:
Dual Beacon
Dual CTS Protection
Non-AP STA
STBC
Null
RTS
DATA
CF-End
SIFS
SIFS
SIFS
SIFS
SIFS
SIFS
SIFS
SIFS
STBC frame
Optional AP
CTS
CTS
ACK
CF-End
CF-END
Non-STBC frame
NAV for 3-party STBC STA near non-AP STA
NAV for 3-party STBC STA near AP
NAV for 3-party STBC STA near AP
Dual CTS Mechanism
L. Chu et al

12. STBC Operation (Cont’d)
September 2008
STBC Operation (Cont’d)
It is not clear if STBC operation is allowed in mesh network.
Dual CTS protection needs master/client relationship.
It is possible to use STBC transmission through indicating a master (ownership of a TXOP) between two peer MPs.
But this may make protocol complicated:
Two STBC capable peer MPs may have different rules comparing with only one peer MP with STBC capability.
So 11s should disallow the STBC transmission.
L. Chu et al

13. 20/40MHz Operation Channel scanning and channel management
September 2008
20/40MHz Operation
Channel scanning and channel management
An AP can switch the infrastructure BSS between 20/40MHz BSS and 20MHz BSS
A 20/40MHz IBSS can not be changed to a 20MHz IBSS and a 20MHz IBSS can not be changed to a 20/40MHz BSS
40MHz transmission restriction
STA CCA sensing in a 20/40MHz BSS
NAV assertion in 20/40 MHz BSS
Switching between 40 MHz and 20 MHz
L. Chu et al

14. 20/40MHz Operation (Cont’d)
September 2008
20/40MHz Operation (Cont’d)
11s should support 20/40MHz operation
11s should allow a mesh network to switch between 20/40MHz mode and 20MHz mode
11s defines a robust distributed channel switch mechanism which can support
When a mesh network switch between 20/40MHz mode and 20MHz mode, peer MPs may renegotiate MDA duration parameters
When a mesh network switch between 20/40MHz mode and 20MHz mode, each MP may do path maintenance to find the optimized path.
L. Chu et al

15. PCO Operation AP Establishes separate 20MHz and 40MHz operating phases
September 2008
PCO Operation
AP Establishes separate 20MHz and 40MHz operating phases
20MHz phase: allows independent BSS activity on control channel and OBSS on extension channel
40MHz phase: 40MHz transmissions across 40MHz channel with no 20MHz interference
Beacon Or
Set PCO Phase
CTS2self
CF-End
40MHz
phase
Set PCO
Phase
CF-End
20MHz
phase
Ch_a
(primary)
Ch_b
(secondary)
20MHz STA
in ch_a
NAV
20MHz STA
in ch_b
NAV
PCO active
STA
NAV
NAV
L. Chu et al

16. PCO Operation (Cont’d)
September 2008
PCO Operation (Cont’d)
40MHz HT frames in 40MHz phase are not protected perfectly with 20MHz hidden MP.
Undetectable duration in 20MHz MP may create more collision.
The new rules need to be defined if two neighbour MPs are allowed to have overlap target 40MHz phase:
20Mz frames are allowed to be transmitted in a 40Mz phase.
If MP1 and MP2 have overlapped 40MHz phases, when MP1 ends its 40MHz phase using CF-end, MP2’s MP 40MHz phase may also be ended.
So 11s should disallow the PCO operation.
MP0
MP1
MP2
MP3
MP4
40MHz PCO Capable MP
20MHz HT MP
Duration
MP1
40MHz HT Data
40MHz HT Data
40MHz Phase
Duration
MP2
ACK
MP3
Data to MP2
EIFS
Backoff
Backoff
L. Chu et al

17. September 2008
Conclusion
After 11s HT capability element being added to Beacon frame, Peer Link action frames, the following features can be used in a mesh network without further work:
A-MSDU
A-MPDU
Enhanced BlockAck
RD (reverse direction)
Beamforming
ASEL (antenna selection)
11s should define new rules to decide how to use the following HT features if the following features are allowed in 11s mesh network
HT protection,
STBC operation,
Transition between 20/40MHz channel and 20MHz channel in 20/40MHz operation.
11s should disallow the following HT features to be used in a mesh network
PSMP,
PCO Operation.
L. Chu et al

18. September 2008
Straw Poll
Do you think 11s should add more text to integrate HT features in its draft?
Yes No Do not know
Do you think 11s should use whatever HT features used by IBSS in its draft?
Do you think 11s should define new rules to integrate HT features into 11s draft?
Yes No Do not know
L. Chu et al