1. Slot-based Power Save Improvement
February 2008
doc.: IEEE /2376r0
Date: July, 2012
Slot-based Power Save Improvement
Date:
Authors:
Liwen Chu Etc.
Liwen Chu, STMicroelectronics

2. Slot-Based Power Save in [1]
Date: May, 2012
Slot-Based Power Save in [1]
The time slot duration T may be chosen to be large enough to complete PS-Poll/trigger+SIFS+DATA+SIFS+ACK (e.g. 20 ~ 40 mS).
A STA randomly chooses a time slot to start its uplink channel access, or an AP can assign a time slot for a STA.
Each STA starts to access the channel at the assigned time slot following EDCA.
STAs do not collide with each other during slot-based access period. This can solve hidden-node problem.
Issues:
Group frames may transmitted before the slot-based transmission. The flexibility should be provided to when to start the slot-based transmission.
Each STA in the BSS should be able to decide the slot-based access period. Otherwise, there may be collision.
OBSS STAs can collide with the slot transmission.
It is difficult to harmonize slot-based PS-Poll access and non-slot-based PS-Poll access.
TIM bitmap
Total N time slots, each time slot duration = T 1 1 1 1
STA1
STA12
STA 1, 2, 3, 4, 5, 6, 7, 8, 9, 10

3. Collision Avoidance in the Same BSS
Date: May, 2012
Collision Avoidance in the Same BSS
With slot-based access, to avoid hidden node problem in the BSS, a non-power-save STA needs to be able to figure out the slot-based access period start time and length.
Solution:
In the beacon frame, slot-based access period is indicated by the AP implicitly or clearly.
Start time of slot-based access period (if slot-based access period always starts after beacon transmission, this is not required),
length of slot-based access period (in ms/TU etc).
During slot-based access period in the received beacon, a STA shall set its NAV and does not try to access the wireless medium.
TIM bitmap
Total N time slots, each time slot duration = T 1 1 1 1
STA1
STA12
STA 1, 2, 3, 4, 5, 6, 7, 8, 9, 10
Slot-based access period …

4. OBSS Collision Avoidance
Date: May, 2012
OBSS Collision Avoidance
OBSS STAs do not decode TIM element. They do not know the slot-based access period start time and length.
OBSS’s hidden node problem is still there.
Solution:
In the beacon frame, slot-based access period is indicated by the AP.
Start time of slot-based access period (if slot-based access period always starts after beacon transmission, this is not required),
length of slot-based access period (in ms/TU etc).
During slot-based access period in the received beacon, a STA shall set its NAV and does not try to access the wireless medium.
TIM bitmap
Total N time slots, each time slot duration = T 1 1 1 1
STA1
STA12
STA 1, 2, 3, 4, 5, 6, 7, 8, 9, 10
Slot-based access period …

5. Harmonization of Non-slot-based and Slot-based PS-Poll
Date: May, 2012
Harmonization of Non-slot-based and Slot-based PS-Poll
A power save STA may not support slot-based PS-Poll to simplify its design.
If the AP allocates slot for such power save STA, the slot is waste.
Solution:
A slot-based power save indication is defined.
Slot-based Power Save Indication can be in Extended Capabilities element or a new element.
An AP shall not allocate slots to power save STAs without slot-based power save capabilities.
STA1 to STA10 have slot-based PS-Poll capabilities.
STA11 and STA12 don’t have slot-based PS-Poll capabilities.
TIM bitmap
Total N time slots, each time slot duration = T 1 1 1 1 1
STA1
STA12
STA 1, 2, 3, 4, 5, 6, 7, 8, 9, 10
Slot-based access period

6. Date: May, 2012
Reference
[1] /606r0, Uplink Channel Access.