1. Power Efficient PS Poll
February 2008
doc.: IEEE /2376r0
Date: May, 2012
Power Efficient PS Poll
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.
AP can assign a time slot for a STA.
Each STA starts to access the channel at the assigned time slot following EDCA.
Issues:
If a STA in the BSS can not receives the Beacon frames, It may still create hidden node problem.
OBSS STAs can collide with the slot transmission.
PS-Poll creates overhead.
TIM bitmap
Total N time slots, each time slot duration = T 1 1 1 … 1 1
STA1
STA10
STA 1, 2, 3, 4, 5, 6, 7, 8, 9, 10

3. Recap of 802.11 Power Save Medium Access
Date: May, 2012
Recap of Power Save Medium Access
Medium access with trigger frame: before trigger frame (PS-Poll, Data frame), a power save STA may be asleep
PS-Poll based power save in CP.
Unscheduled APSD.
Beacon
Data
Beacon PS
Poll
ACK
Awake
Doze
Doze
Awake
Doze
Beacon
ACK
Data
Data
Data
ACK
ACK
Awake
Doze
Awake
Doze
Doze

4. Recap of 802.11 Power Save Medium Access (Cont’d)
Date: May, 2012
Recap of Power Save Medium Access (Cont’d)
Medium access without trigger frame: a power save STA wakes up at the time that is known to both the AP and the STA.
CFP Power Save: A CFP-capable STA in power save mode wakes up at the start time of a CFP if the bit in TIM matching it is 1. No PS-Poll is required before the AP transmits frame to it during the CFP.
S-APSD: A S-APSD STA wakes up at the start time of its scheduled SP. No trigger frame is required before the AP transmits frame to it during the scheduled SP.
DTIM group frame transmission.
Beacon
Data
ACK
Awake (CFP)
Beacon
Data
Data
Data
ACK
ACK
ACK
Awake (SP of S-APSD)
Awake
Doze
Doze

5. Slot-Based Power Save Without PS-Poll
Date: May, 2012
Slot-Based Power Save Without PS-Poll
When the AP knows the which slot or which slots that a STA uses, the AP knows that the STAs will wake up at the slot/slots.
The AP can transmit the buffer frame to it without using Ps-Poll.
By this improvement:
An AP can select to send a CTS-to-self with the Duration value covering whole Slots. Only one backoff before CTS-to-Self is required.
A OBSS STA will set its NAV after receiving the CTS-to-Self. Collision from OBSS can be avoided.
A STA in the BSS which does not receive the beacon will set its NAV after receiving the CTS-to-Self.
When no cts_to_self is used, the AP starts to access the channel at the each time slot following EDCA.
The AP can select one backoff before the first time slot.
Each slot can be shorter.
TIM bitmap
Total N time slots, each time slot duration = T 1 1 1 … 1 1
STA1
STA10
STA 1, 2, 3, 4, 5, 6, 7, 8, 9, 10

6. Slot-Based Power Save Without PS-Poll (Cont’d)
Date: May, 2012
Slot-Based Power Save Without PS-Poll (Cont’d)
A new TIM action frame is defined which includes TIM IE.
When some STAs in power save mode do not receive frames correctly.
After finishing the total time slot access, the AP may transmit TIM frame to start another slot based access.
Beacon
TIM bitmap
TIM frame 1 1 1 … 1 1
STA1
STA10
STA 1, 2, 3, 4, 5, 6, 7, 8, 9, 10
STA 1, 3, 6, 8
Total N=10 time slots, each time slot duration = T
The AP does not receives ACKs from STA 1, 3, 6, 8.

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