Date: 2010-11-07

1. Date:2010-11-07 DL MU TXOP Power Save Authors:

2. Introduction Not all of the stations in a BSS have frames to receive during downlink Multi-User Transmission Opportunity (MU-TXOP). It is possible for a station to save power when it knows that there are no frames for it during a downlink MU-TXOP. In this presentation, a power saving scheme is proposed which allows stations to save power during downlink MU-TXOP.

3. Basic Idea of Proposal Allow STAs that are not targeted by the AP during a downlink TXOP to enter doze state, and sleep until the end of the TXOP; Group ID and NSTS is used by the STAs to determine whether they are targeted STAs NSTS = 0 indicates no data frames will be transmitted to this STA until the end of TXOP (only when STA is a member of received Group ID) Allow STAs that do not have anymore frames for reception during a downlink MU-TXOP to enter doze state, and sleep until the end of that TXOP. The “More Data” bit in the frame control field and the NSTS field in VHT-SIG-A are used to indicate whether there are data frames at AP for transmission More Data = 0 indicates no more data frames to be transmitted to this STA until the end of this TXOP

4. Details of proposal To implement this idea AP needs to announce in Beacon /Probe Response frames whether or not it supports power saving at STAs during downlink MU-TXOPs. The AP needs to know whether STA is capable and willing to save power during a TXOP TXOP Power Management modes defined. A STA needs to inform AP to switch between TXOP PM modes. One bit in HT Control field or in VHT Control field can be used. At the beginning of a downlink MU-TXOP, AP needs to indicate whether or not STAs are allowed to enter Doze state during that TXOP. Method of indication is TBD. The AP needs to buffer data frames for STAs that entered Doze state until the end of the TXOP. A STA that is in TXOP PS mode needs to know the TXOP duration This is done by checking the Duration field of the RTS frames (requiring RTS/CTS exchange at the beginning of a TXOP).

5. TXOP Power Management modes [No change and hence omitted here] TXOP PM Mode = 0 TXOP PM Mode = 1 TXOP non-PS Mode TXOP PS Mode - Received Group ID indicates not a member of group OR - Received a frame with NSTS = 0 for its position in VHT-SIG A OR - Received a frame with More Data Bit = 0 in MAC header End of downlink MU TXOP (NAV duration) The TXOP Power Management mode is only meaningful within a downlink multi-user TXOP.

6. Illustration Frame 1 (NSTS = 1, 1) Frame 2 (NSTS= 1, 1) Frame 3 (NSTS = 1, 0) Frame 4 (NSTS = 1, 1) STA 1, More Bit = 0 STA3, More Bit = 0 STA 6, More Bit = 0 GID=1 GID =2 GID=3 GID=4 STA 2, More Bit = 1 STA 5, More Bit = 1 STA 2, More Bit = 1 STA2, More Bit = 0 AP STA 1 BA BA BA BA STA 2 BA STA 3 STA 4 BA STA 5 BA STA 6 TXOP Duration

7. MU-TXOP Power save

8. Conclusion The proposed scheme allows stations to save power significantly if they do not have frames to receive during downlink MU-TXOP. For power saving during downlink MU-TXOP, station has to inform AP and enter an additional mode called “TXOP power save” mode. AP begins a downlink MU-TXOP with NAV-set sequence when it allows power saving during that TXOP. AP buffers the frames addressed to stations that are Doze state till the end of downlink MU-TXOP.

9. Pre-motion #1 • Do you support the operation of MU TXOP power save capable STA during downlink MU TXOP for power save as follows and update the spec framework accordingly? • STA saves power till the end of DL MU TXOP after it finds that it is not a member of Group ID received in VHT-SIG-A. • STA saves power till the end of DL MU TXOP after receiving VHT-SIG-A with corresponding NSTS = 0 for its position in Group ID. • STA saves power till the end of DL MU TXOP after sending BA in response to frame with “More Data” bit =0. • Note that support for DL MU TXOP power save is optional at both STA and AP. • Y: • N: • A:

10. Pre-motion #2 • Do you support that if AP chooses to allow MU TXOP power saving as described in Pre-motion #1 for certain downlink MU TXOP, then AP shall include NAV-set sequence at the beginning of that TXOP? • Y: • N: • A:

11. Pre-motion #3 • Do you support to add a TXOP PM modes at STA that is used as mentioned in slide #5 and update the spec framework accordingly? • Y: • N: • A:

12. Pre-motion #4 • Do you support to add a bit that indicates whether or not AP allows STAs in TXOP PS mode to do power save during a downlink MU TXOP as mentioned in Pre-motion #1 and update the spec framework accordingly? • Exact bit to be used is TBD. • Y: • N: • A:

13. Appendix

14. Power save calculations Parameters N = Number of uniform sized frames possible in 3.008 ms = floor(3.008ms/(TFrame+TBA)) TTXOP = TXOP Duration TFrame = Frame duration = 0.5ms, 1ms, 1.5ms TVHT-SIG-A = Time to decode VHT SIG-A = 28µs n = Value between 0 to N, that represents the number of frames in TXOP destined to this STA. TOFF = Duration for which STA is in power save mode TON = Duration for which STA is awake TBA = Time for transmission of corresponding BA during uplink phase = 2 *(28µs + SIFS) As an average value for case 1, duration till second uplink BA frame is considered after which station sleeps TMU_BA = Duration of uplink MU-BA phase = 4 * (28µs + SIFS) + 1*SIFS One extra SIFS used between end of BA and start of next frame Assumptions For ease of evaluation and to have a common framework for comparison Frame length is constant within a TXOP duration Time taken to transition between Awake and Doze state is negligible

15. Power save calculations TTXOP TVHT-SIG-A TMU_BA TFrame Frame N … Data MU-BA Data MU-BA Data MU-BA Data MU-BA Frame 1 Frame 2 Frame n TOFF2 TOFF1 Case 1: More Data bit = 0; Entered Doze state Case 2: GID don’t match OR NSTS = 0; Entered Doze state No. of frames (N) No. of frames (N) Case 1: When More data bit = 0 (1) When GID not matching OR NSTS = 0 When GID not matching OR NSTS = 0 Case 2: (2) TMU_BA = 4 *(BA frame duration)+5*SIFS TBA = Time till the corresponding BA transmission(Here , it is second BA frame in uplink = 2 * (BA frame duration) + 2 * SIFS)