Overlapping BSS Co-Existence

1. Overlapping BSS Co-Existence Authors: Date: 2010-11-11 Jarkko Kneckt (Nokia)

2. Abstract The presentation describes enhancements for overlapping BSS co-existence • The presentation provides guidance how to detect the OBSS and the overlapping channels • The presentation describes rules and related methods for transmissions in overlapping channels Jarkko Kneckt (Nokia)

3. Problem Statement • The 802.11ac will increase number of cases in which the primary and the non-primary channels overlap • Currently 802.11ac has inadequate rules and means to protect a primary channel over a non-primary channel Jarkko Kneckt (Nokia)

4. Targets of the Channels Protection • The legacy 802.11a/n with 20 MHz have NAV protection present in all TXOPs • Protection level of 802.11ac should be at similar level or better even with capacity enhancements • The protection mechanism should be scalable • The protection overhead should be minimized • The maximum protection level is not needed in every scenario • Scalability is w.r.t, the number of the operating channels and the number of OBSSs Jarkko Kneckt (Nokia)

5. Detecting OBSS Background Information • Each device calculates backoff and receives transmissions in its primary channel • The AP receives a 802.11ac PLCP header with unallocated Group ID or Nsts values • The STA receives a 802.11ac PLCP header but cannot decode a frame • The STA receives a frame with a foreign BSSID • Through scanning: • Scanning enables STA to detect BSSs that overlap only at non-primary channel(s) • AP may scan or command associated STA(s) to scan OBSS Jarkko Kneckt (Nokia)

6. OBSSs Examples Background Information • Single 802.11ac BSS in 80 MHz mode has the following possibilities to overlap: • The primary channels of the BSSs are at the same channel • The NAV information is shared between BSSs • The other BSS transmits at the non-primary channels • Legacy 802.11a/n BSS may transmit only in non-primary channels • Non-congested 802.11ac BSS may transmit only in subset, i.e. primary and secondary , channels • The other 802.11ac BSS transmitting at the same operating channels • The other 802.11ac BSS operating in 160 MHz mode • 160 MHz BSS may have its primary channel at separate 80 MHz channel Jarkko Kneckt (Nokia)

7. Primary Channel Selection Rules Background Information • The local BSS or OBSS may change its primary channel • The operating channels of the BSS are derived from the primary channel • AP may change the BSS primary channel to the same primary channel as OBSS and thus NAVs are visible to each other • AP should select the BSS primary channel to avoid overlapping operating channels Jarkko Kneckt (Nokia)

8. Problem Statement • The 802.11ac will increase the number of cases in which the primary and the non-primary channels overlap • Currently 802.11ac has inadequate rules and means to protect a primary channel over a non-primary channel • The enhanced RTS CTS scheme only considers CCA of the transmitter and recipient: • NAV protection of other BSS transmissions at non-primary channels cannot be received Jarkko Kneckt (Nokia)

9. Challenge: Protecting Primary Channel over a Non-primary Channel • The operating channels of the BSSs may partially overlap • The primary channel of the 160 MHz mode BSS may be outside the 80 MHz operating channel of other BSS • The NAV information is received if transmitted in primary channel • The BSS should have means to protect its transmissions at primary channel • The 160 MHz mode BSSs should have means to share non-primary 80 MHz channels Jarkko Kneckt (Nokia)

10. Improved Co-existence: NAV to Protect Primary Channel • TXOP holder commands a selected STA to send a frame(TBD) to other BSS’s primary channel • The primary channel of other BSS doesn’t overlap with channels reserved by TXOP holder • The frame sets NAV for the channels reserved by TXOP holder • Justification: • Enables BSS to protect its primary channel over secondary channels • Avoids hidden terminal problems • Improves the efficiency of the non-primary channels sharing Jarkko Kneckt (Nokia)

11. Example Protecting Primary Channel • The BSS2 operates in 80MHz mode and BSS1 operates in 160 MHz mode • A TXOP holder in BSS2 commands a STA to send a frame to primary channel of BSS1 to protect the ongoing transmission and indicate presence of its primary channel • The recipients shall not use the channels for the duration as indicated in the frame Jarkko Kneckt (Nokia)

12. Example Sharing Non-Primary Channels • Two BSSs are operating in a non-contiguous 160 MHz transmission mode • The TXOP holder commands with a frame the reserving STA to send a frame to primary channel of BSS2 • The recipients shall not use the channels for the duration as indicated in the frame Jarkko Kneckt (Nokia)

13. Challenge:Increasing TXOP Bandwidth • The TXOP holder may test availability of the non-primary channels for a PIFS duration • If energy level of the channels is below the CCA threshold, the TXOP holder may transmit at the channels • In theory this is similar to allowing a STA to issue active scanning after a PIFS • The operation is prone to hidden terminal problems • the TXOP holder has no knowledge of the virtual carrier sensing at the non-primary channels Jarkko Kneckt (Nokia)

14. Improved Co-Existence: Virtual Carrier Sensing by Reserving STA • Reserving STA reserves more non-primary channels • The TXOP holder specifies the reserving STA, the channels to be reserved and the time of reporting • The reserving STA follows the orders of the TXOP holder • The reserving STA temporarily contends at the channels it is reserving • The reserving STA synchronizes to the channel to be reserved • The reserving STA transmits a frame to reserve the channel • At the reporting time, the reserving STA signals success to TXOP holder Jarkko Kneckt (Nokia)

15. Example Increasing TXOP Bandwidth • The TXOP holder commands with a frame the reserving STA to reserve the secondary channel • The reserving STA is capable to reserve the secondary channel and indicates the success at the time reporting time, specified by the TXOP holder • Similar operation is repeated for tertiary and quaternary channels • Please note: • The timing of the OK transmission may be freely decided by AP, i.e. no need to overlap with ACK transmission time • Any reservation mechanism may be used, i.e. not limited by proposal Jarkko Kneckt (Nokia)

16. Summary • The problems are: • Blind entry to the channel • Primary channel transmissions colliding with blind secondary channel transmissions • This presentation provides the solutions to the problems: • Synchronization with the channel to be reserved • Protecting and prioritizing primary channel transmissions over secondary channel transmissions Jarkko Kneckt (Nokia)

17. Strawpoll • Do you agree with the targets of channels protection as listed below? • The Protection level of 802.11ac should be at similar level or better than the legacy 802.11a/b/g/n even with capacity enhancements • The protection mechanism should be scalable • The protection overhead should be minimized • The maximum protection level is not needed in every scenario • Scalability is w.r.t, the number of the operating channels and the number of OBSSs? • Yes • No Jarkko Kneckt (Nokia)

18. Strawpoll • Are you in favor of having NAV protection available as described in slides 10 -12 in this presentation? • Yes • No Jarkko Kneckt (Nokia)

19. Strawpoll • Are you in favor of using reserving STA to enable better co-existence? • Yes • No Jarkko Kneckt (Nokia)