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Overlapping BSS Co-Existence

Overlapping BSS Co-Existence

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Overlapping BSS Co-Existence

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  1. Overlapping BSS Co-Existence Authors: Date: 2010-11-8 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 the 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/b/g/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 • 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 • 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 • 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. Reception of PLCP Headers • A copy of PLCP header is present at each 20 MHz channel of the transmission • PLCP Header is received, if any of the transmitted 20 MHz channels is the primary channel of the recipient • PLCP headers contain length, modulation and coding information of the PPDU • The duration of the PPDU may be calculated Jarkko Kneckt (Nokia)

  9. Protecting TXOP Continuation with PLCP • The PLCP headers provide simple protection for acknowledgment transmission and TXOP continuation • Nsts and Group ID fields of VHT-SIG A indicate is the transmission addressed to individual or group address • The channel access after individually addressed PPDU should start after acknowledgment transmission • The channel access after group addressed PPDU should start after PPDU transmission Jarkko Kneckt (Nokia)

  10. PLCP to Protect TXOP Jarkko Kneckt (Nokia)

  11. Single STA Limitations • Single STA is not capable to: • Use virtual carrier sensing when it reserves non-primary channels • Set NAV protection to its transmission when the primary channel of other BSS is not part of the transmission • These operations are possible by using reserving STA • The reserving STA follows orders of the TXOP holder Jarkko Kneckt (Nokia)

  12. Co-existence Challenge 1: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 • The operation is prone to hidden terminal problems • the TXOP holder has no knowledge of the ongoing transmissions at the non-primary channels it is reserving Jarkko Kneckt (Nokia)

  13. 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 changes its primary channel to a channel it is reserving • The reserving STA maintains NAV and PLCP protection for the channels it is reserving • The reserving STA transmits a frame to reserve the channels • At the reporting time, the reserving STA signals success to TXOP holder Jarkko Kneckt (Nokia)

  14. Example • The TXOP holder commands with PMP 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 Jarkko Kneckt (Nokia)

  15. Co-Existence Challenge 2: Partially Overlapping Channels • The operating channels of the BSSs may partially overlap • The primary channels of the 160 MHz mode BSS may be outside the 80 MHz operating channel of other BSS • The NAV or PLCP information are received if they are transmitted to primary channel • The BSS should have means to protect its transmissions at primary channel • Similarly the non-contiguous 160 MHz BSSs should have means to multiplex non-primary 80 MHz channels Jarkko Kneckt (Nokia)

  16. Improved Co-existence: NAV to Partially Overlapping Channels • TXOP holder commands selected STA(s) to send a frame to other BSS primary channel(s) • 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 • The CTSS frame enables BSSs to multiplex the same resources • The NAV information of the reserved channels avoid hidden terminal problems Jarkko Kneckt (Nokia)

  17. Example • Two BSSs are operating in non-contiguous 160 MHz transmission mode • The TXOP holder commands with PMP frame the reserving STA to send TXOP holder channel reservation information to IEEE channels 60 – 63 • The reserving STA transmits CTSS frame to set NAV for channels reserved by the TXOP holder • The BSS2 shall not use IEEE channels 100 – 115 during the period indicated in CTSS Jarkko Kneckt (Nokia)

  18. Thank You Questions, suggestions and comments are welcome. Jarkko Kneckt (Nokia)

  19. Strawpoll • Do you agree with the targets of channels protection as listed below? • The legacy 802.11a/b/g/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? • Yes • No • Abstain Jarkko Kneckt (Nokia)

  20. Pre-Motion • Do you support adding the following text into specification framework document, 11-09/992? • Clause 5.2 Backoff procedure: • When the Group ID and Nsts fields of the VHT-SIG A field of the PLCP header indicate that transmission is addressed to individual address, the channel shall be considered busy for duration of the PPDU, SIFS and ACKDuration(44ys). • Yes • No • Abstain Jarkko Kneckt (Nokia)

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

  22. Strawpoll • Are you in favor of having NAV protection available for partially overlapping channels? • Yes • No • Abstain Jarkko Kneckt (Nokia)