Improved CCA for 80 and 160 MHz BSSs

1. Improved CCA for 80 and 160 MHz BSSs • Authors: • Date: 2010-07-12 Hart et al (Cisco)

2. Outline • Problem Statement • Coexistence mechanisms • Let’s choose a CCA better suited to wider bandwidths • Let’s explore a new coexistence technique: Receiver CCA • Summary Hart et al (Cisco)

3. Problem: WiFi can experience overlapping BSSs (OBSSs) anywhere, any time, on any channel • Three sources of difficulty • WiFi is increasingly used for QoS applications • 80 and 160 MHz BSSs affect, and are affected by, a larger proportion of the available spectrum; e.g. adjacent home/office legacy APs • Personal/mobile usage of WiFi is increasing, driven by smartphone uptake and 3G backhaul, using ad hoc mode, 3G/WiFi bridges (e.g. “MiFi”) and soon WiFi-Direct. • It is harder to locally harmonize: • the Primary channel of mobile devices • the Primary channel of legacy 11a/11n APs with 80/160 MHz APs • Basically, WLANs can experience overlapping BSSs (OBSSs) anywhere, any time, on any channel • Yet customers want reliable very-high throughput • rather than unreliable ultra-high throughput Hart et al (Cisco)

4. 11n coexistence was optimized for a mostly-free secondary channel • Unequal CCA protection by channel • -82/-79 dBm CCA sensitivity for valid 20/40 MHz packets that include the Primary • Only -62 dBm CCA protection on secondary • More hidden nodes on the secondary • No virtual carrier sense on the secondary • A mostly free secondary and tertiary and quaternary etc is much less probable with a mixture of 80/160 MHz, mobile and legacy BSSs • We want to do better in 11ac Hart et al (Cisco)

5. Unequal CCA thresholds introduce unfairness • The default extension of 11n to 11ac is: • -82/-79/-76/-73 dBm CCA sensitivity for valid 20/40/80/160 MHz packets that include the primary • But only -62 dBm per 20 MHz for all other channels • This leads to CCA unfairness for devices on non-Primary channels • A and B are close enough for error-free communications even when colliding with C or D transmissions, yet far enough that the -62 dBm secondary CCA isn’t triggered by C or D. • C and D see A and B and each other on their Primary so defer to everyone using -82 dBm CCA • Full details of 20/40 MHz CCA unfairness in 07/3000r2 • The 20/40 MHz problem is very similar to the 40/80 MHz problem (and 40/160 and 80/160) Hart et al (Cisco)

6. Does CSMA/CA on the Primary channel only approach ALOHA? • Imagine: • Lots of OBSSs, a mixture of legacy and 160 MHz 11ac, on uncoordinated Primary channels with STAs at moderate distances (<-65 dBm) • RTS/CTS and virtual carrier sense respected only on 20 MHz out of 160 MHz • Sensitive CCA only for signals that include the Primary, else -62 dBm CCA • (and -62 dBm is not triggered in this example) • On the non-primary channels, there is not much CS, nor much CA. • Do we approach ALOHA-like efficiency and/or instability? Hart et al (Cisco)

7. Overview of Selected Coexistence Mechanisms • PHY (physical carrier sense aka CCA) • Ensures transmitter doesn’t collide with nearby transmitters • Variants that exclude or include PLCP decoding per channel • May say little about the responder’s environment; no virtual carrier sense • Receiver CCA • Attempt to learn something about responder’s CCA environment via a frame exchange • Variants that exclude or include PLCP decoding per channel • No virtual carrier sense • MAC (virtual carrier sense aka NAV) • All variants require one PPDU decoder per channel • Loses NAV during transmissions, although mitigation techniques exist • In this presentation we only address techniques that do not require multiple PLCP or PPDU decoders • Techniques that involve more than one PLCP or PPDU decoder require careful analysis to determine if the benefit justifies the complexity Hart et al (Cisco)

8. MOSTLY PHY Coexistence Mechanisms • Non-PLCP decoding • Energy Detection, on each channel • Parallel filters and energy detection – low complexity • More false alarms if threshold is below -72 dBm • Preamble Detection, on each channel • Parallel filters and short symbol detectors - low complexity • Blinded while transmitting on a subset of channels • Mid-packet Detection, on each channel • Parallel filters and cyclic extension detectors - low complexity • Resynchronizes quickly even after transmitting on a subset of channels • (PLCP decoding, on each channel) • High complexity Hart et al (Cisco)

9. MOSTLY Example Scheme for Mid-Packet CCA for OFDM • OFDM looks like Gaussian noise yet can be identified by its regular cyclic extension • Obscured by carrier frequency offsets and delay spread • No complicated processing here • Many improvements and/or simplifications are possible Hart et al (Cisco)

10. MOSTLY Two independent sim studies show effectiveness Ch C/E, 1x1 Ch D, 1x1 • See 07/3001r2 (Hart) and 10/0012r0 (Kim) for full results • 10/0012r0 shows that Pmiss is similar to or better than HT-SIG error rate of primary channel • Summary: sensitive non-primary, non-PLCP CCA is feasible Hart et al (Cisco)

11. Problem with duplicated RTS/CTS • Initiator sends duplicated RTS to 40/80/160 MHz responder • 40/80/160 MHz responder detects RTS on primary, and sends duplicated CTS • Without regard to conditions on the non-primary channels – e.g. if one or more are busy • There is actually no collision detection/avoidance on non-primary channels when the responder only considers the primary • The purpose of RTS/CTS is collision detection with hidden nodes • The responder could try to detect the RTS on the non-primary, but has higher implementation complexity, still may not detect collisions, and was not required by 11n Hart et al (Cisco)

12. Receiver CCA Coexistence Mechanism • How it works • TXOP begins with an initFrame/initResponse • Responder does non-PLCP CCA (ED/preamble/mid-pkt) on all non-primary channels • initResponse indicates which channels were clear during the PIFS leading up to the initFrame (e.g. via 3 bits for ch2, ch3-4, ch5-8, each indicating all-clear or any-busy/incapable) • initResponse could be: • A CTS or Ack that includes 3 bits of multichannel CCA busy/free state in Service field or PHY padding • A new control frame, etc • initFrame/initResponse are duplicated packets Responder reports “ch1 & 2 clear; 3 & 4 busy” Responder reports “ch1, 2, 3 & 4 clear” ch1 Dup-initResp Initiator data to responder BA Responder is performing non-PLCP CCA on non-Primary channels ch2 Initiator data to responder … BA Dup-initResponse Dup-initFrame Dup-initFrame ch3 … OBSS data BA ch4 … Hart et al (Cisco)

13. Benefits of Receiver CCA Mechanism • Low PHY complexity • Only requires ED or mid-packet CCA • Low MAC complexity • Integrates well with existing RTS/CTS or initial Data/Ack exchanges • Better than just CCA at the transmitter • Enables RTS/CTS or initial Data/Ack to detect collisions on non-Primary channels • This reduces collisions with OBSS(s) • Especially valuable as the bandwidth of the signal gets wider, since then: • there is more potential for OBSS(s) and • the NAV state of the Primary channel is a much less complete picture. Hart et al (Cisco)

14. Summary • We should do better than 11n • It is feasible to do better than 11n • PLCP decoding on primary • Plus multichannel ED or mid-packet CCA • implementer’s choice • spec just defines a threshold such as TBD dBm for each 20 MHz channel • In order to avoid overlapped transmissions in neighborhood of transmitter • No known, reasonable virtual carrier sense solution exists • Parallel PPDU decoders is a tough requirement • The proposed Receiver CCA mechanism enables detection and avoidance of non-Primary-channel collisions at receiver • Complexity is modest • Enables duplicated RTS/CTS or initial Data/Ack exchanges to perform non-Primary collision detection • Recommended to the group for further study Hart et al (Cisco)

15. Questions? • ? Hart et al (Cisco)

16. Strawpoll Add to the Spec Framework Document a new requirement: An 11ac device shall provide a CCA per 20 MHz channel, for all 20 MHz channels that the device is presently capable of transmitting over. The CCA sensitivity shall be: TBD (<-62) dBm for valid 802.11 signals -62 dBm for any signal. • Y • N • A Hart et al (Cisco)