Presentation on theme: "Doc.: IEEE 802.11-10/0744r1 Submission July 2010 Hart et al (Cisco)Slide 1 Improved CCA for 80 and 160 MHz BSSs Date: 2010-07-12 Authors:"— Presentation transcript:
doc.: IEEE /0744r1 Submission July 2010 Hart et al (Cisco)Slide 1 Improved CCA for 80 and 160 MHz BSSs Date: Authors:
doc.: IEEE /0744r1 Submission July 2010 Hart et al (Cisco)Slide 2 Outline Problem Statement Coexistence mechanisms Lets choose a CCA better suited to wider bandwidths Lets explore a new coexistence technique: Receiver CCA Summary
doc.: IEEE /0744r1 Submission July 2010 Hart et al (Cisco)Slide 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
doc.: IEEE /0744r1 Submission July 2010 Hart et al (Cisco)Slide 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
doc.: IEEE /0744r1 Submission July 2010 Hart et al (Cisco)Slide 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 isnt 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 07/3000r207/3000r2 The 20/40 MHz problem is very similar to the 40/80 MHz problem (and 40/160 and 80/160)
doc.: IEEE /0744r1 Submission July 2010 Hart et al (Cisco)Slide 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?
doc.: IEEE /0744r1 Submission July 2010 Hart et al (Cisco)Slide 7 Overview of Selected Coexistence Mechanisms PHY (physical carrier sense aka CCA) Ensures transmitter doesnt collide with nearby transmitters Variants that exclude or include PLCP decoding per channel May say little about the responders environment; no virtual carrier sense Receiver CCA Attempt to learn something about responders 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
doc.: IEEE /0744r1 Submission July 2010 Hart et al (Cisco)Slide 8 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 MOSTLY
doc.: IEEE /0744r1 Submission July 2010 Hart et al (Cisco)Slide 9 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 MOSTLY
doc.: IEEE /0744r1 Submission July 2010 Hart et al (Cisco)Slide 10 Two independent sim studies show effectiveness See 07/3001r2 (Hart) and 10/0012r0 (Kim) for full results07/3001r210/0012r0 10/0012r0 shows that P miss is similar to or better than HT-SIG error rate of primary channel Summary: sensitive non-primary, non-PLCP CCA is feasible Ch D, 1x1Ch C/E, 1x1 MOSTLY
doc.: IEEE /0744r1 Submission July 2010 Hart et al (Cisco)Slide 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
doc.: IEEE /0744r1 Submission July 2010 Hart et al (Cisco)Slide 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 ch1 ch3 ch2 ch4 Initiator data to responder OBSS data Responder reports ch1 & 2 clear; 3 & 4 busy Initiator data to responder Responder is performing non- PLCP CCA on non-Primary channels BABA BABA Dup-initResp Dup-initResponse Responder reports ch1, 2, 3 & 4 clear BABA Dup-initFrame … … …
doc.: IEEE /0744r1 Submission July 2010 Hart et al (Cisco)Slide 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.
doc.: IEEE /0744r1 Submission July 2010 Hart et al (Cisco)Slide 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 –implementers 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
doc.: IEEE /0744r1 Submission July 2010 Hart et al (Cisco)Slide 15 Questions? ?
doc.: IEEE /0744r1 Submission July 2010 Hart et al (Cisco)Slide 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 signals -62 dBm for any signal. Y15 N0 A21