1. Improved CCA for 80 and 160 MHz BSSs
September 2006
doc.: IEEE /1458r0
July 2010
Improved CCA for 80 and 160 MHz BSSs
Date:
Authors:
Hart et al (Cisco)
Joonsuk Kim, Broadcom Corp.

2. Outline Problem Statement Coexistence mechanisms
September 2006
doc.: IEEE /1458r0
July 2010
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)
Joonsuk Kim, Broadcom Corp.

3. September 2006
doc.: IEEE /1458r0
July 2010
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)
Joonsuk Kim, Broadcom Corp.

4. 11n coexistence was optimized for a mostly-free secondary channel
September 2006
doc.: IEEE /1458r0
July 2010
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)
Joonsuk Kim, Broadcom Corp.

5. Unequal CCA thresholds introduce unfairness
September 2006
doc.: IEEE /1458r0
July 2010
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)
Joonsuk Kim, Broadcom Corp.

6. Does CSMA/CA on the Primary channel only approach ALOHA?
September 2006
doc.: IEEE /1458r0
July 2010
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)
Joonsuk Kim, Broadcom Corp.

7. Overview of Selected Coexistence Mechanisms
September 2006
doc.: IEEE /1458r0
July 2010
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
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)
Joonsuk Kim, Broadcom Corp.

8. PHY Coexistence Mechanisms
September 2006
doc.: IEEE /1458r0
July 2010
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
Hart et al (Cisco)
Joonsuk Kim, Broadcom Corp.

9. Example Scheme for Mid-Packet CCA for OFDM
September 2006
doc.: IEEE /1458r0
July 2010
Example Scheme for Mid-Packet CCA for OFDM
MOSTLY
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)
Joonsuk Kim, Broadcom Corp.

10. Two independent sim studies show effectiveness
September 2006
doc.: IEEE /1458r0
July 2010
Two independent sim studies show effectiveness
MOSTLY
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)
Joonsuk Kim, Broadcom Corp.

11. Problem with duplicated RTS/CTS
September 2006
doc.: IEEE /1458r0
July 2010
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)
Joonsuk Kim, Broadcom Corp.

12. Receiver CCA Coexistence Mechanism
September 2006
doc.: IEEE /1458r0
July 2010
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
ch2
Initiator data to responder …
Responder is performing non-PLCP CCA on non-Primary channels BA
Dup-initFrame
Dup-initFrame
Dup-initResponse
ch3 …
OBSS
data BA
ch4 …
Hart et al (Cisco)
Joonsuk Kim, Broadcom Corp.

13. Benefits of Receiver CCA Mechanism
September 2006
doc.: IEEE /1458r0
July 2010
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)
Joonsuk Kim, Broadcom Corp.

14. Summary July 2010 We should do better than 11n
September 2006
doc.: IEEE /1458r0
July 2010
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)
Joonsuk Kim, Broadcom Corp.

15. ? Questions? July 2010 September 2006 doc.: IEEE 802.11-06/1458r0
Hart et al (Cisco)
Joonsuk Kim, Broadcom Corp.

16. Strawpoll Add to the Spec Framework Document a new requirement:
September 2006
doc.: IEEE /1458r0
July 2010
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
Hart et al (Cisco)
Joonsuk Kim, Broadcom Corp.