1. CCA Threshold Levels Date: 2011-01-17 Authors: January 2011 Month Year
doc.: IEEE yy/xxxxr0
January 2011
CCA Threshold Levels
Date:
Authors:
Ron Porat
Broadcom
16340 West Bernardo Dr., San Diego, CA 92127
Vinko Erceg
Robert Stacey
Intel
Eldad Perahia
Ning Zhang
Atheros
1700 Technology Drive, San Jose, CA 95110
Youhan Kim
Aheros
Brian Hart
Cisco
Reza Hedayat
Hongyuan Zhang
Marvell
5488 Marvell Ln,
Santa Clara, CA 95054
Raja Banerjea
Hemanth Sampath
Qualcomm
San Diego
VK Jones
Yongho Seok
LG Electronics, Inc.
533, Hogye-1dong, Dongan-Gu, Anyang-Shi, Kyungki-Do, ,
Daewon Lee
LG Electronics, Inc
Minho Cheong
ETRI
161 Gajeong-dong, Yuseong-gu,
Jae Seung Lee
Ron Porat, Broadcom
John Doe, Some Company

2. Authors: January 2011 Ron Porat, Broadcom Youngsoo Kim
Samsung Electronics
Mt Nongseo-Ri, Giheung-Eup,
Yongin-Si, Gyeonggi-Do, Korea
Chunhui (Allan) Zhu
Samsung
Electronics
75 W Plumeria Dr.
San Jose CA 95134
USA
Vish Ponnampalam
MediaTek
Sean Coffey
Realtek
George Vlantis
ST-Ericsson
Edward Au
Huawei
Bo Sun
ZTE
Matthew Fischer
Broadcom
Joshua Zhao
Atheros
1700 Technology Drive, San Jose, CA 95110
Kai Shi
Ron Porat, Broadcom

3. January 2011
Summary
We present system simulation results to determine and propose the CCA levels on secondary 20/40/80MHz channels
Two sets of different simulations are presented
The first set assumes that all STAs are sharing same bandwidth
The second set assumes that STAs share a mix of 40MHz and 80MHz bandwidths
Slide 3
Ron Porat, Broadcom

4. System Simulation Parameters – First Set
January 2011
System Simulation Parameters – First Set
N APs and M STA per AP are dropped in an area of size SxS ft (numbers shown in plots)
APs are placed regularly with 5ft std (results with 15ft are similar)
STAs are associated with the closest AP according to path loss (including random shadowing)
One valid transmission per BSS is assumed
BSSs are chosen randomly that meet CCA rules (50% probability to choose an AP as a transmitter)
After all transmitters were chosen, SINR is calculated at each receiver and mapped to MCS
SISO links over one 40MHz channel are assumed with 15dBm transmit power
CCA levels are varied from -90dBm to -50dBm (-90dBm just above noise floor for 40MHz)
Simulation uses 50 drops and 250 TXOP per drop
All plots show average and five percentile per-link throughput (Tput) , average and five percentile sum Tput in TXOP, and the number of concurrent transmissions
Slide 4
Ron Porat, Broadcom

5. A note on Performance with Multiple Frequencies
January 2011
A note on Performance with Multiple Frequencies
The results in this simulation assume that all BSS use the same frequency representing a worst case scenario.
If multiple frequencies are used, adjacent BSS will not cause as much interference to each other since they will be operating on different frequencies.
BSS that share the same frequency will be farther apart and will not suffer as much from increased CCA levels.
Slide 5
Ron Porat, Broadcom

6. Result 1 – very low density (400ft x 400ft, 4 APs)
January 2011
Result 1 – very low density (400ft x 400ft, 4 APs)
Due to the large distance between AP the individual links Tput is low. CCA levels above -75dBm achieve most of the network throughput
Optimal point maximizing network throughput at -65dBm
Optimal point maximizing per-link Tput at noise level
Slide 6
Ron Porat, Broadcom

7. Result 2 – low density (200ft x 200ft, 4 APs)
January 2011
Result 2 – low density (200ft x 200ft, 4 APs)
Similar trends seen with an optimal point maximizing network throughput at -60dBm
Optimal point maximizing per-link Tput is at approximately -85dBm
Slide 7
Ron Porat, Broadcom

8. Result 3 – high density I (200ft x 200ft, 9 APs)
January 2011
Result 3 – high density I (200ft x 200ft, 9 APs)
Optimal point maximizing network throughput at -50dBm
Optimal point maximizing per-link Tput at noise level
Slide 8
Ron Porat, Broadcom

9. Result 4 – high density II (300ft x 300ft, 16 APs)
January 2011
Result 4 – high density II (300ft x 300ft, 16 APs)
High number of AP and clients enables many more transmitters to operate concurrently and maximum network throughput is achieved at CCA level -55dBm
Optimal point maximizing per-link Tput at noise level
Slide 9
Ron Porat, Broadcom

10. January 2011
Result 4 – cont.
In each drop we sum the number of times each BSS had a transmission (out of 250 x 50 TXOP) and plot the ratio relative to the average of al BSS
Low CCA level cause unfairness for inner BSS
Slide 10
Ron Porat, Broadcom

11. System Simulation Parameters – Second Set
January 2011
System Simulation Parameters – Second Set
System-level simulations that include all the MAC behaviors would provide more insight on the interplay of PHY and MAC
VHT STA receiver with 80 MHz operating bandwidth shall provide CCA on all 20 MHz channels constituting the 80 MHz channel.
In the following slides the CCA choices for secondary 40 MHz is investigated, and would expect equivalent CCA values to be extended to secondary 80 MHz channel
CCA threshold -79dBm
Primary 40 MHz
CCA threshold TBD
Secondary 40 MHz
Ron Porat, Broadcom

12. System Simulation Parameters – Second Set: Cont
January 2011
System Simulation Parameters – Second Set: Cont
UDP traffic, packet size of 1500 bytes
CWmin=7, CWmax=63
Data rate: 3/4, 64QAM
Control rate: 1/2, 16QAM
TXOP limit: 3 ms
Bandwidth 40 MHz: 108/6 data/pilots tones
Bandwidth 80 MHz: 234/8 data/pilot tones
Ron Porat, Broadcom

13. Case I January 2011 BSS40MHz (blue) BSS80MHz (red) BW=40MHz
Channel 44(P)-48
BSS80MHz (red)
BW=80MHz
Channel 36(P)-48
APs distance ~22 m
Ron Porat, Broadcom

14. Case I January 2011 -79dBm on Secondary 40MHz APs distance ~10 m
Ron Porat, Broadcom

15. Case II January 2011 BSS40MHz (blue) BSS80MHz (red) BSS40MHz (green)
BW=40MHz
Channel 36(P)-40
BSS80MHz (red)
BW=80Mhz
Channel 36(P)-48
BSS40MHz (green)
Channel 44(P)-48
Blue and green curves are same
APs Distance ~20-30 m
Ron Porat, Broadcom

16. Case II January 2011 -79dBm on Secondary 40MHz APs Distance ~8 m
Ron Porat, Broadcom

17. Conclusions from the Results
January 2011
Conclusions from the Results
Optimal CCA levels may depend on the density of the networks and the achievable per-link throughput without interference (which is a function of the MIMO configuration, transmit power and path loss)
Higher CCA levels are preferable to maximize network throughput
Lower CCA levels are preferable to maximize single link throughput
Fairness:
High CCA levels
May result in some STAs having low per-link throughput
Link Adaptation may be important
Low CCA levels
Shut down adjacent BSS for several TXOPs even though AP and clients in that BSS have high SNR
Impact QoS since delay in channel access is increased
Fairness with mixed bandwidths needs to be taken into account
Slide 17
Ron Porat, Broadcom

18. Conclusions from the Results: Cont
January 2011
Conclusions from the Results: Cont
Dynamic selection of CCA levels may be optimal but more work may be needed to understand how to choose the levels
Assuming one CCA level is chosen we think a reasonable compromise is to adopt CCA levels in the middle of the range at -72 dBm for 40MHz BW adjusted proportionally for different BWs.
However, as shown in the next slide, for the case of 20 MHz secondary channels we propose a higher level of -72dBm to improve fairness with n
Note that the proposed CCA levels are dB more stringent than the n levels
Slide 18
Ron Porat, Broadcom

19. HT and VHT STAs in Adjacent 40MHz Channels
Month Year
doc.: IEEE yy/xxxxr0
January 2011
HT and VHT STAs in Adjacent 40MHz Channels
WiFi devices operate with an average of 25dB adjacent channel leakage
Large discrepancy in CCA level for secondary20 between 11n and 11ac may lead to unfairness between HT and VHT STAs in adjacent channels using the same bandwidth in some cases
11n has -62 dBm CCA sensitivity for the secondary20
There is extra incentive to reduce the discrepancy in CCA level between 11n and 11ac for secondary20
VHT STA in pink channel, HT STA in blue channel
VHT STA transmitting
HT STA transmitting
40 MHz
40 MHz
40 MHz
40 MHz P S P S
-62 dBm
-82 dBm
Secondary20 of HT40 STA is IDLE
Secondary20 of VHT40 STA is BUSY
Ron Porat, Broadcom
John Doe, Some Company

20. January 2011
Pre motion #1
Move to accept the following CCA levels on the secondary channels:
20MHz: dBm
40MHz: dBm
80MHz : dBm
Slide 20
Ron Porat, Broadcom