1. doc.: IEEE /0190r0
March 2005
March 2005
Performance Analysis on 20/40 MHz coex. methods taking in legacy in BSS
Date:
Authors:
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Tomoko Adachi, Toshiba Corporation
Tomoko Adachi, Toshiba Corporation

2. March 2005
Abstract
Simulations are performed under the condition that there are associated legacy STAs in addition to 40MHz HT STAs and OBSS in the extension channel.
Simulation results are provided to compare the proposed 20/40 MHz coexistence methods.
Results show the effectiveness of the TGn Sync 20 MHz-base method.
Tomoko Adachi, Toshiba Corporation

3. March 2005
20/40 MHz coexistence
There will be cases when a HT AP takes in 20MHz legacy STAs in its BSS.
Unmanaged or un-scheduled CSMA/CA operation has the following issues:
20 MHz legacy STAs may prevent 40 MHz capable HT STAs from transmitting 40 MHz data frames.
Channel switching is required every time when the bandwidth capability of destination STAs changes.
Tomoko Adachi, Toshiba Corporation

4. Approach for 20-base managed mixed operation
March 2005
Approach for 20-base managed mixed operation
The first goal is to reduce the probability of 20MHz STAs (legacy and HT) and 40MHz STAs contending for the medium simultaneously.
The second goal is to remove inefficiency in channel switching.
The 20-base managed mixed mode provides well-scheduled traffic distribution and efficiency in channel switching.
Tomoko Adachi, Toshiba Corporation

5. TGn Sync 20 MHz-base managed mixed operation
March 2005
TGn Sync 20 MHz-base managed mixed operation
AP divides one Beacon interval into 40MHz period and 20MHz period according to the traffic conditions.
40 MHz capable HT STAs have less probability contending with 20 MHz legacy STAs.
40 MHz period
20 MHz period
ch_a (control)
Carrier
Sense
(CS)
Bcn/
ICB
CF-
End
40 MHz frame
exchange
DCB
CF-
End
Bcn/
ICB
Ch.
Sw.
20 MHz
frame
exchange t
CTS
self
/Bcn
CF-
End
Ch.
Sw.
Busy CS t
ch_b (extension)
NAV
ch_a
NAV
NAV
ch_b
NAV
NAV
ch_a+ch_b
NAV
NAV
Tomoko Adachi, Toshiba Corporation

6. March 2005
WWiSE Operation -1-
Legacy STAs prevent from transmitting 40 MHz frame by 40 MHz HT STAs.
40 MHz HT STAs have higher probability contending with Legacy STAs.
Delay in both 40 MHz and 20 MHz transmission will increase.
ch_a (primary)
Carrier
Sense
(CS)
20 MHz frame
transmission
20 MHz frame
transmission
40 MHz
frame
transmission CS CS t CS t
ch_b (secondary)
NAV
ch_a
NAV
NAV
NAV
ch_b
NAV
ch_a+ch_b
NAV
Tomoko Adachi, Toshiba Corporation

7. March 2005
WWiSE Operation -2-
Channel switching from 20 MHz to 40 MHz, vice versa are required every time when the bandwidth of destination STA is different from that of 40 MHz STA.
Frequent channel switching will degrade efficiency in using bandwidth.
ch_a (primary)
Carrier
Sense
(CS)
40 MHz
frame
transmission
20 MHz frame
transmission
Ch.
Sw.
40 MHz
frame
transmission CS CS t CS
Ch.
Sw. CS t
ch_b (secondary)
NAV
ch_a
NAV
NAV
ch_b
NAV
ch_a+ch_b
NAV
NAV
Tomoko Adachi, Toshiba Corporation

8. Performance evaluation
March 2005
Performance evaluation
Four kinds of simulations are performed to evaluate the performance of 20/40 MHz coexistence methods.
STA type in target BSS
Legacy transmission by 40 MHz HT STA
OBSS in the extension channel
Purpose
40 MHz HT STA
Legacy
Case 1
2 STAs No
Evaluate the impact of 20 MHz legacy transmission against 40 MHz HT transmission in terms of throughput.
Case 2
Yes
Examine the effect of channel switching in WWiSE method in addition to conditions in Case 1.
Case 3
Varied
Evaluate the effect of increase in the number of legacy STAs.
Case 4
Evaluate the effect of increase in the number of legacy STAs under the circumstance where OBSS exists.
Tomoko Adachi, Toshiba Corporation

9. Common parameters March 2005 Beacon interval 100 TU
Number of aggregated frames 16
MPDU size
1500 bytes
Transmission rate in 40 MHz capable HT STA
216 Mbps
Transmission rate in legacy STA (for Beacon, control frames for 20-base, and OBSS)
54 Mbps
Channel switching time
1 ms
CWmin 15
AIFS
SIFS + 3*slottime
Maximum retry count 4
Tomoko Adachi, Toshiba Corporation

10. Simulation parameters --- Case 1---
March 2005
Simulation parameters --- Case 1---
Number of 40 MHz HT STAs
2 (including 1 AP)
Number of legacy STAs 2
Offered load on 40 MHz capable HT STA (only sent in 40 MHz)
30 Mbps
Offered load on legacy STA
10 Mbps
Ratio of 40 MHz period and 20 MHz period (20-base)
Varied
Tomoko Adachi, Toshiba Corporation

11. Network topology --- Case 1 ---
March 2005
Network topology --- Case 1 ---
Legacy STA
(-7,7)
Target BSS
Offered load
10 Mbps
40 MHz AP
(0,0)
Legacy STA
(5,0)
Offered load
30 Mbps
40 MHz HT STA
(0,-5)
40MHz capable HT STA
Legacy STA
This topology is a portion taken from SS1 in TGn UM.
Tomoko Adachi, Toshiba Corporation

12. Simulation results --- Case 1---
March 2005
Simulation results --- Case 1---
Throughput depends on the ratio between 40 MHz and 20 MHz periods
HT throughput of WWiSE degrades compared with that of Sync 20-base because of contention among HT and legacy STAs.
By the selection of the ratio between 40 MHz and 20 MHz periods, high throughput can be achieved not only for 40 MHz but also for 20 MHz.
Tomoko Adachi, Toshiba Corporation

13. Simulation parameters --- Case 2---
March 2005
Simulation parameters --- Case 2---
Number of 40 MHz HT STAs
2 (including 1 AP)
Number of legacy STAs 2
Offered load on 40 MHz capable HT STA for 40 MHz traffic
30 Mbps
Offered load on legacy STA
10 Mbps
Offered load on 40 MHz capable HT STA for legacy traffic
Ratio of 40 MHz period and 20 MHz period (TGn Sync)
Varied
Channel switching time
1 ms
Tomoko Adachi, Toshiba Corporation

14. Network topology --- Case 2 ---
March 2005
Network topology --- Case 2 ---
Legacy STA
(-7,7)
Target BSS
Offered load
10 Mbps
40 MHz AP
(0,0)
Legacy STA
(5,0)
Offered load
30 Mbps
40 MHz HT STA
(0,-5)
40MHz capable HT STA
This topology is a portion taken from SS1 in TGn UM.
Legacy STA
Tomoko Adachi, Toshiba Corporation

15. Simulation results --- Case 2---
March 2005
Simulation results --- Case 2---
Throughput depending on the ratio between 40 MHz and 20 MHz periods in the case that 40 MHz capable HT STAs exchange frames with legacy STAs.
Both 40 MHz HT and 20 MHz legacy throughput in WWiSE lower because of frequent channel switching between 40 MHz and 20 MHz at 40 MHz HT STAs.
Required throughput in 40 MHz HT transmission still achieved while 20 MHz legacy throughput gained by the optimal scheduling in Sync 20-base.
Tomoko Adachi, Toshiba Corporation

16. Simulation parameters --- case 3---
March 2005
Simulation parameters --- case 3---
Number of 40 MHz HT STAs
2 (including 1 AP)
Number of legacy STAs
Varied
Offered load on 40 MHz capable HT STA for 40 MHz traffic
30 Mbps
Offered load on legacy STA
2 Mbps
Offered load on 40 MHz capable HT STA for legacy traffic
Ratio of 40 MHz period and 20 MHz period (TGn Sync)
6:4
Tomoko Adachi, Toshiba Corporation

17. Network topology --- Case 3 ---
March 2005
Network topology --- Case 3 ---
40MHz capable HT STA
Legacy STA
1.5 m
40MHz AP
(-0.5, 0)
40MHz HT STA
(0.5, 0)
Legacy STA
( , )
The number of legacy STAs
increased from 0 to 16.
i : index of legacy STA
Nlegacy: number of legacy STAs
Target BSS
Tomoko Adachi, Toshiba Corporation

18. Simulation results --- Case 3---
March 2005
Simulation results --- Case 3---
Throughputs and transmission delay depending on the number of 20 MHz legacy STAs in the control channel
Throughput vs. Number of 20 MHz legacy STAs in the target BSS
Delay in 40 MHz HT transmission vs.
Number of 20 MHz legacy STAs
in the target BSS
Sync 20-base can achieve high throughput in 40 MHz HT transmission against the increasing number of 20 MHz legacy STAs in the control channel.
Less delay in Sync 20-base method, whereas delay in WWiSE increases in proportion to the number of legacy STAs.
Tomoko Adachi, Toshiba Corporation

19. Simulation parameters --- case 4---
March 2005
Simulation parameters --- case 4---
Number of 40 MHz HT STAs
2 (including 1 AP)
Number of legacy STAs in the target BSS
Varied
Number of legacy STAs in the extension channel 2
Offered load on 40 capable MHz HT STA for 40 MHz traffic
30 Mbps
Offered load on legacy STA in the target BSS
2 Mbps
Offered load on 40 MHz capable HT STA for legacy traffic
Offered load on legacy STAs in the extension channel
5 Mbps
Ratio of 40 MHz period and 20 MHz period (TGn Sync)
6:4
Distance between target BSS and OBSS
20 m
Tomoko Adachi, Toshiba Corporation

20. Network topology --- Case 4 ---
March 2005
Network topology --- Case 4 ---
40MHz capable HT STA
Legacy STA
Legacy STA in OBSS
20 m
40MHz AP
(-0.5, 0)
40MHz HT STA
(0.5, 0)
Legacy STA
(19.5, 0)
1 m
Legacy STA
(20.5, 0)
Legacy STA
( , )
i : index of legacy STA
Nlegacy: number of legacy STAs
OBSS in the extension channel
The number of legacy STAs in the control
channel increased from 0 to 16.
Target BSS in the control channel
Tomoko Adachi, Toshiba Corporation

21. Simulation results --- Case 4---
March 2005
Simulation results --- Case 4---
Throughput and transmission delay depending on the number of 20 MHz legacy STAs in the target BSS under the circumstance where OBSS exists in the extension channel.
Throughput vs. Number of 20 MHz legacy STAs in the target BSS
Delay in 40 MHz HT transmission vs.
Number of 20 MHz legacy STAs
in the target BSS
Sync 20-base can provide higher performance under the circumstance where there is OBSS in the extension channel in terms of 40 MHz throughput and transmission delay.
Tomoko Adachi, Toshiba Corporation

22. March 2005
Conclusion
TGn Sync 20 MHz-base method lowers the probability of collision with 20 MHz legacy STAs in the control channel resulting in higher throughput in 40MHz communication compared with the WWiSE method.
High throughput in 40 MHz transmission is achieved; besides 20 MHz legacy throughput is gained by optimal scheduling in TGn Sync 20 MHz-base method.
Frequent channel switching suppressed in TGn Sync 20 MHz-base method leads to higher throughput for 40 MHz capable HT STAs when they also transmit in 20 MHz.
TGn Sync 20 MHz-base method can provide high throughput in 40 MHz transmission in the environment that 20 MHz legacy STAs exist in both control and extension channels.
Tomoko Adachi, Toshiba Corporation

23. References 11-05-1633-00 Comparison of 20/40 MHz coexistence methods
doc.: IEEE /0190r0
March 2005
March 2005
References
Comparison of 20/40 MHz coexistence methods
Tomoko Adachi, Toshiba Corporation
Tomoko Adachi, Toshiba Corporation