July 2014
doc.: IEEE 11-14/0801r0
Envisioning 11ax PHY Structure - Part II
Date: 2014-07-14
Authors:
Name
Affiliations
Address
Dongguk Lim
LG Electronics
Gyeonggi-do, Korea +82-2-69126580
Eunsung Park
LG Electronics
Wookbong Lee
LG Electronics
Jinsoo Choi
LG Electronics
[email protected]
om
[email protected]
m
wookbong.lee@lge.
com
[email protected]
Jinyoung Chun
LG Electronics
[email protected]
HanGyu Cho
LG Electronics
[email protected]
Submission
Phone
Slide 1
email
Dongguk Lim, LG Electronics
July 2014
doc.: IEEE 11-14/0801r0
Introduction
• 802.11ax supports the following scopes in PAR[1]
 Improvement in average throughput per station
 Robustness in outdoor propagation environments
• To satisfy the suggested targets, increased FFT size and proper
cyclic prefix (CP) length are proposed as a new 11ax frame
structure in [2].
• In this contribution, we analyze the impact of increased FFT size
on carrier frequency offset (CFO) and the impact of CP length on
outdoor robustness.
Submission
Slide 2
Dongguk Lim, LG Electronics
July 2014
doc.: IEEE 11-14/0801r0
Issues with PHY change in 11ax
• 4 times larger FFT size than that of 11ac is proposed in 11ax for
both of average throughput enhancement and outdoor robustness
[2][3][4]
• Regarding increased FFT size, there are two issues:
 Issue 1. Carrier Frequency Offset (CFO): Since carrier spacing is reduced
with larger FFT size, we need to check how sensitive it is to CFO.
 Issue 2. CP length: Since symbol duration is increased with larger FFT size,
we need to analyze which CP length is necessary for outdoor robustness.
• This contribution provides answers on the above two issues.
Submission
Slide 3
Dongguk Lim, LG Electronics
July 2014
doc.: IEEE 11-14/0801r0
Issue 1. Carrier Frequency Offset:
Performance with increased FFT (1/3)
• 4 times larger FFT size does not show performance degradation
compared to 11ac
• But, 8 times larger FFT size show significant loss due to higher
sensitivity on CFO.
TGac
B channel with LS channel estimation, data size:100bit, MCS0, CP:0.8us
0
10
0
UMi channel with LS channel estimation, data size:100bit, MCS0, CP:0.8us
10
conv. w/o CFO
conv. w/ CFO & w/ comp.
4times w/o CFO
4times w/ CFO & w/ comp.
8times w/o CFO
8times w/ CFO & w/ comp.
frame error rate
frame error rate
8times w/ CFO & w/comp.
-1
10
-1
10
8times w/ CFO & w/comp.
conv. w/o CFO
conv. w/ CFO & w/ comp.
4times w/o CFO
4times w/ CFO & w/ comp.
8times w/o CFO
8times w/ CFO & w/ comp.
-2
10
-2
0
2
Submission
4
6
8
10
12
SNR [dB]
14
16
18
20
10
Slide 4
0
2
4
6
8
10
12
SNR [dB]
14
16
18
20
Dongguk Lim, LG Electronics
July 2014
doc.: IEEE 11-14/0801r0
Issue 1. Carrier Frequency Offset:
Performance with increased FFT (2/3)
• With 4 times larger FFT size, CFO is compensated well
irrespective of MCS both in indoor and outdoor channel.
TGac B channel with LS channel estimation, 100bit data and CP:0.8us
0
10
frame error rate
frame error rate
UMi channel with LS channel estimation, 100bit data and CP:0.8us
0
10
MCS8
MCS0
-1
10
w/o CFO
w/ CFO & w/o comp.
w/ CFO & w/ comp.
MCS0
MCS8
-1
10
w/o CFO
w/ CFO & w/o comp.
w/ CFO & w/ comp.
-2
10
-2
0
5
Submission
10
15
20
25
SNR [dB]
30
35
40
45
10
0
Slide 5
5
10
15
20
25
SNR [dB]
30
35
40
45
Dongguk Lim, LG Electronics
July 2014
doc.: IEEE 11-14/0801r0
Issue 1. Carrier Frequency Offset:
Performance with increased FFT (3/3)
• With 4 times larger FFT size (78.125kHz subcarrier spacing), CFO
is compensated well up to about 500kHz of CFO
LS channel estimation with 100bit data and 0.8us CP at SNR = 12dB
0
10
frame error rate
TGac B
UMi
-1
10
-2
10
Submission
0
100
200
300
400
500
600
CFO [KHz]
Slide 6
700
800
900
1000
Dongguk Lim, LG Electronics
July 2014
doc.: IEEE 11-14/0801r0
Issue 2. CP length:
Performance with increased FFT (1/2)
• Performance gain of 1/4 CP compared to 1/16 CP
•
It is small in low SNR region but increases in higher SNR region
•
It increases significantly with MCS
UMi channel with 256FFT, 2 OFDM symbol,LS
0
UMa channel with 256FFT, 2 OFDM symbol,LS
0
10
10
MCS7
MCS4
-1
MCS4
-1
10
MCS7
FER
FER
10
MCS0-0.8us
MCS0-3.2us
MCS4-0.8us
MCS4-3.2us
MCS7-0.8us
MCS7-3.2us
-2
10
-5
0
Submission
5
CP
CP
CP
CP
CP
CP
MCS0
10
15
SNR(db)
20
-2
10
25
30
35
-5
Slide 7
MCS0-0.8us
MCS0-3.2us
MCS4-0.8us
MCS4-3.2us
MCS7-0.8us
MCS7-3.2us
0
5
CP
CP
CP
CP
CP
CP
MCS0
10
15
SNR(db)
20
25
30
35
Dongguk Lim, LG Electronics
July 2013
doc.: IEEE 11-14/0801r0
Conclusion
• When 4 times larger FFT is adopted in 11ax,
• We have confirmed that CFO is not a critical issue.
• But, CFO can be critical when 8 times larger FFT is adopted
• We have confirmed that conventional CP length (0.8us) does not
secure the robustness in outdoor.
• Severe performance degradation for higher MCS
Submission
Slide 8
Dongguk Lim, LG Electronics
July 2014
doc.: IEEE 11-14/0801r0
Appendix
Submission
Slide 9
Dongguk Lim, LG Electronics
July 2014
doc.: IEEE 11-14/0801r0
Simulation Assumption for CFO
Conventional
4 times FFT
FFT size
64
256
Used data subcarriers
52
242
Symbol length with CP
4us
13.6 us
CP length
0.8us
0.8us
Center frequency
2.4GHz
Bandwidth
20MHz
Channel model
Tgac B, Umi
Channel Estimation
Least Square
Data size
MCS
Carrier frequency
offset
Submission
100bit
0, 8
±20ppm (uniformly generated within ± 48KHz in 2.4Ghz)
Slide 10
Dongguk Lim, LG Electronics
July 2014
doc.: IEEE 11-14/0801r0
Simulation Assumption for CP length
conventional
CP1 of 4 times FFT
CP2 of 4 times FFT
FFT size
64
256
256
Used data subcarriers
52
242
242
Symbol length with CP
4.0us
13.6us
16.0us
CP length
0.8us
0.8us
3.2us
Center frequency
2.4GHz
Bandwidth
20MHz
Channel model
Channel Estimation
Data bit
MCS
Submission
Tgac B and D, UMi, UMa
Least Square
300bit or 2 symbol
0, 4, 7
Slide 11
Dongguk Lim, LG Electronics
July 2014
doc.: IEEE 11-14/0801r0
References
[1] 11-14/0165-01-0hew-hew-sg-proposed-par
[2] 11-14-xxxx-00-0ax-envisioning-11ax-frame-structure-p
art-I
[3] 11-13-0536-00-0hew-hew-sg-phy-considerations-foroutdoor-environment
[4] 11-13-0843-00-0hew-further-evaluation-on-outdoor-wi
-fi
Submission
Slide 12
Dongguk Lim, LG Electronics