doc.: IEEE 802.11-10/0771r0 Submission Phase Tracking During VHT-LTF Date: 2010-07-10 Youhan Kim, et al. Slide 1 N am e A ffiliations A ddress Phone em ail K aiShi A theros 5480 G reatA m erica Pkw y Santa C lara, CA 95054,U SA +1-408-720-5574 [email protected]Y ouhan K im A theros 5480 G reatA m erica Pkw y Santa C lara, CA 95054,U SA +1-408-830-5835 youhan.kim@ atheros.com N ing Zhang A theros 5480 G reatA m erica Pkw y Santa C lara, CA 95054,U SA +1-408-773-5363 [email protected]Richard van N ee Qualcom m Straatw eg 66S, Breukelen, The Netherlands +31-346-259-650 rvannee@ qualcomm .com A llertvan Zelst Qualcom m Straatw eg 66S, Breukelen, The Netherlands +31-346-259-663 allert@ qualcomm.com H em anth Sam path Qualcom m 5775 M orehouseD rive, San D iego, CA , 92121, U SA hsam path@ qualcomm .com V K Jones Qualcom m 3105 K iferRoad, Santa C lara, CA , 95051,U SA vkjones@ qualcomm.com V inko Erceg Broadcom 16340 W Bernardo D r; San D iego CA 92127,U SA verceg@ broadcom.com Y oungsoo K im Sam sung M t. 14-1 N ongseo-Ri, G iheung-Eup Yongin-Si, G yeonggi-Do, Korea449-712 +82-31-280-9614 kimyoungsoo@ samsung.com Eldad Perahia Intel 2111 N E 25 th A ve H illsboro, O R 97124, U SA [email protected]Raja Banerjea M arvell 5488 M arvellLane Santa C lara, C A 95054,U SA rajab@ marvell.com Authors: July 2010
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Doc.: IEEE 802.11-10/0771r0 Submission Phase Tracking During VHT-LTF Date: 2010-07-10 Youhan Kim, et al.Slide 1 Authors: July 2010.
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doc.: IEEE 802.11-10/0771r0
Submission
Phase Tracking During VHT-LTFDate: 2010-07-10
Youhan Kim, et al.Slide 1
Name Affiliations Address Phone email Kai Shi Atheros 5480 Great America Pkwy
• Carrier frequency offset causes EVM degradation at RX– Carrier frequency offset estimation error due to phase noise– Carrier frequency drift
• 11a/n has pilot tones in data symbols to track phase per symbol– Compensate residual frequency offset error and phase noise– But no pilot tones in HT-LTF
• No phase tracking during HT-LTF
• 11ac supports max. 8 spatial streams (c.f. 4 in 11n)– Much longer VHT-LTF (e.g. 8 VHT-LTF symbols)
• More susceptible to phase rotations– Simulation results show significant channel estimation performance
degradation w/o phase tracking during VHT-LTF• 11ac requires higher channel estimation quality and EVM
– Higher order MIMO, 256-QAM, DL MU-MIMO
July 2010
Youhan Kim, et al.Slide 2
doc.: IEEE 802.11-10/0771r0
Submission
Initial Carrier Frequency Offset Estimation Accuracy
• Frequency offset estimation usingL-LTF
• 4x4, NLOS B, HT40
• SNR = 40 dB
• Carrier frequency = 5 GHz
• IEEE phase noise (both at TX and RX)
July 2010
Youhan Kim, et al.Slide 3
-0.2 -0.1 0 0.1 0.2 0.30
20
40
60
80
100
120
140IPN=-41dBc
ppm error
# o
f ev
ents
-0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.40
20
40
60
80
100
120
140IPN=-36dBc
# o
f ev
ents
ppm error
IPN: -41 dBc
IPN: -36 dBc
Integrated Phase Noise (IPN)
Standard Deviation of freq. offset estimation
error [ppm]
-41 dBc 0.074
-36 dBc 0.13
doc.: IEEE 802.11-10/0771r0
Submission
Carrier Frequency Drift
• TX carrier frequency may drift during a packet due to various reasons– Supply voltage change due to various circuits (e.g. PA) being
turned on
– Temperature change
– Etc.
• WLAN is going into all types of systems– Very little control over quality of reference crystal, etc.
• Pilot tones in data symbols allow tracking carrier frequency drift– Need similar mechanism to track drift during VHT-LTF
July 2010
Youhan Kim, et al.Slide 4
doc.: IEEE 802.11-10/0771r0
Submission
Impact on Channel Estimation
• Significant degradation in channel estimation performance observed due to residual carrier frequency offset for high order MIMO transmissions
July 2010
Youhan Kim, et al.Slide 5
MIMO dimension: Ntx x Nrx 1x1 4x4 8x8
Number of VHT-LTF symbols: 1 4 8
AverageChannel
EstimationSNR (dB)
Residual freq. offset =250Hz(0.05ppm @ 5 GHz)
36.8 35.4 33.9
Residual freq. offset =500Hz(0.1ppm @ 5 GHz)
36.6 33.6 29.9
Residual freq. offset =1kHz(0.2ppm @ 5 GHz)
36.0 29.7 24.5
• AWGN channel• -41 dBc integrated phase noise at both TX and RX
doc.: IEEE 802.11-10/0771r0
Submission
Proposed Solution• Insert pilot tones in VHT-LTF symbols
– Pilot tone locations identical to those in data symbols– The pilot tones shall use the element of the VHT-LTF sequence corresponding to that tone index– Identical pilot values for all space-time streams
• All tones in VHT-LTF symbols, except pilot tones, are multiplied by the PVHTLTF matrix (VHT-LTF mapping matrix) as in 11n
• Pilot tones are multiplied by a row-repetition matrix RVHTLTF instead– Dimension of RVHTLTF = Dimension of PVHTLTF (NSTS x NLTF)– All rows in RVHTLTF is the same as the 1st row of PVHTLTF
• Avoid spectral line
– Allows phase tracking during VHT-LTF w/o MIMO channel estimation• Simple digital solution to mitigate carrier frequency offset and drift
July 2010
Youhan Kim, et al.Slide 6
CSD
xkVHTLTF
x
1,
kVHTLTF nA
STS
k NQ
IFFT
IFFT
,STS
kVHTLTF N nA
, if is a pilot tone
, otherwise
VHTLTFkVHTLTF
VHTLTF
R kA
P
XnmX nm matrix of column and rowin element ,
doc.: IEEE 802.11-10/0771r0
Submission
Proposed Solution (Cont’d)
• Recall 11n– Different pilot sequence values for different space-time streams in
data symbols• Allows per-stream phase tracking
• Propose to have identical pilot sequence values for all space-time streams in data symbols in 11ac– Allows phase tracking w/o MIMO channel estimation on pilot tones
• Pilot tones in VHT-LTF symbols not multiplied by P matrix– Receiver may still choose to do per-stream phase tracking during data
symbols if desired• MIMO channel estimation for pilot tone locations can be obtained via
frequency domain interpolation
• For each pilot subcarrier, the same per-stream CSD and spatial mapping shall be applied across VHT-LTF and data symbols
July 2010
Youhan Kim, et al.Slide 7
doc.: IEEE 802.11-10/0771r0
Submission
Proposed Solution (Cont’d)
• Proposed pilot patterns for data symbols– 11n pattern for NSTS = 1 used for 20 and 40 MHz transmissions
[1]
– See [1] for details on pattern for 80 MHz
– Non-contiguous 160 MHz consisting of two 80 MHz frequency segments• Each frequency segment shall use the 80 MHz pattern
– Pattern for 160 MHz is obtained by repeating the 80 MHz pattern twice in frequency [3]• Contiguous and non-contiguous devices shall be capable of
transmitting and receiving frames between each other [2]
July 2010
Youhan Kim, et al.Slide 8
doc.: IEEE 802.11-10/0771r0
Submission
PER Simulation
• Parameters– 40MHz, NLOS B
– 2000 bytes / packet
– Phase noise added at both TX and RX (IEEE phase noise model)
– Initial carrier frequency offset estimation using L-LTF
• Proposed to multiply pilot tones in VHT-LTF by a row-repetition matrix RVHTLTF – To avoid spectral line
• However, RVHTLTF is all ones for the case of 6 VHT-LTFs, because the 1st row of the 6x6 P matrix [2] consists of ones only– Results in spectral line on pilot tones
July 2010
Youhan Kim, et al.Slide 12
)6/2exp( where
1
1
1
1
1
111111
252015105
20161284
1512963
108642
54321
x66
jw
wwwww
wwwww
wwwww
wwwww
wwwww
P
111111
111111
111111
111111
111111
111111
x66R
doc.: IEEE 802.11-10/0771r0
Submission
6x6 P Matrix (Cont’d)
• Propose to fix this by multiplying 2 columns of P by -1– Proposed modified 6x6 P matrix
– First row is equal to first row of 4x4 P matrix {1,-1,1,1}, with the first 2 values repeated at the end
– Notice multiplying any column by -1 does not change the orthogonality of P
July 2010
Youhan Kim, et al.Slide 13
)6/2exp( ere wh
1
1
1
1
1
111111
252015105
20161284
1512963
108642
54321
x66 jw
wwwww
wwwww
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wwwww
wwwww
P
doc.: IEEE 802.11-10/0771r0
Submission
Summary
• VHT-LTF more susceptible to carrier frequency offset than HT-LTF– VHT-LTF potentially much longer than HT-LTF– 11ac requires higher channel estimation quality (256-QAM, DL
MU-MIMO)
• Propose to– Insert pilot tones in VHT-LTF
• Do not multiply pilot tones by P matrix
– Use identical pilot values for all space-time streams for both VHT-LTF and data symbols• Allows phase tracking w/o MIMO channel estimation on pilot tones
– Modify 6x6 P matrix• Avoid spectral line at VHT-LTF pilot tones
July 2010
Youhan Kim, et al.Slide 14
doc.: IEEE 802.11-10/0771r0
Submission
Straw Poll #1
• Do you support adding the following items into of the specification framework document, 11-09/0992?(Note: Refer to solution provided on slides 6 and 7)– 3.2.3.2.4 VHT-LTF definition
• The VHT-LTF symbols shall have the same number of pilot subcarriers as the data symbols. The pilot subcarrier indices of the VHT-LTF symbols shall be identical to the pilot subcarrier indices of the data symbols. The pilot subcarriers shall use the element of the VHT-LTF sequence corresponding to that subcarrier index.
• The VHT-LTF mapping matrix P shall be applied to all subcarriers in the VHT-LTF symbols except for the pilot subcarriers. Instead, a row-repetition matrix R shall be applied to all pilot subcarriers in the VHT-LTF symbols. The row-repetition matrix R has the same dimensions as the matrix P (NSTS x NLTF), with all rows of the matrix R being identical to the first row of the matrix P of the corresponding dimension. This results in all space-time streams of the pilot subcarriers in VHT-LTF symbols to have the same pilot values.
• For each pilot subcarrier, the same per-stream CSD and spatial mapping shall be applied across VHT-LTF and data symbols
July 2010
Youhan Kim, et al.Slide 15
doc.: IEEE 802.11-10/0771r0
Submission
Straw Poll #2
• Do you support modifying the VHT-LTF mapping matrix P for six VHT-LTFs in section 3.2.3.2.4 of the specification framework document, 11-09/0992, as follows?
July 2010
Youhan Kim, et al.Slide 16
)6/2exp( where
1
1
1
1
1
111111
252015105
20161284
1512963
108642
54321
x66
jw
wwwww
wwwww
wwwww
wwwww
wwwww
P
doc.: IEEE 802.11-10/0771r0
Submission
References
• [1] Van Zelst, A. et al., Pilot Sequence for VHT-DATA, IEEE 802.11-10/0811r0, July 2010
• [2] Stacey, R. et al., Specification Framework for TGac, IEEE 802.11-09/0992r11, May 2010
• [3] Kim, Y. et al., 160 MHz Transmission, IEEE 802.11-10/0774r0, July 2010
Youhan Kim, et al.Slide 17
July 2010
doc.: IEEE 802.11-10/0771r0
Submission
Backup
July 2010
Youhan Kim, et al.Slide 18
doc.: IEEE 802.11-10/0771r0
Submission
Channel Interpolation for Pilot Tones
• 4x4, 40MHz
• -41dBc integrated phase noise on both Tx and Rx sides