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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
Santa Clara, CA 95054, USA +1-408-720-5574 [email protected]
Youhan Kim Atheros 5480 Great America Pkwy Santa Clara, CA 95054, USA
+1-408-830-5835 [email protected]
Ning Zhang Atheros 5480 Great America Pkwy Santa Clara, CA 95054, USA
+1-408-773-5363 [email protected]
Richard van Nee Qualcomm Straatweg 66S, Breukelen, The Netherlands
+31-346-259-650 [email protected]
Allert van Zelst Qualcomm Straatweg 66S, Breukelen, The Netherlands
+31-346-259-663 [email protected]
Hemanth Sampath Qualcomm 5775 Morehouse Drive, San Diego, CA, 92121, USA
VK Jones Qualcomm 3105 Kifer Road, Santa Clara, CA, 95051, USA
Vinko Erceg Broadcom 16340 W Bernardo Dr; San Diego CA 92127, USA
Youngsoo Kim Samsung Mt. 14-1 Nongseo-Ri, Giheung-Eup Yongin-Si, Gyeonggi-Do, Korea 449-712
+82-31-280-9614 [email protected]
Eldad Perahia Intel 2111 NE 25th Ave Hillsboro, OR 97124, USA
Raja Banerjea Marvell 5488 Marvell Lane Santa Clara, CA 95054, USA
Authors:
July 2010
doc.: IEEE 802.11-10/0771r0
Submission
Motivation
• 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
– ML MIMO receiver
– Phase tracking always enabled for data symbols
July 2010
Youhan Kim, et al.Slide 9
doc.: IEEE 802.11-10/0771r0
Submission
No Frequency Drift
• 4x4, 4 streams, 256-QAM 3/4• IPN = -41 dBc
• 8x8, 8 streams, 64-QAM 5/6• IPN = -41 dBc
July 2010
Youhan Kim, et al.Slide 10
0.0100
0.1000
1.0000
-59 -57 -55 -53
PE
R
RSSI (dBm)
w/o tracking
w/ tracking
0.01
0.1
1
-59 -57 -55 -53 -51
PE
R
RSSI (dBm)
w/o tracking
w/ tracking
doc.: IEEE 802.11-10/0771r0
Submission
With Frequency Drift
• 4x4, 4 streams, 64-QAM 5/6• IPN = -36 dBc• Freq. drift = 50 Hz/us
• 8x8, 8 streams, 64-QAM 5/6• IPN = -41 dBc• Freq. drift = 25 Hz/us
July 2010
Youhan Kim, et al.Slide 11
0.0100
0.1000
1.0000
-59 -57 -55 -53 -51 -49
PE
R
RSSI (dBm)
w/o tracking
w/ trackin
0.1
1
-59 -57 -55 -53 -51 -49 -47
PE
R
RSSI (dBm)
w/o tracking
w/ tracking
doc.: IEEE 802.11-10/0771r0
Submission
6x6 P Matrix
• 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
wwwww
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
July 2010
Youhan Kim, et al.Slide 19
15 20 25 30 35 4014
16
18
20
22
24
26
28
30
32
34
channel SNR
chan
nel
est
imat
ion
SN
R
NLOS B
15 20 25 30 35 4016
18
20
22
24
26
28
30
32
channel SNR
chan
nel
est
imat
ion
SN
R
NLOS D
