1. Phase Tracking During VHT-LTF
Month Year
doc.: IEEE yy/xxxxr0
July 2010
Phase Tracking During VHT-LTF
Date:
Authors:
Youhan Kim, et al.
John Doe, Some Company

2. Motivation Carrier frequency offset causes EVM degradation at RX
July 2010
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
Youhan Kim, et al.

3. Initial Carrier Frequency Offset Estimation Accuracy
July 2010
Initial Carrier Frequency Offset Estimation Accuracy
IPN: -41 dBc
Frequency offset estimation using L-LTF
4x4, NLOS B, HT40
SNR = 40 dB
Carrier frequency = 5 GHz
IEEE phase noise (both at TX and RX)
IPN: -36 dBc
Integrated Phase Noise (IPN)
Standard Deviation of freq. offset estimation error [ppm]
-41 dBc
0.074
-36 dBc
0.13
Youhan Kim, et al.

4. Carrier Frequency Drift
July 2010
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
Youhan Kim, et al.

5. Impact on Channel Estimation
July 2010
Impact on Channel Estimation
Significant degradation in channel estimation performance observed due to residual carrier frequency offset for high order MIMO transmissions
MIMO dimension: Ntx x Nrx
1x1
4x4
8x8
Number of VHT-LTF symbols: 1 4 8
Average
Channel
Estimation
SNR (dB)
Residual freq. offset =250Hz
5 GHz)
36.8
35.4
33.9
Residual freq. offset =500Hz
5 GHz)
36.6
33.6
29.9
Residual freq. offset =1kHz
5 GHz)
36.0
29.7
24.5
AWGN channel
-41 dBc integrated phase noise at both TX and RX
Youhan Kim, et al.

6. Proposed Solution Insert pilot tones in VHT-LTF symbols July 2010
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
Youhan Kim, et al.

7. Proposed Solution (Cont’d)
July 2010
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
Youhan Kim, et al.

8. Proposed Solution (Cont’d)
July 2010
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]
Youhan Kim, et al.

9. PER Simulation Parameters 40MHz, NLOS B 2000 bytes / packet
July 2010
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
Youhan Kim, et al.

10. No Frequency Drift 4x4, 4 streams, 256-QAM 3/4 IPN = -41 dBc
July 2010
No Frequency Drift
4x4, 4 streams, 256-QAM 3/4
IPN = -41 dBc
8x8, 8 streams, 64-QAM 5/6
IPN = -41 dBc
Youhan Kim, et al.

11. With Frequency Drift July 2010 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
Youhan Kim, et al.

12. July 2010
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
Youhan Kim, et al.

13. July 2010
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
Youhan Kim, et al.

14. July 2010
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
Youhan Kim, et al.

15. July 2010
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)
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
Youhan Kim, et al.

16. July 2010
Straw Poll #2
Do you support modifying the VHT-LTF mapping matrix P for six VHT-LTFs in section of the specification framework document, 11-09/0992, as follows?
Youhan Kim, et al.

17. July 2010
References
[1] Van Zelst, A. et al., Pilot Sequence for VHT-DATA, IEEE /0811r0, July 2010
[2] Stacey, R. et al., Specification Framework for TGac, IEEE /0992r11, May 2010
[3] Kim, Y. et al., 160 MHz Transmission, IEEE /0774r0, July 2010
Youhan Kim, et al.

18. July 2010
Backup
Youhan Kim, et al.

19. Channel Interpolation for Pilot Tones
July 2010
Channel Interpolation for Pilot Tones
4x4, 40MHz
-41dBc integrated phase noise on both Tx and Rx sides
Youhan Kim, et al.