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Phase Tracking During VHT-LTF

Phase Tracking During VHT-LTF. Authors:. Date: 2010-07-10. 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

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Phase Tracking During VHT-LTF

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  1. Phase Tracking During VHT-LTF Authors: Date: 2010-07-10 Youhan Kim, et al.

  2. 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 IPN: -41 dBc • Frequency offset estimation usingL-LTF • 4x4, NLOS B, HT40 • SNR = 40 dB • Carrier frequency = 5 GHz • IEEE phase noise (both at TX and RX) IPN: -36 dBc Youhan Kim, et al.

  4. 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 • Significant degradation in channel estimation performance observed due to residual carrier frequency offset for high order MIMO transmissions • 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 • 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) • 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) • 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 • 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 • 8x8, 8 streams, 64-QAM 5/6 • IPN = -41 dBc Youhan Kim, et al.

  11. 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 Youhan Kim, et al.

  12. 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. 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. 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. 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 Youhan Kim, et al.

  16. 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? Youhan Kim, et al.

  17. 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.

  18. Backup Youhan Kim, et al.

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

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