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**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

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**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.

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**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.

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**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.

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**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.

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**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.

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**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.

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**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.

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**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.

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**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.

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**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.

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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.

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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.

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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.

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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.

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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.

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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.

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July 2010 Backup Youhan Kim, et al.

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**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.

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