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Project: IEEE P802.15 Working Group for Wireless Personal Area Networks (WPANs)
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  1. Project: IEEE P802.15 Working Group for Wireless Personal Area Networks (WPANs) Submission Title: [Comment resolutions for OFDM scattered pilots] Date Submitted: [July 15, 2010] Source: [Tim Schmidl, Texas Instruments] Re: [TG4g comment resolution] Abstract: [OFDM scattered pilots] Notice: This document has been prepared to assist the IEEE P802.15. It is offered as a basis for discussion and is not binding on the contributing individual(s) or organization(s). The material in this document is subject to change in form and content after further study. The contributor(s) reserve(s) the right to add, amend or withdraw material contained herein. Release: The contributor acknowledges and accepts that this contribution becomes the property of IEEE and may be made publicly available by P802.15.

  2. Summary This document describes the proposed resolution on some comments related to OFDM regarding the scattered pilots. The following comments are addressed: OFDM scattered pilots: CID# 1421, 1423

  3. Comment resolution for OFDM scattered pilots The current scattered pilots have an uneven spacing between pilot subcarriers, so this increases the implementation complexity for channel estimation. The proposed resolution is to make the spacing between the pilot subcarriers uniform and to set the spacing to four subcarriers for Options 1 and 2 and two subcarriers for Options 3, 4, and 5.

  4. Comment resolution for OFDM scattered pilots (text 1) 6.12b.2.6 Pilot Tones / Null Tones Replace the remainder of the section after Table 75l with the following: The pilot tones shall be transmitted with different shifts in the frequency domain in order to enable channel estimation when the channel is changing due to Doppler. Immediately after the second LTF, the pilot shifts change every OFDM symbol to the next set. For Options 1, 2, 3, 4, and 5 there are 13, 7, 7, 4, and 2 pilot sets, respectively. Figure 65q illustrates how the pilot sets cycle through the sets for Option 1. Figure 65r illustrates how the pilot sets cycle through the sets for Options 2 and 3. Figures 65s and 65t illustrate how the pilot sets cycle through the sets for Options 4 and 5, respectively. The pilot sets for each option are unique to that option. The long vertical lines show visually when each cycles through the pilots sets is complete.

  5. Comment resolution for OFDM scattered pilots (text 2) time Set1 Set2 Set3 Set4 Set5 Set6 Set8 Set5 Set9 Set10 Set11 Set12 Set13 Set1 Set2 Set4 LTF LTF Set7 Set3 Set5 Set4 Set7 LTF LTF Set1 Set2 Set3 Set4 Set5 Set6 Set7 Set1 Set2 Set3 Set6 Set3 Set4 Set2 Set1 LTF Set2 Set1 Set2 Set3 Set4 Set1 Set2 Set3 Set4 Set1 Set2 Set3 Set4 Set1 Set2 Set3 Set1 LTF Set1 LTF Set2 LTF Set1 Set2 Set1 Set2 Set2 Set2 Set1 Set1 Set1 Set2 Set1 Set2 Set1 Set2 Set1 Set4 Set2 time time time Figure 65q—Pilot tone sets for Option 1. Figure 65r—Pilot tone sets for Options 2 and 3. Figure 65s—Pilot tone sets for Option 4. Figure 65t—Pilot tone sets for Option 5.

  6. Comment resolution for OFDM scattered pilots (text 3) For Option 1, the device shall use 13 sets of pilot tones: The subcarriers for pilot and data are numbered as -52 to 52 with the DC tone unused. Pilot Set 1: -38 -26 -14 -2 10 22 34 46 Pilot Set 2: -46 -34 -22 -10 2 14 26 38 Pilot Set 3: -42 -30 -18 -6 6 18 30 42 Pilot Set 4: -50 -38 -26 -14 -2 10 22 50 Pilot Set 5: -46 -34 -22 -10 2 14 34 46 Pilot Set 6: -42 -30 -18 -6 6 18 26 38 Pilot Set 7: -50 -38 -26 -14 -2 30 42 50 Pilot Set 8: -46 -34 -22 -10 10 22 34 46 Pilot Set 9: -42 -30 -18 -6 2 14 26 38 Pilot Set 10: -50 -38 -26 6 18 30 42 50 Pilot Set 11: -46 -34 -14 -2 10 22 34 46 Pilot Set 12: -42 -30 -22 -10 2 14 26 38 Pilot Set 13: -50 -18 -6 6 18 30 42 50 For Option 2, the device shall use 7 sets of pilot tones: The subcarriers for pilot and data are numbered as -26 to 26 with the DC tone unused. Pilot Set 1: -14 -2 10 22 Pilot Set 2: -22 -10 2 14 Pilot Set 3: -18 -6 6 18 Pilot Set 4: -26 -14 -2 26 Pilot Set 5: -22 -10 10 22 Pilot Set 6: -18 -6 2 14 Pilot Set 7: -26 6 18 26

  7. Comment resolution for OFDM scattered pilots (text 4) For Option 3, the device shall use 7 sets of pilot tones: The subcarriers for pilot and data are numbered as -13 to 13 with the DC tone unused. Pilot Set 1: -7 7 Pilot Set 2: -11 3 Pilot Set 3: -3 11 Pilot Set 4: -9 5 Pilot Set 5: -5 9 Pilot Set 6: -13 1 Pilot Set 7: -1 13 For Option 4, the device shall use 4 sets of pilot tones: The subcarriers for pilot and data are numbered as -7 to 7 with the DC tone unused. Pilot Set 1: -3 5 Pilot Set 2: -7 1 Pilot Set 3: -5 3 Pilot Set 4: -1 7 For Option 5, the device shall use 2 sets of pilot tones: The subcarriers for pilot and data are numbered as -3 to 3 with the DC tone unused. Pilot Set 1: -3 1 Pilot Set 2: -1 3

  8. Comment resolution for OFDM scattered pilots (text 5) The data carried on the pilot tones shall be determined by a pseudo-noise sequence PN9 with the seed “111111111.” The first output bit is assigned to most negative index in Set 1. For example, for Option 3, the first output bit from the PN9 sequence is assigned to the pilot symbol with index -7 and the second output bit is assigned to the pilot symbol with index 7. Table 75l shows the mapping from PN9 bits to the pilot BPSK symbols. Index n starts after the LTF from zero and is increased by one every pilot subcarrier. Table 75m—Mapping from PN9 sequence to Pilot BPSK Symbols

  9. Pilots for Option 1 Pilot Set 1: -38 -26 -14 -2 10 22 34 46 Pilot Set 2: -46 -34 -22 -10 2 14 26 38 Pilot Set 3: -42 -30 -18 -6 6 18 30 42 Pilot Set 4: -50 50 -38 -26 -14 -2 10 22 Pilot Set 5: -46 -34 -22 -10 2 14 34 46 Pilot Set 6: -42 -30 -18 -6 6 18 26 38 Pilot Set 7: 30 42 -50 50 -38 -26 -14 -2 Pilot Set 8: -46 -34 -22 -10 10 22 34 46 Pilot Set 9: -42 -30 -18 -6 2 14 26 38 Pilot Set 10: 6 18 30 42 -50 50 -38 -26 Pilot Set 11: -46 -34 -14 -2 10 22 34 46 Pilot Set 12: -42 -30 -22 -10 2 14 26 38 Pilot Set 13: -18 -6 6 18 30 42 -50 50 Pilots for Option 1 are spaced evenly every 4 tones from -50:4:50 and repeat every 3.25 symbol.

  10. Pilot distribution in frequency Pilots in 802.15.4g draft Pilots in 474r1

  11. Problems with Proposal in doc 577 r1, Option 1 Spacing in symbols between consecutive pilots within the same subcarrier is 5 symbols, which means poor Doppler performance compared with a spacing of 3 symbols.

  12. Problems with Proposal in doc 577 r1, Option 2 The number of data subcarriers varies from 48 to 48 to 47 to 47. This complicates the data path because there is no longer an integer number of data bits per OFDM symbol

  13. Problems with Proposal in doc 577 r1, Option 3 The performance of the edge tones is poor because the pilot locations are too far from the edge tones.