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Project: IEEE P802.15 Working Group for Wireless Personal Area Networks (WPANs)

Project: IEEE P802.15 Working Group for Wireless Personal Area Networks (WPANs) Submission Title: [More Suggested Improvements for SUN OFDM] Date Submitted: [December 2009] Source: [T. Schmidl, A. Batra, S. Hosur] Company [Texas Instruments] Address [12500 TI Blvd, Dallas, TX 75243 USA]

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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: [More Suggested Improvements for SUN OFDM] Date Submitted: [December 2009] Source: [T. Schmidl, A. Batra, S. Hosur] Company [Texas Instruments] Address [12500 TI Blvd, Dallas, TX 75243 USA] Voice:[+1 214-480-4460], FAX: [+1 972-761-6966], E-Mail:[schmidl@ti.com] Abstract: [This presentation gives more suggested improvements for SUN OFDM] Purpose: [For information] 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. Tim Schmidl, Texas Instruments Inc.

  2. Real versus Complex for Lower MCS Levels • We still need to decide whether to use real signals or complex signals for the lower MCS levels • Advantage of real signals is that 1 DAC chain can be eliminated for devices which only support the lowest MCS levels • Advantages of complex signals are better frequency diversity (duplicate tones have large frequency separation) when frequency spreading is used and lower peak-to-average power ratio • For Option 1 the PAR for real signals (BPSK with spreading factor of 2, average of PAR for each of 10000 OFDM symbols) was 9.1 dB, while for complex signals the PAR was 7.2 dB (with phase rotations shown on the next slide) • For the reason of lower PAR and better frequency diversity, it is preferable to use complex signals for all MCS levels Tim Schmidl, Texas Instruments Inc.

  3. Frequency Spreading by 4x for Option 1 • Option 1 with 96 data subcarriers supports frequency spreading by 4x • Data is generated on 24 data subcarriers, and each data subcarrier is copied to 3 other subcarriers • Simply copying the data subcarriers increases the peak-to-average ratio versus having independent data on all 96 tones • PAR for BPSK with simple copying for diversity subcarriers • with no spreading = 7.3 dB • with frequency spreading by 2 = 9.1 dB • with frequency spreading by 4 = 10.7 dB • The 3 copies can be phase-rotated so that the peak-to-average ratio does not increase • PAR for BPSK with phase rotation for diversity subcarriers • with no spreading = 7.3 dB • with frequency spreading by 2 = 7.2 dB • with frequency spreading by 4 = 7.2 dB Tim Schmidl, Texas Instruments Inc.

  4. Frequency Spreading by 4x for Option 1 (cont’d) • Number the subcarriers from -52 to 52 including both the data and the pilots. These can be denoted d -52 to d52. The DC subcarrier is not used. • Data is generated for subcarriers 1 to 26, including 2 pilots • Subcarrier 1 is copied to subcarriers 27, -52, and -26 (with phase rotations) so that maximum frequency spacing is maintained between copies • dk+26 = dk * exp[j*2*pi*((k-1)/4] for k = 1:26 • dk-53 = dk * exp[j*2*pi*((2*k-1)/4] for k = 1:26 • dk-27 = dk * exp[j*2*pi*((3*k-1)/4] for k = 1:26 Tim Schmidl, Texas Instruments Inc.

  5. Frequency Spreading by 2x for Option 1 • Number the subcarriers from -52 to 52 including both the data and the pilots. These can be denoted d -52 to d52. The DC subcarrier is not used. • Data is generated for subcarriers 1 to 52, including 4 pilots • Subcarrier 1 is copied to subcarrier -52 (with phase rotations) so that maximum frequency spacing is maintained between copies • dk-53 = dk * exp[j*2*pi*((2*k-1)/4] for k = 1:52 • The same phase rotations can be used for all 5 Options Tim Schmidl, Texas Instruments Inc.

  6. PAR’s for all Options with Phase Rotations Tim Schmidl, Texas Instruments Inc.

  7. Design of STF and LTF Sequences PAR (dB) for STF PAR (dB) for LTF Tim Schmidl, Texas Instruments Inc.

  8. Complex LTF Sequences LTF Sequences LTF_freq(Option-1)= [0, 1,-1, 1,-1, 1, 1,-1,-1, 1,-1, 1, 1, 1, 1,-1, 1, 1, 1, 1, 1,-1, 1,-1, 1, -1, 1,-1, 1, 1,-1, 1,-1,-1,-1, 1, 1, 1, 1, 1, 1,-1,-1,-1,-1,-1,-1, 1,-1, 1, 1,-1, 1,zeros(1,23), -1, 1, 1,-1,-1,-1,-1, 1, 1,-1,-1, 1, 1, 1,-1,-1, 1, 1,-1,-1,-1,-1,-1, 1, 1,-1,-1,-1,-1,-1, 1, 1,-1, 1,-1,-1, 1,-1, 1, 1, 1, 1,-1,-1, 1, 1,-1, 1, 1,-1, 1, 1] LTF_freq(Option-2)= [0,1,-1, 1, 1,-1, 1,-1,-1, 1,-1, 1, 1,-1,-1, 1, 1,-1,-1,-1,-1,-1, 1,-1,-1,-1, 1, zeros(1,11), -1,-1,-1,-1, 1, 1, 1,-1, 1,-1, 1,-1, 1, 1,-1,-1,-1, 1, 1,-1, 1, 1, 1,-1,-1,-1] LTF_freq(Option-3)= [0, -1,-1, 1,-1, 1, 1,-1,-1, 1, 1,-1,-1, 1, zeros(1,5), 1,-1, 1,-1, 1, 1, 1, 1, 1, 1, 1, 1,-1] LTF_freq(Option-4)= [0, -1, 1, 1, 1,-1,-1,-1, 0, 1,-1, 1, 1,-1, 1, 1] LTF_freq(Option-5)= [0, -1, 1,-1, 0, 1, 1, 1] Tim Schmidl, Texas Instruments Inc.

  9. Complex STF Sequences STF Sequences STF_freq(Option-1) = sqrt(104/24)*[0, 0, 0, 0,-1, 0, 0, 0,-1, 0, 0, 0, 1, 0, 0, 0, 1, 0, 0, 0, 1, 0, 0, 0,-1, 0, 0, 0,-1, 0, 0, 0, 1, 0, 0, 0,-1, 0, 0, 0, 1, 0, 0, 0,-1, 0, 0, 0, 1, 0, 0, 0,-1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,-1, 0, 0, 0,-1, 0, 0, 0,-1, 0, 0, 0,-1, 0, 0, 0,-1, 0, 0, 0,-1, 0, 0, 0,-1, 0, 0, 0, 1, 0, 0, 0, 1, 0, 0, 0,-1, 0, 0, 0, 1, 0, 0, 0, 1, 0, 0, 0,-1, 0, 0, 0] STF_freq(Option-2) = sqrt(52/12)*[0, 0, 0, 0, 1, 0, 0, 0,-1, 0, 0, 0, 1, 0, 0, 0, 1, 0, 0, 0,-1, 0, 0, 0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,-1, 0, 0, 0,-1, 0, 0, 0,-1, 0, 0, 0, 1, 0, 0, 0, 1, 0, 0, 0, 1, 0, 0, 0] STF_freq(Option-3) = sqrt(26/6)*[0, 0, 0, 0,-1, 0, 0, 0, 1, 0, 0, 0,-1, 0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0, 1, 0, 0, 0, 1, 0, 0, 0] STF_freq(Option-4) = sqrt(14/6)*[0, 0,-1, 0, 1, 0,-1, 0, 0, 0, 1, 0, 1, 0, 1, 0] STF_freq(Option-5) = sqrt(6/2)*[0, 0,-1, 0, 0, 0, 1, 0] Tim Schmidl, Texas Instruments Inc.

  10. Possible Solutions to Doppler within a Packet • Differential modulation introduces new MCS levels which require extra implementation effort and testing, interoperability issues, and up to 3 dB loss in SNR • Decision-directed channel estimation is shown to work for IEEE 802.11a with the same fixed pilot structure (4 pilots) with over 1000 Hz Doppler. Channel estimates need to be updated throughput the packet. The loss due to channel estimation is < 0.5 dB even with 460 Hz Doppler. See http://publik.tuwien.ac.at/files/PubDat_110691.pdf • Pilot hopping introduces no additional pilot overhead and does not require decision-directed channel estimation. • If we use pilot hopping, then the implementer can choose to use either decision-directed or pilot-only channel estimation for coherent demodulation. Tim Schmidl, Texas Instruments Inc.

  11. Simulations for IEEE 802.11a System • Coherent demodulation is shown to work for IEEE 802.11a with the same fixed pilot structure (4 pilots) with over 1000 Hz Doppler. Channel estimates need to be updated throughput the packet. The loss due to channel estimation is < 0.5 dB even with 460 Hz Doppler. The paper also shows that if the channel estimates are not updated during the packet the performance is poor. See http://publik.tuwien.ac.at/files/PubDat_110691.pdf Simulation for 802.11a with 460 Hz Doppler. Channel estimates not updated throughput packet. Channel estimates are updated throughput packet. Tim Schmidl, Texas Instruments Inc.

  12. Set3 Set2 LTF Set1 Set1 Set1 Set2 Set2 Set3 Set3 Set3 Set1 Set1 Set1 Set2 Set2 Set2 Set3 Set3 Staggered Pilot Structure • The cyclic prefix is 1/4 of the useful part of the OFDM symbol, so this limits the delay spread that can be tolerated in the system • For Option 2 we can use 3 sets of pilot tones: • If we number the subcarriers for pilot/data as -26 to 26 with the DC unused • Pilot Set 1: -20 -6 6 20 • Pilot Set 2: -24 -11 2 15 • Pilot Set 3: -15 -2 11 24 • The pilot sets can be repeated for 3 OFDM symbols to enable averaging over interference/noise before computing new channel estimates • Limiting the total number of pilot tones minimizes the computational complexity and memory required for channel estimation time LTF Tim Schmidl, Texas Instruments Inc.

  13. Staggered Pilot Structure for All Options • For Option 1 we can use 3 sets of pilot tones • If we number the subcarriers for pilot/data as -52 to 52 with the DC unused • Pilot Set 1: -46 -33 -19 -6 6 19 33 46 • Pilot Set 2: -50 -37 -24 -11 2 15 28 41 • Pilot Set 3: -41 -28 -15 -2 11 24 37 50 • For Option 2 we can use 3 sets of pilot tones: • If we number the subcarriers for pilot/data as -26 to 26 with the DC unused • Pilot Set 1: -20 -6 6 20 • Pilot Set 2: -24 -11 2 15 • Pilot Set 3: -15 -2 11 24 • For Option 3 we can use 3 sets of pilot tones: • If we number the subcarriers for pilot/data as -13 to 13 with the DC unused • Pilot Set 1: -7 7 • Pilot Set 2: -11 2 • Pilot Set 3: -2 11 • For Option 4 we can use 2 sets of pilot tones: • If we number the subcarriers for pilot/data as -7 to 7 with the DC unused • Pilot Set 1: -6 -2 • Pilot Set 2: 2 6 • For Option 5 we can use 2 sets of pilot tones: • If we number the subcarriers for pilot/data as -3 to 3 with the DC unused • Pilot Set 1: -3 1 • Pilot Set 2: -1 3 Tim Schmidl, Texas Instruments Inc.

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