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PAPR reduction of Legacy portion of VHT PLCP Preamble

PAPR reduction of Legacy portion of VHT PLCP Preamble. Authors:. Date: 2010-07-13. Abstract.

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PAPR reduction of Legacy portion of VHT PLCP Preamble

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  1. PAPR reduction of Legacy portion of VHT PLCP Preamble Authors: Date: 2010-07-13 Daewon Lee, LG Electronics

  2. Abstract We discussion PAPR of legacy portion of the VHT PLCP preamble. We share simulation results and analysis of PAPR of L-STF, L-LTF, and L-STF+L-LTF OFDM symbols. We also share some proposals on how to mitigate high PAPR Daewon Lee, LG Electronics

  3. PAPR reduction in VHT PLCP preamble • PAPR reduction is important in reducing implementation cost of Tx Power Amplifier (PA) and Rx receiver dynamic range related function blocks • PAPR is known to be especially high if sequence is repeated in frequency domain. • this results in high PAPR for legacy portion of VHT PLCP preamble if simply repeated • Methods of reducing PAPR for legacy portion of VHT PLCP preamble in case of 40/80/160 MHz should be strongly considered Daewon Lee, LG Electronics

  4. PAPR reduction sequence for 40 MHz in 11n • +1 is multiplied to lower 20 MHz subchannel • +j is multiplied to upper 20 MHz subchannel Daewon Lee, LG Electronics

  5. Analysis of 11n PAPR of 40 MHz PLCP preamble • PAPR is mostly important to analog domain signals which is the operation domain of Tx PA and Rx AGC/ADC • this means PAPR should be calculated with oversampled time domain signals (with pulse shaping) • PAPR calculation should also include GI of OFDM symbols • Observation : PAPR reduction using {+1, +j} does not improve lowering PAPR Daewon Lee, LG Electronics

  6. Potential PAPR sequences for 80 MHz • One method is to multiply sequence { +a, +b, +c, +d } to each 20 MHz subband in each 80 MHz transmission band Daewon Lee, LG Electronics

  7. Analysis of potential 11ac PAPR of 80 MHz PLCP preamble (1/3) • L-STF PAPR analysis Daewon Lee, LG Electronics

  8. Analysis of potential 11ac PAPR of 80 MHz PLCP preamble (2/3) • L-LTF PAPR analysis Daewon Lee, LG Electronics

  9. Analysis of potential 11ac PAPR of 80 MHz PLCP preamble (3/3) • Observation • { +1, +1, +1, -1 } has lowest PAPR for STF, { +1, +1, -1, +1 }, { +1, +j, 1, -j }, ... is second lowest PAPR for STF • difference between the three are extremely marginal (~0.13 dB) • { +1, +1, -1, +1 } is lowest PAPR for LTF, { +1, +j, 1, -j } is 4th lowest PAPR, and { +1, +1, +1, -1 } is the 6th lowest PAPR for LTF • difference between the lowest and others are small (~0.26 dB, ~0.36 dB) • difference between simple repetition { +1 +j, +1, +j } and lowest PAPR sequence is 1.6 dB for both STF and LTF • See attached file in Annex for detailed results Daewon Lee, LG Electronics

  10. Observations on PAPR metric (1/3) • Absolute PAPR metric depends on several factors • how PAPR is calculated (e.g. 100% PAPR, 99.99% PAPR, or 99.9% PAPR) • the oversampled rate, • whether GI was considered as part of the OFDM symbol when calculating PAPR for a particular PAPR • whether pulse shaping was applied to time domain signals • a particular PAPR metric only can give rough estimate value time domain signals • there are some marginal differences between PAPR values calculated in different methods • the slight differences in different PAPR calculation methods result in different optimal time domain PAPR sequences Daewon Lee, LG Electronics

  11. Observations on PAPR metric (2/3) • Comparison between 99.9% PAPR metric with 4x and 8x oversampled time domain signals > the best PAPR reduction sequence is different for different oversampled values Daewon Lee, LG Electronics

  12. Observations on PAPR metric (2/3) • PAPR does not give a strictly accurate picture of required PA backoff [1] • researches have shown that PAPR and actual PA required backoff are not strictly linear depending on what type of signal waveform PA was designed for and exact type of PA • “Cubic Metric” seems to be a better metric to compare actual required PA backoff in case of PA distortion due to high dynamic range for GaAs HBT or pHEMT PAs. • PAPR does gives a good estimate (but not extremely strict) picture of actual backoff and dynamic ranges the implementations needs to take into account Daewon Lee, LG Electronics

  13. PAPR Reduction Sequence and OBSS support (1/2) • In case 40MHz and 80MHz BSS overlap • 40MHz BSS entities (AP and STAs) need to perform carrier sense • 40 MHz BSS applies sequence { +1, +j } • Assume 80 MHz BSS applied sequence {+a,+b,+c,+d} to each subband of the 80 MHz • AP and STA in BSS #2 is force to detect two different type of sequence {+a,+b} or {+c,+d} depending on deployment scenario Daewon Lee, LG Electronics

  14. PAPR Reduction Sequence and OBSS support (2/2) • Case #1 • PAPR sequence { a, ja, b, jb } is applied • Simple extension of 11n, no co-existence detection issues • PAPR of sequence { a, ja, b, jb } is known to be high • Case #2 • PAPR sequence { a, ja, c, d } is applied, where d≠jc • Potential co-existence detection issues • Potential possibility of PAPR reduction with particular sequences Daewon Lee, LG Electronics

  15. Potential issues at the receiver (1/4) • In case #2 • Legacy (including 11n) STA, and APs may have co-existence problems (e.g. difficulty in decoding L-SIG for CCA) • In order for other BSS#2 AP/STAs to use L-SIG information from BSS #1 80 MHz signal transmission, following procedures needs to be functional • Timing Acquisition, Coarse Frequency Offset (L-STF) • Fine Frequency Offset, Fine Timing Acquisition (L-LTF) • Channel Estimation (L-LTF) • L-SIG demodulation Daewon Lee, LG Electronics

  16. PAPR reduction method for Legacy Portion of VHT PLCP Preamble • The problem is repeated sequence across frequency • No repeated sequence across frequency solves PAPR issues • small (~30ns<) cylic delay signals do not effect receiver much • alternative solution is use cyclic delayed sequence across subchannel Daewon Lee, LG Electronics

  17. PAPR analysis of 40/80MHz with additional -25ns CSD of half of BW (1/3) • For 40MHz BW, substantially improves(lowers) PAPR • From 5.2429 dB to 4.3818 dB for L-STFs • From to 6.2399 dB to 5.3455 dB for L-LTFs Daewon Lee, LG Electronics

  18. PAPR analysis of 40/80MHz with additional -25ns CSD of half of BW (2/3) • For 80MHz BW, additionally lowers PAPR on top of γ sequence for L-STF • lowest PAPR from 4.348 dB to 3.9027 dB • PAPR of simple repetition {+1,+j,+1,+j} from 5.8219dB to 4.2865dB • Note : simple repetition {+1,+j,+1,+j} with CSD is better than any γ sequence without CSD Daewon Lee, LG Electronics

  19. PAPR analysis of 40/80MHz with additional -25ns CSD of half of BW (3/3) • For 80MHz BW, additionally lowers PAPR on top of γ sequence for L-LTF • lowest PAPR from 5.4271dB to 4.9883dB • PAPR of simple repetition {+1,+j,+1,+j} from 7.0554dB to 5.3241B • Note : simple repetition {+1,+j,+1,+j} with CSD is better than any γ sequence without CSD Daewon Lee, LG Electronics

  20. PAPR reduction mechanism for 160 MHz • If continuous 160 MHz is being processed by single RF chain (i.e. single PA) for a given antenna port, the PAPR may also be an issue • Same PAPR reduction methods can be applied to 160 MHz • γ sequence for across 160 MHz • apply small CSD on half of the BW Daewon Lee, LG Electronics

  21. PAPR of 20MHz VHT-Data portion Daewon Lee, LG Electronics

  22. PAPR of 40MHz VHT-Data portion Daewon Lee, LG Electronics

  23. PAPR of 80MHz VHT-Data portion Daewon Lee, LG Electronics

  24. PAPR of 20MHz L-SIG portion Daewon Lee, LG Electronics

  25. PAPR of 40MHz L-SIG portion Daewon Lee, LG Electronics

  26. PAPR of 80MHz L-SIG portion Daewon Lee, LG Electronics

  27. PAPR bottleneck of VHT PLCP Preamble (1/2) + : median value of possible PAPR values for random data Note: for ‘small CSD’ case the PAPR values of phase rotation can be applied Daewon Lee, LG Electronics

  28. PAPR bottleneck of VHT PLCP Preamble (2/2) + : median value of possible PAPR values for random data Note: for ‘small CSD’ case the PAPR values of phase rotation can be applied Daewon Lee, LG Electronics

  29. Observation Summary • Observation • 40MHz 11n phase rotation γ sequence does not seem to give any benefits in terms of PAPR reduction • 80MHz phase rotation γ sequence may have co-existence issues • Applying small CSD on half of the BW improves PAPR for 40/80/160 MHz legacy portion of the PLCP preamble • High PAPR results in higher PA backoff (in order to reduce Adjacent Channel Leakage Power) • general Low PAPR sequence gives improvement in performance for initial timing acquisition and frequency offset estimation • PAPR bottlenck seems to be in the legacy portion of the Preamble • Annex contains detailed analysis of PAPR of legacy portion of VHT PLCP preamble in 10/0847r0 Daewon Lee, LG Electronics

  30. Proposal • Recommended Conclusion for 11ac • strongly consider to separate PAPR reduction methods for Legacy portion and VHT portion in the PLCP preamble • consider methods of lower PAPR for the legacy portion as the first priority since analysis show the legacy portion seems to the PAPR bottleneck in which the Tx PA backoff will work on and effects timing acquisition performance Daewon Lee, LG Electronics

  31. References • [1] Motorola, “Comparison of PAR and Cubic Metric for Power De-rating,” 3GPP TSG RAN WG4 R4-040367 Daewon Lee, LG Electronics

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