1. PAPR reduction of Legacy portion of VHT PLCP Preamble
July 2010
doc.: IEEE yy/xxxxr0
July 2010
PAPR reduction of Legacy portion of VHT PLCP Preamble
Date:
Authors:
Daewon Lee, LG Electronics
Daewon Lee, LG Electronics

2. July 2010
doc.: IEEE yy/xxxxr0
July 2010
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
Daewon Lee, LG Electronics

3. PAPR reduction in VHT PLCP preamble
July 2010
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
July 2010
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
July 2010
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
40 MHz Legacy STF 4x Oversampled
PAPR [dB] g0 g1
4.7126 1 -1
5.0997
5.2497 +j -j
40 MHz Legacy LTF 4x Oversampled
PAPR [dB] g0 g1
5.8876 1 -1
6.1793 -j
6.2348
6.2399 +j
Daewon Lee, LG Electronics

6. Potential PAPR sequences for 80 MHz
July 2010
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)
July 2010
Analysis of potential 11ac PAPR of 80 MHz PLCP preamble (1/3)
L-STF PAPR analysis
80 MHz Legacy STF 4x Oversampled
PAPR [dB] g0 g1 g2 g3
4.348 1 -1
4.4722 -j +j
4.7126 …
5.8219
8.11
Daewon Lee, LG Electronics

8. Analysis of potential 11ac PAPR of 80 MHz PLCP preamble (2/3)
July 2010
Analysis of potential 11ac PAPR of 80 MHz PLCP preamble (2/3)
L-LTF PAPR analysis
80 MHz Legacy LTF 4x Oversampled
PAPR [dB] g0 g1 g2 g3
5.4271 1 -1
5.4566
5.5931 -j +j
5.6875
5.7191
5.783
5.8324 …
7.0554
9.2451
Daewon Lee, LG Electronics

9. Analysis of potential 11ac PAPR of 80 MHz PLCP preamble (3/3)
July 2010
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)
July 2010
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)
July 2010
Observations on PAPR metric (2/3)
Comparison between 99.9% PAPR metric with 4x and 8x oversampled time domain signals
80 MHz Legacy STF 4x Oversampled
PAPR [dB] g0 g1 g2 g3
4.348 1 -1
4.4722 -j +j
4.7126
80 MHz Legacy STF 8x Oversampled
PAPR [dB] g0 g1 g2 g3
4.5471 1 -j +j -1
4.5562
4.6802
4.7126
> the best PAPR reduction sequence is different for different oversampled values
Daewon Lee, LG Electronics

12. Observations on PAPR metric (2/3)
July 2010
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)
July 2010
doc.: IEEE yy/xxxxr0
July 2010
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
Daewon Lee, LG Electronics

14. PAPR Reduction Sequence and OBSS support (2/2)
July 2010
doc.: IEEE yy/xxxxr0
July 2010
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
Daewon Lee, LG Electronics

15. Potential issues at the receiver (1/4)
July 2010
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. Co-existence γ sequence for 80 MHz
July 2010
Co-existence γ sequence for 80 MHz
In order to align γ sequence between different BW BSS, { +1, +j, +1, -j } and { +1, -j, +1, +j } sequences
exact sequence depends on location of primary/secondary 20MHz subchannel within 80 MHz
Daewon Lee, LG Electronics

17. Additional PAPR reduction methods
July 2010
Additional PAPR reduction methods
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

18. July 2010
PAPR analysis of 40/80MHz with additional -25ns CSD of half of BW (1/3)
For 40MHz BW, substantially improves(lowers) PAPR
From dB to dB for L-STFs
From to dB to dB for L-LTFs
40 MHz Legacy STF 4x Oversampled + -25ns CSD
PAPR [dB] g0 g1
4.3818 1 -j +j
4.5502 -1
40 MHz Legacy LTF 4x Oversampled+ -25ns CSD
PAPR [dB] g0 g1
5.3455 1 +j
5.5627
5.7637 -j
5.9833 -1
Daewon Lee, LG Electronics

19. 80 MHz Legacy STF 4x Oversampled+ -25ns CSD
July 2010
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 dB to dB
PAPR of simple repetition {+1,+j,+1,+j} from dB to dB
Note : simple repetition {+1,+j,+1,+j} with CSD is better than any γ sequence without CSD
80 MHz Legacy STF 4x Oversampled+ -25ns CSD
PAPR [dB] g0 g1 g2 g3
3.9027 1 -1
4.1989 +j -j
4.2507
4.2865 …
7.5605
Daewon Lee, LG Electronics

20. 80 MHz Legacy LTF 4x Oversampled+ -25ns CSD
July 2010
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 dB to dB
PAPR of simple repetition {+1,+j,+1,+j} from dB to B
Note : simple repetition {+1,+j,+1,+j} with CSD is better than any γ sequence without CSD
80 MHz Legacy LTF 4x Oversampled+ -25ns CSD
PAPR [dB] g0 g1 g2 g3
4.9883 1 +j -j
5.0405 -1
5.0543
5.1935
5.2665
5.2731
5.2841
5.2978
5.3079
5.3241
5.3581 …
8.2812
Daewon Lee, LG Electronics

21. PAPR reduction mechanism for 160 MHz
July 2010
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

22. July 2010
Summary
Summary
40MHz 11n γ sequence does not seem to give any benefits in terms of PAPR reduction
80MHz γ sequence may have co-existence issues
Switching between 2 sequence may help with the co-existence issues for the 80 MHz
Applying small CSD on half of the BW improves PAPR for 40/80/160 MHz legacy portion of the PLCP preamble
Annex contains detailed analysis of PAPR of legacy portion of VHT PLCP preamble
Daewon Lee, LG Electronics

23. Proposal Recommended Conclusion for 11ac
July 2010
Proposal
Recommended Conclusion for 11ac
support applying additional small CSD value on half of the BW (only for the legacy portion of the to improve PAPR for all supported BW (i.e. 40, 80, 160 MHz) for the legacy portion of the VHT PLCP header
have an baseline of simple repetition of 11n gamma sequence for the legacy portion of the VHT PLCP header
either study further on this issue and confirm baseline in next meeting
If non-{+a, +ja, +b, +bj } sequence deemed to be the best during the study then further consider having sequence hopping depending on Primary and Secondary channel of the 80 MHz bandwidth
have further study whether to have the same gamma sequence for legacy portion or the data portion of the VHT PLCP header
Daewon Lee, LG Electronics

24. July 2010
References
[1] Motorola, “Comparison of PAR and Cubic Metric for Power De-rating,” 3GPP TSG RAN WG4 R
Daewon Lee, LG Electronics

25. Annex. PAPR detailed results
July 2010
Annex. PAPR detailed results
Daewon Lee, LG Electronics