1. Phase Rotation for the 80 MHz 802.11ac Mixed Mode Packet
Month Year
doc.: IEEE yy/xxxxr0
Phase Rotation for the 80 MHz ac Mixed Mode Packet
Date:
Authors:
John Doe, Some Company

2. Month Year
doc.: IEEE yy/xxxxr0
Abstract
Low PAPR symbols in ac is beneficial for lower PA backoff as well as lower dynamic range in digital domain algorithms.
In n, a phase rotation of 90 degrees was applied to the upper 20MHz band of the 40MHz packet in order to reduce the PAPR of non-oversampled systems[1].
In [2], we proposed phase rotations for every 20MHz band in the 80MHz packet that achieves the same effect.
We will show theoretical proofs, additional simulations, and make a straw poll for our proposal
John Doe, Some Company

3. Optimal Phase Shifts (1/5)
-128
-64 64
127
20MHz
S20 can be (L-STF, L-LTF, L-SIG, VHT-SIG)
subcarrier symbol vector for the 20MHz packet
θi = phase shift for the ith 20MHz band

4. Optimal Phase Shifts (2/5)
rearranging
Oversampled IDFT of S20
It is clear that duplication of the 20MHz subcarrier bits to 80MHz band is equal to a 4x oversampled of the 20MHz signal modulated by

5. Optimal Phase Shifts (3/5)
To minimize the PAPR of , it is necessary to ensure unity magnitude of
IDFT of repeating periodically
Constant amplitude
To minimize the PAPR of , we need to choose whose IDFT has low PAPR
can be chosen from any CAZAC (Constant Amplitude Zero Autocorrelation)
sequence (e.g. Frank Zadoff Chu) to ensure unity magnitude of

6. Optimal Phase Shifts (4/5)
To maintain compatibility with 40MHz legacy devices,
Unfortunately, there are no CAZAC sequences of this form. The closest and easiest to implement sequence we found is
where are any phase shifts
(e.g )
Including its cyclic shifts and scaled versions

7. Optimal Phase Shifts (5/5)
Magnitude
Magnitude
PAPR=0dB
PAPR=3dB
No PAPR increase from S20
Expect increase of 3dB from S20

8. Proposed Phase Rotation
For 40MHz Packet, the phase rotation is similar to 11n: Re-use of preamble for 40MHz 11n is possible.
with [1 j 1 –j] phase rotation
-128
-64 64
127
3rd 20MHz: No Phase rotation
4th 20MHz: -90 Degree Phase rotation
1st 20MHz: Base
2nd 20MHz: 90 Degree Phase rotation
Slide 8

9. Simulations Results – PAPR (1/3)
20MHz Sampling Rate
Fields
PAPR
L-STF
2.09dB
L-LTF
3.17dB
L-SIG
5.28dB
HT-SIG
6.28dB
PACKET
9.58dB
Basis PAPR
20MHz 11n Mixed Mode packet

10. Simulations Results – PAPR (2/3)
80MHz Sampling Rate
Fields
PAPR
L-STF
2.28dB
L-LTF
3.20dB
L-SIG
7.98dB
VHT-SIG-A
8.12dB
PACKET
9.34dB
Except for the effect of oversampling for L-STF and L-LTF, change of subcarrier bits in SIG fields, the PAPR is unchanged
80MHz Single user 11ac Mixed Mode Packet

11. Simulations Results – PAPR (3/3)
80MHz Sampling Rate
Fields
PAPR
L-STF
5.14dB
L-LTF
6.21dB
L-SIG
11dB
VHT-SIG-A
10.87dB
PACKET
~+3dB
~+3dB
~+3dB
~+3dB
~+2dB
PAPR increase due to
80MHz Single user 11ac Mixed Mode Packet

12. Simulations Results – Backward Compatibility (1/2)jS S
-jS S jS S
-jS
80MHz
40MHz
40MHz
40 MHz device
40 MHz device
80 MHz device
11n devices at the UPPER 40 MHz channel expects [1 j] subcarrier rotation and has a possible compatibility issue when Cross Correlation is used for frame synchronization

13. Simulations Results – Backward Compatibility (2/2)
SIG Field Symbol Error Rate
TGn Channel D
10000 iterations
Impairments: CFO, Phase noise, PA nonlinearity,
40MHz legacy devices at both sides of the 80MHz channel that uses only Frame Synchronization has similar performance

14. Problems with [1 -1 -1 -1] phase rotation [3]
80MHz
40MHz
40MHz
40 MHz 11n device
40 MHz 11n device
80 MHz device
Both 40MHz devices at each side of the 80MHz channel has potential compatibility issue
Each 40MHz receivers receive signals phase rotated by [1 -1] and [-1 -1]. Both are non-CAZAC sequence by themselves hence legacy 11n may receive unexpectedly high PAPR signals　for non-oversampled receivers.
Slide 14

15. Conclusions
In order to minimize the PAPR of the 80MHz packet, phase rotation for every 20MHz band is necessary.
CAZAC sequences are optimal sequence for the phase rotation
[1 j 1-j] is the closest CAZAC sequence to extended 11n phase rotation of [1 j 1 j] hence has the least potential issue in all possible usable optimal sequences.
Simulation results show that [1 j 1 –j] has no visible backward compatibility issue compared with [1 j 1 j] in Channel D and B [2] models.
The phase rotation at the lower 40MHz is also a CAZAC sequence and hence can be re-used for 40MHz packet 11n/ac implementation.
Similar to n, we propose that this phase rotation be applied to all the symbols in the ac Mixed Mode Packet

16. Straw Poll #1
Do you support to have 90 degrees phase rotation for the 2nd 20MHz subchannel and -90 degrees phase rotation for the 4th 20MHz subchannel, of L-STF, L-LTF, L-SIG and VHT-SIG-A in VHT mixed format packet as described in slide 8 and edit the specification framework accordingly?
A Yes:
B No:
C Abs:
Slide 16

17. Straw Poll #2
Do you support to have 90 degrees phase rotation for the 2nd 20MHz subchannel and -90 degrees phase rotation for the 4th 20MHz subchannel, of VHT-STF, VHT-LTF, VHT-SIG-B, and DATA field in VHT mixed format packet as described in slide 8 and edit the specification framework accordingly?
A Yes:
B No:
C Abs:
Slide 17

18. References
[1] Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specification. Amendment 5: Enhancements for Higher Throughput, IEEE Std n, 2009.
[2] doc.: IEEE /0847r1. IEEE802.11ac Preamble with Legacy a/n Backward Compatibility
[3] doc.: IEEE /0802r0. VHT-LTF sequence for 80 MHz