Phase Rotation for the 80 MHz 802.11ac Mixed Mode Packet

1. Phase Rotation for the 80 MHz 802.11ac Mixed Mode Packet Authors: Date: 2010-07-12

2. Abstract • Low PAPR symbols in 802.11ac is beneficial for lower PA backoff as well as lower dynamic range in digital domain algorithms. • In 802.11n, 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

3. Optimal Phase Shifts (1/5) -128 -64 0 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. To minimize the PAPR of , it is necessary to ensure unity magnitude of Optimal Phase Shifts (3/5) 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. 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 Optimal Phase Shifts (4/5) 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 0 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 Basis PAPR 20MHz 11n Mixed Mode packet

10. Simulations Results – PAPR (2/3) 80MHz Sampling Rate 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 ~+3dB ~+3dB ~+3dB ~+3dB ~+2dB PAPR increase due to 80MHz Single user 11ac Mixed Mode Packet

12. Simulations Results – Backward Compatibility (1/2) jS -jS S S jS -jS S S 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] -S -S -S S -S -S -S S 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 signalsfor non-oversampled receivers. Slide 14

15. 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 802.11n, we propose that this phase rotation be applied to all the symbols in the 802.11ac Mixed Mode Packet Conclusions

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. 802.11n, 2009. [2] doc.: IEEE 802.11-09/0847r1. IEEE802.11ac Preamble with Legacy 802.11a/n Backward Compatibility [3] doc.: IEEE 802.11-10/0802r0. VHT-LTF sequence for 80 MHz