1. On Amplitude Drift Compensation
Month Year
doc.: IEEE yy/xxxxr0
September 2017
On Amplitude Drift Compensation
Date:
Authors:
Name
Affiliation
Address
Phone
Jianhan Liu
Frank Hsu
Thomas Pare
Mediatek
Hongyuan Zhang
Marvell
Bin Tian
Qualcomm
Ron Porat
Broadcom
Xiaogang Chen
Intel
Ross Yu
David Yang
Huawei
Jinsoo Choi
LGE
Yujin Noh
Minho Cheong
Newracom
Bo Sun
ZTE
Tianyu Wu
Samsung
Jianhan Liu, etc., Mediatek
John Doe, Some Company

2. EVM test with amplitude drift
September 2017
EVM test with amplitude drift
It is noticed that EVM degradation caused by amplitude drift can be pretty large.
HT and VHT devices on the market allow amplitude drift compensation in the EVM test.
For HE products, it is reasonable to reduce EVM degradation caused by amplitude drift.
Therefore 11ax added the following requirement to the EVM test :
The text in the 11ax draft 1.2 ( Transmitter modulation accuracy (EVM) test) says:
d) Symbols in a PPDU shall be derotated according to estimated frequency offset. Sampling offset drift shall be also compensated. Note that amplitude drift shall not be compensated by the testing instrument.
Jianhan Liu, etc., Mediatek

3. EVM degradation caused amplitude drift
September 2017
EVM degradation caused amplitude drift
Amplitude drift is caused by PA self-heating. Even with enhanced PA design, it is hard to reduce amplitude drift to zero.
For MCS 10 and 11, EVM is below -35dB.
It can be seen that the lower the EVM, the more sensitive to amplitude drift.
For example, for 0.05dB delta amplitude drift:
EVM degrades from -38dB to -35dB
EVM degrades from -31dB to -30dB.
It is not a easy task to control the delta amplitude drift smaller than 0.05dB!
Jianhan Liu, etc., Mediatek

4. More reasons on relaxing EVM requirements caused amplitude drift
September 2017
More reasons on relaxing EVM requirements caused amplitude drift
WiFi receivers can have Amplitude/Phase/Frequency tracking capability. Completely disallowing the amplitude tracking in the test does not reflect the true condition of some receivers.
Completely disallowing amplitude drift compensations increase the cost and power consumptions of PA, for example, PA need to be built with very accurate temperature measurement and gain compensation circuits to avoid the amplitude drift caused by PA self-heating.
HT and VHT devices on the market allows amplitude drift compensation in the EVM test and we did not see inter-operable problem caused by amplitude drift compensation .
Jianhan Liu, etc., Mediatek

5. Minimize the change of the 11ax spec.
September 2017
Minimize the change of the 11ax spec.
Only change the EVM test for MCS with amplitude drift:
Only low EVMs are sensitive to amplitude drift;
Avoid change of -27dBm for HE TB PPDU.
Jianhan Liu, etc., Mediatek

6. Suggested changes on EVM test
September 2017
Suggested changes on EVM test
Putting an requirement on EVM degradation caused by amplitude drift for HE products is a good move, but making the requirement too strict may hurt the industry.
Given there are no HE products on the mass production yet, it is hard to guarantee that HE products can reduce the amplitude drift to zero. Note that even 0.01dB amplitude drift can cause 0.5dB EVM change in very low EVM regions.
To reduce the risk for HE products and also avoid huge EVM degradation caused by amplitude drift, we propose to relax the Transmitter modulation accuracy (EVM) test) for MCS10-11 (1024QAM).
Jianhan Liu, etc., Mediatek