[ath9k-devel] TX power and TPC

[Benoit Papillault <benoit.papillault@free.fr>]

Hello there,

I'm working on EN 301 893 certification. There is a part regarding the 
RF Output Power to make sure that the RF Output Power is within some 
limits. Let's say 27 dBm at 5600 MHz for instance.

In 4.4.1.1, the RF Output Power is defined as the "mean EIRP during a 
transmission burst". This is exactly the RF power at the connector + the 
antenna gain.

So, I started with an AR9280 card / ath9k and hookep up to a Spectrum 
Analyzer with a Channel Power measure. It has been configured with RMS 
detector / Average trace over 100 samples. In order to be close to 100% 
duty cycle, I did a "ping -I wlan0 -b 255.255.255.255 -f -s 1472" (OK, 
this is not perfect, but it has been compared with continuous transmit 
on some other chipset and gives a 0.1 dBm difference).

I also did a Peak power measure using the Peak detector / Max hold trace 
of the spectrum analyzer. I made another measure using a USB power 
sensor over 10MHz - 12.4 GHz. Here are the raw results (iwconfig is the 
reported TX power, BTW, any way to do that with iw ? ) :

  mBm | iwconfig | Peak/Max Hold | RMS/Average | USB Power Sensor
    0 |    3 dBm |      -3.8 dBm |   -14.5 dBm | -14.2 dBm
  100 |    3 dBm |      -3.2 dBm |   -14.4 dBm | -14.1 dBm
  200 |    3 dBm |      -3.3 dBm |   -14.4 dBm | -14.1 dBm
  300 |    3 dBm |      -3.1 dBm |   -14.5 dBm | -14.1 dBm
  400 |    4 dBm |       0.6 dBm |   -11.4 dBm | -10.0 dBm
  500 |    5 dBm |       2.9 dBm |    -9.5 dBm |  -7.9 dBm
  600 |    6 dBm |       4.8 dBm |    -7.8 dBm |  -6.0 dBm
  700 |    7 dBm |       6.2 dBm |    -6.5 dBm |  -4.6 dBm
  800 |    8 dBm |       7.4 dBm |    -5.6 dBm |  -3.5 dBm
  900 |    9 dBm |       8.6 dBm |    -4.6 dBm |  -2.3 dBm
1000 |   10 dBm |       9.7 dBm |    -3.6 dBm |  -1.1 dBm
1100 |   11 dBm |      10.8 dBm |    -2.7 dBm |  -0.1 dBm
1200 |   12 dBm |      12.0 dBm |    -2.0 dBm |   0.9 dBm
1300 |   13 dBm |      12.7 dBm |    -1.2 dBm |   1.6 dBm
1400 |   14 dBm |      13.5 dBm |    -0.2 dBm |   2.8 dBm
1500 |   15 dBm |      15.0 dBm |     1.0 dBm |   3.9 dBm
1600 |   16 dBm |      16.3 dBm |     2.2 dBm |   5.2 dBm
1700 |   17 dBm |      18.1 dBm |     2.8 dBm |   6.1 dBm
1800 |   18 dBm |      18.4 dBm |     3.5 dBm |   6.9 dBm
1900 |   19 dBm |      19.7 dBm |     4.2 dBm |   7.6 dBm
2000 |   20 dBm |      20.1 dBm |     5.2 dBm |   8.8 dBm
2100 |   21 dBm |      21.0 dBm |     6.0 dBm |   9.7 dBm

I am not 100% confident in the way I did all those measures. If you know 
a better way to achieve them, please stand up.

Questions are :

1. since regulatory limitation are based on "mean EIRP over burst" ... 
why the driver is calibrated over "peak power" ? We are just loosing 11 
dBm or so.

2. What would explain the difference between the RMS power measured from 
the SA and the power measured by the power sensor ?

Regards,
Benoit