1. IEEE 802.11 Regulatory AHC Power Measurement Method
September 2009
doc.: IEEE /1053r0
September 2009
IEEE Regulatory AHC Power Measurement Method
Date:
Authors:
R. Roy, SRA
Dick Roy, SRA

2. September 2009
doc.: IEEE /1053r0
September 2009
Abstract
This submission presents a general method for measuring/estimating (transmitted or received) time-averaged power (equivalently total energy) over a specified time interval.
Dick Roy, SRA

3. September 2009
Proposed Power Restriction Approach (from R. Kennedy’s submission 0997r1)
We should offer a variation of the current formula that we believe will suit not only the short range devices, but also protect devices, like devices, that have optimized active sharing mechanism
Medium Utilization = Energy in millijoules measured over any 1 second of the complete duty cycle; the limit is
MU < 4 millijoules
Examples
100 mW for 100 milliseconds (out of 1 second) = 10 millijoules
10 mW for 1 second (DC – 100%) = 10 millijoules
Compared to the current agreement, the 4 Millijoules represents a lower threshold, but on the other hand, apart from the measuring period of 1 second, no further restrictions are applied on modulation e.g. the Tx-on time and Tx-off time
Joules preferred over ‘Power x Time’ to allow non-constant envelope modulations, or technologies that operate at variable power to benefit from those transmissions where they do not operate at full power.

4. Time-averaged Power Basic procedure:
September 2009
Time-averaged Power
Basic procedure:
Obtain a waveform proportional to the square (complex envelope if it’s a single-sideband waveform) of the voltage (RF electric field into a matched load, suitably bandpass filtered and downconverted to baseband) of the signal of interest as a function of time.
Convolve this waveform with a unit-pulse (square-wave) waveform whose width in the time-domain is the time period over which it is desired to calculate the time-averaged power (cf. energy within the specified time interval).
Take care to get the normalization factors correct for comparison to an absolute threshold (eg. a threshold in millijoules).
Compare the filter output to a threshold if desired.
This can be all be done digitally if baseband IQ output sample of the signal of interest are available.

5. Time-averaged Power For the example in the previous slide:
September 2009
Time-averaged Power
For the example in the previous slide:
The time interval = 1 sec; so use a 1 second wide unit-pulse as the impulse response of the power measurement filter and process a sufficient amount of the signal of interest so all its transmit modes are captured (this may mean several minutes for very low duty cycle outputs).
Set the threshold to 4 millijoules and compare the filter output to this threshold.
Slide 5

6. September 2009
Conclusions
The basic idea is to use a sliding window “integrator” to estimate (calculate) the time-averaged signal power over any time-interval of a specified length. This is a well-known technique an dis easily implemented in a variety of forms.