Power Measurement Method - IEEE Standards Association
September 2009 doc.: IEEE 802.11-09/1053r0 IEEE 802.11 Regulatory AHC Power Measurement Method Date: 2009-09-23 Authors: Name Affiliations Richard Roy SRA Submission Address Slide 1 Phone
email +1-650-861-3351 [email protected] R. Roy, SRA September 2009 doc.: IEEE 802.11-09/1053r0 Abstract This submission presents a general method for measuring/estimating (transmitted or received) timeaveraged power (equivalently total energy) over a specified time interval. Submission Slide 2 September 2009 doc.: IEEE 802.11-09/1053r0
Proposed Power Restriction Approach (from R. Kennedys 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 802.11 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. Submission Slide 3 September 2009 doc.: IEEE 802.11-09/1053r0 Time-averaged Power Basic procedure: 1. Obtain a waveform proportional to the square (complex envelope if its 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. 2. 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). 3. Take care to get the normalization factors correct for comparison to an absolute threshold (eg. a threshold in millijoules).
4. 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. Submission Slide 4 September 2009 doc.: IEEE 802.11-09/1053r0 Time-averaged Power For the example in the previous slide: 1. 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). 2. Set the threshold to 4 millijoules and compare the filter output to this threshold. Submission
Slide 5 September 2009 doc.: IEEE 802.11-09/1053r0 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. Submission Slide 6
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