3.0 Measurement Results
3.1 Ambient Fields
Measurements were initially performed todetermine the RF exposure levels in the ambient environment when the Wi-Fidevices under test, and all other wireless devices in the boardroom, were non-operational.This section summarizes the main findings for test cases 1a through 4a.However, more detailed measurement results are presented in Annex E.
1a) Ambient levels from 50 MHzto 6 GHz at each measurement location with the tri-axis antenna positionedat a height of 1.75 metres
Location P6 had the highest instantaneous(maximum) recorded value, which was 0.013% of the SC6 limits. The average RF exposurelevel recorded at the same location was 0.003% of the SC6 limits. The averagefield strength values for the 12 locations were similar, with a standarddeviation of 0.0001%.
2a) Ambient levels from 50 MHzto 6 GHz at one measurement location with the tri-axis antenna positioned at aheight of 1.25 metres
Because RF exposure levels were similar forall locations, location P11 was arbitrarily chosen, with the tri-axis antenna positionedat a height of 1.25 metres. The RF exposure levels were similar to the levelsmeasured with the tri-axis antenna positioned at a height of 1.75 metres. Theaverage RF exposure level recorded at P11 was 0.003% of the SC6 limits.
3a) Ambient levels from 50 MHzto 6 GHz at one measurement location with the tri-axis antenna positioned at aheight of 1.25 metres for a sweep time of 6 minutes
Location P11 was chosen for the 6-minutetime-averaging measurement with the tri-axis antenna positioned at a height of1.25 metres. The average RF exposure level recorded was 0.003% of the SC6limits.
4a) Ambient levels from 50 MHzto 6 GHz at one measurement location with a nine-point matrixrepresenting a cross-section of the human body
Spatial averaging was performed at locationP11 by using a nine-point matrix representing a cross-section of thehuman body. Because the ambient RF exposure levels were well below the SC6limits, a quick scan was performed at each point of the nine-point matrix. Thevalue obtained for the spatial averaging was 0.003% of the SC6 limits. Theambient average is the same for all locations.
3.2 Test Results with the Wi-Fi Access Points and Wi-Fi-Enabled DevicesOperational
Measurements were performed to determinethe RF exposure levels when the Wi-Fi devices under test, and all 24 Wi-Fi-enableddevices, were operational. This section summarizes the main findings for testcases 1b through 9b. However, more detailed measurement results are presentedin Annex E.
1b) RF exposure levels from 2.4 to 5.825 GHz at each preselectedmeasurement location with the tri-axis antenna positioned at a height of 1.75 metres
The measurements were performed at each preselectedlocation (see Figure 4) with the tri-axis antenna positioned at a height of1.75 metres. Information was captured for approximately 50 to 70 sweeps (~1 min)at each location. The Wi-Fi-enabled devices were downloading a large file froma host server via AP2 while AP1 was in continuous-transmission test mode (dutycycle ~100%).
According to the measurement results, locationP8 had the highest RF exposure level. Location P8 was the preselected locationclosest to AP1 (distance ~1.1 m). The average RF exposure level for the Wi-Fibands at this location was 0.232% of the SC6 limits (431 times below thelimits). The 5150-5350 MHz band provided the largest contribution tothe average RF level, which was 0.229% of the SC6 limits (436 times below thelimits).
2b) RF exposure levels from 2.4 to 5.825 GHz at each preselectedmeasurement location with the tri-axis antenna positioned at a height of 1.25 metres
The measurements were again performed ateach preselected location with the tri-axis antenna positioned at a height of1.25 metres. Information was captured for approximately 50 to 70 sweeps (~1 min)at each location. The Wi-Fi-enabled devices were downloading a large file byconnecting to AP2 while AP1 was in continuous-transmission test mode (dutycycle ~100%).
According to the measurement results, locationP5 had the highest RF exposure level. Location P5 was the preselected locationclosest to AP2 (distance ~0.43 m). The average RF exposure level for the Wi-Fibands at this location was 0.234% of the SC6 limits (427 times below thelimits). The 2400-2483.5 MHz band provided the largest contribution tothe average RF level, which was 0.221% of the SC6 limits (452 times below thelimits).
3b) Spatial- and time-averaged RF exposure levels from 2.4to 5.825 GHz at the location with the highest levels found from 1b and 2b
Health Canada's Safety Code 6 states that forsituations in which the exposure intensity varies significantly with timewithin a period of 6 minutes, time-averaging values must be calculated frommultiple measurements. Safety Code 6 also states that spatial averaging (from anine-point matrix) over the projected surface area (flat plane) equivalent tothe head and the body region of a person shall be measured if the localizedvalues vary by more than 20%.
In compliance with the requirements set forth in Industry Canada's GL-01 (basedon SC6), spatial and time averaging were performed at the location with thehighest RF exposure value from 1b and 2b. Location P5, the location nearest toAP2, was selected for the spatial- and time-averaging measurements. Measurementswere logged for 6 minutes at each point of the nine-point matrix. As in testcases 1b and 2b, both Wi-Fi access points were operational, and all Wi-Fi-enableddevices were in downloading mode.
The percentages of the SC6 limits weremeasured from 2400 to 5825 MHz. The spatial- and time-averaging value was0.099% of the SC6 limits (1010 times below the limits).
4b) RF exposure levels for eight laptops connected to AP2 inuploading mode at a preselected measurement location with the tri-axis antennapositioned at a height of 1.25 metres
Test cases 4b through 7b determined thevariation in the RF exposure levels when laptops were in uploading or downloadingmode, or both, at one selected measurement location. The measurement tri-axisantenna was positioned at a height of 1.25 metres. The measurement location was50 cm from location P7, closer to location P8. This location was selected becauseit was centered between the eight uploading laptops (see Figure 6). Themeasurement location and the tri-axis antenna height remained the same for testcases 4b, 5b, 6b and 7b. Data was captured for 6 minutes for the four uploadingand/or downloading scenarios.
Figure 6 – Tri-axis antenna locationfor the uploading and/or downloading scenarios
In test case 4b, eight laptops (Nos. 1 to 4and 13 to 16) were uploading a large file of approximately 3.6 GB to aserver connected to AP2 while the remaining 16 laptops were turned off. Theaverage RF exposure level for the 2400-2483.5 MHz band was measured at 0.006% ofthe SC6 limits.
5b) RF exposure levels for eight laptops connected to AP2 inuploading mode and 16 laptops in downloading mode at a preselected measurementlocation with the tri-axis antenna positioned at a height of 1.25 metres
In this test case, eight laptops (Nos.1 to 4and 13 to 16) were uploading a large file of approximately 3.6 GB to aserver connected to AP2 while the remaining laptops were downloading a largefile of approximately 14.4 GB from AP2. The average RF exposure level for the2400-2483.5 MHz band was 0.005% of the SC6 limits.
6b) RF exposure levels for one laptop connected to AP2 inuploading mode at a preselected measurement location with the tri-axis antennapositioned at a height of 1.25 metres
In this test case, one laptop (No. 3) wasuploading a large file of approximately 3.6 GB to a server connected to AP2while the remaining laptops were turned off. Laptop 3 was selected because itwas closest to the measurement tri-axis antenna. The average RF exposure levelfor the 2400-2483.5 MHz band was 0.004% of the SC6 limits.
7b) RF exposure levels for one laptop connected to AP2 indownloading mode at a preselected measurement location with the tri-axisantenna positioned at a height of 1.25 metres
In this test case, laptop 3 was downloadinga large file from Wi-Fi AP2 while the remaining laptops were turned off. Theaverage level for the 2400-2483.5 MHz band was 0.007% of the SC6 limits.
8b) RF exposurelevels from 5150 to 5350 MHz with the tri-axis antenna positioned at 20 cm fromAP1
The tri-axis antenna was positioned at 20cm17 from AP1 for a total sweeptime of approximately 70 seconds when AP1 was transmitting in the continuousmode. The maximum RF exposure level obtained during this time frame was 10.59%of the SC6 limits.
9b) RF exposure levels from 2400 to 2483.5 MHz with the tri-axisantenna positioned at 20 cm from AP2
The tri-axis antenna was positioned at 20 cm above AP2 (this access point has built-inantennas) for a total sweep time of approximately 70 seconds. All laptops werein downloading mode. The built-in antennas of this access point were small, so thedistance of 20 cm was well in the far-field zone of each one (see Annex B). Themaximum RF exposure level obtained during this time frame was 7.73% of the SC6limits.
4.0 Discussion
4.1 Applicationof Equipment-Related Uncertainty to Spatial- and Time-Averaged Measurements
Industry Canada's GL-01 states that inorder to verify compliance with the SC6 limits, measurement equipmentuncertainty must be considered. Table 6 represents the spatial- andtime-averaged RF exposure levels at location P5 in the configuration of testcase 3b, with all Wi-Fi devices active, along with the minimum and maximum expectedvalues,18 based on a confidence level of 95%. (See Annex C for the expandeduncertainty of the Narda SRM 3006 and its corresponding antennas,which constitute the measurement system.)
| Measured Value – Exp. Uncert.(% SC6 limits)a | Measured % SC6 limits for the uncontrolled environment | Measured Value + Exp. Uncert.(% SC6 limits)a |
|---|---|---|
| 0.037% | 0.099% | 0.194% |
a Includes the expanded measurementuncertainty (Exp. Uncert.) of the equipment.
4.2 Impact of Wi-Fi Protocol on RF Exposure
Table 7summarizes the contribution of the 2400-2483.5 MHz band when Wi-Fi-enableddevices operate in different configurations of uploading or downloading, orboth. The highest average RF exposure level obtained from the fourconfigurations was 0.007% of the SC6 limits, for the test case with a single laptopin downloading mode.
| Locationa | Lower frequency (MHz) | Upper frequency (MHz) | Configurationb | Average (% SC6 limits) |
|---|---|---|---|---|
| 50 cm from P7 | 2400 | 2483.5 | Eight laptops uploading | 0.006 |
| 2400 | 2483.5 | Eight laptops uploading; 16 laptops downloading | 0.005 | |
| 2400 | 2483.5 | One laptop uploading | 0.004 | |
| 2400 | 2483.5 | One laptop downloading | 0.007 |
a SeeFigure 4 for measurement location.
b All configurations were statisticallydifferent (at 95% confidence level), except for the configuration with eightlaptops uploading versus the configuration with one laptop downloading.
When numerous devices are uploading and/ordownloading at the same time, a collision avoidance (CA) mechanism is used toimprove the performance of carrier sense multiple access (CSMA). To avoid acollision when a packet is sent, the node must first "listen" to confirm that achannel is clear. When the full bandwidth (e.g. 20 MHz) of a node is devoted toonly one client device, such as a laptop, the node can transmit more oftenwithout deferring the transmission. However, when the bandwidth is divided intonumerous channels of similar size because the node is connected to many clientdevices, the node must listen more often before a packet can be transmitted. Whena channel is sensed as busy, transmission is deferred and the RF exposure leveldecreases. A configuration of one laptop uploading produces a lower level of RFexposure than a configuration of one laptop downloading, because the power ofthe Wi-Fi module in a Wi-Fi-enabled device is typically lowerthan the power of the Wi-Fi access points.
5.0 Conclusion
In this study, the measured results wereexpressed as percentages of the SC6 limits (see Section 3.0).
When the Wi-Fi access points and Wi-Fi-enableddevices were operating, the RF exposure levels were higher when the tri-axisantenna was nearer to the Wi-Fi access points (locations P5 and P8). Forlocation P8, which was near AP1, the highest average RF exposure level obtainedwas 0.232% of the SC6 limits, for a measurement time of approximately 1 minute(see test case 1b in Section 3.2). The largest contribution came from the 5150-5350MHz band, with a value of 0.229%. For location P5, which was near AP2, thehighest average RF exposure level obtained was 0.234% of the SC6 limits, forthe same measurement time of approximately 1 minute (see test case 2b inSection 3.2). The largest contribution came from AP2 in the 2400-2483.5 MHzband, with a value of 0.221%.
In compliance with the procedures set forthin Industry Canada's GL-01 spatial- and time-averaging measurementswere performed at location P5 near AP2, the location with the highest measuredfield strength levels (see test case 3b in Section 3.2). The resulting valuewas 0.1% of the SC6 limits. With the measurement equipment uncertainty added, themeasured RF exposure level was 0.19% of the SC6 limits (515 times below thelimit).
With the tri-axis antenna positioned at adistance of 20 cm from the Wi-Fi access points,19 the maximum RF exposure levelsobtained for AP1 and AP2 were 10.59% and 7.73% of the SC6 limits, respectively.In a typical scenario in which a person is located several metres from theaccess point and surrounded by other users, the RF exposure level is thousandsof times below the SC6 limits.
Based on the results of this case study,the aggregated RF exposure of multiple Wi-Fi access points and Wi-Fi-enableddevices in this indoor location was well below the SC6 limits. In addition, theWi-Fi access points selected for this study were operating at higher powercompared with the majority of the Wi-Fi devices currently available on theCanadian market. Therefore, the results of this study are likely higher thantypical equivalent setups in public and private environments, such as homes,schools and businesses.