Case Study: Measurements of Radio Frequency Exposure from Wi-Fi Devices

3.0 Measurement Results

3.1 Ambient Fields

Measurements were initially performed to determine the RF exposure levels in the ambient environment when the Wi-Fi devices 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 MHz to 6 GHz at each measurement location with the tri-axis antenna positioned at 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 exposure level recorded at the same location was 0.003% of the SC6 limits. The average field strength values for the 12 locations were similar, with a standard deviation of 0.0001%.

2a) Ambient levels from 50 MHz to 6 GHz at one measurement location with the tri-axis antenna positioned at a height of 1.25 metres

Because RF exposure levels were similar for all locations, location P11 was arbitrarily chosen, with the tri-axis antenna positioned at a height of 1.25 metres. The RF exposure levels were similar to the levels measured with the tri-axis antenna positioned at a height of 1.75 metres. The average RF exposure level recorded at P11 was 0.003% of the SC6 limits.

3a) Ambient levels from 50 MHz to 6 GHz at one measurement location with the tri-axis antenna positioned at a height of 1.25 metres for a sweep time of 6 minutes

Location P11 was chosen for the 6-minute time-averaging measurement with the tri-axis antenna positioned at a height of 1.25 metres. The average RF exposure level recorded was 0.003% of the SC6 limits.

4a) Ambient levels from 50 MHz to 6 GHz at one measurement location with a nine-point matrix representing a cross-section of the human body

Spatial averaging was performed at location P11 by using a nine-point matrix representing a cross-section of the human body. Because the ambient RF exposure levels were well below the SC6 limits, a quick scan was performed at each point of the nine-point matrix. The value obtained for the spatial averaging was 0.003% of the SC6 limits. The ambient average is the same for all locations.

3.2 Test Results with the Wi-Fi Access Points and Wi-Fi-Enabled Devices Operational

Measurements were performed to determine the RF exposure levels when the Wi-Fi devices under test, and all 24 Wi-Fi-enabled devices, were operational. This section summarizes the main findings for test cases 1b through 9b. However, more detailed measurement results are presented in Annex E.

1b) RF exposure levels from 2.4 to 5.825 GHz at each preselected measurement location with the tri-axis antenna positioned at a height of 1.75 metres

The measurements were performed at each preselected location (see Figure 4) with the tri-axis antenna positioned at a height of 1.75 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 from a host server via AP2 while AP1 was in continuous-transmission test mode (duty cycle ~100%).

According to the measurement results, location P8 had the highest RF exposure level. Location P8 was the preselected location closest to AP1 (distance ~1.1 m). The average RF exposure level for the Wi-Fi bands at this location was 0.232% of the SC6 limits (431 times below the limits). The 5150-5350 MHz band provided the largest contribution to the average RF level, which was 0.229% of the SC6 limits (436 times below the limits).

2b) RF exposure levels from 2.4 to 5.825 GHz at each preselected measurement location with the tri-axis antenna positioned at a height of 1.25 metres

The measurements were again performed at each preselected location with the tri-axis antenna positioned at a height of 1.25 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 by connecting to AP2 while AP1 was in continuous-transmission test mode (duty cycle ~100%).

According to the measurement results, location P5 had the highest RF exposure level. Location P5 was the preselected location closest to AP2 (distance ~0.43 m). The average RF exposure level for the Wi-Fi bands at this location was 0.234% of the SC6 limits (427 times below the limits). The 2400-2483.5 MHz band provided the largest contribution to the average RF level, which was 0.221% of the SC6 limits (452 times below the limits).

3b) Spatial- and time-averaged RF exposure levels from 2.4 to 5.825 GHz at the location with the highest levels found from 1b and 2b

Health Canada's Safety Code 6 states that for situations in which the exposure intensity varies significantly with time within a period of 6 minutes, time-averaging values must be calculated from multiple measurements. Safety Code 6 also states that spatial averaging (from a nine-point matrix) over the projected surface area (flat plane) equivalent to the head and the body region of a person shall be measured if the localized values vary by more than 20%.

In compliance with the requirements set forth in Industry Canada's GL-01 and TN-329 (based on SC6), spatial and time averaging were performed at the location with the highest RF exposure value from 1b and 2b. Location P5, the location nearest to AP2, was selected for the spatial- and time-averaging measurements. Measurements were logged for 6 minutes at each point of the nine-point matrix. As in test cases 1b and 2b, both Wi-Fi access points were operational, and all Wi-Fi-enabled devices were in downloading mode.

The percentages of the SC6 limits were measured from 2400 to 5825 MHz. The spatial- and time-averaging value was 0.099% of the SC6 limits (1010 times below the limits).

4b) RF exposure levels for eight laptops connected to AP2 in uploading mode at a preselected measurement location with the tri-axis antenna positioned at a height of 1.25 metres

Test cases 4b through 7b determined the variation in the RF exposure levels when laptops were in uploading or downloading mode, or both, at one selected measurement location. The measurement tri-axis antenna was positioned at a height of 1.25 metres. The measurement location was 50 cm from location P7, closer to location P8. This location was selected because it was centered between the eight uploading laptops (see Figure 6). The measurement location and the tri-axis antenna height remained the same for test cases 4b, 5b, 6b and 7b. Data was captured for 6 minutes for the four uploading and/or downloading scenarios.

Figure 6 – Tri-axis antenna location for the uploading and/or downloading scenarios

In test case 4b, eight laptops (Nos. 1 to 4 and 13 to 16) were uploading a large file of approximately 3.6 GB to a server connected to AP2 while the remaining 16 laptops were turned off. The average RF exposure level for the 2400-2483.5 MHz band was measured at 0.006% of the SC6 limits.

5b) RF exposure levels for eight laptops connected to AP2 in uploading mode and 16 laptops in downloading mode at a preselected measurement location with the tri-axis antenna positioned at a height of 1.25 metres

In this test case, eight laptops (Nos.1 to 4 and 13 to 16) were uploading a large file of approximately 3.6 GB to a server connected to AP2 while the remaining laptops were downloading a large file of approximately 14.4 GB from AP2. The average RF exposure level for the 2400-2483.5 MHz band was 0.005% of the SC6 limits.

6b) RF exposure levels for one laptop connected to AP2 in uploading mode at a preselected measurement location with the tri-axis antenna positioned at a height of 1.25 metres

In this test case, one laptop (No. 3) was uploading a large file of approximately 3.6 GB to a server connected to AP2 while the remaining laptops were turned off. Laptop 3 was selected because it was closest to the measurement tri-axis antenna. The average RF exposure level for the 2400-2483.5 MHz band was 0.004% of the SC6 limits.

7b) RF exposure levels for one laptop connected to AP2 in downloading mode at a preselected measurement location with the tri-axis antenna positioned at a height of 1.25 metres

In this test case, laptop 3 was downloading a large file from Wi-Fi AP2 while the remaining laptops were turned off. The average level for the 2400-2483.5 MHz band was 0.007% of the SC6 limits.

8b) RF exposure levels from 5150 to 5350 MHz with the tri-axis antenna positioned at 20 cm from AP1

The tri-axis antenna was positioned at 20 cm17 from AP1 for a total sweep time of approximately 70 seconds when AP1 was transmitting in the continuous mode. 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-axis antenna positioned at 20 cm from AP2

The tri-axis antenna was positioned at 20 cm above AP2 (this access point has built-in antennas) for a total sweep time of approximately 70 seconds. All laptops were in downloading mode. The built-in antennas of this access point were small, so the distance of 20 cm was well in the far-field zone of each one (see Annex B). The maximum RF exposure level obtained during this time frame was 7.73% of the SC6 limits.

4.0 Discussion

4.1 Application of Equipment-Related Uncertainty to Spatial- and Time-Averaged Measurements

Industry Canada's GL-01 states that in order to verify compliance with the SC6 limits, measurement equipment uncertainty must be considered. Table 6 represents the spatial- and time-averaged RF exposure levels at location P5 in the configuration of test case 3b, with all Wi-Fi devices active, along with the minimum and maximum expected values,18 based on a confidence level of 95%. (See Annex C for the expanded uncertainty of the Narda SRM 3006 and its corresponding antennas, which constitute the measurement system.)

Table 6 – Percentage of SC6 limits (uncontrolled environment) including the measurement equipment uncertainty for the spatial- and time-averaged RF exposure value obtained at location P5
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 measurement uncertainty (Exp. Uncert.) of the equipment.

4.2 Impact of Wi-Fi Protocol on RF Exposure

Table 7 summarizes the contribution of the 2400-2483.5 MHz band when Wi-Fi-enabled devices operate in different configurations of uploading or downloading, or both. The highest average RF exposure level obtained from the four configurations was 0.007% of the SC6 limits, for the test case with a single laptop in downloading mode.

Table 7 – Percentage of SC6 limits (uncontrolled environment) for the 2400-2483.5 MHz band between locations P7 and P8 with the tri-axis antenna at a height of 1.25 metres for different configurations
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 See Figure 4 for measurement location.

b All configurations were statistically different (at 95% confidence level), except for the configuration with eight laptops uploading versus the configuration with one laptop downloading.

When numerous devices are uploading and/or downloading at the same time, a collision avoidance (CA) mechanism is used to improve the performance of carrier sense multiple access (CSMA). To avoid a collision when a packet is sent, the node must first "listen" to confirm that a channel is clear. When the full bandwidth (e.g. 20 MHz) of a node is devoted to only one client device, such as a laptop, the node can transmit more often without deferring the transmission. However, when the bandwidth is divided into numerous channels of similar size because the node is connected to many client devices, the node must listen more often before a packet can be transmitted. When a channel is sensed as busy, transmission is deferred and the RF exposure level decreases. A configuration of one laptop uploading produces a lower level of RF exposure than a configuration of one laptop downloading, because the power of the Wi-Fi module in a Wi-Fi-enabled device is typically lower than the power of the Wi-Fi access points.

5.0 Conclusion

In this study, the measured results were expressed as percentages of the SC6 limits (see Section 3.0).

When the Wi-Fi access points and Wi-Fi-enabled devices were operating, the RF exposure levels were higher when the tri-axis antenna was nearer to the Wi-Fi access points (locations P5 and P8). For location P8, which was near AP1, the highest average RF exposure level obtained was 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 5150-5350 MHz band, with a value of 0.229%. For location P5, which was near AP2, the highest average RF exposure level obtained was 0.234% of the SC6 limits, for the same measurement time of approximately 1 minute (see test case 2b in Section 3.2). The largest contribution came from AP2 in the 2400-2483.5 MHz band, with a value of 0.221%.

In compliance with the procedures set forth in Industry Canada's GL-01 and TN-329, spatial- and time-averaging measurements were performed at location P5 near AP2, the location with the highest measured field strength levels (see test case 3b in Section 3.2). The resulting value was 0.1% of the SC6 limits. With the measurement equipment uncertainty added, the measured RF exposure level was 0.19% of the SC6 limits (515 times below the limit).

With the tri-axis antenna positioned at a distance of 20 cm from the Wi-Fi access points,19 the maximum RF exposure levels obtained 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 the access point and surrounded by other users, the RF exposure level is thousands of 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-enabled devices in this indoor location was well below the SC6 limits. In addition, the Wi-Fi access points selected for this study were operating at higher power compared with the majority of the Wi-Fi devices currently available on the Canadian market. Therefore, the results of this study are likely higher than typical equivalent setups in public and private environments, such as homes, schools and businesses.

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