Joseph, Wout

Martens, Luc

Verloock, Leen

Vermeeren, Günter

English

Ghent University Academic Bibliography

Measurement of the duty cycle of WLAN in different environments Günter Vermeeren*, Leen Verloock, Wout Joseph, and Luc Martens Department of Information Technology, Ghent University / IBBT, Ghent, Belgium *Corresponding author e-mail: Gunter.Vermeeren@intec.UGent.be INTRODUCTION Nowadays, Wireless Local Area Networks (WLANs) are commonly deployed in office buildings and at home. As a consequence, many people are exposed to the electromagnetic fields irradiated by these networks during long periods of time. Exposure assessment of WLAN using Wi-Fi is only rarely investigated. In a WLAN data packets are transmitted in bursts. According to international safety guidelines, the exposure is to be averaged over 6 min or 30 min time period. Hence, the correct assessment of the exposure requires the knowledge of the duty-cycle. The duty cycle can be measured using a spectrum analyzer (SA) in zero span. In this study, the duty cycle is measured in an office building and at a total of 151 locations in Belgium and the Netherlands. MATERIALS AND METHODS In an in-situ assessment of the exposure to radio-frequency electromagnetic fields, the SA often measures in max-hold mode until the signal stabilizes. In this way the maximum field level during the measurement time is determined. In a WLAN data is transmitted in bursts (packets, frames). Thus, the maximum field level measured with a SA in maximum-hold mode overestimates largely the time averaged field level. Therefore, the duty cycle must be taken into account. The following method to assess correctly Wi-Fi exposure is used [1]: first, the active WLAN channels are determined with a WLAN-packet analyzer. Secondly, a maximum-hold measurement of the electric field of the different channels is performed using a SA and a tri-axial measurement probe. Thirdly, the duty cycle of the active channels is determined. The duty cycle is also measured using the SA, but now the SA is set in zero-span mode. The different settings of the SA to correctly measure the duty cycle according to [1] are listed in Table 1. Finally, the total averaged field is determined from the duty cycle and the maximum-hold field strength as follows:  ܧtotavg ൌ √ܦܧtotmax‐hold (1) with D the measured duty cycle. Table 1: Optimal SA settings for measuring correctly the duty cycle in a WLAN. Parameter Value Span 0 MHz Center frequency Channel frequency  Detector RMS  SWT (sweep time) 1 ms RBW (resolution bandwidth) 1 MHz VBW (video bandwidth) 10 MHz Number of sweeps 2200  RESULTS  Figure 1 shows the cumulative distribution function (cdf) of the duty cycle measured at the UGent-INTEC / IBBT offices in Ghent, Belgium and the overall duty cycle measured during a large measurement campaign performed in Belgium and the Netherlands. Figure 1 shows the cumulative distribution function (cdf) of the duty-cycle. At the UGent-INTEC / IBBT offices, measurements were executed at 33 different locations spread over the whole building. The median or 50th percentile (p50) equals 2.5 % and the 95th percentile is 2.7 %. In the measurement campaign performed in Belgium and the Netherlands, the duty cycle was measured at 151 different locations. The median or 50th percentile (p50) equals 1.4 % and the 95th percentile is 11.1 %. These duty-cycles might be worst-case estimates.  Figure 1: Cumulative distribution function (cdf) of the duty-cycle D measured at the UGent-INTEC / IBBT office building and in 151 locations spread over Belgium and the Netherlands. CONCLUSIONS The duty cycle has been measured in different environments using a spectrum analyzer in zero-span mode. In an office building the 50th percentile of the duty cycle in a WLAN equals 2.5 %. In measurements performed in Belgium and the Netherlands the 50th percentile of the overall duty cycle equaled 1.4 %. REFERENCES [1] Verloock L, Joseph W, Vermeeren G, and Martens L. Procedure for assessment of general public exposure from WLAN in offices and in wireless sensor network testbed. Health Physics 98:628-638, 2010.  ACKNOWLEDGEMENT The authors wish to thank the GSMA and WiMAX forum for the financial support.  “The research leading to these results has received funding from the European Union's Seventh Framework Programme ([FP7/2007-2013]) under grant agreement n° 244149”  

Measurement of the duty cycle of WLAN in different environments

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Measurement of the duty cycle of WLAN in different environments

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