Topic

Determination of the public's exposure caused by WiMAX transmitters

Start

10.01.2007

End

09.04.2008

Project Management

IMST GmbH, Kamp-Lintfort

Background

WiMAX (Worldwide Interoperability for Microwave Access) is a new standard for regional radio networks, promising much better coverage and data rates (theoretically several kilometers and Mbit/s)than current WLAN technology. Especially in rural areas, where cable-bound DSL connections to the Internet are missing, WiMAX is seen as an alternative for network providers to bridge the so-called "last mile" to the customer.

Objective

The goal of the research project was to gather information on possible exposures of the general public due to WiMAX radio signals. Both contributions of base stations and terminals were to be investigated. For this purpose, measurement and calculation methods had to be developed or modified, respectively. Then maximum and average exposures in different realistic scenarios had to be determined. The results had to be evaluated according to relevant safety standards or guidelines before finally they had to be compared to data describing exposures induced by competitive radio services.

Results

In a first step the researchers compiled a summary of the WiMAX standardization according to IEEE 802.16. The emphasis in the survey was on information about parameters relevant for radiation protection, such as frequencies, transmitting power, modes of operation, modulations, channel access procedures etc. Furthermore the licensing procedure in Germany was described. Finally the results of a market survey are presented which include equipment already put into service or at least being already available at present.

Part I of the final report, which contains the relevant sections of the first interim report, is available as a PDF-file in German (524 KB)

The second part of the final report contains a presentation of procedures, which are suitable for assessing exposures of the general public to electromagnetic radiation emitted by WiMAX base stations and terminals respectively. Both measurements and computational methods are described and discussed in detail. Different aspects are covered: procedures that are appropriate to check whether exposure limits are met, and on the other side procedures that can be used to assess spatial and time averaged exposures. A comprehensive analysis of signal characteristics is used to select suitable aerials for measurements and proper settings of measurement equipment in general. WiMAX signals at frequencies of both 3.5 GHz and 5.5 GHz are investigated.

Part II of the final report is available as a PDF-file in German (4.2 MB)

The third part of the final report contains results of measurements conducted in the vicinity of WiMAX base stations and terminal equipment. The results are compared with theoretically and practically determined data of competing radio services (WLAN and UMTS) and evaluated with regard to relevant exposure standards. Furthermore, this part of the report contains an analysis of computational exposure estimations carried out employing different software packages.

The range of exposures over all scenarios being investigated, extrapolated to the maximal operational state of the base stations, was found to be on the order of 50 dB. This is comparable to findings in exposure surveys regarding competing radio services. The highest value was found to be 0.84 V/m electric field strength (corresponding to 0.003 % of the field strength limit in Germany) or 1.9 mW/m² power density, respectively. The smallest value was about 0.002 V/m (corresponding to 0.003 % of the field strength limit) or 0.01 µW/m². The median value of all measurements yielded about 0.028 %, the mean value 0.26 % of the field strength limit. The difference between median and mean value is mainly due to different maximum transmit powers of sites utilizing frequencies in the licensed 3.5 GHz frequency band and those utilizing the license-free 5.5 GHz band. The latter ones typically use lower transmit power. Time and space averaged power densities are a factor of about 6 to 11 smaller than the exposures given above. Long term measurements over at least 24 hours showed variations in exposure of about 6 dB.

Exposure due to terminal equipment

It was found that WiMAX indoor terminal equipment in typical user scenarios induces a significantly higher exposure than radio signals emitted by fixed WiMAX base stations. Measurements carried out showed a maximal field strength of 10.1 V/m (17 % of the recommended field strength reference level) in 50 cm distance, which declined rapidly with distance and reached maximal 1.2 V/m (2 % of the reference level) in 5 m distance. In contrast, exposure induced by terminal user equipment with the transmitting antenna being separated from the modem (“outdoor unit”) was found negligible, because the high directivity antenna of these devices is mounted outside the dwelling and is not directed to the user. Measurements of the SAR of an indoor unit transmitting at maximal operation conditions has shown a strong dependence of exposure on the orientation of the device and its distance to the user or phantom, respectively. Whereas at distances from the antenna in the centimeter range the SAR was found to reach only percentages of the basic restriction, exceeding of the basic restrictions is possible for prolonged contact to the antenna. The maximum SAR in situations like this was found to be 3.91 W/kg averaged over 10 g of tissue simulating fluid.

Part III of the final report is available as a PDF-file in German (8.7 MB)

Conclusions

Measurements and computations conducted within the scope of this project indicate that radio signals of WiMAX base stations contribute only to a low degree to the exposure of the general public in terms of the field strength limits. At least at present exposure is often lower than exposure from mobile phone base stations. However, as the WiMAX networks are not fully deployed yet, it is seen appropriate to monitor the situation in a medium term. As expected, exposure attributed to WiMAX base stations has been found much lower than exposure due to WiMAX terminal equipment. Regarding terminal equipment, outdoor units cause much lower exposure than indoor units. The latter ones may induce significant exposures, even exceeding the recommended basic restrictions, if disadvantageous conditions are taken into account. Not least due to higher transmit power exposure induced by WiMAX terminal equipment is expected to be higher compared to common WLAN routers. Therefore, as with other radio signals emitting devices, users should pay attention to the minimum operation distances recommended in the manual, for compliance with the recommended basic restrictions. WiMAX modems may feature an additional WLAN radio interface to connect to personal or laptop computers, inducing additional exposure. Precaution measures recommended by the BfS with regard to installation and positioning of WLAN routers (installation not close to places, where people may stay for extended periods of time) can be extended to WiMAX terminals. With respect to potential exposures, outdoor units should be preferred. For distributing signals indoor, devices without WLAN interface but with cable connections should be preferred.