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There are many ways to build a mobile communications network to obtain good coverage and capacity in an indoor environment. One possibility is a dedicated in-building network with antennas placed inside the building. Another alternative is to use pico cell base stations with integrated antennas. Still other solutions are to put outdoor antennas nearby or to extend an existing network using a repeater. Each alternative has different advantages regarding coverage, capacity and cost. Four different approaches are shown to the left.
Radio wave exposure from antennas
In-building systems with distributed antennas can be planned in various ways. The more antennas used the lower the output power level needed for good coverage and capacity. The antennas can transmit in one (directional), two (bi-directional) or all directions (omni-directional). The maximum input power for in-building antennas, and pico radio base stations with integrated antennas, is usually less than 30 dBm (1 W). Another antenna solution makes use of leaky cables. This solution is mainly used in basements and culverts.
The Effective Isotropic Radiated Power (EIRP) is typically between 0 dBm and 20 dBm (1 mW–100 mW) in a distributed in-building antenna system. The EIRP can be higher (up to one or a few watts) for solutions with few antennas. Usually the systems are balanced in such a way that all antenna output power levels are more or less the same.
For distributed antenna configurations with 1 W input power, the RF exposure compliance distance for the general public is less than 10 cm. For the typical output power levels mentioned above, the basic restrictions will not be exceeded even at the surface of the antenna cover. The compliance distances for occupational exposure are even shorter than those for the general public.
Due to the very short compliance distances, these antennas do not require any special RF exposure safety instructions. Installation and maintenance personnel can work close to antennas in operation without being exposed to levels exceeding the basic restrictions. Touching an antenna for short times will not lead to exposure levels that exceed the SAR limits. However, it is advisable that the antenna should be placed where it cannot be easily reached by the general public.
Taking an example from the real world, Ericsson has measured the output power levels in a modern office building in Gothenburg, Sweden, with a typical in-building network consisting of more than 100 distributed antennas of the three types mentioned above. In the GSM 1800 MHz frequency band the average output power level for the mobile phones was reduced considerably from the maximum to about 15 mW, and the base station antennas transmitted with an average EIRP level of 10 mW and a maximum level of 50 mW.