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Elektromagnetische Felder

GSM standard

  • Between 1992 and 1995, the first completely digital mobile telephone networks went into operation in Germany.
  • These networks, which are still used today, operate according to the GSM standard (Global System for Mobile Communications).
  • They employ radio frequencies in the range of 900 MHz and 1800 MHz.

Radio network cells Radio network cellsRadio network cells

In order to provide wide area coverage with mobile radio applications, the areas to be served are divided into radio network cells, each of which is covered by stationary radio transmitter stations (base stations or base transceiver stations). Usually three sectors, which each cover an angle of 120°, are each provided with one or several radio channels from one location. Three antennas, staggered against each other, are installed at one antenna location.

Base stations and radio network cells

The base stations communicate with the mobile terminals using high-frequency electromagnetic fields. The base stations are connected to a mobile switching centre via cable or radio link systems. The radiated powers of the stations typically range from 10 to 50 watts. They depend, among other things, on the size of the relevant cell whose typical radii can vary between a few tens of metres for so-called pico cells and 35 kilometres for macro cells in rural areas, and on the number of people making calls at the same time. Nowadays, well-developed mobile network coverage with small cell networks is provided especially in cities. New base stations are continued to be built to increase the capacity and to cover dead zones. However, new stations mostly use more advanced mobile communications technologies such as UMTS or LTE.

Distinguishing the cells

As several phone calls are usually conducted at the same time within the same cell, their radio signals have to be distinguishable. Initially they are distinguished by the frequencies of the radiation.

Every mobile network operator is assigned a frequency range by the Federal Network Agency (formerly: Regulatory Authority for Telecommunications and Posts RegTP) which is divided into several narrow frequency bands (frequency channels). Adjacent cells use different frequency bands to avoid interference between phone calls.

Time slot technique

In order to increase the number of possible simultaneous conversations in a cell, the so-called time slot technique is employed. A period of 4.62 milliseconds is divided into 8 time slots of 0.577 milliseconds each. A particular mobile phone uses only one of the 8 time slots for sending information to the base station during a phone call. In the remaining time slots, the base station can receive information from other mobile phones. For an individual mobile phone this results in a pulsed transmission signal with one pulse per 4.62 milliseconds, which equates to a pulse repetition frequency of 217 Hz. This is referred to as a low-frequency pulsed high-frequency signal. The pulsing of the base station signals depends on the number of simultaneous conversations in each frequency channel and can vary significantly.

State of 2018.01.17

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