Timing advance

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Timing Advance ( TA ) is a value in the GSM - and LTE - mobile radio standard for the synchronization between uplink and downlink is used. The value indicates the time offset by which a mobile station must send earlier so that, taking into account the signal propagation time between mobile station and base station, the signal arrives at the base station in the correct time slot and does not overlap with other signals.

Timing advance in the GSM cellular network

Timing Advance - Shifting the transmission time at the mobile station by twice the signal propagation time for correct reception at the base station

The GSM standard uses both frequency division multiplexing ( FDMA) and time division multiplexing (TDMA). Each frequency channel is also divided into eight time slots so that several users can be served with one frequency. A user is allocated a specific time slot in the TDMA frame so that with the repetition of the TDMA frame every 4.616 ms, a time slot of 577 μs duration is available to send information. The base station sends in a time slot and expects the response from the mobile station exactly three time slots later.

Since the mobile stations or users can now be at different distances from the base station (in GSM up to 35 km), the different signal propagation times between the base and mobile station and back must be taken into account in order to ensure reception at the base station in the correct time slot. This means that the mobile station has to send the signal transit time (there and back) twice earlier between the base and mobile station in order to compensate for the distance to the base station (see also the schematic diagram on the right).

The quantized value Timing Advance (TA) is used to inform the mobile station how much earlier it has to transmit. TA can have values ​​between 0 and 63 and specifies the offset by which the mobile station must send earlier in multiples of the bit duration of 3.7 μs (156.25 bits in a time slot with a length of 577 μs). The possible TA values ​​from 0 to 63 thus correspond to a time offset of 0 to a maximum of 233 μs. This maximum value also results in the maximum range in GSM networks, as a maximum of 35 km can be compensated with the highest TA value of 63. A TA value of 0 corresponds to a distance between the mobile station between 0 and 550 m, a TA value of 1 to a distance between 550 m and 1,100 m, etc. The distance of 550 m corresponds to a signal transit time of 3.7 μs (Hin - and return, 1100 m) when the signal propagates at the speed of light .

However, it should often be noted that in GSM networks, especially in cities, there is no line of sight between the mobile station and the base station. Therefore, higher TA values ​​can also be measured, since signal detours resulting from reflection and diffraction have a longer transit time than the direct distance to the base station would suggest.

The value of the timing advance is determined by the base station and communicated to the mobile station.

In the GSM 400 band, the maximum value of the timing advance is increased from 63 to 219, which corresponds to a maximum signal delay of 809 μs, and thus theoretically a range of 121 km.

Extended range for GSM

The suppliers of GSM mobile radio infrastructure offer the option of achieving TA values ​​up to 219 in the GSM900 mobile radio network under the term "Extended Range" (Ericsson) or "Extended Range Cell (ERC)" (Motorola). For this purpose, two time slots are used simultaneously for a telephone call. TA values ​​of 219 allow ranges of up to around 120 kilometers under optimal radio conditions, if there is a line of sight between the transmitting and receiving antenna (s) and the first Fresnel zone is free of any obstacles. The use of TA values> 64 only makes sense if the cell phone antenna is on a mountain or hill that towers above the plain or water surface by at least 100 meters. In general, the maximum range of a radio cell is limited by the curvature of the earth . The formula for geodetic visibility can be used to calculate the maximum range of a radio cell, which is limited by the curvature of the earth.

Timing advance in the LTE cellular network

Timing Advance limits the range of an LTE cellular network to 100 kilometers.

Extended range for EAN

For the implementation of EAN , the range limitation had to be increased to over 150 kilometers through timing advance.

See also

literature

  • Michel Mouly, Marie-Bernadette Pautet: The GSM System for Mobile Communications. Cell & Sys, 1992, ISBN 2-9507190-0-7 .

Individual evidence

  1. ↑ 3GPP TS 45.010: Radio subsystem synchronization. (ZIP / DOC; 209 KB) 3GPP, December 18, 2009, p. 11 , accessed on January 12, 2010 (English): “The maximum timing advance value TAmax shall be 63 ... In the case of GSM 400 the extended timing advance information element is supported and the maximum timing advance value TAmax shall be 219. "
  2. ↑ https://www.huawei.com/mediafiles/CORPORATE/PDF/Magazine/communicate/45/HW_082723.pdf Huawei - Vodafone Iceland - For the sake of Fire and Ice
  3. ↑ https://howltestuffworks.blogspot.com/2014/07/timing-advance-and-time-alignment-timer.html How LTE Stuff Works? - Timing Advance and Time Alignment Timer

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