Timing Advance

Timing Advance (TA ) is a value that is used in the GSM standard for the synchronization between uplink and downlink. The value specifies ( in bit periods of about 3.7 microseconds ) the time offset to order must send to a mobile station earlier so taking into account the propagation delay between the mobile station and base station counts the signal in the correct time slot at the base station and not with other signals overlaps.

The GSM standard uses both a frequency division multiple access ( FDMA ) and time division multiplexing (TDMA). In this case, each frequency channel is additionally divided into eight time slots in order to operate at a frequency multiple users. Here a user a certain time slot is allocated in the TDMA frame so that it all has 4,616 ms with repetition of the TDMA frames a time slot of 577 microseconds duration available to send information. In this case, the base station transmits in a time slot and wait for a response from the mobile station exactly three time slots later.

Since the mobile stations or users may now located at different distances to the base station (in the GSM to 35 km), has the difference in signal propagation time between the base and mobile station, and returned into account in order to ensure reception at the base station in the correct time slot. That the mobile station has twice the signal propagation time ( there and back ) between base and mobile station send earlier to compensate for the distance to the base station ( see also schematic diagram right).

In order to tell the mobile station to how much earlier they must send the quantized value Timing Advance ( TA) will be used. TA can take values ​​between 0 and 63 and are in multiples of the bit period of 3.7 microseconds ( 156.25 bits in a time slot with 577 microseconds duration length) the offset in to the mobile station needs to send earlier. The possible TA values ​​from 0 to 63 so that a time lag of 0 to a maximum of 233 microseconds. This maximum value also results in the maximum range in GSM networks, since a maximum of 35 km could be compensated with the highest TA value of 63. A TA value of 0 corresponds to a distance between the mobile station 0-550 m, a TA value of 1 at a distance of between 550 m and 1100 m, etc. The distance of 550 m corresponds to a propagation delay of 3.7 microseconds (round - and back, 1100 m) in the signal propagation speed of light. Frequently, however, to note that in GSM networks, especially in cities, no line of sight between the mobile station and the base station is given. Therefore, higher TA values ​​measured values ​​can, as produced by reflection and diffraction signal detours have a higher duration than the direct distance could be expected to base station.

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 propagation delay of 809 microseconds, and thus theoretically a range of 121 km.

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