For this purpose, the parameter Timing Advance (TA) was introduced. The mobile stations receive from base station on the SACCH downlink the TA value it must use; it reports the actually used value on the SACCH uplink. There are 64 steps for the timing which is coded 0 to 63 (TAmax). One step corresponds to one bit period. Step 0 means no timing advance and the frames are transmitted with a time shift of 3 slots or 468.75 bit durations with regard to the downlink. At other steps, the timing advance is given by 468.75-TA bit periods, and for maximum value 63, that goes to 405.75 bit period duration. The required adjustment always corresponds to twice the propagation time or is equal to the round-trip delay (Fig. 2). In this way, the available range of values allows compensation over a maximum propagation time of 31.5 bit periods.
s = c x f = 3x108 x 31.5 x 3.69 x 10-6 = 34870.5[m]
This corresponds to maximum distance between mobile and base station of about 35 km. A GSM cell may therefore have a maximum diameter of 70 km. The distance from the base station of the currently valid TA value is an important handover criterion for a mobile station in GSM networks.
According to GSM technical specification 63 bit periods is the maximum value for TA, whitch corresponds to 35 km. In a standard GSM cell, the mobile station is not allowed to transmit if it is more than 35 km away from the base station, since the burst would be received in the following time slot which may be assigned to another connection. In Ericsson equipment that limitation has been defined with two parametars TALIM and MAXTA. The parameter TALIM is used to specify the TA limit for urgency handover and can be 0-62 (default value is 62). When a mobile station reaches that value, urgency handover procedure is trrigered. If there is no neighbour cell that can take over that call, TA value goes to 63, equal to MAXTA value and the call is disconected.
Extending the receiving window in base station to two time slots instead of one for each connection and allowing the base station to receive the bursts in both time slots, the mobile station can transmit at a distance larger than 35 km (TA > 63 bit period) fiom the cell. Refemng to Fig3., two time slots assigned for each connection allows the base station to receive the bursts in both time slots, so it is possible to serve mobile stations with a TA larger than 63. The allowed value range for TALJM and MAXTA for an extended range cell is 0 - 2 19 (bit periods), MAXTA>TALIM. Round-trip propagation delay for Extended Range equals to 219 corresponds to a distance between the site and the mobile of 121 kilometres. Since two time slots will be assigned to each mobile station, there is a trade off between capacity and coverage range.